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Warming, Climate Changes Part II and Impacts in the 21st Century and Beyond In order to respond Fortunately, scientists have made great strides in effectively to the risks predicting the amount of temperature change that posed by future climate can be expected for different amounts of future greenhouse gas emissions and in understanding change, decision makers how increments of globally averaged need information on the temperatures--increases of 1°C, 2°C, 3°C and types and severity of so forth--relate to a wide range of impacts. impacts that might Many of these projected impacts pose serious be expected. risks to human societies and things people care about, including water resources, coastlines, infrastructure, human health, food security, and land and ocean ecosystems. 20
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How do scientists project future climate change? T he biggest factor in determining future global warming is projecting future emissions of CO2 and other greenhouse gases--which in turn depend Each model uses a slightly different set of mathe- matical equations to represent how the atmosphere, oceans, and other parts of the climate system inter- W a r m i n g , C l i m at e C h a n g e s , a n d I m pa c t s i n t h e 2 1 s t C e n t u r y a n d B e y o n d on how people will produce and use energy, what act with each other and evolve over time. Models national and international policies might be imple- are routinely compared with one another and tested mented to control emissions, and what new tech- against observations to evaluate the accuracy and nologies might become available. Scientists try to robustness of model predictions. account for these uncertainties by developing differ- The most comprehensive suite of modeling ex- ent scenarios of how future emissions--and hence periments to project global climate changes was climate forcing--will evolve. Each of these scenarios completed in 2005.1 It included 23 different models is based on estimates of how different socioeco- from groups around the world, each of which used nomic, technological, and policy factors will change the same set of greenhouse gas emissions scenarios. over time, including population growth, economic Figure 19 shows projected global temperature activity, energy-conservation practices, energy tech- changes associated with high, medium-high, and nologies, and land use. low future emissions (and also the "committed" Scientists use climate models (see Box 4, p.14) 1The modeling experiments were part of the World Cli- to project how the climate system will respond to mate Research Programme's Coupled Model Intercom- different scenarios of future greenhouse gas concen- parison Project phase 3 (CMIP3) in support of the Inter- trations. Typically, many different models are used, governmental Panel on Climate Change (IPPC) Fourth each developed by a different modeling team. Assessment Report. Projected temperature change for three emissions scenarios Models project global mean temperature change during the 21st century for different scenarios of future emissions--high (red), medium-high (green) and low (blue)-- each of which is based on different assumptions of future population growth, economic development, life-style choices, technological change, and avail- ability of energy alternatives. Also shown are the results from "constant concentrations commitment" runs, which assume that atmo- spheric concentrations of greenhouse gases remain constant after the year 2000. Each solid line represents the average of model runs from different modeling using the same scenario, and the shaded areas provide a measure of the spread (one standard devia- tion) between the temperature changes projected by the different models. Source: FIGURE 19 National Research Council, 2010a 21
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warming--warming that will occur as a result of 2100, relative to the late 20th century, ranging from greenhouse gases that have already been emitted). less than 2°F (1.1°C) for the low emissions scenario Continued warming is projected for all three future to more than 11°F (6.1°C) for the high emissions emission scenarios, but sharp differences in global scenario. These results show that human decisions average temperature are clearly evident by the end can have a very large influence on the magnitude of of the century, with a total temperature increase in future climate change. How will temperatures be affected? L ocal temperatures vary widely from day to day, week to week, and season to season, but how will they be affected on average? Climate modelers ing amount of global average warming and then scaled to show what pattern of warming would be expected. Warming is greatest in the high latitudes have begun to assess how much of a rise in average of the Northern Hemisphere and is significantly temperature might be expected in different regions larger over land than over ocean. (Figure 20). The local warmings at each point As average temperatures continue to rise, the on the map are first divided by the correspond- number of days with a heat index above 100°F FIGURE 20 Projected warming for three emissions scenarios Models project the geographical pattern of annual average surface air temperature changes at three future time periods (relative to the average temperatures for the period 19611990) for three different scenarios of emissions. The projected warming by the end of the 21st century is less extreme in the B1 scenario, which assumes smaller greenhouse gas emissions, than in either the A1B scenario or the A2 "business as usual" scenario. Source: National Research Council 2010a 22
