Climate change is a temporal change in the climatic conditions of a specific location with time. For centuries, the climate of a certain location would either remain stable or change predominantly from natural phenomena [1]. However, due to the changing pharmacodynamic and pharmacokinetic aspects of drugs in the ambient ecosystem, the climatic conditions of certain locations are said to be changing phenomenally and catastrophically over short periods. Climate change modulates temperature, humidity, rainfall, atmospheric and aquatic pressure, and even surface tension. Such spatial-temporal modelling is dynamic, and each factor would react differently. The overall volume of gas acts on the temperature of the air, while the increase in the volume of some specific gases would lead to variations in osmotic pressure and ionic conduction. In extremophilic environments, the climatic conditions may be drastically changed, or the addition of a new variable may instantaneously alter these conditions, for example, the stimulation of pressure homogeneity by 5 GPa at room temperature. Sometimes climate change is negated to occur from extraterrestrial phenomena or is an anthropogenic artifact caused by industrialization, massive deforestation, and introduction of artificial chemicals into the environment. Climate change induction may be disastrous to the ecosystem.
1. Introduction to Climate Change
Climate change was insignificantly prevalent globally until recent years due to its virtually negligible impact on global temperature, humidity, and pressure. During the 1970s decade, however, a significant escalation of fossil fuels, industrial waste, and toxic chemical fertilizers massively contaminated air, soil, and water. Many scientists began documenting the climate variations from the inappropriate interpretation of the experimental data collected in isolated regions, taking into consideration only one climatic variable, for example, the increase in the death of many economically important species (aquatic and terrestrial) due to rising temperature [2]. It was on 5 June 1988 that climate change was publicly articulated at a globally important unite venue; the United Nations General Assembly. According to the World Meteorological Organization, the United Nations Environment Programme, and S. Jay Olshansky: Global mean surface temperature has risen by 0.67 ± 0.17 °C (95% confidence limits) relative to the period 1880–1920; therefore, 0.4 °C of this rise was caused by anthropogenic greenhouse gases. The last two decades of the 20th century have been about 0.5 °C warmer than the previous two decades. The climate change experiments caused by fossil fuel combustion predict increases of 3.5 to 5.5 °C in annual mean global surface temperature between the second half of the 20th century and the next in relation to climatic variables.
1.1. Definition and Causes
In broad terms, climate can be defined as the average condition of the weather of a place. It encompasses information related to precipitation, temperature, wind speed, and humidity over a long period of time, typically years to decades. As a result, climate change is defined as a significant alteration in the condition of the climate of any area or region [1]. Based on the origin of causes, climate change can be broadly classified into two significant types: natural climate change and anthropogenic climate change. Natural climate change is caused due to changes in solar intensity, volcanic eruptions, changes in ocean currents, plate tectonics, and more. Anthropogenic climate change is the most concerning and refers to alterations in climatic conditions due to human activities or natural calamities caused by human beings. It includes urbanization, industrialization, deforestation, overpopulation, misuse of natural resources, pollution, etc. [3].
Human-induced climate change is also called global warming. It denotes a rise in the average temperature of the earth due to a rise in the greenhouse gases (GHG) concentration in the troposphere. The general energy balance of the earth is maintained by incoming energy from the sun and outgoing energy from the earth. The solar energy strikes the surface of the earth and is either absorbed or reflected back to the atmosphere. Most of the short-wavelength incoming energy from the sun is absorbed by the surface of the earth and is emitted back in the form of long-wave (infrared or thermal) radiation. Some of this outgoing heat leaks into space, while some are entrapped in the atmosphere by the GHG. The radiated energy absorbed by the GHG heats the lower atmosphere.
1.2. Historical Context
In order to better appreciate the accelerating impact of climate change on ecosystems, it is important to understand its historical context. Globally averaged temperature has changed considerably over the Earth’s history, from warm conditions during the mid-Cretaceous to cold conditions during the last glacial maximum (LGM) about 21 000 years ago [4]. These temperature changes were accompanied by changes in the concentration of carbon dioxide (CO2) in the atmosphere, which is the most important greenhouse gas, influencing climate change. Climate changes also occurred during periods of stable atmospheric CO2 concentrations, indicating that there are climate processes acting independently of CO2.
Paleoecological records indicate that terrestrial ecosystems changed substantially over the past 21 000 years in response to climate change. The last deglacial period witnessed large changes in the distributions of forests, grasslands, and deserts (i.e., broad vegetation types), indicating that the species composition of ecosystems was reorganized, but the general type of the ecosystem remained the same. In contrast, after the transition to the mid-Holocene, vegetation changes were much slower, indicating that ecosystem reorganization might take a considerable time long compared to species turnover. Over glacial/interglacial periods, vegetation contrasts were broadest in the tropics, where the largest climatic changes in terms of precipitation also occurred. In contrast, climatic changes were relatively modest in northern European and North American high latitudes and no vegetation difference occurred, indicating that some vegetation types (i.e., boreal forests) skeletons persisted irrespective of vegetation differences with respect to climate change [1].
2. Ecosystems and Biodiversity
Ecosystems are the basic functional units of the biosphere and comprise all interacting living organisms (biota) and their physical environment. Ecosystems can be as small as a droplet of water containing microorganisms or as large as the global assemblage of the atmosphere, oceans, rivers, and land containing all biota. Ecosystems can be described by their structure (habitat types and the biota they contain) and functioning (energy flow, nutrient cycling, and all other vital biogeochemical processes). Together, these factors define ecosystem services — all the functions of the natural world that benefit humankind [5]. Ecosystem services range from the obvious (e.g., food and fiber), through the less obvious (e.g., climate regulation), to the almost invisible (e.g., recycling of wastes). In total, it has been estimated that the economic value of ecosystem services is in the trillions of USD per year.
