warmıng impacts by degree world - PDF

warmıng impacts world by degree Based on the National Research Council report, Climate Stabilization Targets: Emissions, Concentrations, and Impacts over Decades to Millennia (2011) 2 1 Emissions of carbon

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warmıng impacts world by degree Based on the National Research Council report, Climate Stabilization Targets: Emissions, Concentrations, and Impacts over Decades to Millennia (2011) 2 1 Emissions of carbon dioxide from the burning of fossil fuels have ushered in a new epoch during which human activities will largely determine the evolution of the Earth s climate. 2 Because carbon dioxide is long-lived in the atmosphere, increases in this key gas can effectively lock the Earth and many future generations into a range of impacts, some of which could be severe. 3 Therefore, emission reduction choices made today matter in determining impacts that will be experienced not just over the next few decades, but also into the coming centuries and millennia. 4 Policy choices can be informed by recent advances in climate science that show the relationships among increasing carbon dioxide, global warming, related physical changes, and resulting impacts. These impacts include changes in streamflow, wildfires, crop productivity, extreme hot summers, and sea-level rise, along with associated risks and vulnerabilities. Society faces important choices in this century regarding emissions of heat-trapping (green house) gases and the resulting effects on the Earth s climate, ecosystems, and people. Human activities are responsible for the observed increases in atmospheric concentrations of several important greenhouse gases. These added gases carbon dioxide in particular very likely account for most of the globally averaged warming since There is now more carbon dioxide in the air than at any time in at least 800,000 years. This amount could double or nearly triple by 2100, greatly amplifying the human impact on climate. There is widespread interest in reducing emissions of carbon dioxide and other greenhouse summary gases to stabilize atmospheric concentrations. One way to gauge the implications of any such approach is to identify particular concentrations or stabilization targets and assess the emissions reductions necessary to achieve them, as well as the climate impacts that would result. This booklet summarizes the findings of a report from the National Research Council, Climate Stabilization Targets: Emissions, Concentrations, and Impacts over Decades to Millennia (2011). The report evaluates the implications of different stabilization targets, with particular emphasis on avoiding serious or irreversible impacts on the Earth s climate. Each stabilization target results in a different future climate, with changes that may be difficult or impossible to reverse for millennia, such as melting of the Greenland Ice Sheet. Some impacts will take hundreds or even thousands of years to emerge because of inherent lags in the climate system and because of the long atmospheric lifetime of carbon dioxide. This report also evaluates impacts expected to occur in the next few decades to centuries. 3 The impacts of human activities particularly emissions of carbon dioxide, but also including other greenhouse gas emissions, land use, and population growth are so vast that they will largely control the future of the Earth s climate system. This future could bring a relatively mild change in climate, or it could deliver an extreme change from today s climate to entirely different climate conditions that will last many thousands of years. The eventual course of the climate system over millennia will be determined largely by the actions taken this century by governments, businesses, and individuals around the world. The human contribution to global warming is due to increases in the concentration of greenhouse gases and aerosol particles, which alter the Earth s energy budget. In the special case of the greenhouse gas carbon dioxide, cumulative emissions are also an important metric or measure of the effect of humans on the climate system. The best estimate is that 1,000 gigatonnes of human-emitted carbon emissions leads to about 1.75 C (3.15 F) increase in global average temperature. Cumulative carbon emissions to date (2010) are about 500 gigatonnes, and the rate of global emissions is increasing. Based on current understanding, this warming is expected to be nearly irreversible for more than 1,000 years. 