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Projections of Hotter Days Model projections sug- gest that, relative to the 1960s and 1970s, the number of days with a heat index above 100°F will increase markedly across the United States. Image courtesy U.S. Global Climate Research Program (the heat index combines temperature and humid- ity to determine how hot it feels) is projected to increase throughout this century (Figure 21). By the end of the century, the center of the United States is expected to experience 60 to 90 ad- ditional days per year in which the heat index is W a r m i n g , C l i m at e C h a n g e s , a n d I m pa c t s i n t h e 2 1 s t C e n t u r y a n d B e y o n d more than 100°F. Heat waves also are expected to last longer as the average global temperature increases. It follows that as global temperatures rise, the risk of heat-related illness and deaths also should rise. Similarly, there is considerable con- fidence that cold extremes will decrease, as will cold-related deaths. The ratio of record high tem- peratures to record low temperatures, currently 2 to 1, is projected to increase to 20 to 1 by mid- century and 50 to 1 by the end of the century for a mid-range emissions scenario. FIGURE 21 How is precipitation expected to change? G lobal warming is ex- pected to intensify regional contrasts in precipi- Using the same general approach as for tempera- tures, scientists can project tation that already exist: dry regional and seasonal per- areas are expected to get centage change in precipita- even drier, and wet areas tion expected for each 1°C even wetter. This is because (1.8°F) of global warming warmer temperatures tend (Figure 22). The results show to increase evaporation from that the subtropics, where oceans, lakes, plants, and most of the world's deserts soil, which, according to both theory and observa- are concentrated, are likely to see 5-10% reductions tions, will boost the amount of water vapor in the in precipitation for each degree of global warming. atmosphere by about 7% per 1°C (1.8°F) of warm- In contrast, subpolar and polar regions are expected ing. Although enhanced evaporation provides more to see increased precipitation, especially during win- atmospheric moisture for rain and snow in some ter. The overall pattern of change in the continental downwind areas, it also dries out the land surface, United States is somewhat complicated, as it lies which exacerbates the impacts of drought in some between the drying subtropics of Mexico and the regions. Caribbean and the moistening subpolar regions of 23
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FIGURE 22 Precipitation Patterns per Degree Warming Higher temperatures increase evaporation from oceans, lakes, plants, and soil, putting more water vapor in the atmosphere and, in turn, producing more rain and snow in some areas. However, increased evaporation also dries out the land surface, which reduces precipitation in some regions. This figure shows the projected percentage change per 1°C (1.8°F) of global warming for winter (DecemberFebru- ary, left) and summer (JuneAugust, right). Blue areas show where more precipitation is predicted, and red areas show where less precipitation is predicted. White areas show regions where changes are uncertain at present, be- cause there is not enough agreement among the models used on whether there will be more or less precipitation in those regions. Source: National Research Council, 2011b Canada. Most models suggest increased drying in throughout most of the United States, except for the southwestern United States. parts of the Northwest and Northeast, with particu- Observations in many parts of the world show larly sharp drops in the Southwest. A decrease in a statistically significant increase in the intensity of runoff of 5-10% per degree of warming is expected heavy rainstorms. Computer models indicate that in some river basins, including the Arkansas and the this trend will continue as Earth warms, even in Rio Grande (Figure 23). This decrease would be due subtropical regions where overall precipitation will mainly to increased evaporation because of higher decrease. In those regions, the projections show an temperatures, which will not be offset by changes increase in dry days between rainstorms with the av- in precipitation. Globally, streamflow in many tem- erage rainfall over seasons going down. In general, perate river basins outside Eurasia is likely to de- extreme rainstorms are likely to intensify by crease, especially in arid and semiarid regions. 5-10% for each 1°C (1.8°F) of global Rising temperatures and increased warming, with the greatest intensi- evaporation and drought can also be fication in the tropics, where rain expected to boost the risk of fire is heaviest. in some regions. In general, for- Changes in precipitation ests that are already fire-prone, will affect annual streamflow, such as the evergreen forests of which is roughly equal to the the western United States and amount of runoff--the water Canada, are likely to become from snow or rain that flows even more vulnerable to fire as into rivers and creeks. Global temperatures rise. The average climate models indicate that area burned by wildfire per year in future runoff is likely to decrease parts of the western United States is 24