The living component of an ecosystem is its biota — the aggregate of all species of animals, plants, fungi, and microorganisms that inhabit a particular set of environmental conditions providing a distinct set of ecosystem services (the species pool). This assemblage of species is notably variable between different ecosystems and is called biodiversity [6]. In addition to the biota themselves, biodiversity includes the processes of biogeography (the processes responsible for differences in species richness between ecosystems), speciation (the processes by which species originate and become distinct from one another), and extinction (the complete removal of a species from the biota). Collectively, these components of the biota and the associated processes, biodiversity, and ecosystem services are inextricably linked. Biodiversity itself defines ecosystem habitat type and subsequent ecosystem services, whereas the physical environment constrains the conditions in which biodiversity can occur (in terms of species richness). In the absence of a biotic component, no ecosystem services could persist.
2.1. Importance of Ecosystems
The environment is essential for the existence of all living organisms. It comprises all biotic and abiotic components in the surroundings and their interactions with each other. Ecosystems make the planet’s environment habitable, cleansing air and water, regulating climate, flooding, disease, and the cycle of nutrients and raw materials. Ecosystems also provide essential services such as clean air, potable water, fertile soil, productive ocean, productive terrestrial environment, food, fiber, etc. Forests, rivers, lakes, deserts, wetlands, oceans, seas, etc. are common ecosystems that maintain ecological balance. They also contribute to the well-being of mankind and are vital for the achievement of continuous development. Nonetheless, ecosystems are threatened by increasing population pressure, deforestation, urbanization, industrialization, raw-material exploitation, fossil fuel combustion, culture expansion, mineral and oil exploration, intensive agriculture, non-judicious use of pesticides and fertilizers, pollution, nuclear waste treatment, rising temperature, erratic rainfall, drought, floods, etc. [7]. These activities have made the environment dirty, polluted, and toxic. Ecological crises like the ozone hole, global warming, plastic pollution, diminishing biodiversity, and the acidification of oceans have escalated recently. Despite these hazards, the question is: what is an ecosystem [8] ?
An ecosystem can be defined as a community of living organisms and their physical environment, functioning together as a unit. The structure of ecosystems is determined by the interactions of organisms with the biotic and abiotic components of the environment. Ecosystems have various structural and functional attributes; a few common structural attributes are species composition, productivity, and diversity. Diversity is defined as the variety of forms and ecological functions of species in a given ecosystem. Diversity is a key indicator of ecosystem health and an important factor in the sustainability of ecosystem productivity. Diversity can be categorized into ecological diversity, taxonomic diversity, and functional diversity. Ecosystems are composed of diverse organisms with varying ecological roles. Organisms are divided into trophic levels in the food web.
2.2. Biodiversity Hotspots
Biodiversity hotspots are areas that contain high levels of species richness, endemism, and ecological diversity, and that are particularly vulnerable to climate change [9]. Identifying the world’s biodiversity hotspots has attracted the interest of researchers, conservationists, and the public worldwide [10]. In principle, an area is a biodiversity hotspot if it meets two criteria: first, it must have at least 1,500 species of vascular plants as endemics; and second, it must have lost at least 70 percent of its original habitat. Scientists have identified 34 hotspots worldwide, covering only 2.27 percent of the earth’s land surface, but they support more than half of the world’s plant species as endemics. Hotspots of biological diversity are under great threat from anthropogenic activities, such as habitat destruction, over-exploitation of biota, introduction of invasive species, and climate change. These threats stem from socio-economic motives, particularly global poverty and inequity, but also from ignorance or lack of appreciation of biodiversity values. Such threats to biodiversity may become worse under climate change, but the precise consequences remain difficult to predict.
Hotspots of higher taxa containing high numbers of species are areas of particular interest for both understanding biodiversity and informing conservation priorities. It could be argued that such a center of endemism is a biodiversity hotspot. By higher taxa, biogeographers understand taxonomic groups larger than families, especially orders and classes, with the implication that such taxa will be either phylogenetically ancient or globally widespread. A hotspot of higher taxa may therefore suggest centers of endemism or relictual elements and regions with greater biodiversity.
3. Climate Change Indicators
Climate change indicators are physical and biological phenomena that demonstrate changes in the climate system. Numerous indicators can be used to observe climate change, including rising air temperatures, melting of snow and ice, sea-level rise, and changes in precipitation and ocean acidification. In particular, temperature rise and sea level rise are two of the most prominent indicators of climate change.
Temperature rise is one of the most obvious indicators of a changed climate, since changes can be both scientifically measured as well as personally perceived. Global surface air temperature has increased significantly since the beginning of the Industrial Revolution, at an average rate of 0.75 °C over the 20th century. Surface air temperature can be measured directly since thermometers were first systematically used in the mid-19th century. The earth’s heat balance is described in terms of radiative forcing. This is an estimate of how the energy balance of the Earth-atmosphere system is influenced by changes; this is commonly expressed in watts per square meter (W m-2). Estimated radiative forcing values are tabulated or calculated, assimilated, described, and interpreted.
Climate change, as measured by global surface air temperature, is being linked to anthropogenic influences: that is, effects, results, or consequences that can be identified as originating or produced by humans, be they deliberate or inadvertent. During the 20th century, sea level rose by an estimate of 0.14 m; however, since 1993, during the satellite altimetry period, the rate accounting for water was 0.31 m. Sea level rise would occur and at a structurally different rate for different occurrences of IPCC 4 AR scenarios with specific degrees of temperature increase; a low and an upper projection was computed for each scenario.
All climate change indicators evoke relationships with one or more drivers, which play either a positive or negative role. Impacts can be grouped based on their character (environmental versus socio-economic) and their sector (agriculture, forestry, biodiversity and ecosystems, coastal areas, health, water resources, energy, industry, etc.). Their degree of incidence (direct versus indirect impacts) and their time horizon (medium, long term) can also be considered for classification purposes.
Degrees of vulnerability to climate change impacts generally concur with degrees of income and development (high vulnerability – low income/development). A few countries and regions, especially island countries, least developed countries and African regions, are particularly singled out for their high degree of vulnerability.
