Biodiversity is the number of distinct varieties or types within a group of living systems: distinct genes in a species, species in an ecosystem, or ecosystems in a biome. The term is often used to mean the total number of species living in a given ecosystem or on Earth as a whole. Climate change affects biodiversity primarily by shifting the boundaries of ecosystems, by altering the timing of seasonal events such as hatching and budding, and altering the temperature and chemical characteristics of lakes, rivers, and oceans. Atmospheric carbon dioxide (CO2), which is a main cause of global warming, also has direct effects on ecosystems, acidifying the oceans and encouraging the growth of some plants more than others. Such stresses inevitably cause extinctions, that is, loss of biodiversity.
Only evolution can create new species, and this occurs only over geologic time (usually millions of years). On human or historical time scales, extinction decreases biodiversity irreversibly. Some extinctions from climate change have already been recorded, although to date most have been caused by other human activities such as pollution, over-hunting, and deforestation. The rate of extinctions caused by climate change is predicted to be greater later in the twenty-first century than today.
Historical Background and Scientific Foundations
The modern systematic classification of species was invented by Swedish naturalist Carl Linnaeus (1707– 1788) in the eighteenth century. In following decades, European naturalists scoured Earth looking for and classifying new species of plants and animals, greatly expanding scientific knowledge of just how diverse life on Earth is. Such knowledge was crucial to the development of evolutionary biology by Charles Darwin (1809–1882) and others in the mid-nineteenth century. Today, biologists estimate that there are more than 280,000 species of plants Page 125 | Top of Articleand over 1,250,000 species of animals (including insects). Nineteenth- and twentieth-century paleontology has shown that extinction is a normal process: over 99% of all species that have ever lived are now extinct. The typical lifespan of a species is between 1 and 10 million years.
Awareness that species biodiversity is a key feature of ecosystems was not common until the 1970s, however, when the term “biological diversity” first came into frequent use. The word “biodiversity,” a shortening of “biological diversity,” first appeared in print in 1988. In the 1990s and beyond, most biologists have agreed that human beings are causing the first mass extinction in 65 million years. This modern pulse of extinctions is sometimes called the Holocene extinction event. (The Holocene is the geological term for the era from 11,500 years ago to the present.)
Humans are causing extinctions through land development, destruction of rainforests, over-hunting and over-fishing, pollution, and other activities. Some of the more famous extinctions of recent times include the dodo bird, the passenger pigeon, and (announced in 2007) the baiji, a white river dolphin of China. Hunting has caused about 23% of known animal extinctions since 1600; the introduction of invasive species, 39%; and habitat destruction, 36%. These numbers are approximate because many of the species that are being destroyed are uncatalogued insects and plants living in rainforests. Estimates of how many extinctions have already occurred due to human activities range from several tens of thousands to over a million.
Although impacts by large asteroids have caused or at least contributed to some ancient mass extinctions, climate change has been the most common cause of natural mass extinction. Ancient climate changes were brought about by shifts in Earth's orbit, continental drift, volcanism, and other processes. Today's episode of climate change is unique in being caused by a single species, humans. Moreover, today's mass extinction is unique in that human beings, by taking action to mitigate (reduce the severity of) climate change, can influence the overall severity of the event.
Mechanisms by which Climate Change Affects Biodiversity
As warming continues, other forms of human pressure on biodiversity will continue and will be, in most cases, amplified by the effects of climate change. Although effects may vary from region to region, the overall effect of global warming is to cause the cooler zones of the world—the regions around the poles (especially the North Pole) and on mountains—to shrink. Shrinkage of habitat puts species at risk because smaller habitats support smaller populations, and smaller populations are always at higher risk of extinction.
Climate change also has many other effects on ecosystems. Some are not immediately obvious. For example, Lake Tanganyika in Africa, the world's second-largest and second-deepest lake, harbors at least 350 species of fish, most unique to its own waters. Lake Tanganyika is not typical of ecosystems most vulnerable to climate change: being near the equator it is likely to see less drastic warming than, say, the Arctic, while its large thermal mass (4,526 cubic mi of water; 18,900 cubic km) will resist temperature shifts and thus might be expected to moderate climate-change impacts on the lake ecosystem.