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% change in runoff per degree warming W a r m i n g , C l i m at e C h a n g e s , a n d I m pa c t s i n t h e 2 1 s t C e n t u r y a n d B e y o n d (relative to 1971-2001) FIGURE 23 Changes in Runoff per Degree Warming Enhanced evaporation caused by warming is projected to decrease the amount of runoff--the water flowing into rivers and creeks-- in many parts of the United States. Runoff is a key index of the availability of fresh water. The figure shows the percent median change in runoff per degree of global warming relative to the period from 1971 to 2000. Red areas show where runoff is expected to decrease, green where it will increase. Source: National Research Council, 2011a expected to increase annually by two to four times per degree of warming (Figure 24). At the same time, areas dominated by shrubs and grasses, such as parts of the Southwest, may experience a reduction in fire over time as warmer temperatures cause shrubs and grasses to die out. In this case, the potential societal benefits of fewer fires would be countered by the loss of existing ecosystems. FIGURE 24 Increased Risk of Fire Rising temperatures and in- creased evaporation are expected to increase the risk of fire in many regions of the West. This figure shows the percent increase in burned areas in the West for a 1°C increase in global average temperatures relative to the median area burned during 1950-2003. For example, fire damage in the northern Rocky Mountain forests, marked by region B, is expected to more than double annually for each 1°C (1.8°F) increase in global average temperatures. Source: National Research Council, 2011a 25
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How will sea ice and snow be affected? A s global warming continues, the planet's many forms of ice are decreasing in extent, thickness, banned the use of ozone-depleting chemicals. Still, Antarctic sea ice may decrease less rapidly than and duration. Models indicate Arctic ice, in part because the that seasonally ice-free condi- Southern Ocean stores heat tions in the Arctic Ocean are at greater depths than the likely to occur before the end Arctic Ocean, where the of this century and suggest heat can't melt ice as easily. about a 25% loss in Septem- In many areas of the ber sea-ice extent for each globe, snow cover is expect- 1°C (1.8°F) in global warming. ed to diminish, with snowpack In contrast to the Arctic, sea building later in the cold season ice surrounding Antarctica has, on and melting earlier in the spring. average, expanded during the past According to one sensitivity analysis, several decades. This increase may be linked each 1°C (1.8°F) of local warming may lead to the stratospheric "ozone hole" over the Antarc- to an average 20% reduction in local snowpack in tic, which developed because of the use of ozone- the western United States. Snowpack has impor- depleting chemicals in refrigerants and spray cans. tant implications for drinking water supply and The ozone hole allows more damaging UV light to hydropower production. In places such as Siberia, get to the lower atmosphere and, in the Antarctic, parts of Greenland, and Antarctica, where tem- may have also resulted in lower temperatures as peratures are low enough to support snow over more heat escapes to space. However, this effect is long periods, the amount of snowfall may increase expected to wane as ozone returns to normal levels even as the season shortens, because the increased by later this century, due in part to the success of amount of water vapor associated with warmer the Montreal Protocol, an international treaty that temperatures may enhance snowfall. How will coastlines be affected? S ome of Earth's most densely populated regions Quantifying the future threat posed to particular lie at low elevation, making rising sea level coastlines by rising seas and floods is challeng- a cause for concern. Sea-level rise is projected ing. Many nonclimatic factors are involved, such to continue for centuries in response to human- as where people choose to build homes, and the caused increases in greenhouse gases, with an risks will vary greatly from one location to the next. estimated 0.5-1.0 meter (20-39 inches) of mean Moreover, infrastructure damage is often triggered sea-level rise by 2100. However, there is evidence by extreme events, for example hurricanes and that sea-level rise could be greater than expected earthquakes, rather than gradual change. However, due to melting of sea ice. Recent studies have there are some clear "hot spots," particularly in shown more rapid than expected melting from large urban areas on coastal deltas, including those glaciers and ice sheets. Observed sea-level rise has of the Mississippi, Nile, Ganges, and Mekong rivers. been near the top of the range of projections that If average sea level rises by 0.5 meters (20 inches) were made in 1990 (Figure 25). relative to a 1990 baseline, coastal flooding could 26
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W a r m i n g , C l i m at e C h a n g e s , a n d I m pa c t s i n t h e 2 1 s t C e n t u r y a n d B e y o n d FIGURE 25 Comparison of Projected and Observed Sea-Level Rise Observed sea-level change since 1990 has been near the top of the range projected by the Intergov- ernmental Panel on Climate Change Third Assessment Report, published in 1990 (gray-shaded area). The red line shows data derived from tide gauges from 1970 to 2003. The blue line shows satellite observations of sea- level change. Source: National Research Council, 2011a affect 5 million to 200 million people worldwide. Up where reductions in sea ice and melting permafrost to 4 million people could be permanently displaced, allow waves to batter and erode the shoreline. and erosion could claim more than 250,000 square Coastal erosion effects at 1.0 meter of sea-level kilometers of wetland and dryland (98,000 square rise would be much greater, threatening many miles, an area the size of Oregon). Relocations are parts of the U.S. coastline (Figure 26). already occurring in towns along the coast of Alaska, Projected Effects of Sea-Level Rise on the U.S. East and Gulf Coasts If sea level were to rise as much as 1 meter (3.3-feet), the areas in pink would be susceptible to coastal flooding. With a 6-me- ter (19.8-foot) rise in sea level, areas shown in red would also be susceptible. The pie charts show the percentage area of some cities that are potentially susceptible at 1-meter and 6-meter sea-level rise. Source: National Research Council, 2010a FIGURE 26 27