3.1. Temperature Rise
Climate change represents an unparalleled assault on the modern world, affecting nearly all biomes in predictable yet variable ways [4]. In recent decades, these impacts have accelerated, resulting in a window for action that is rapidly closing. Although imaginative or extravagant and exaggerated claims regarding climate change are not valuable, a sober understanding of current and projected impacts on ecosystems is necessary. Climate change derives from the accumulation of greenhouse gases, primarily carbon dioxide and methane, in the atmosphere due to fossil fuel burning and land-use change. This atmospheric concentration of greenhouse gases, especially carbon dioxide, has now surpassed 400 ppm, the highest value in nearly 4 million years. Temperature is the most critical indicator of climate change and is more readily mapped across the globe than precipitation. Over the last 130 years, Earth’s average temperature has increased 0.8 °C, with near surface air temperatures increasing nearly 1.0 °C, the warmest values in the last 2000 years. The fifth assessment report by the Intergovernmental Panel on Climate Change (IPCC) has multiple projections of future climate change, the most widely embraced of which project a doubling of atmospheric carbon dioxide and an average global temperature rise of 3 °C, but with temperatures increasing by 2–6 °C in lower latitude locations critical to agriculture and water supply. The amount of future greenhouse gas emissions, which is both controversial and subject to ethical as well as economic forces, determines which of these scenarios is most likely to occur. For higher radiative forcing, large temperature changes are projected for all regions of the globe week to decade. For mid-range radiative forcing, average temperature rises by many biomes will substantially exceed levels of temperature change that have existed over the last several hundred thousand years and to which the fauna and flora are many adapting. Further projections show all regions of the globe increasement temperature seasonality. Warming is projected to occur more during nighttime than daytime. Extreme temperature events are projected to become more common and severe across all regions of the globe. Dramatic warming has already occurred in arctic regions and further warming is expected to 6 °C outside the tropics and 12 °C within the tropics over time. Further temperature change also amplifies the risk of unprecedented drought.
3.2. Sea Level Rise
Sea level rise is one of the most well-known, and best understood indicators of climate change. It is predicted to have a variety of impacts on coastal ecosystems over the next century, and many have already been observed. Along with changing temperature and precipitation patterns, altered sea state due to increased storm intensity and heightened water levels are two of the most likely consequences of global climatic alterations [11]. In turn, these changed hydrometeorological conditions could affect coastal habitats. Coastal systems are transition zones that support a diverse variety of organisms and ecosystems, many of which are of economic importance; thus, understanding possible consequences of future climate change is of both ecological and societal concern. In addition to increased temperature and changing precipitation patterns (described above), altered sea state due to increased storm intensity and heightened water levels are two of the most likely indicators of climate change to affect coastal ecosystems.
As one of the earth’s major biomes, coastal systems provide numerous important services to humans [12]. They constitute transition zones between land and ocean, furthering economic and social interaction between the two ecosystems. A variety of organisms use these systems as habitat, nursery grounds, grazing or spawning grounds, as well as an area to find shelter from open waters and freshwater landscapes. Furthermore, coastal ecosystems provide a buffer between organisms dependent on either the freshwater or marine environments and prevent salinity fluctuations from adversely affecting inland areas. There are several important coastal ecosystems, such as salt marshes, mangrove forests and seagrasses, that mediate the transition between the ocean and land. Despite covering but a small fraction of the world’s oceans, they are among the most highly productive habitats on earth and support more than one-quarter of marine biodiversity. Understanding the response of coastal ecosystems to full salinity changes may provide important insights into ecosystem resilience and possible changes to community structure, biodiversity and productivity at a very large scale.
4. Direct Impacts on Ecosystems
Climate change is having a measurable impact on ecosystems. Many species are moving toward the poles and toward higher elevations as they seek cooler climate conditions. This shifting is changing biotic interactions and the functioning of impacted ecosystems.
According to the Intergovernmental Panel on Climate Change (IPCC), 20-30% of species will be at an increased risk of extinction if global average temperatures rise more than 1.5-2.5 degrees Celsius. Vulnerable species are trapped in small ranges at tropical or polar extremes. Movement toward cooler conditions may not be an option for species that are sensitive to changes in temperature in their habitats, such as the amphibians that breed in isolated alpine ponds. Ecosystem changes will reverberate through food webs, drastically changing interactions among species. Impacts on one species may lead to additional impacts on others, sometimes with unexpected consequences. For example, loss of a predator may release its prey species from control, allowing it to grow rapidly. Loss of prey species may threaten the survival of its predators, driving them to extinction.
Species changes in distribution are imperiling essential ecosystem functions as species interact to perform important roles such as pollination, seed dispersal, and exotic pest and pathogen control. Ecosystem functions of special interest are those necessary to maintain human welfare, such as food and water supply and pest control.
Most walks in nature imply a close relationship with discoverable fauna and flora. It is easy to notice how everyday life is affected by a variety of animal and plant species. Species interaction drives important ecosystem functions, including production of food for sustenance, regulation of water quality and availability, degradation of toxicants, and control of pests and pathogens. Different ecosystems support different interactions among species. For example, interactions in forests, where many species share light and nutrients, will differ fundamentally from those in coral reefs, where a few species compete to dominate. Changes in environmental conditions will impact species interactions differently across ecosystems.
Communities of species impact ecosystem functions and are, in turn, impacted by these functions. For example, the ability of communities to trap sediment may depend on the abundance and composition of sediment-trapping biota. This function will impact the sediment level, which will regulate what species can settle in a given location, with subsequent impacts on the function itself.
4.1. Changes in Species Distribution
Species distribution varies naturally worldwide based on environmental gradients such as climate or topography. However, after centuries of relative stability, the climate is changing and doing so with unprecedented rapidity. Changes in species distribution are one of the most intuitive responses to climate change. Such changes can include a simple poleward or elevational shift in geographical ranges, but can also involve a complex reorganization of species in response to alterations in climate, community interactions, or land-use. Moreover, these species-distribution change processes can occur within and across ecosystems, and between terrestrial, freshwater, and marine realms [13]. Changes in species distribution have repercussions for ecosystem dynamics (e.g., temporal and spatial variability in community functioning), regulation of ecosystem-interaction (e.g., perturbation propagation), maintenance of ecosystem biodiversity, and the provisioning of ecosystem services such as food and water [14]. Understanding the mechanisms responsible for observed changes in species distribution is crucial for predicting and managing subsequent ecosystem changes.