However, regional climate warming by 1.08°F (0.6°C), along with lessened wind speeds, has had rapid effects on Tanganyika's ecosystem. Warming of surface waters more than deep waters has decreased mixing between the two: warm water is less dense than cool water, so the bigger the temperature difference between the layers, the more stably the warm upper water floats on top of the cool deeper water. Since the deeper waters are more nutrient-rich, reduced mixing has meant that fewer planktonic organisms (tiny, floating organisms, both plants and animals) can thrive in the upper water, where energy from the sun is abundant but nutrients are poor. As of 2003, plankton density in Lake Tanganyika had declined to less than one third what it was 25 years before; algae density had declined by 30% from values 80 years before. Since plankton are the basis of the marine food chain, fish stocks declined along with the plankton: fish stocks in the lake were 30% smaller than they were 80 years earlier.
The reduction of water mixing due to climate warming has made other changes in the lake's chemistry: for one, oxygen dissolved from the air no longer mixes as well in deeper waters. As a result, the habitat has shrunk for some of the lake's endemic species, such as the snail Tiphobia horei, which in 1890 lived at depths down to Page 126 | Top of Article1,000 ft (300 m) but as of 2003 lived only down to 330 ft (100 m). The snail's habitat has thus shrunk by about two thirds, even though the lake itself has not shrunk and its bulk average temperature has changed only slightly.
Tanganyika surface plankton loss has been reflected in declining food-fish harvests from the lake (about 400,000 tons per year in 2003). Since Tanganyika supplies 25–40% of the protein needs of the four nations bounding the lake, such declines can have direct impacts on human populations as well as on biodiversity. Losses in biodiversity have not yet been measured directly in Tanganyika, but smaller populations will put some species at risk of extinction, especially as warming in the region continues to about 2.7°F (1.5°C), possibly higher, with even more drastic stabilization of the lake's waters and consequent effects on its ecosystem.
Slight changes in climate can lead to pressures on biodiversity by other mechanisms. For example, a 2006 study by J. Alan Pounds and colleagues found that global warming has almost certainly caused the recent extinction of about 67% of the 110 or so species of the Monteverde harlequin tree frog of the mountains of Costa Rica. The scientists saw the extinctions as validating the climate-linked epidemic hypothesis, according to which shifts in temperature, rainfall, and other climate variables make populations more vulnerable to disease and therefore to extinction. In the case of the Monteverde frogs, more frequent warm years shifted conditions toward the growth optimum of the Bactrachochytrium fungus, which infects the frogs. The researchers found that extinctions of the frogs consistently followed temperature peaks that were favorable to growth of the fungal disease.
Other effects are not strictly changes in climate, in the sense of temperature or precipitation, but chemical changes to air and water. Increased carbon dioxide (CO2) in theatmospherehas twomajor effectsthatare likelyto decrease biodiversity: 1) Heightened atmospheric CO2 causes increased levels of dissolved CO2 in the ocean. When CO2 dissolves in water, it produces a weak acid, carbonic acid. Rising atmospheric CO2 thus acidifies the oceans. 2) Green plants extract carbon from the air by breaking up CO2, constructing their tissues using the carbon, and releasing the oxygen. CO2 is plant food. Thus, increasing atmospheric CO2 tends to cause more rapid growth in most plant species, an effect called CO2 enrichment.
Acidification of the oceans by dissolved excess CO2 will impact biodiversity by making survival more difficult for organisms that form shells of calcium carbonate. This includes bivalves such as clams, mollusks such as periwinkles and conches, microscopic plankton species, and corals. Corals, which are also subject to bleaching in excessively warm waters, form large, shallow communities in tropical waters that have been compared to rainforests because of their high level of biodiversity. A typical large reef may support on the order of a million species of plants and animals.
Over the last two centuries, the average pH of the oceans has fallen by 0.1, corresponding to a rise in acidity and a 30% reduction in the number of carbonate ions (CO32–) available to shell-making organisms as building material. When carbonate ions fall below a certain level, corals have difficulty making their skeletons. This threshold may be reached if the atmospheric CO2 level, today about 375 parts per million, rises to over 500 parts per million, as may occur by the end of the twenty-first century.
