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22 Examples of Ecological Impacts of Climate Change in the United States Climate change is global in scope, but ecological impacts are often quite localized. Although most of the evidence of the ecological impacts of climate change stems from trends observed among hundreds of species rather than a particular species, there are compelling examples of how climate change has affected individual species and ecosystems. The following examples review just a few of the ecological changes that have been documented in regions across the United States. Future projections of the effects of climate change on these areas are also explored, although it should be noted that such projections are based on the continuation of current trends in anthropogenic contributors to climate change. If human activities change, so too may these projections. The Pacific Coastline Edith’s and Quino checkerspot butterfly We know some species are very sensitive to climate which allows them to act as early warning indicators for climate change. One such species is Edith's checkerspot butterfly (Euphydryas editha), a species with a marked range shift over the past 100 years that has been attributed to climate change. Forty years of research have documented strong responses of wild populations of Edith’s checkerspot butterfly to the vagaries of weather and to climates with strong seasonal variation. Weather extremes cause local extinctions but this is a natural part of Edith’s checkerspot biology (Singer & Ehrlich 1979, Singer & Thomas 1996). Using museum records to determine where Edith’s checkerspot lived in the past, an asymmetrical pattern of population extinctions on a continental scale was revealed. Population extinctions were four times as high at the southern end of the butterflies' range (in Baja, Mexico) than at the northern end (in Canada), and nearly three times as high at lower elevations (below 2400 m (8,000 ft)) than at higher elevations (from 2400 to 3800 m (8,000 to 12,500 ft)) (Parmesan 1996). This extinction process has effectively shifted the range of E. editha both northward and upward in elevation since the beginning of the 20th century—a shift in concert with temperature increases resulting from climate change. Separate analyses showed that other factors (such as proximity to large urban areas) were not associated with the observed extinction patterns. Since the only strong associations were between the extinction patterns and various climate trends, regional climate warming was by default the most likely cause of the observed shift in the butterfly’s range. The Quino checkerspot (E. editha quino) is a federally listed endangered subspecies of Edith’s checkerspot whose case highlights the conservation implications of climate change. Although habitat destruction is the primary cause of the decline of the Quino checkerspot, climate change poses problems for its recovery. Quino checkerspot populations along the southernmost range (in Mexico) face the lowest degree of threat from development. Unfortunately, these habitats are at the greatest risk from continuing warming and drying climate trends. By contrast, Quino habitat that might have been available farther north has been destroyed by development in the Los Angeles/San Diego corridor. The case of the Quino checkerspot has resulted in the first habitat recovery plan to list climate change not only as a current threat but also as a factor that should be considered in designing habitat reserves and recovery management (Anderson et al. 2001).
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Examples of Ecological Impacts of Climate Change in the United States 23 Pacific Ocean and fisheries With seafood providing the primary source of protein for more than 1 billion people worldwide, and demand for seafood growing exponentially, the future of the world’s fisheries is of critical importance. There is, however, very limited understanding of how global climate change might affect whole ocean ecosystems. Some of what we have learned about how changes in climate affect marine ecosystems comes from what has been observed during periodic climate cycles, such as the El Niño-Southern Oscillation, the Pacific Decadal Oscillation, and the North Pacific Gyre Oscillation, natural climatic fluctuations generated by ocean-atmosphere interactions over the Pacific that can have important effects on weather conditions globally For example, some species’ distributions change with El Niño cycles. Other ecosystem changes, however, appear to be unlinked to those cycles, and instead seem to have arisen as novel, unexpected perturbations. One such anomaly is a new dead zone that has appeared off the Pacific Northwest coasts of Washington and Oregon. A dead zone is an area of the ocean with insufficient oxygen to support most marine life. Most animals that cannot swim or scuttle away suffocate. The zone of low (or no) oxygen along the Pacific Northwest is different from most of Earth's other approximately 400 dead zones that are caused by runoff of excess nutrients from the land, usually from agricultural lands (Diaz and Rosenberg 2008). The Pacific Northwest dead zone first appeared in the summer of 2002 and has appeared