4° Turn Down the Heat – Why a 4°C Warmer World Must be Avoided
Ich habe wichtige Aussagen aus dem Report hier zusammenkopiert:
A Report for the World Bank by the Potsdam Institute for Climate Impact Research and Climate Analytics
This report provides a snapshot of recent scientific literature and new analyses of likely impacts and risks that would be associated with a 4° Celsius warming within this century. This report spells out what the world would be like if it warmed by 4 degrees Celsius, which is what scientists are nearly unanimously predicting by the end of the century, without serious policy changes.
The 4°C scenarios are devastating: the inundation of coastal cities; increasing risks for food production potentially leading to higher malnutrition rates; many dry regions becoming dryer, wet regions wetter; unprecedented heat waves in many regions, especially in the tropics; substantially exacerbated water scarcity in many regions; increased frequency of high-intensity tropical cyclones; and irreversible loss of biodiversity, including coral reef systems.
And most importantly, a 4°C world is so different from the current one that it comes with high uncertainty and new risks that threaten our ability to anticipate and plan for future adaptation needs.
The science is unequivocal that humans are the cause of global warming, and major changes are already being observed: global mean warming is 0.8°C above pre industrial levels; oceans have warmed by 0.09°C since the 1950s and are acidifying; sea levels rose by about 20 cm since pre-industrial times and are now rising at 3.2 cm per decade; an exceptional number of extreme heat waves occurred in the last decade; major food crop growing areas are increasingly affected by drought.
While the global community has committed itself to holding warming below 2°C to prevent “dangerous” climate change, the sum total of current policies—in place and pledged—will very
likely lead to warming far in excess of this level. Indeed, present emission trends put the world plausibly on a path toward 4°C warming within this century.
Despite the global community’s best intentions to keep global warming below a 2°C increase above
pre-industrial climate, higher levels of warming are increasingly likely. Finding ways to avoid that scenario is vital for the health and welfare of communities around the world.
While every region of the world will be affected, the poor and most vulnerable would be hit hardest.
A 4°C world can, and must, be avoided. But with action, a 4°C world can be avoided and we can likely hold warming below 2°C.
This report is a stark reminder that climate change affects everything.
Even with the current mitigation commitments and pledges fully implemented, there is roughly a 20 percent likelihood of exceeding 4°C by 2100.
If they are not met, a warming of 4°C could occur as early as the 2060s. Such a warming level and associated sea-level rise of 0.5 to 1 meter, or more, by 2100 would not be the end point: a further warming to levels over 6°C, with several meters of sea-level rise, would likely occur over the following centuries.
Indeed, present emission trends put the world plausibly on a path toward 4°C warming within the century.
No nation will be immune to the impacts of climate change.
Warming of 4°C can still be avoided: numerous studies show that there are technically and economically feasible emissions pathways to hold warming likely below 2°C. Thus the level of impacts that developing countries and the rest of the world experience will be a result of government, private sector, and civil society decisions and choices, including, unfortunately, inaction.
The concentration of the main greenhouse gas, carbon dioxide (CO2), has continued to increase from its preindustrial concentration of approximately 278 parts per million (ppm) to over 391 ppm in September 2012, with the rate of rise now at 1.8 ppm per year.
The present CO2 concentration is higher than paleoclimatic and geologic evidence indicates has occurred at any time in the last 15 million years.
Emissions of CO2 are, at present, about 35,000 million metric tons per year (including land-use change) and, absent further policies, are projected to rise to 41,000 million metric tons of CO2 per year in 2020.
It is also useful to recall that a global mean temperature increase of 4°C approaches the difference between temperatures today and those of the last ice age, when much of central Europe and the northern United States were covered with kilometers of ice and global mean temperatures were about 4.5°C to 7°C lower. And this magnitude of climate change—human induced—is occurring over a century, not millennia.
The global oceans have continued to warm, with about 90 percent of the excess heat energy trapped by the increased greenhouse gas concentrations since 1955 stored in the oceans as heat.
The average increase in sea levels around the world over the 20th century has been about 15 to 20 centimeters. Over the last decade the average rate of sea-level rise has increased to about 3.2 cm per decade. Should this rate remain unchanged, this would mean over 30 cm of additional sea-level rise in the 21st century.
The warming of the atmosphere and oceans is leading to an accelerating loss of ice from the Greenland and Antarctic ice sheets, and this melting could add substantially to sea-level rise in the future. If ice sheet loss continues at these rates, without acceleration, the increase in global average sea level due to this source would be about 15 cm by the end of the 21st century.
Overall, the rate of loss of ice has more than tripled since the 1993–2003 period as reported in the IPCC AR4.
As for Arctic sea ice, it reached a record minimum in September 2012, halving the area of ice covering the Arctic Ocean in summers over the last 30 years.
The effects of global warming are also leading to observed changes in many other climate and environmental aspects of the Earth system.
Preliminary estimates for the 2010 heat wave in Russia put the death toll at 55,000, annual crop failure at about 25 percent, burned areas at more than 1 million hectares, and economic losses at about US$15 billion (1 percent gross domestic product).
The 2012 drought in the United States impacted about 80 percent of agricultural land, making it the most severe drought since the 1950s.
An MIT study used historical fluctuations in temperature within countries to identify its effects on aggregate economic outcomes. It reported that higher temperatures substantially reduce economic growth in poor countries and have wide-ranging effects, reducing agricultural output, industrial output, and political stability.