1 main findings The higher the total or cumulative carbon dioxide emitted and the higher the resulting atmospheric concentration, the higher the warming will be for the next thousand years. Higher emissions would lead to more warming over many thousands of years, allowing more time for key but slow components of the Earth system to act as amplifiers of climate change. For example, warming of the deep ocean over many centuries will release additional carbon stored in deep-sea sediments, and the Greenland ice sheet could shrink or even disappear if global warming remained in the range of C ( F) for several thousand years, raising global sea level by about meters (13-24 feet). Many aspects of climate are expected to change in a linear fashion as temperatures rise. A growing body of research suggests that many important physical changes and impacts in the climate system during the next few decades to centuries will be proportional to global temperature increase. It is now possible to utilize increments of change in globally averaged temperature increases of 1 C, 2 C, 3 C, and so forth as a tool for examining a wide range of climate impacts. In turn, each increase in temperature also can be linked to a carbon dioxide emissions stabilization target around which emission policies could be structured. This framework helps decision makers weigh the potential risks of climate change; however, the costs of achieving particular emission reductions are not addressed. 1 Approaches to geoengineer future climate, e.g., to actively remove carbon from the atmosphere or reflect sunlight to space using particulate matter or mirrors are topics of active research. If effective, these may be able to reduce or reverse global warming that would otherwise be effectively irreversible. This study does not evaluate geoengineering options, and statements throughout this report regarding the commitment to climate change over centuries and millennia from near term emissions should be read as assuming no geoengineering. Reforestation or other methods of sequestration of carbon are also not considered. 4 In general, each degree C of global temperature increase can be expected to produce: 5-10% changes in precipitation across many regions 3-10% increases in the amount of rain falling during the heaviest precipitation events 5-10% changes in streamflow across many river basins 15% decreases in the annually averaged extent of sea ice across the Arctic Ocean, with 25% decreases in the yearly minimum extent in September 5-15% reductions in the yields of crops as currently grown % increases in the area burned by wildfire in parts of the western United States However, many other impacts remain difficult to quantify, in part because they depend on additional factors besides climate change. For example, changes in the risk of flood damage depend not only on precipitation but also on urbanization and other changes in land cover. In addition, some phenomena beyond the next few centuries such as the potential large-scale release of methane from deep-sea sediments could act as amplifiers that would greatly increase the size and duration of human impact on climate. Much recent attention has focused on thresholds or tipping points that might trigger widespread change. However, while The impacts of human activities particularly emissions of carbon dioxide are so vast that they will largely control the future of Earth s climate system. thresholds could be important for some phenomena, many potentially serious changes in physical climate and related impacts increase gradually, in line with the perdegree (linear) estimates outlined above. While this study did not find evidence for tipping points that could be related explicitly to particular stabilization targets, the possibility of surprises increases the larger the warming becomes. Some uncertainty remains in the relationships among the total amount of carbon dioxide emitted over time, the portion that accumulates in the atmosphere, and the resulting climate changes and their impacts. One uncertainty is that, as temperatures warm, the ability of seawater to absorb carbon dioxide is expected to decrease, and the percentage absorbed by land-based ecosystems may also decline. However, these processes will be driven by a number of interacting variables that are not yet well-quantified. Further, the amount of global temperature increase likely to result from a given increase in carbon dioxide ranges from about 30% below the best estimate to 40% above it. Thus, each stabilization target encompasses a range of potential temperature change and associated risks that must be taken into account in evaluating stabilization targets. 5 What Impacts Can Be Expected? SOME CLIMATE CHANGES AND IMPACTS OF NEXT FEW DECADES AND CENTURIES FOR 1 4 C WARMING RAIN 5 10% less rainfall per degree in Mediterranean, SW North America, southern Africa dry seasons 5 10% more rainfall per degree in Alaska and other high latitude NH areas 3 10% more heavy rain per degree in most land areas RIVERS 5 10% less streamflow per degree in some river basins, including the Arkansas and Rio Grande FOOD 5 15% reduced yield of US corn, African corn, and Indian wheat per degree SEA ICE 15% and 25% reductions in Arctic sea ice area per degree, in the annual average and September (respectively) FOR 1 2 C WARMING FIRE % increase in area burned per degree in parts of western US FOR 3 C COASTS Loss of about 250,000 square km of wetlands and drylands Many millions more people at risk of coastal flooding EXTREMES About 9 out of 10 summer seasons expected to be warmer than all