each summer since that time. This dead zone is not a result of fertilizer use or sewage from land-based sources. Its ultimate cause is still under investigation, but three immediate causes have been documented, each of which is possibly linked to climate change. (1) Warmer ocean waters hold less oxygen at the surface and slow the resupply of oxygen to deeper waters (Stramma et al. 2008). (2) Changes in the coastal winds that control a process called coastal upwelling have been documented (Barth et al. 2007). (3) Changes in ocean circulation that bring waters with abnormally low oxygen and high nutrient levels to the surface during upwelling have been recorded. Reliable and comparable oxygen measurements in this coastal ocean date back 60 years. Researchers who compared dissolved oxygen content in coastal waters along the Oregon shore conclude that the recent seven years (starting in 2002) have been dramatically different from the previous fifty years (Chan et al. 2008). Analysis of over 10,000 individual dissolved oxygen measurements indicate that prior to the early 2000s only one record showed severe hypoxia (low oxygen of less than 0.5 ml of oxygen per liter of water), and none showed anoxia (no oxygen) in nearshore coastal waters. Starting in 2002 the dead zone has appeared repeatedly each summer. The most severe low-oxygen event on record was in 2006 along the Pacific Northwest coast; that dead zone lasted four months and occupied up to two-thirds of the water column, oxygen levels dropped to zero, and there were widespread die-offs of seafloor life that could not get away quickly enough. The implications for fisheries in the region are under investigation. Other ecosystems off the coasts of Peru, Chile, Namibia, South Africa, and Morocco also seem to be undergoing changes involving dead zones, although the specifics vary across these systems. These large marine ecosystems all depend on coastal upwelling; they collectively represent 1 percent of the surface area of oceans, but have historically provided around 20 percent of the fisheries (Pauly and Christensen 1995). If catches diminish and ecosystem functioning is disrupted as a consequence of emerging or expanding dead zones, the consequences for many of the world’s key fisheries could be substantial.
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24 Examples of Ecological Impacts of Climate Changein the United States Wine Quality in California Climate change affects managed ecosystems like vineyards just as it affects natural ecosystems, and thereby can have major economic and social effects. Wine is one of California’s most important agricultural products. The industry takes in billions of dollars per year and is a critical part of the State’s cultural fabric. Wine grapes can grow in a wide range of climates and soils, but the quality of the crop and its value for producing high-quality wine depends on something the growers call “terroir,” a subtle balance of climate, soils, and landforms. Terroir can be so important that the price of grapes from a premium wine region typically fetch more than 10 times the price of the same variety grown elsewhere. Climate changes from 1950 to 1997 generally improved conditions for growing grapes in California’s premium wine regions (Nemani et al. 2001). A modest warming, especially at night, decreased the incidence of frost and advanced the start of the growing season. Further warming would, however, be unlikely to aid the industry. One study concluded that the warming associated with “business as usual” emissions would, by the last decades of the 21st century, degrade California’s premium wine regions to marginal from their current status of optimal (Figure 10) (Hayhoe et al. 2004). Another study concluded that the area with the potential to produce premium wines could decrease by up to 81 percent (White et al. 2006). 2020-2049 2070-2099 1961-1990 PCM model HadCM3 model PCM model HadCM3 model Wine lower higher lower r higher lower higher lower higher Grape emissions emissions emissions emissions emissions emissions emissions emissions growing B1 A1fi B1 A1fi B1 A1fi B1 A1fi region Wine Optimal Optimal Optimal Optimal Optimal Impaired Impaired Marginal Impaired Country (mid) (mid) (high) (mid) (high) Cool Optimal Optimal Optimal Optimal Optimal Optimal Optimal Optimal Impaired Coastal (low) (mid) (mid) (mid) (mid) (mid-high) (high) (mid-high) Central Optimal Optimal Optimal Optimal Optimal Marginal Marginal Marginal Impaired Coast (mid-high) (high) (high) (high) (high) Northern Central Marginal Impaired Impaired Impaired Impaired Impaired Impaired Impaired Impaired Valley Southern Central Impaired Impaired Impaired Impaired Impaired Impaired Impaired Impaired Impaired Valley FIGURE 10 Projected conditions for grape growing in certain regions of California, from two different climate models, the low sensitivity PCM model and the medium sensitivity HadCM3 model. Each model was used to project conditions for grape growing under conditions of higher or lower CO2 emissions. Both models project that conditions will improve in some regions in the medium term (2020-2049), but that by 2070-2099 only the cool coastal regions will still have optimal conditions for growing grapes (Hayhoe et al. 2004).