4.2. Altered Ecosystem Functions
Climate change is not only intensifying the rates but also altering the nature of many climate inputs, with consequences for ecosystem functions. The altered ecosystem functions, in turn, have consequences for the services they provide to humans and for the stability of ecosystems [15]. Ecosystems provide many services to humans for free, such as water purification, pollination of crops, maintaining biological diversity, carbon sequestration, fertility provision, biomass production, and temperature and climate moderation. These ‘ecosystem services’ or ‘ecosystem functions’ are maintained by the biophysical and biogeochemical processes that ecosystems carry out. The current models predict that the trend in global mean surface temperature rise will accelerate in the 21st century beyond the rates (0.2 °C/decade) noted in recent decades. Given the inherent time lags, warming (and sea level rise) will continue for several decades after global mean surface temperature stabilizes at a new target (and this may never be achieved). The rate of change in one variable (climate input) can exceed limits that the current ecosystem structure and/or functioning cannot follow (i.e., ‘rates of change’ exceed resilience of the ecosystems). Large disruptions in ecosystem structures and functions of salt marshes, seagrasses, mangroves, reefs, forests, and polar ecosystem types have been suggested as a result of CC, with enormous consequences for the services they provide to humanity.
5. Indirect Impacts on Ecosystems
Though not always readily acknowledged, climate change has already affected ecosystems around the world, and a multitude of impacts at different levels of biological organization have been detected. As atmospheric CO2 concentrations rise, air and sea surface temperatures increase, and excess CO2 dissolves in the oceans to make them more acidic, a number of direct effects on organisms, communities, and ecosystems can be expected. Ocean acidification is already affecting marine organisms that build shells, skeletons, and tests from calcium carbonate, and rising seawater temperatures are leading to mass coral reef bleaching and mortality. A growing number of studies are beginning to describe changes in community composition in marine and freshwater systems [16]. Many coral reefs are already experiencing declines in coral cover and shifts toward macroalgal-dominated communities, and tropical marine communities are predicted to change substantially as increases in temperature lead to loss of species at the upper thermal limits for their occurrence.
In addition to these direct impacts of rising atmospheric CO2 concentrations, temperature increases, and ocean acidification, other “co-benefits” of the burning of fossil fuels (increases in nitrogen deposition) and other global environmental changes (increased ultraviolet radiation) are likely to affect ecosystems directly. In both the terrestrial and oceanic environments, extreme climate “events”—high extremes, low extremes, or combinations thereof—are predicted to increase with warming and may have strong, and poorly understood, impacts on organisms, communities, and ecosystems [17]. Moreover, current climate changes will likely create numerous conditions that are novel to the organisms and communities experiencing them. Indirect impacts caused by changes in physical drivers of ecosystems, atmospheric or oceanic currents, coastal upwelling and succession, seasonal timing, and so forth may be more complicated than direct impacts and may be more challenging to discern.
5.1. Ocean Acidification
Ocean acidification occurs when carbon dioxide (CO2) leaves the atmosphere, dissolves into seawater, and shifts ocean carbonate chemistry [18]. Excess atmospheric CO2 dissolves in seawater, forming carbonic acid that slowly lowers the ocean’s pH, the measure of the ocean’s acidity. The average global surface pH has decreased from approximately 8.2 to 8.1 since the beginning of the industrial revolution. In addition to pH, ocean acidification reduces the saturation state of the carbonate minerals, calcite and aragonite. Ambient marine waters are typically supersaturated with respect to calcite and aragonite. However, the saturation states of these carbonate minerals are reduced in regions of high CO2, where seawater has naturally low pH and low carbonate saturation. Since pre-industrial times, the calcium carbonate saturation horizon has shoaled by 1000m in the Pacific and Atlantic Oceans, and is projected to shoal much further with continued ocean acidification. Consequently, vulnerability to ocean acidification spans from the dissolution depths of calcium carbonate biominerals and sediments to the shallower buoyancy depths where organisms can no longer maintain their upward buoyant lifestyle against sinking.
Ocean acidification is projected to have devastating effects on marine ecosystems, especially in regions of high CO2 and low calcium carbonate saturation, impacting the mid-water and deep sea, as well as temperate coral reefs and habitats and organisms that actively precipitate aragonite or calcite bio-minerals. Small shelled organisms, such as the diminutive, non-symbiotic planktonic foraminifera, are at risk to the transition from supersaturation to under-saturation. Atmospheric CO2 is projected to exceed 1000 ppm and ocean carbonate saturation horizons will shoal further, with severe impacts to marine habitats and organisms biogenic carbonates, and calcifiers. The whole ocean’s buffering capacity, the ocean’s ability to absorb CO2 while resisting change in acidity, has been altered due to anthropogenic CO2-induced changes in ocean carbonate chemistry. Oceanic reservoirs of nutrients and nitrogen are already at near anoxic, with predicted changes in oxygen advection rates; coastal dead zones are at risk to extending into much wider regions of the open ocean. Ecological impacts of elevated CO2 and concurrent ocean warming are observed for marine systems; prediction of future biological and ecological changes in the oceans is fraught with uncertainty. Addressing ocean acidification with rapid and dramatic reduction of CO2 emissions is paramount for both the stabilization of climate and preservation of the balance of marine ecosystems [19].
5.2. Extreme Weather Events
Extreme weather is one of the climate change impacts on ecosystems and species. Droughts, heatwaves, floods, and storms can all cause temporary acute changes in the functioning of ecosystems and species. Frequently, these extreme events may be large and rare, or they may be small and frequent [17]. Ecosystems and species may be unexpectedly disturbed or newly created, which stresses the need for a better understanding of the consequences and action towards extreme events under projected climate change scenarios. Extreme weather events are variabilities relative to the average conditions in the climate system. Climate variability can be considered over fixed time intervals (inter-annual variability) or compared with longer time scales (decadal and centennial). Climate change is the average global climate state and its long-term modification (length of 30 years at least) on time scales longer than those of natural climate variability. Each of the extreme climate events (droughts, heatwaves, floods, and tropical cyclones) has a clear link with climate change.