but 1 summer out of 20 in the last decades of the 20 th century over nearly all land areas FOR 4 C EXTREMES About 9 out of10 summers warmer than the warmest ever experienced during the last decades of the 20 th century over nearly all land areas FOR 5 C FOOD Yield losses in most regions and potential doubling of global grain prices 1600 CO 2 -equivalent (ppmv) Stabilization Transient Warming Equilibrium Warming AND DAMAGE TO CORALS AND SHELL-FORMING MARINE LIFE DUE TO INCREASED ACIDITY AND WARMING Global Average Warming ( C) 2 C 3 C 4 C 5 C 430 ppmv 540 ppmv 670 ppmv 840 ppmv ( ) ( ) ( ) ( ) Figure 1. As atmospheric concentrations of greenhouse gases rise, global average temperatures also rise. The graph shows that as atmospheric S TA B I carbon L I Z AT dioxide I O N C concentrations O N C E N T R AT increase, I O N there will be near-term or transient warming (range of projected temperatures in blue) which is only about half as large as the total or equilibrium warming (range of projected temperatures in red) that will eventually occur if concentrations are stabilized at those values. The black stabilization arrow R01705 shows the Climate difference fig.eps in transient and equilibrium temperatures if the atmospheric concentration of carbon dioxide were stabilized at about 580 ppm. The top portion of the figure lists key impacts for given temperature increases. The impacts listed for 1-4 C warming and for 1-2 C warming are given per degree of warming. For example, streamflow is projected to be reduced by 5-10% per 1 C warming, by 2-20% at 2 C, by 15-30% at 3 C, and 20-40% at 4 C. Because equilibrium warming is about twice as large as transient warming, the impacts experienced as temperatures rise are expected to double for stabilization. 6 Large reductions in carbon dioxide emissions would be needed in order to stabilize carbon dioxide concentrations at any chosen target level. Carbon dioxide is the dominant greenhouse gas driving the observed changes in the Earth s climate today and is expected to become even more dominant in the future. The challenge of stabilizing carbon dioxide concentrations is a daunting one. Global emission rates of carbon dioxide have increased in every decade of the industrial era. About 55% of the carbon dioxide emitted by human activities each year is absorbed by oceans, plants, and soil. However, today s emissions are much greater than natural removals. Even if society managed to hold emission rates steady, carbon dioxide would continue to accumulate in the atmosphere, and warming would continue to increase. To keep atmospheric concentrations of carbon dioxide roughly steady for a few decades and avoid increasing impacts, global emissions would have to be reduced by at least 80% (see Figure 2). Even greater emission reductions would be required to maintain stability in the longer term, as the Earth system continues to respond to emissions already added. Illustrative Example of the Relationship of Emissions to Carbon Dioxide Concentrations Uncertainties about human behavior affect our ability to project future climate. These uncertainties become more and more important over time. The higher the total or cumulative carbon dioxide emitted and the higher the resulting atmospheric concentration, the higher the warming will be for the next thousand years. Increasing emissions Increasing concentra4ons Stable emissions 80% less emissions Stable concentra4on Figure 2. Large reductions in greenhouse gas emissions are needed to stop the rise in atmospheric concentrations of carbon dioxide and meet any chosen stabilization target. The graphs show how changes in greenhouse gas emissions (top panel) are related to changes in atmospheric concentrations (bottom panel). It would take an 80% reduction in greenhouse gas emissions (green line in top panel) to stabilize atmospheric concentrations (green line, bottom panel); this is due to the carbon cycle and does not depend on the value of the chosen stabilization target. Stabilizing emissions (blue line, top panel) would result in a continued rise in atmospheric concentrations (blue line, bottom panel), but not as steep a rise as if emissions continue to increase (red lines). Nations, organizations, and individuals could take a multitude of actions whether intentional, inadvertent, or both that influence emissions in the coming years. We do not yet know how these actions will combine to shape global emissions. Several possible pathways, or potential rates of emission change over time, have been developed by researchers, but we do not yet know which pathway will prove most accurate. This uncertainty increases in importance over time. 7 Earth and its residents are entering a new geological epoch one now beginning to be called the Anthropocene in which human activities are a primary force affecting climate. Our actions this century to reduce or increase greenhouse gas emissions will determine whether the Anthropocene is a relatively mild event or a severe transition extending over many thousands of years. A variety of human-produced