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Examples of Ecological Impacts of Climate Change in the United States 25 Marine species along the Pacific coastline Changing climate has already affected the distribution of marine organisms from south to north along the coasts. For example, in one long-term study of animals and plants that inhabit rocky shores along the central California coasts, many southern species became more common while northern species became more rare over the 60-year period ending in the mid-1990s (Sagarin et al. 1999). During that period, shoreline ocean temperatures increased by 0.8°C (1.4°F) and summer maximum temperatures by nearly 2.2°C (4°F). Other changes in species abundances that are consistent with expectations of climate change have been reported in this system (Smith et al. 2006) as well as for rocky shores in Europe (Southward et al. 2005). Very rapid shifts in geographic distribution have been recorded for bottom-dwelling species that are important to fisheries in the Bering Sea (Mueter and Litzow 2008). A clear northerly migration of snow crab, rock sole, halibut, and pollock has been reported with rates of movement 2-3 times faster than the average rate found for terrestrial species (Parmesan and Yohe 2003). These species appear to be shifting northward in response to the northward movement of the extent of seasonal ice. The Rocky Mountains Range of the American pika Once havens for cold-adapted species, mountaintops around the world are now showing signs of warming stress. Consider the case of the American pika, a small mammal that looks like a hamster but is actually more closely related to rabbits. Paleoecological records show that it lived in the lowlands during the last ice age. As the ice retreated, the pika that had once lived across the entire landscape gradually shifted uphill—an easy move. It now survives in isolated mountaintop islands on various mountain ranges throughout the western United States. Populations below about 7000 ft are rapidly going extinct, with past physiological studies suggesting stress from high temperature is the cause (Beever et al. 2003; Smith 1974). Trout habitat Earlier springs and warmer summers are beginning to restrict trout habitat severely in some of the small headwater streams of the Rocky Mountains, home to legendary trout fisheries. As snowpacks melt earlier in the spring, late summer streamflows of cool snowmelt water are declining, and some small rivers, like the Big Hole in Montana, cease to flow in late summer, becoming isolated pools until replenished by fall rains. Trout die at water temperatures above 26°C (78°F), and some stream temperatures are now reaching lethal levels in July and August. If current trends continue, coldwater species like trout could increasingly be restricted to the most permanent streams. Late summer stream flow in seven Montana rivers has dropped an average of 30 percent since 1950 as a result of increasing irrigation demand, earlier snowmelt, and warmer summer temperatures. State officials have had to temporarily close recreational trout fishing during August in recent years on certain streams because of low stream flow and high water temperatures. From 18 to 92 percent of bull trout habitat could be lost in the northern Rocky Mountains in the next half century due to global warming influences on stream temperatures (Rieman et al. 2007). Rocky Mountain lakes will likely see an increase in the abundance of warmwater fish like yellow perch and smallmouth bass but a decrease in coldwater species.
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26 Examples of Ecological Impacts of Climate Changein the United States Spring emergence of yellow-bellied marmots High in the Colorado Rockies, some important members of the animal community are responding to warmer springtime temperatures. The yellow-bellied marmot, a large burrowing mammal of the squirrel family, very common in the western mountains of the United States, emerged from hibernation 23 days earlier (around April 1) in 1999 than in 1976, apparently in response to warmer late-winter temperatures. This unfortunately means they are present and active when snow still covers their normal food; in this Rocky Mountain location warmer temperatures were not, by 1999, associated with earlier melting of the winter snowpack. Cut off from food the marmots need to survive longer on stored reserves, potentially decreasing their ability to reproduce (Inouye et al. 2000). Forests: a deadly combination of drought, wildfire, and insect pests In much of the country winter temperatures are not as severe as before, which may be more convenient for humans but throughout the western mountains we now see more wintertime precipitation falling as rain instead of snow (Knowles 2006). Thus the winter snowpack, which is crucial for summer water resources, is no longer providing as much free natural storage as before. Winter snowpack used to peak around April 1, when snow hydrologists would take a measure of peak snow depth for each year’s water management planning. Now the April 1 snowpack is 10 to 20 percent lower than 50 years ago, partly because of less snow but also because the spring melt begins on average 2 to 4 weeks earlier (Mote et al. 2005; Barnett et al. 2008). In climates with adequate summer rainfall, earlier springs mean a longer growing season, which may result in greater plant growth. But in the arid climates of the West, earlier snowmelt and warmer spring temperatures mean that the annual summer drought may now begin in the spring. Western valleys have always been dry, but now the