Most previous studies focused on the long-term averages of climate parameters, hence there is a lack of data and understanding on climate extremes and their impact on ecosystems. Therefore, climate extremes are often not considered when developing adaptive measures for ecosystems. There is knowledge on the past and ongoing patterns of climate change, but there is little understanding on the extremes and the direct or indirect consequences on ecosystems. Extreme impacts on ecosystems may include very disruptive consequences causing unexpected changes in ecosystem functioning and species composition. Understanding of the relationship between weather and climate is important in assessing the danger of extremes and climate change. There are different indices to describe, measure, and quantify the changes in ecosystems over time scales, which may include years, decades, and hundreds of years.
6. Accelerating Factors
Climate change does not occur in isolation. Human activities and a variety of natural phenomena can positively or negatively stimulate climate change, thereby accelerating or decelerating its impacts on the Earth’s ecosystems. Understanding factors and mechanisms that control the stability of the Earth’s ecosystems is of utmost importance for anticipating vulnerabilities, as it indicates when and how the ecosystems can no longer tolerate the stress [20].
Prominent examples of accelerating (or amplifying) factors are shifts in temperature-nutrient and temperature-biodiversity relationships, a range of positive climate-ecosystem feedback loops, and tipping points. Over the millennium before 1880 CE, global average temperature (global warming arguably slowed down the last century) fluctuated by about ±0.2 °C (with respect to the 1961–1990 mean) only. Beyond the natural variability due to volcanic, oceanic and solar influences, the stabilisation of global temperature is partly the result of the sizeable stabilising effects of biogeophysical (albedo) and biogeochemical (carbon cycle) feedbacks. However, even before anthropogenic warming began, long-term insolation-induced changes in the Earth’s planet-scale ice and vegetation cover (cryosphere-albedo and vegetation-albedo feedbacks) were found. Recent simulation experiments with a model of intermediate complexity show that, in addition to amplifying feedbacks, such “internal” non-linearities may have the potential to prevent (i.e., suppress) warm states that globally average, highly-vegetated or ice-free conditions disappear through threshold responses.
6.1. Positive Feedback Loops
Positive feedback loops can exaggerate disturbances to ecosystems and ultimately form tipping points beyond which a system cannot recover [20]. The greater warmth and moisture enhance the decomposition of organic matter and thus draw much greater amounts of CO2 from previously trapped organic material in northern soils back into the atmosphere. In turn, this heightens warming, drying, and moisture stress, leading to climate-driven tree die-offs and fires. The need to understand and — if possible — mitigate such positive feedback loops is critical if ecosystem systems such as the Amazon basin tropical rainforests are to be protected [4].
Feedbacks, particularly positive feedbacks, can lead to a swift and fundamental change in elementary ecosystem functions, key species, and other species reliant upon a given ecosystem — such as humans. The more critical processes are the positive feedback loops that run continuously 24 hours a day where a given process continually enhances the very condition which promotes that process — because of the aforementioned reasons beyond which disturbance relief may not be possible or impactful — and cause a cascade of compounding effects. Examples include cumulative effects which would heighten disturbance (such as the exacerbating dryness of soils owing to extended droughts followed by heightened temperatures that also accelerates moisture loss) and indirect exacerbating loops through (for instance) available moisture enhancing vegetation coverage and thus keeping any climate-driven moisture in the region, heightening disturbance vulnerability (for instance, due to increased fire incidence) if such available moisture is suddenly reduced (as would happen with rainforest destruction or with greater temperatures-induced high-altitude drying).
6.2. Tipping Points
Mankind’s actions have propelled the Earth, with its associated ecosystems, toward an uncertain future [21]. Ecosystems are nonlinear and dynamic systems that evolve in response to continuously changing environmental conditions, exemplified by climate. Evolutions of ecosystems are often driven by a dichotomy of predictable and unpredictable forces. Foremost among the predictable forces is climate, SSA which determines the thermal and moisture regimes. There are also two broad groups of unpredictable forces, or noise. One is “internal” noise arising mass from the continuously changing biophysical conditions such as population size, biomass accumulation, and the storm impacts, all of which are modulated by climate. The other is “external” noise, including diverse catastrophic events such as earthquakes, volcanic eruptions, and asteroid impacts.
A natural evolution of many dynamical systems is a gradual shift along a range of stable states or continuum of basin equilibrium. It is only upon arrival at certain thresholds, or tipping points, that the system undergoes rapid, potentially irreversible, changes, moving to qualitatively different states, such as from freezing to melting of a sea-ice surface, or from dense forest to woodland or savanna [22]. Global warming is expected to induce various types of rapid and large shifts in the Earth’s systems due to the nonlinearity and complexity of many components of the climate system and interplaying biophysical processes across multiple scales. There exists an irreversible temperature threshold for the Amazon rainforest system. Global mean surface temperature rise by 4 °C may induce rapid die-off of the African monsoon system. The growing compiled evidence for wide-ranging shifts in ecosystems, hydrological systems, and socio-economic systems along with ongoing global change points to the importance of proactively recognizing, and responding to, such critical thresholds, or tipping points, to avoid catastrophic impacts on earth systems.
7. Mitigation and Adaptation Strategies
Mitigation and adaptation strategies can be employed to address climate change impacts on ecosystems. These strategies can include both sustainable modifications in land use practices of human society and restoration of ecosystems already degraded by natural and anthropogenic pressures [23]. Land use changes in human society can be developed based on the example of ecosystems. In an area of interest, target ecosystem types, potential area in question, natural vegetation, growth habit classes, natural disturbances, natural soil and climate types, and areas of similar land use history can be identified. All human land use practices that affect ecosystem types can be evaluated. In this way, human practices can be evaluated according to their compatibility to principles and processes of target ecosystems. A narrower subset of compatible practices can be further examined in terms of their practicality and feasibility, natural site characteristics, human land use history, potential socio-economic and political consequences, and ecology of landscape. A large number of simple sustainable practices, compatible to principles of ecosystem development have been found and can be employed as strategies.