Increasing atmospheric CO2 will also affect plant growth. Farmers today often add CO2 to the air inside greenhouses, because under indoor conditions, extra CO2 speeds plant growth and increases crop yields. Under outdoor conditions, however, the gain in yield is about half as much and the foods produced are significantly lower in protein and minerals. In the wild, rising CO2 will favor some species over others, depending on rooting depth, woodiness, and photosynthetic chemistry; this will impact biodiversity by altering competitive balances. The CO2 fertilization effect will be strongest in biomes where plant growth is limited by water availability, such as grasslands, savanna, and desert. The biodiversity impact of a 2.5-fold increase in CO2 would likely be only about a third as great on a boreal (northern pine) forest as on savanna or grassland, and half as great as on desert.
Impacts and Issues
Most plants require a specific range of temperature, moisture, and seasonal change to thrive; most animals require certain plants or other animals to thrive, and also have a limited range of tolerance for temperature and moisture. As climate warms, a typical ecosystem will tend to migrate away from areas where it was at the warm edge of its tolerance range and toward places where it was formerly at the cool edge. The most general effect of global climate change is thus to move ecological zones toward the poles and toward higher altitudes. For each 1.8°F (1°C) of warming, Page 127 | Top of Articleterrestrial (on-land) ecosystems typically shift poleward by 100 mi (160 km): for example, if climate warms by 5.4°F (3°C) by 2100, plant and animal communities in the Northern Hemisphere will migrate an average of 300 mi (480 km) northward—if they can—to stay in a suitable climate zone. This effect is observed, not only predicted.
In the Northern Hemisphere, terrestrial animal and plant ranges have been observed to shift northward, on average, by 3.8 mi (6.1 km) per decade over the last 50 years. In mountainous terrain, plant and animal ranges have shifted upward by 20 ft (6.1 m) over the same time period. Fragmentation of landscapes by human activity such as agriculture and city-building makes ecosystem migration more difficult today than during past climatic shifts, such as glacial periods. Species that fail to colonize new areas as the climate changes may go extinct.
Although climate change has so far been most intense in the Arctic and the West Antarctic Peninsula, where warming has been about twice the global average and dramatic effects such as retreating sea ice and melting tundra are readily visible, biodiversity is low in these regions compared to the tropics, where rainforests, coral reefs, and other particularly diverse communities are found. Thus, a smaller climate shift can have a greater impact on biodiversity in the tropics than a larger shift in boreal or temperate regions.
For marine ecosystems, changes in circulation patterns, ocean temperature, and ocean chemistry all influence biodiversity. For example, over the last 40 years or so, warm-water plankton species have shifted about 620 mi (1,000 km) in the North Atlantic due to warming.
Not all observed climate effects on biological systems are consistent with climate warming; a few are consistent with cooling. Also, some observed effects are consistent with natural climate shifts rather than those attributed to human-caused (anthropogenic) global warming. However, mathematical analysis shows that the very great majority of changes are consistent with warming trends, and that a combination of natural and anthropogenic climate changes describe observed changes in physical and biological systems better than either natural or anthropogenic changes alone. Anthropogenic changes have been added to or laid over those caused by natural processes, and are gradually becoming more dominant.
The five main drivers of biodiversity change, ranked from most severe impact to least severe between now and 2100, are: 1) land-use changes (including deforestation);2) climate change; 3) nitrogen deposition (from fertilizer use); 4) biotic exchange (the introduction of invasive species); and 5) direct effects of increasing atmospheric CO2, apart from climate change.
The result of these combined, continuing, and growing pressures will be an irreversible loss of biodiversity in many parts of the world. However, many uncertainties remain. Ecologists do not understand the relationship between ecosystem structure and rapid climate change well enough to predict the exact effects of current climate changes on biomes. It is also unknown whether efforts to mitigate climate change will occur or succeed, and if so, to what extent.
Despite these uncertainties, scientists have estimated the likely impact that climate change will have on biodiversity. In 2004, Chris Thomas and colleagues published their study of an unbiased or representative sample of 1,103 animal and plant species. They found that climate change was likely to commit 15–37% of all species examined to extinction by 2050. “Committed to extinction” does not mean that a species would necessarily be extinct by that time, but that the population of each species would be so reduced that its species' extinction becomes highly likely. In many or most ecological regions, climate change will become the greatest threat to biodiversity by 2050. There are 5 to 15 million species of creatures on Earth (the large range arises from the difficulty of counting insect, bacterial, and fungal species). If only 15% of all species are committed to extinction by climate change—the lower end of the range given by Thomas and colleagues—then 750,000 to 2,250,000 million species will eventually become extinct as a result of global climate change.