higher mountain forests, from 1200 m (4000 ft) and higher in Montana to over 3600 m (12,000 ft) in Arizona are no longer as protected by a slowly melting snowpack as before. The longer, more intense spring-summer drought is stressing mountain forests (Logan et al. 2003). Wildfire occurrence and extent are also dramatically escalating in western forests (not only in the U.S. Rockies but also in western Canada and Alaska), a legacy of both a changing climate and decades of total fire suppression that has resulted in a dramatic buildup of dead fuels. In the last 20 years the western fire season has expanded by over 2.5 months (Westerling et al. 2006). In 2007 California had wildfires burning in November, and Billings, Montana, had a wildfire burning on January 8, 2008. There are now four times as many wildfires exceeding 4 km2 (1.5 mi2) as there were 30 years ago, and these frequent large fires are burning six times as much area. The national wildfire-fighting budget (already strained for other reasons such as increased development in fire vulnerable areas) now exceeds $1 billion almost every year, and an ominous interaction is now emerging: as insect epidemics kill vast areas of forest, they leave standing dead fuels for even larger wildfires. Ecologists now expect that some of these areas will not recover as forests; they will instead return as more open savannah or grassland ecosystems. The climate is becoming too dry to support some of our nation’s forests. An unprecedented bark beetle epidemic has affected 47,000 km2 (18,000 mi2) of forest in western North America over the last 10 years (Raffa et al. 2008). The epidemic is an example of the complicated interactions that characterize ecosystem dynamics: in this case the interplay between a changing climate, vulnerable trees, and opportunistic insects. Mild overwintering temperatures have allowed more insect larvae to survive the winter (Logan et al. 2003). At the
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Examples of Ecological Impacts of Climate Change in the United States 27 same time longer, dryer, and warmer, summers have both accelerated beetle life cycles and stressed the trees upon which these beetles feed. The stressed trees produce less pitch, which makes them more susceptible to beetle damage. Many of the affected forests are made up of trees that are all the same age because of earlier wildfire damage. These uniform forest age structures provide contiguous landscapes allowing rapid population dispersal and successful beetle attacks on the stressed trees. The Breadbasket: Central United States Agricultural Shifts The central part of the United States is one of the world’s great agricultural regions. With rich soils and a favorable climate, the region produces some of the world’s highest yields of corn, soybeans, and wheat. Production of corn and soy beans are centered east of the plains, with the highest production in Iowa, Illinois, Minnesota, and Indiana. Wheat is grown mainly in the western part of the region, especially in the Dakotas and Kansas. For these three crops, yields have increased steadily over the last century, reflecting the combined effects of improved seed stocks and improved management (National Agricultural Statistics Service 2008). Continuing efforts by farmers and scientists to increase yields make it difficult to assess whether climate changes to date have had an effect. In general, the rate of plant growth increases with warming, up to a point, but it decreases when temperatures get above that point. For each crop, varieties have adapted or been bred to thrive in a range of temperatures, but there are limits to the temperature range for each crop. When temperatures get too warm, crops tend to mature early, completing growth before the end of the season. Under extreme conditions crops can be killed by either high or low temperatures. Projecting crop growth in a changing climate is further complicated by the fact that for most plant species, growth increases in response to an increase in the CO2 concentration of the atmosphere. Plants grow by combining CO2 from the atmosphere with water, using energy from light, to make carbohydrates, in the process called photosynthesis. Under open-field conditions, increasing atmospheric CO2 concentrations (but not changing temperature) from the current ambient level to a level that may occur in 30 to 50 years increases the yield of plants like soybeans or wheat an average of about 15 percent (Long et al. 2006). Plants like corn and sugarcane have a mechanism that concentrates CO2 in the leaves, and they generally do not grow more when exposed to elevated CO2. Unhappily, some pest plants (like poison ivy) grow faster and produce more of their characteristic irritant when atmospheric CO2 concentration is higher (Mohan 2006). A number of models estimate changes in crop yields in response to changing climate and CO2 concentrations. At the global scale these models suggest that if warming is modest, yields in hot regions will decrease, while yields in cool regions will increase. For the temperate Great Plains, most models conclude that warming up to 2oC will probably increase average yields by 5 to 20 percent (Field et al. 2007b). Depending on the amount of warming that occurs, these yield increases may persist through the century. The actual impact of climate change on crop yields in the future depends on a number of factors. The balance between effects of warming (which can increase or decrease yields) and effects of increased CO2 (which increases yields for some crops but not for corn) will mean increased yields for some crops and different varieties within crops and decreases for others. Higher temperatures can interfere with pollination and seed set, resulting in reductions in productivity; the acceleration of plant life cycles can result in crops that are smaller when they