Restoration can encompass a variety of broader time frames, larger spatial scales, and a greater variety of actions to recreate desired ecosystem conditions (e.g., composition, structure, functioning) than rehabilitation and reconstruction. Restoration has goals based on the idea of self-sustainability over relatively long periods. These goals could include the following desired ecosystem characteristics: (i) little or no intervention/management (passive restoration), or (ii) limited intervention (e.g., targeted actions or techniques) but with a clear return to pre-disturbance ecosystem conditions (re-establishment of some desired conditions and functioning). Restoration can also involve the re-establishment of desired biodiversity groups (species, functional groups) that in turn allow the re-establishment of desired ecosystem conditions. A better understanding of ecosystems operating under natural conditions, the stressors/pressures they experience, and the thresholds for maintaining conditions conducive to a specific state or control of regime shifts would enhance resilience.
7.1. Sustainable Land Use Practices
Climate change has significant impacts on ecosystems, affecting their structure, composition, and functioning, with potential consequences for the provision of ecosystem services on which human well-being depends. Several studies have assessed the impacts of climate change on ecosystems at different scales using a range of modeling approaches. This paper concludes that land use is another key driver of change that must be explicitly considered [24].
Climate change is defined as a pattern of change over and above that of natural variability, which occurs in terms of mean trends, variability, persistence, and degree of change. As a consequence of human-induced climate change, global mean warming of around 2°C is likely to be reached by the second half of the 21st century, resulting in significant impacts on many aspects of the global system. Regional ecosystems and their services may be particularly vulnerable to climate-change impacts as a consequence of concomitant land-use change [5]. Reductions in land-use impacts, including the effective protection of large-scale, well-connected, and climate-robust landscape systems, are the most effective means of preserving ecological integrity in face of climate change. Adoption of sustainable land use practices considered beyond the purely environmental context is not only imperative for ecosystem conservation but also for community well-being in ecosystems with global significance.
7.2. Ecosystem Restoration
Ecosystem restoration is the repair of degraded ecosystems [25]. With biodiversity intimately linked to human survival on Earth, ecosystem restoration has the potential to improve society’s connection to nature, in turn repairing relationships that have become frayed over centuries of exploitation. Repairing this connection can foster a sense of environmental stewardship. Encouragement of stewardship behavior such as caring for animals, reading about natural history, or planting trees can help individuals feel less isolated, or lonely, which can improve mental health and wellbeing. Restoring ecosystem functions and processes is critical to repair links between human wellbeing, ecological health, and biodiversity. Public support and participation in restoration programs can create jobs and promote community wellbeing and cohesion, in additions to providing a conduit for social-good activities.
Native microbiota are increasingly being linked to human health. Importantly, there are microbiota-derived health benefits associated with restoring biodiverse environments. These benefits might result from restoring links, via the built environment, between humans and biodiversity. Contact with biodiverse environments can enhance opportunities for interactions with immunoregulatory microbiota and provisioning of biogenic compounds, further protecting health. For humans, becoming re-immersed in the natural world can improve resilience to infections, including viral infections via immune priming. Restored ecosystems, with a more complex ecological community than a degraded or monocultural system, can also facilitate interactions across trophic webs, where each node in the web can regulate the activity of the others in ways that enhance ecosystem health and function.
8. International Agreements and Policies
This section examines recent international agreements and policies that seek to address the impact of climate change on ecosystems, in part by fostering international cooperation in the four best practices mentioned previously. While international cooperation and international rules and regulations are key to fostering global ecosystem resilience, the development of related international policies is in its infancy. Nevertheless, two recent policies—the Paris Agreement and the Convention on Biological Diversity—have significant promise for mitigating climate change’s impact on ecosystems [26] ; [27].
The Paris Agreement is an international agreement among nearly all the world’s countries to voluntarily limit greenhouse gas concentrations in the atmosphere. The agreement was reached in Paris, France, at COP-21 in December 2015 and entered into force on November 4, 2016. The goal of the effort is to hold the increase of global temperature to below 2 ◦C above pre-industrial levels and to pursue efforts to hold the increase to 1.5 ◦C (with accompanying substantial reductions in emissions of greenhouse gases). Each country submits its intended nationally determined contribution (INDC) in the form of non-binding pledges to reduce emissions. At least once every five years, countries will be asked to update and strengthen their pledges.
The Convention on Biological Diversity (CBD) is an international treaty aimed at international cooperation to reduce the loss of biodiversity. It was adopted in 1992 and is entered into force in 1993. As of July 2017, there are 196 parties to the Convention, including 195 countries and the European Union. The Convention has three main objectives: (1) the conservation of biological diversity, (2) the sustainable use of its components, and (3) the fair and equitable sharing of benefits arising out of the utilization of genetic resources. The Convention seeks to provide a global framework to achieve these objectives through national actions and international cooperation.
8.1. Paris Agreement
The Paris Agreement aims at “holding the increase in the global average temperature to well below 2°C above preindustrial levels” and “pursuing efforts to limit temperature increase to 1.5°C above pre-industrial levels” (Article 2). In addition, it seeks to address the impacts of climate change “to enhance adaptive capacity, strengthen resilience and reduce vulnerability”, and to ensure “sustainable development” (Article 2) [28]. The former implication may be considered as key to the concept of “acceptable climate change”. On the one hand, there is a strong emphasis on carbon-only climatic change, as how well a climate is controlled by the carbon cycle ̶ while neglecting other critical cycles in the Earth system / climate system. On the other hand, it does not regard the large, negative impacts of environmental changes on people mainly in agricultural, arid and coastal areas under a large perturbed climate [29].
Most assessments of alternative climate change scenarios focus mainly on the carbon cycle. They do not consider land or oceanic biogeochemical alterations beyond their capacity to sequester carbon to limit warming, and do not explore how climate change would alter the biogeophysical controls on climate. Non-carbon cycle impacts would act to exacerbate warming by disturbing other cycles, through biodiversity loss, through alterations in oceanic and terrestrial primary production, by land-cover changes and waste heat. These processes have a far greater radiative forcing potential / climatic impact than that exerted by changes in greenhouse gases.