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How will ecosystems be affected? W hether marine or terrestrial, all organisms The American pika is a cold-adapted attempt to acclimate to a changing environ- species that is ment or else move to a more favorable location-- being isolated but climate change threatens to push some species on mountaintop "islands" by rising beyond their ability to adapt or move. Special temperatures. stress is being placed on cold-adapted species on Image courtesy mountain tops and at high latitudes. Shifts in the of J. R. Douglass, Yellowstone timing of the seasons and life-cycle events such as National Park. blooming, breeding, and hatching are causing mis- matches between species that disrupt patterns of feeding, pollination, and other key aspects of food webs. The ability of species to move and adapt also are hampered by human infrastructural barriers (e.g., roads), land use, and competition or interac- tion with other species. In the ocean, circulation changes will be a key driver of ecosystem impacts. Satellite data show that warm surface waters are mixing less with cooler, deeper waters, separating near-surface marine life from the nutrients below and ultimately reducing the amount of phytoplankton, which forms the base of the ocean food web (Figure 27). Climate change will exacerbate this problem in the tropics and subtropics. However, in temperate and polar waters, vertical mixing of waters could increase, especially with expected losses in sea ice. At the same time, ocean warming will continue to push the ranges of many marine species toward the poles. Changing ocean chemistry can result in other impacts--warmer waters could lead to a decline FIGURE 27 in subsurface oxygen, boosting the risk of "dead Effects on the Ocean Food Web The growth rate zones," where species high on the food chain are of marine phytoplankton, which form the base of largely absent because of a lack of oxygen. Ocean the ocean food web, is likely to be reduced over time because of higher ocean surface temperatures. This acidification, brought on as the oceans take in more creates a greater distance between warmer surface of the excess CO2 will threaten many species over waters and cooler deep waters, separating upper ma- time, especially mollusks and coral reefs. But not all rine life from nutrients found in deep water. The figure shows changes in phytoplankton growth (vertically life forms will suffer: some types of phytoplankton integrated annual mean primary production, or PP), and other photosynthetic organisms may benefit expressed as the percentage difference between 2090- from increases in CO2. Ocean acidification will 2099 and 1860-1869) per 1°C (1.8°F) of global warm- ing. Source: National Research Council, 2011a continue to worsen if CO2 emissions continue un- abated in the decades ahead. 28
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How will agriculture and food production be affected? T he stress of climate change on farming may threaten global food security. Although water resources from glacial melt and snowpack. Modeling indicates that the an increase in the amount of CO2-related benefits for some CO2 in the atmosphere favors crops will largely be outweighed the growth of many plants, it by negative factors if global tem- W a r m i n g , C l i m at e C h a n g e s , a n d I m pa c t s i n t h e 2 1 s t C e n t u r y a n d B e y o n d does not necessarily translate into perature rises more than 1.0°C more food. Crops tend to grow (1.8°F) from late 20th-century values more quickly in higher temperatures, (Figure 28), with the following project- leading to shorter growing periods ed impacts: and less time to produce grains. In addition, · or each degree of warming, yields of corn in F a changing climate will bring other hazards, the United States and Africa, and wheat in including greater water stress and the risk of higher India, drop by 5-15% temperature peaks that can quickly damage crops. · Crop pests, weeds, and disease shift in Agricultural impacts will vary across regions and by geographic range and frequency crop. Moderate warming and associated increases in CO2 and changes in precipitation are expected · If 5°C (9°F ) of global warming were to be to benefit crop and pasture lands in middle to high reached, most regions of the world would latitudes but decrease yield in seasonally dry and experience yield losses, and global grain prices low-latitude areas. In California, where half the would potentially double nation's fruit and vegetable crops are grown, climate Growers in prosperous areas may be able to adapt change is projected to decrease yields of almonds, to these threats, for example by varying the crops walnuts, avocados, and table grapes by up to 40 which they grow and the times at which they are percent by 2050. Regional assessments for other grown. However, adaptation may be less effective parts of the world consistently conclude that climate where local warming exceeds 2°C (3.6°F) and will be change presents serious risk to critical staple crops limited in the tropics, where the growing season is in sub-Saharan African and in places that rely on restricted by moisture rather than temperature. Loss of Crop Yields per Degree Warming Yields of corn in the United States and Africa, and wheat in India, are projected to drop by 5-15% per de- gree of global warming. This figure also shows projected changes in yield per degree of warming for U.S. soybeans and Asian rice. The expected impacts on crop yield are from both warming and CO2 increases, assuming no crop adap- tation. Shaded regions show the likely ranges (67%) of projections. Values of global temperature change are relative to the preindustrial value; current global temperatures are roughly 0.7°C (1.3°F) above that value. Source: National Re- FIGURE 28 search Council, 2011a 29