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28 Examples of Ecological Impacts of Climate Changein the United States mature (Hatfield et al. 2008). Effects of climate change on the competitive ability of weeds and other pests, on the susceptibility of weeds to herbicides, and on the frequency of severe weather all potentially complicate the challenge of sustaining historical rates of yield increases (Hatfield et al. 2008). Another critical factor in future yields is the level of adaptation by farmers. Aggressive action to adjust farming methods, planting dates, and the crops or varieties grown can play a large role in yields. The potentially large contrast between yields with and without farmers’ taking steps towards adaptation underscores the importance of good information for coping effectively with climate change. Migratory Waterways The natural ecosystems in the central United States are also affected by climate change. Millions of migratory birds fly back and forth across the central United States, many of them resting, feeding, and mating in temporary lakes called playa lakes in the south and prairie potholes in the north. The region is especially critical for mallard ducks and other waterfowl, with their annual population numbers corresponding closely to the number of these temporary wetlands available at the beginning of the breeding season. The health of the prairie potholes for waterfowl habitat depends on whether future precipitation increases sufficiently to offset warmer temperatures. A combination of higher temperatures and lower rainfall could dry up potholes in a region covering six U.S. states and three Canadian provinces, home to the most productive waterfowl habitat on Earth (Johnson et al. 2005). These shallow water holes are already under pressure because they are used to provide water for irrigation, filled to provide more land for crops and houses, and often subject to runoff of nutrients and pesticides. Climate change could further stress these essential but transient habitats (Covich 1997). The Northeastern United States Northeast Fisheries New England fisheries have for many decades been based on cod and lobster. Stocks of Georges Bank cod, flounder, and haddock have collapsed due to overfishing. It is increasingly clear that the cod fishery is also vulnerable to stress related to warming (Fogarty et al. 2008). Cod require average bottom-water temperatures cooler than 12°C (54°F) and cooler than 8°C (46°F) for growth and survival of young. If future warming of bottom waters is limited to the low end of projected increases by the end of this century, cod may still survive over much of its current range from Long Island to the Gulf of Maine, but more substantial warming will likely push temperatures south of Cape Cod above the 12°C (54°F) threshold. Cod could survive in cooler pockets north of Cape Cod and the cooler, historically rich waters of Georges Bank. Lobsters tolerate a wider range of water temperatures, however in warmer water lobsters need more oxygen to survive, and warmer water holds less oxygen. As temperatures approach 26°C (79°F) the concentration of oxygen in the water becomes insufficient for lobsters. Since the late 1990s, lobster populations in Long Island Sound have fallen rapidly, with harvests in 2003 only 20 to 30 percent of their earlier size. While many factors may contribute to this decline, warming is probably part of the mix, as water temperatures have exceeded 26°C with increasing frequency (Frumhoff et al. 2007). In the Gulf of Maine warmer conditions in the future will probably improve lobster habitat, providing a longer growing season, more rapid growth, and more area suitable for the growth and survival of juveniles. However, such warmer temperatures may have other indirect effects on lobsters. Since the late 1990s, lobster-shell disease, caused by
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Examples of Ecological Impacts of Climate Change in the United States 29 a bacterium, has been prevalent in southern New England, making the affected lobsters unmarketable. While temperature increases have not been demonstrated to be the cause of greater disease prevalence, the spatial pattern of disease occurrence suggests temperature is a factor (Glenn and Pugh 2006). We are also seeing animal parasites moving northward. For example, the oyster parasite (Perkinsus marinus) extended its range northward from the Chesapeake Bay to Maine, a 500 km (310 mi) shift with potentially major implications for oyster fisheries. Censuses from 1949 to 1990 showed a stable distribution of the parasite from the Gulf of Mexico to its northern boundary at the Chesapeake Bay. A rapid northward expansion of the parasite in 1991 has been linked to above-average winter temperatures, rather than to human-driven introduction or genetic change (Ford 1996). Florida and the Southern United States Northward movement of tropical species Just as cold-adapted species are moving toward the poles and up the mountains, many tropical species are also moving into the United States. Former migrants like the rufous hummingbird (Selasphorus rufus) and the Mexican green jay (Cyanocorax yncas) have become year-round residents in Alabama and Texas, respectively (Hill et al. 1998). Florida has five new species of tropical dragonfly (Paulson 2001). Many tropical