Primary Source Connection
Human activities and the resulting global climate change have had, and will continue to have, a major impact on biodiversity across the globe. Biodiversity is the variety of all living organisms that exist in an ecosystem. This paper from the Intergovernmental Panel on Climate Change (IPCC) discusses the effect of human activity on biodiversity and possible adaptation and mitigation strategies.
The IPCC is a scientific panel that was founded by the United Nations in 1988 as part of the United Nations Environment Program and the U.N.'s World Meteorological Organization.
CLIMATE CHANGE AND BIODIVERSITY
At the global level, human activities have caused and will continue to cause a loss in biodiversity through, inter alia, land-use and land-cover change; soil and water pollution and degradation (including desertification), and air pollution; diversion of water to intensively managed ecosystems and urban systems; habitat fragmentation; selective exploitation of species; the introduction of non-native species; and stratospheric ozone depletion. The current rate of biodiversity loss is greater than the natural background rate of extinction. A critical question for this Technical Paper is how much might climate change (natural or human-induced) enhance or inhibit these losses in biodiversity?
Changes in climate exert additional pressure and have already begun to affect biodiversity. The atmospheric concentrations of greenhouse gases have increased since the pre-industrial era due to human activities, primarily the combustion of fossil fuels and land-use and land-cover change. These and natural forces have contributed to Page 128 | Top of Articlechanges in the Earth's climate over the 20th century: Land and ocean surface temperatures have warmed, the spatial and temporal patterns of precipitation have changed, sea level has risen, and the frequency and intensity of El Niño events have increased. These changes, particularly the warmer regional temperatures, have affected the timing of reproduction in animals and plants and/or migration of animals, the length of the growing season, species distributions and population sizes, and the frequency of pest and disease outbreaks. Some coastal, high-latitude, and high-altitude ecosystems have also been affected by changes in regional climatic factors.
Climate change is projected to affect all aspects of biodiversity; however, the projected changes have to take into account the impacts from other past, present, and future human activities, including increasing atmospheric concentrations of carbon dioxide (CO2). For the wide range of Intergovernmental Panel on Climate Change (IPCC) emissions scenarios, the Earth's mean surface temperature is projected to warm 1.4 to 5.8°C by the end of the 21st century, with land areas warming more than the oceans, and the high latitudes warming more than the tropics. The associated sea-level rise is projected to be 0.09 to 0.88 m. In general, precipitation is projected to increase in high-latitude and equatorial areas and decrease in the subtropics, with an increase in heavy precipitation events. Climate change is projected to affect individual organisms, populations, species distributions, and ecosystem composition and function both directly (e.g., through increases in temperature and changes in precipitation and in the case of marine and coastal ecosystems also changes in sea level and storm surges) and indirectly (e.g., through climate changing the intensity and frequency of disturbances such as wild-fires). Processes such as habitat loss, modification and fragmentation, and the introduction and spread of non-native species will affect the impacts of climate change. A realistic projection of the future state of the Earth's ecosystems would need to take into account human land- and water-use patterns, which will greatly affect the ability of organisms to respond to climate change via migration.
The general effect of projected human-induced climate change is that the habitats of many species will move pole-ward or upward from their current locations. Species will be affected differently by climate change: They will migrate at different rates through fragmented landscapes, and ecosystems dominated by long-lived species (e.g., long-lived trees) will often be slow to show evidence of change. Thus, the composition of most current ecosystems is likely to change, as species that make up an ecosystem are unlikely to shift together. The most rapid changes are expected where they are accelerated by changes in natural and anthropogenic non-climatic disturbance patterns.
Changes in the frequency, intensity, extent, and locations of disturbances will affect whether, how, and at which rate the existing ecosystems will be replaced by new plant and animal assemblages. Disturbances can increase the rate of species loss and create opportunities for the establishment of new species.
Globally by the year 2080, about 20% of coastal wetlands could be lost due to sea-level rise. The impact of sea-level rise on coastal ecosystems (e.g., mangrove/coastal wetlands, seagrasses) will vary regionally and will depend on erosion processes from the sea and depositional processes from land. Some mangroves in low-island coastal regions where sedimentation loads are high and erosion processes are low may not be particularly vulnerable to sea-level rise.