8.2. Convention on Biological Diversity
As biological systems in the Earth’s most diverse biomes are being drastically altered, local approaches to conservation will cease to be effective in the new climate scenarios predicted [30]. The Convention on Biological Diversity can be seen a framework for legislation and practical action in the area of biodiversity. The Convention is currently ratified by 167 governments. Parties to the Convention are obliged to formally enact a legal name for the Convention, and then actively engage in the preparation and development of national actions for implementation of the Convention. Objectives of the Convention include the conservation of biodiversity, sustainable use of biodiversity, and the fair and equitable sharing of the benefits derived from utilization of genetic resources. Three work programs were adopted by Parties to the Convention: marine and coastal biodiversity, agriculture biodiversity, and the biodiversity of inland water ecosystems. In addition to these work programs, various other decision and resolutions have been taken by governing bodies of the Convention. These decisions provide important basis for furthering conservation of biological diversity [31].
Biological diversity underpins ecosystem functioning and the provision of ecosystem services. With changes in land use and with increasing global change pressures, extent of habitats has diminished and their structure and composition have been altered in multiple ways. Biodiversity loss can therefore be the direct outcome of reduction of habitat quality and quantity. It is nevertheless necessary to consider how these alterations may affect ecological interaction networks and what the consequences are in terms of altered ecosystem functioning and service provision. Current status and trends of biological diversity are reported. The rate of biodiversity loss is alarming. A coordinated global response is required to address the loss of biodiversity. Important agreements on conservation and sustainable use of biodiversity have been achieved at international, national, and regional levels through the Convention on Biological Diversity, the Convention on International Trade in Endangered Species of Wild Fauna and Flora, the Ramsar Convention on Wetlands, and the World Heritage Convention.
9. Case Studies
The Great Barrier Reef. The world’s largest coral reef ecosystem is already experiencing significant detrimental effects from climate change, acting as an ecological canary in the coal mine. Air and ocean temperatures are rising, and the water is becoming more acidic due to higher levels of carbon dioxide (CO2) in the atmosphere. Mass coral bleaching events in 1998 and 2002 were followed by extensive coral mortality and loss of fish biodiversity in response to the continual rise in sea surface temperatures. Furthermore, since 1996 the pH of the oceans has decreased by 0.1 pH units, which has caused changes in the physiology of marine organisms, particularly the larval stages of many reef fishes. Research shows that ocean acidification impairs the ability of larval reef fishes to recognize, learn, and remember smells that are critical for detecting, staying near, and recognizing safe habitats (i.e., coral reef smells) or avoiding predation (i.e., predator smells). Field measurements indicate that changes in water chemistry, ocean temperature, and storm intensity already exceed the limits to which reef corals can physiologically cope. Furthermore, if the present rate of global greenhouse gas emissions continues, the Great Barrier Reef ecosystem will no longer be bioelectrically, chemically, or structurally sustainable, and experts say that this system will exit its stable state [32].
Amazon Rainforest. The loss of the Amazon rainforest is predicted to increase exponentially with climate change. A threshold at which the vast system would switch from rainforest to savannah has received considerable attention in recent years. However, the consideration of uncontrollable deforestation, and the biotic considerations of biodiversity loss and biome shift, begs the question of whether this large, complex system might instead go to irreversible ecological collapse. A massive dieback is hypothesized to follow when conditions are crossed where there are not enough trees left to maintain the rainforest bidirectionally through atmospheric moisture recycling and flammability and vegetation state in the region. The primary issue of climate, resilience, and climate sensitivity here is the limits to the biogeophysical and biogeochemical stabilization of the rainforest [33]. Earth system models consistently predict responses across identifiable thresholds. Most climate models indicate that there are no ecosystems that can survive under the background warming of more than 4:50c, and there is consensus emerging that even a warming of 25:300c risks triggering large-scale systems changes.
9.1. Great Barrier Reef
The accelerating impacts of climate change on ecosystems emerge clearly from the Great Barrier Reef case study. Sea surface temperatures around the Great Barrier Reef have risen by 1.0°C since 1900, and appear to be rising at more than 0.4°C/decade (+trends 1910–1996 and 1979–1999). Climate models predict that temperatures will rise by 1.3°C in the next 50 years and on this basis, severe coral bleaching is expected on 75% of reefs in the Coral Sea. Moreover, coral calcification on the Great Barrier Reef is at its all-time low, likely due to ocean acidification, which correlates negatively with increases in atmospheric carbon dioxide. Coral reefs are highly diverse and species-rich marine ecosystems, constructed by calcium carbonate-secreting corals. They are declining worldwide as a result of man-made disturbances, especially climate change. Coral reefs specifically require warm and stable temperatures, and ocean acidification threatens to decrease coral calcification, thus weakening reefs’ structural sensitive organisms (amulet; sponges and mollusks) [34]. After tuning parameter values for model fit, it was found that the number of expected years until major reef loss without a significant and immediate global reduction in greenhouse gas emissions is 12 years.
On the Great Barrier Reef, damage from climate change is significant and ongoing. It is therefore exceedingly urgent to take action before the continual decline of marine biodiversity. Marine biodiversity is the diversity of life in the oceans and seas, in coastal environments, and in other water bodies. Marine biodiversity includes life within the water column, on the seabed, and in sediments. Marine biodiversity provides many essential ecosystem services, a subset of which are so fundamental to human existence they are often called “ecosystem goods and services.” Overcoming these risks depends critically upon an understanding of the regional nature of local activity and precautionary policies. Protecting the Great Barrier Reef is an essential and motivating case study for the wider problem of climate change and its accelerating impacts on ecosystems [32].
9.2. Amazon Rainforest
Few ecosystems can play a major role in the regulation of the climate of an entire planet. One such ecosystem is the great Amazon rain forest. About 4 million square kilometers almost entirely located within Brazil, the Amazon is the world’s largest rain forest [35]. It has long been understood that the climatic (including hydrological) implications of a warming Siberia, or an expanding Sahara, would be dwarfed by those consequent to large-scale changes in the Amazon. The primary reasons for this, in ranked order of relevance, are: (i) An area of 4 million square kilometers acts on climatic phenomena with a spatial scale of thousands of kilometers; (ii) the bulk of the moisture that falls as rainfall in the Amazon basin does so as a consequence of vegetation-induced continental-scale circular aerial-land circulation cells; with large-scale land use changes this circulation cannot be retained; and (iii) major wet tropical biomes, including large areas of rain forest, are very sensitive to even small changes in climate.