butterflies that are normally confined to Mexico are starting to breed as far north as Austin, Texas. Having these new species in U.S. backyards has delighted bird and butterfly watchers, and overall species diversity may have actually increased along the southern U.S. border. The observed northward movement of species also has such potentially negative implications as new arrivals competing with local species for scarce resources, bringing with them new diseases, or crowding out native species. Sea-level rise and the Everglades The Florida Everglades are a unique ecosystem: a vast subtropical wetland of sawgrass, mangrove forests, and cypress swamps that are home to wading birds, alligators, wood storks, Florida panthers, and manatees. Beginning in the late 1800s, first to support agriculture and later to protect a rapidly growing human population from flooding, a large part of the natural wetlands have been drained or otherwise managed. These activities shrank the Everglades to half their original size, fresh water flows through the wetlands changed dramatically, pollution from urban areas and fertilizer runoff from agricultural areas increased, and saltwater from the surrounding oceans infiltrated further inland. In response to the deterioration of this natural resource––the Everglades National Park is a designated International Biosphere Reserve, a World Heritage Site and a Wetlands of International Importance––a number of preservation and restoration initiatives are underway. Sea-level rise has already resulted in the landward encroachment of salt-tolerant mangroves (Ross et al. 2000) and the task of restoring the Everglades will be made more difficult by the higher rate of sea-level rise, increase in water temperature, changes in precipitation, and more extreme storms that are expected to result from climate change (Twilley et al. 2001; NRC 2008). In regions of the low-lying southeastern U.S. coast that are undergoing subsidence, a projected relative sea-level rise of 0.6 to 1.2 m (2 to 4 ft) over the 21st century would reconfigure shorelines and fragment barrier islands. Coastal saltmarshes and mangroves would be hard
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30 Examples of Ecological Impacts of Climate Changein the United States pressed to accumulate new soil fast enough to keep pace with the rising water level, and many of them would be lost. The migration of wetlands inland as sea levels rise is an important means of coastal ecosystem adaptation. When human development prevents such migration, adaptation is more difficult. Conversely, protection of regions into which coastal wetlands can migrate would ease adaptation. Even where landward migration is not blocked by development, rapid climate change would make it unlikely that this process could happen fast enough to compensate for the losses. Animal species that depend on coastal marshes and mangroves, ranging from fish species that use them as nursery areas to migratory waterfowl and wading birds will likely be adversely affected. Coral reefs Coral reefs provide many ecosystem services including acting as a habitat for many kinds of fish and a protective barrier for nearby shores. Reefs off the Florida Keys and in other tropical U.S. waters are to varying degrees already in degraded condition due to the effects of overfishing, land-based pollution by nutrients and sediments, and coastal development (Pandolfi et al. 2005). Ocean warming and acidification due to increasing carbon dioxide concentrations pose an additional double threat that will challenge the survival of coral reefs (Hoegh-Guldberg et al. 2007). Heat stress in shallow tropical waters causes corals to expel the colorful symbiotic algae that provide a primary source of nutrition for the coral, leaving only the white “bone structure” of the corals behind. This process, called coral bleaching, can be lethal to the coral if it lasts too long. Bleaching has increased in intensity and frequency in recent decades. Bleaching and coral mortality become progressively worse as unusually high temperatures are experienced over longer periods. Increased acidity is likely to slow or stop the growth of coral over this century, making them less competitive with the seaweeds that overgrow them, and reducing the capacity of corals to build reefs. Not only are the corals themselves in jeopardy but so is the survival of the myriad species found only on coral reefs, which make coral reefs one of the most diverse ecosystems on Earth. The Southwestern Deserts Wildfire and invasive species Until recently the Mojave and Sonoran deserts of the southwestern United States were generally fireproof. There simply was not enough fuel to carry a fire from shrub to shrub or cactus. However, a number of non-native grasses have now become successfully established in much of the area, transforming fireproof desert into highly flammable grassland. Important examples include buffelgrass (Pennisetum ciliare) a non-native grass originally from Africa that is spreading rapidly over large parts of the Sonoran desert, and other grasses (e.g., Bromus rubens) in the Mojave (Brooks and Matchett 2006). Like many fire-prone grasses, these are also fire- adapted, sprouting again quickly and densely following fire, pushing out the native species, including the iconic Saguaro cactus, which is not adapted to frequent fire (Esque et al. 2004). While climate change is not implicated in the spread of buffelgrass, there is concern that warming temperatures will allow this plant to continue to thrive in the desert Southwest, and also extend its range to higher elevations.