The risk of extinction will increase for many species that are already vulnerable. Species with limited climatic ranges and/or restricted habitat requirements and/or small populations are typically the most vulnerable to extinction, such as endemic mountain species and biota restricted to islands (e.g., birds), peninsulas (e.g., Cape Floral Kingdom), or coastal areas (e.g., mangroves, coastal wetlands, and coral reefs). In contrast, species with extensive, non-patchy ranges, long-range dispersal mechanisms, and large populations are at less risk of extinction. While there is little evidence to suggest that climate change will slow species losses, there is evidence it may increase species losses. In some regions there may be an increase in local biodiversity—usually as a result of species introductions, the long-term consequences of which are hard to foresee.
Where significant ecosystem disruption occurs (e.g., loss of dominant species or a high proportion of species, or much of the species redundancy), there may be losses in net ecosystem productivity (NEP) at least during the transition period. However, in many cases, loss of biodiversity from diverse and extensive ecosystems due to climate change does not necessarily imply loss of productivity as there is a degree of redundancy in most ecosystems; the contribution to production by a species that is lost from an ecosystem may be replaced by another species. Globally, the impacts of climate change on biodiversity and the subsequent effects on productivity have not been estimated.
Changes in biodiversity at ecosystem and landscape scale, in response to climate change and other pressures (e.g., changes in forest fires and deforestation), would further affect global and regional climate through changes in the uptake and release of greenhouse gases and changes in albedo and evapotranspiration. Similarly, structural changes in biological communities in the upper ocean could alter the uptake of CO2 by the ocean or the release of precursors for cloud condensation nuclei causing either positive or negative feedbacks on climate change.
Modeling the changes in biodiversity in response to climate change presents some significant challenges. The data and models needed to project the extent and nature of future ecosystem changes and changes in the geographical distribution of species are incomplete, meaning that these effects can only be partially quantified.
Impacts of climate change mitigation activities on biodiversity depend on the context, design, and implementation of these activities. Land-use, land-use change, and forestry activities (afforestation, reforestation, avoided deforestation, and improved forest, cropland, and grazing land management practices) and implementation of renewable energy sources (hydro-, wind-, and solar power and biofuels) may affect biodiversity depending upon site selection and management practices. For example, 1) afforestation and reforestation projects can have positive, neutral, or negative impacts depending on the level of biodiversity of the non-forest ecosystem being replaced, the scale one considers, and other design and implementation issues; 2) avoiding and reducing forest degradation in threatened/vulnerable forests that contain assemblages of species that are unusually diverse, globally rare, or unique to that region can provide substantial bio-diversity benefits along with the avoidance of carbon emissions; 3) large-scale bioenergy plantations that generate high yields would have adverse impacts on biodiversity where they replace systems with higher biological diversity, whereas small-scale plantations on degraded land or abandoned agricultural sites would have environmental benefits; and 4) increased efficiency in the generation and/or use of fossil-fuel-based energy can reduce fossil-fuel use and thereby reduce the impacts on biodiversity resulting from resource extraction, transportation (e.g., through shipping and pipelines), and combustion of fossil fuels.
Climate change adaptation activities can promote conservation and sustainable use of biodiversity and reduce the impact of changes in climate and climatic extremes on biodiversity. These include the establishment of a mosaic of interconnected terrestrial, freshwater, and marine multiple-use reserves designed to take into account projected changes in climate, and integrated land and water management activities that reduce non-climate pressures on biodiversity and hence make the systems less vulnerable to changes in climate. Some of these adaptation activities can also make people less vulnerable to climatic extremes.
The effectiveness of adaptation and mitigation activities can be enhanced when they are integrated with broader strategies designed to make development paths more sustainable. There are potential environmental and social synergies and tradeoffs between climate adaptation and mitigation activities (projects and policies), and the objectives of multilateral environmental agreements (e.g., the conservation and sustainable use objective of the Convention on Biological Diversity) as well as other aspects of sustainable development. These synergies and tradeoffs can be evaluated for the full range of potential activities—inter alia, energy and land-use, land-use change, and forestry projects and policies through the application of project, sectoral, and regional level environmental and social impact assessments—and can be compared against a set of criteria and indicators using a range of decision making frameworks. For this, current assessment methodologies, criteria, and indicators for evaluating the impact of mitigation and adaptation activities on biodiversity and other aspects of sustainable development will have to be adapted and further developed.
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