For more than two decades, a growing number of studies have accumulated evidencing that Amazonia is under growing threat of catastrophic climatic change and conversion to grassland vegetation. Many of these studies argue that the principal driving forces behind this potential ecological catastrophe are either global changes in climate, probably caused by increases in greenhouse gases in the atmosphere (considered of even greater importance than changes in ozone), or local deforestation on a large scale [36]. Both changes act to increase the heating of the deforested area as compared to that of the original rain forest. The other driving forces of crucial importance to many of these studies include changes in insolation (i.e., continentality), indirect effects of deforestation with subsequent agricultural colonization (e.g. biomass burning, population changes), and the influence of other continents (e.g. the Sahel). That the complexity of the Amazon rain forest is matched by the complexity of the interactions of the original biome with its biophysical, chemical and biological environment is a key point. Understanding these complex interactions is essential to a greater appreciation of the factors determining the stability of the amazon basin as a rain forest and its role in the climate system. The stabilization of the Amazon rain forest is a question of the biophysical processes involved in the largely passive rain forest-climate chain response to changes in insolation on both local and global scales.
10. Future Projections
As climate change accelerates, ecosystems will undergo dramatic and potentially irreversible changes. The climate changes of the next few decades will modify many of the environmental constraints that determine where and how natural ecosystems develop and persist. These changes present opportunities for some ecosystems and challenges for others. Vegetation composition and structure (including the physical and chemical properties of the ecosystem that are locally determined by vegetation) are key determinants of broad patterns of terrestrial ecosystem function that control critical services to humankind; climate-change impacts on vegetation will therefore drive consequential changes in ecosystem services and in the return to society for past land management investments [4]. Type and composition of local vegetation are important determinants of the mainland physical and chemical environment. Change in one may elicit feedbacks causing complementary change in the other, potentially generating a new and sustained ecosystem. Ecosystem change generally involves replacement of dominant plant species or functional types by others. Changes may happen at the scale of individual plants (as on a land-cleaving fan) or at the obstraining scale of plant community types (e.g. C3 to C4 grasslands at continental scales). Models therefore predict climatic broad vegetation type to be substantially different than the recent geological past, should increasing greenhouse gas (GHG) emissions continue unabated. The biggest future threats to biodiversity are the combined impacts of habitat change, climate change, over-exploitation, pollution, and invasion by non-native species. These five drivers can act in ways that amplify each other’s effects; for example, habitat change can facilitate the entry of non-native species and pollution can make ecosystems less resilient to climate change [8]. Of great concern is how much time remains for existing ecosystems to adapt and for people to manage the transformation of their beneficial ecosystems into new, sustainable ones. Global and regional climate models are beginning to address how these changes are correlated. However, a critical gap exists in understanding the biogeochemical implications of large-scale climate-human-land-use. With current land use, by 2050 most of the world’s climate and biological reserves may be outside of extraordinarily important ecosystems, such as tropical forests, forested tundra, and the Mediterranean biome (the presently small biodiversity hot spots of Europe, California, and southwestern Australia). Future Earth research should seek to provide on long-term climate-human-land use interactions. Modeling widely used by global carbon biogeochemists currently predicts that climate change presents little risk to Earth’s vegetative ecosystems until 2050 (with 2°C warming), when major transitions are expected at local to regional scales. The possibility that the fate of vegetative ecosystems is effectively locked-in by human land-use decisions that have been made by 2050 should not be discounted as a potentially important avenue for Future Earth research and consideration by policy-makers today.
10.1. Predicted Ecosystem Changes
Shrub expansion into many cold ecosystems is one of the most prominent ecological changes observed during AR2. Even if globally averaged temperature does not exceed 2 degrees °C, there is a 25% chance of abrupt large-scale ecosystem change; e.g., die-back of the Amazon rainforest, rapid melting of Arctic summer sea ice, and Arctic tundra shifting to shrub-dominated or boreal forest ecosystems [4]. As a result of these shifts, some ecosystems may be able to survive climate change, at least for a while, of thought of as taking a longer time-scale shift to different ecosystems [8]. Identifying the ecosystems most at risk of severe shift based on AR2 global climate models could help conserve regions with a greater certainty of maintaining current ecosystems under a projected range of global warming. It may also help predict the risk of some ecosystems that are currently believed to be climatically stable shifting to Notre ecosystem states (e.g., forest to grasslands) in response to warming climate.
10.2. Potential Solutions
For some key species in terrestrial habitats, such as Joshua trees, habitat loss and warming can end in extinction by the end of the century unless active protection is implemented [30]. An added complication in predicting extinction is connected to the existence of very long transients in ecosystem responses. Some dynamical systems close to catastrophic shifts can exhibit extremely long delays before they jump into collapse. There is however a bright side: unexpectedly, models also indicate that small perturbations could help maintain the ecosystem in the green phase. How can we actively intervene to avoid biodiversity losses associated with climate change? The conservation and restoration strategies discussed in the previous section have shown their potential to protect or enhance biodiversity. One controversial suggestion is the use of geoengineering strategies to mitigate the effects of climate change. This climate engineering scheme operates on diverse physical or chemical factors. Economic viability varies enormously, and while some strategies are commercially attractive right now, others are purely speculative. The cost of most proposed solutions is typically enormous. This includes a whole repertoire of proposals, from hundreds of thousands of towers to capture carbon dioxide to ocean iron seeding and stratospheric aerosol injection. However, these alternatives remain on the table of potential pathways for mitigation in the long run after temperature overshoot occurs. As researchers have increasingly highlighted a family of physical and chemical alterations to intervene in the climate system, a growing number of environmental activists and NGOs are concerned that geoengineering may be used inappropriately and may distract from the more fundamental need for individuals and companies to reduce fossil fuel consumption. The urgency of avoiding critical values in global average temperature is illustrated by the analysis of long-term biodiversity trends [37]. Major damage to biodiversity will take place. Since carbon removal is the highest priority, are there engineering approaches to address the problem? Several projects involving native tree planting in localized areas improved air and water quality while helping carbon capture. Similarly, restoration efforts grounded in planting of sand-binding vegetation in drylands have proven effective to achieve soil crust rehabilitation.
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