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Examples of Ecological Impacts of Climate Change in the United States 31 The piñon pine We tend to think of established vegetation, especially in desert regions, as very drought tolerant, but drought tolerance has limits. Severe drought, especially combined with warming, has the potential to push ecosystems past those limits. That is exactly what happened recently across much of the Four Corners region, where New Mexico, Arizona, Colorado, and Utah meet. This region experienced a severe drought from 2000 to 2003, with precipitation levels 25 to 50 percent less than the long-term average. This was not the region’s most severe drought if measured by total precipitation, but it was unusual in combining low precipitation with abnormally hot temperatures. Much of this region is covered with piñon and juniper woodlands, a vegetation type that is between a forest and a shrubland. By 2003 a large fraction of the piñons in the region died, with mortality greater than 90 percent in some areas (Breshears et al. 2005). The main cause of death was infestation by the pine bark beetle (Ips confusus), which often successfully attacks trees weakened by other stresses. The consequence of this mortality is a major change in the ecosystem structure and function over a large area. We do not, in general, know the thresholds for this kind of major change before we see them occur. It is possible that many ecosystems may be subject to dramatic changes at conditions only slightly outside the observed range, especially when they are subject to many interacting stresses. Alaska and the Arctic The Arctic is warming about twice as rapidly as the rest of the planet. Warming at high latitudes causes the sea ice and seasonal snow cover to melt more rapidly (Figure 11), converting white reflective surfaces to darker ocean water or vegetation, respectively. These dark surfaces absorb more solar radiation, transferring the heat to the air, leading to higher air temperatures and more rapid melting—a feedback loop that exposes arctic marine and terrestrial ecosystems to much higher rates of warming than the rest of the planet (Chapin et al. 2005). Permafrost, the permanently frozen ground characteristic of cold regions, contains about as much carbon as the global atmosphere (Zimov et al. 2006). As high-latitude regions warm, the thawing soil will release much of this carbon to the atmosphere, which in turn will cause more warming, in another continuous feedback loop. Exactly how fast this will happen is not known. In addition, as permafrost thaws and ice volume is lost, the ground subsides unevenly, forming small thaw ponds. The organic matter that decomposes in the airless sediments of these ponds produces methane, an even more powerful greenhouse gas than CO2 (Walter et al. 2006). These powerful feedbacks to climate warming from permafrost thaw and many other finer-scale physical processes are not currently incorporated in global models. Uncertainty about the impact of these regional phenomena suggests that climate change could occur even more rapidly than models currently project. Climate change is also affecting the way humans interact with arctic ecosystems. Shorter winters mean that ice roads used for oil exploration may no longer be practical. Alternative construction methods that lay gravel roads over sensitive tundra have much greater ecological impact on tundra than the ice roads and also affect the streams and rivers from which the gravel is taken. For example, female caribou and their calves avoid roads, thereby losing access to grazing areas, and the harvesting of stream gravels disrupt an important spawning habitat for fish.
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32 Examples of Ecological Impacts of Climate Changein the United States FIGURE 11 Average arctic sea ice area for the month of September 2007 (in white) and the average from 1979 to 2000 (pink outline). SOURCE: National Snow and Ice Data Center.