substances affect the Earth s energy budget and thus its climate. These include greenhouse gases, whose molecular structure allows them to capture radiation that would otherwise escape from the Earth to space, and aerosols (airborne particles), which can either reflect or absorb incoming radiation from the Sun. A critical task in assessing future climate is to diagnose how atmospheric concentrations of these substances and their effects are likely to change during the coming decades and centuries. Humans generate greenhouse gases by burning fossil fuels, clearing tropical forests, and other activities. The impact of each type of greenhouse gas on climate depends on the number of molecules emitted, the strength of each molecule in trapping radiation, and the lifetime of each molecule in the atmosphere. Greenhouse gas emissions from human activities are now outstripping the earth s natural ability to remove them, increasing atmospheric concentrations. Examining how global climate may evolve over thousands of years requires analyzing a number of very slow processes triggered by long-lasting the human impact on climate increases in CO 2 concentrations and global temperature. One example of such a process over coming millennia is the growing potential for large-scale release of carbon perhaps from methane compounds stored in deep-sea sediments or permafrost. Although recent methane releases at specific points may appear dramatic, and a major release could have a substantial effect on climate, it is not yet possible to quantify the longterm risk of a major release. Ice sheets are another increasing concern over the very long term. Models indicate that Greenland s ice sheet could shrink or even disappear if global warming remained in the range of C ( F) for several thousand years. All else being equal, this would raise global sea level by about meters (13-24 feet). There is evidence that as little as 5 C (9.0 F) of local oceanic warming over a few thousand years could destabilize and deglaciate the West Antarctic ice sheet, which would raise sea level by about 5 more meters (17 feet). Whether the ice sheets could destabilize more rapidly is a topic of active research. Geological history confirms the long-term risks posed by enhanced concentrations of greenhouse gas. During the Pliocene period (from about 5.3 to 2.6 million years ago), carbon dioxide concentrations were similar to those today; the difference is that the Northern Hemisphere was free of large ice sheets at that time, and global temperatures were about 3 C (5.4 F) above today s levels. Further in the past, the Paleocene-Eocene Thermal Maximum (roughly 56 million years ago) provides an even more dramatic example. Atmospheric carbon dioxide was far higher than today, and the planet was warm enough to be free of ice. Ultimately, there are no historical analogues for the mix of temperatures, ice sheets, and CO 2 concentrations already present and projected for the Anthropocene. It is an open question whether the Earth s climate will stabilize after several 8 three small atoms, lots of warming power The human-produced greenhouse gas of most concern is carbon dioxide. It is emitted in vast amounts. 10 billion metric tons of carbon in 2008 alone Higher emissions cause more warming, and more warming causes greater impacts. 9 thousand years, or whether impacts such as sea-level rise will continue or accelerate. Greenhouse gases and the carbon cycle The most important humanproduced greenhouse gas is carbon dioxide. It is emitted in vast amounts: 10 billion metric tons of carbon (equivalent to 36 billion metric tons of CO 2 ) in 2008 alone. Though the emission rates sometimes temporarily decrease from one year to the next (due to economic downturns, for instance), huge amounts of carbon dioxide continue to be added to the atmosphere every year. About 55% of this total is absorbed relatively quickly by plants, soil, and the ocean. The rest stays in the atmosphere for much longer, decreasing only gradually. More than half of the remainder will be in the air a century later. Some of it will persist for more than a thousand years. (See Figure 3.) Although some other greenhouse gases produced by human activity are stronger absorbers of radiation then carbon dioxide, they are emitted in much smaller amounts, and thus they have less of an impact on climate ( see Figure 4). However, as a group, they are still important. In order to simplify the task of climate analysis, these gases are often characterized by the climate effect they would have if they were in the form of carbon dioxide--their carbon dioxide equivalent concentration. Carbon dioxide is by far the biggest contributor to current global warming. While each molecule of methane has about 25 times the impact of carbon dioxide over a century s time, this is counterbalanced by methane s relative scarcity it is currently less than 1% as prevalent as carbon dioxide. Overall, this means that the CO 2 -equivalent concentration of human-produced metha
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