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Examples of Ecological Impacts of Climate Change in the United States 33 Changes in terrestrial vegetation and effects on the arctic food chain Arctic warming has important consequences for the mixture of plants on the land. Under experimental conditions it has been shown that warming of tundra causes increased shrub growth. This vegetation change has also been observed in the field, based on repeat aerial photography, long-term field measurements, observations by indigenous people, and increased vegetation cover observed from satellites (Figure 12) (Chapin et al. 2005; Sturm et al. 2005; Goetz et al. 2005). If current trends continue, the future will see forests growing in areas previously dominated by shrubs and shrubs taking over in areas that used to hold rushes and sedges. The expansion of shrub habitat is another example of a feedback loop. Because the shrubs are taller than the vegetation they replace, they tend to trap snow, preventing it from blowing away or turning directly from snow into water vapor. This increases the availability of meltwater in the spring, and the thicker blanket of snow insulates the soil, keeping it warmer over much of the winter. The arctic microbes respond dramatically to these warmer conditions, increasing the processing of soil organic matter and making more useable nitrogen. In tundra the shrubs grow faster in response to added nitrogen than do other species (Chapin et al. 2005), thus adding to their capacity to trap snow and further warm the soil (Sturm et al. 2005). The expansion of tundra shrubs also has important ecological and social implications. The entire arctic ecosystem depends on caribou, including bears, wolves, and a range of carrion feeders, whose populations shift with the abundance of caribou. Caribou are also perhaps the most important terrestrial subsistence resource for indigenous peoples across the circumpolar Arctic. Lichens, an important winter food for caribou, are among the species that are crowded out by increased shrub growth (Cornelissen et al. 2001). The deeper snow around shrubs also makes it harder for caribou to reach the lichens beneath. And over the longer term, warming- induced increases in wildfire place an additional stress on this ecosystem because lichens recover from wildfire much more slowly than shrubs (Rupp et al. 2006). A warmer climate may help caribou in the summer. Warmer summer temperatures tend to increase food availability and, as a consequence, survival of calves. But these advantages are countered by more frequent thaw events in winter, which tend to produce a layer of ice on top of the snow, making it difficult for caribou to reach the underlying foliage. Herd sizes have been observed to decrease during periods of frequent icing (Griffith et al. 2002).
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34 Examples of Ecological Impacts of Climate Changein the United States FIGURE 12 Two photographs of the Ayiyak River in Alaska (68° 53'N, 152° 31'W), taken 50 years apart, showing larger individual shrubs, denser shrub patches, and expansion of shrubs into areas that were previously shrub-free. SOURCE: Sturm et al. 2001. Ice-dependent animals: Walrus and polar bear Climate change is having a major impact on the extent of sea ice, and therefore on the animals that depend on it, including walruses and polar bears. Walruses, for example, use ice floes as nursing platforms and as a home base from which they dive to feed on clams and other bottom- dwellers (Ray et al. 2006). Each spring walruses follow their sea-ice perches northward as ice floes in southern latitudes melt. Because of climate change, the range of year-round sea ice is shrinking and the walruses must move farther northward each year (Krupnik and Bogoslovskaya 1999; Grebmeier et al. 2006). In 2007 the sea ice moved beyond the edge of the continental shelf, where the water becomes too deep for the walruses to feed. For the first time in recorded history several thousand walruses—seeking an alternate place to rest between feeding excursions—set up camp along the beaches near the village of Wainwright, Alaska. Over time, such a dense aggregation of animals in a single location could deplete bottom food resources along the coast. This dense aggregation of animals also crushed many calves as adults moved to and from the ocean to feed (Metcalf and Robards 2008). Polar bears rely on sea ice for hunting; when the sea is covered with ice, bears can wait at openings in the ice for their favorite prey (ringed seals) to surface for air. In the open sea, seals can surface anywhere and the polar bears cannot catch them (Laidre et al. 2008).
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Examples of Ecological Impacts of Climate Change in the United States 35 Warmer waters and declines in sea ice (Figure 11) in the Bering Sea between Alaska and Russia are causing massive ecological changes (Grebmeier et al. 2006). Fish species are shifting northward in response to warmer temperatures and greater algal production in the water column. This reduces the organic matter that falls to the seafloor as sediments, reducing the productivity of the seafloor ecosystem on which walrus, crabs, and other species feed. As sea ice continues to retreat this entire ice-dependent ecosystem, including coastal indigenous communities that depend on marine mammals both culturally and nutritionally, will be substantially restructured (Grebmeier et al. 2006).