The largest warming will occur over land and range from 4°C to 10°C. Increases of 6°C or more in
average monthly summer temperatures would be expected in large regions of the world, including the Mediterranean, North Africa, the Middle East, and the contiguous United States.
Projections for a 4°C world show a dramatic increase in the intensity and frequency of high-temperature extremes. Recent extreme heat waves such as in Russia in 2010 are likely to become the new normal summer in a 4°C world. Tropical South America, central Africa, and all tropical islands in the Pacific are likely to regularly experience heat waves of unprecedented magnitude and duration.
In this new high-temperature climate regime, the coolest months are likely to be substantially warmer than the warmest months at the end of the 20th century.
Extreme heat waves in recent years have had severe impacts, causing heat-related deaths, forest fires, and harvest losses. The impacts of the extreme heat waves projected for a 4°C world have not been evaluated, but they could be expected to vastly exceed the consequences experienced to date and potentially exceed the adaptive capacities of many societies and natural systems.
Sea-level rise impacts are projected to be asymmetrical even within regions and countries. Highly vulnerable cities are to be found in Mozambique, Madagascar, Mexico, Venezuela, India,
Bangladesh, Indonesia, the Philippines, and Vietnam.
With extremes of temperature, heat waves, rainfall, and drought are projected to increase with warming; risks will be much higher in a 4°C world compared to a 2°C world.
Maintaining adequate food and agricultural output in the face of increasing population and rising levels of income will be a challenge irrespective of human-induced climate change.
The projected impacts on water availability, ecosystems, agriculture, and human health could lead to large-scale displacement of populations and have adverse consequences for human security and economic and trade systems.
As global warming approaches and exceeds 2°C, the risk of crossing thresholds of nonlinear tipping elements in the Earth system, with abrupt climate change impacts and unprecedented
high-temperature climate regimes, increases.
Projections of damage costs for climate change impacts typically assess the costs of local damages, including infrastructure, and do not provide an adequate consideration of cascade effects (for example, value-added chains and supply networks) at national and regional scales.
Seaports are an example of an initial point where a breakdown or substantial disruption in infrastructure facilities could trigger impacts that reach far beyond the particular location of the loss.
Similarly, stresses on human health, such as heat waves, malnutrition, and decreasing quality
of drinking water due to seawater intrusion, have the potential to overburden health-care systems to a point where adaptation is no longer possible, and dislocation is forced.
Thus, given that uncertainty remains about the full nature and scale of impacts, there is also no certainty that adaptation to a 4°C world is possible. A 4°C world is likely to be one in which
communities, cities and countries would experience severe disruptions, damage, and dislocation, with many of these risks spread unequally.
The projected 4°C warming simply must not be allowed to occur—the heat must be turned down. Only early, cooperative, international actions can make that happen.
Levels greater than 4°C warming could be possible within this century should climate sensitivity be higher, or the carbon cycle and other climate system feedbacks more positive, than anticipated.
Developed countries are also vulnerable and at serious risk of major damages from climate change.
Results show an increase from 316 ppm (parts per million) in March 1958 to 391 ppm in September 2012.
A suite of studies, as reported by the IPCC, confirms that the observed warming cannot be explained by natural factors alone and thus can largely be attributed to anthropogenic influence (for
example, Santer et al 1995; Stott et al. 2000).
In fact, the IPCC (2007) states that during the last 50 years “the sum of solar and volcanic
forcings would likely have produced cooling, not warming”, a result which is confirmed by more recent work (Wigley and Santer 2012).
Between 1955 and 2010 the world’s oceans, to a depth of 2000 meters, have warmed on average by 0.09°C.
Furthermore, warming surface waters can enhance stratification, potentially limiting nutrient
availability to primary producers. Another particularly severe consequence of increasing ocean warming could be the expansion of ocean hypoxic zones, ultimately interfering with global
ocean production and damaging marine ecosystems.
This rise in sea levels is caused by thermal expansion of the oceans and by the addition of water to the oceans as a result of the melting and discharge of ice from mountain glaciers and
ice caps and from the much larger Greenland and Antarctic ice sheets.
A significant fraction of the world population is settled along coastlines, often in large cities with extensive infrastructure, making sea-level rise potentially one of the most severe long-term impacts of climate change, depending upon the rate and ultimate magnitude of the rise.
The rate of sea-level rise was close to 1.7 mm/year (equivalent to 1.7 cm/decade) during
the 20th century, accelerating to about 3.2 mm/year (equivalent to 3.2 cm/decade) on average since the beginning of the 1990s (Meyssignac and Cazenave 2012).
The acceleration of sea-level rise over the last two decades is mostly explained by an increasing
land-ice contribution from 1.1 cm/decade over 1972–2008 period to 1.7 cm/decade over 1993–2008 (Church et al. 2011), in particular because of the melting of the Greenland and Antarctic
ice sheets, as discussed in the next section.
The rate of land ice contribution to sea level rise has increased by about a factor of three since the 1972–1992 period.
There are significant regional differences in the rates of observed sea-level rise because of a range of factors, including differential heating of the ocean, ocean dynamics (winds and currents), and the sources and geographical location of ice melt, as well as subsidence or uplifting of continental margins.
Many tropical ocean regions have experienced faster than global average increases in sea-level
Rignot and colleagues (Rignot et al. 2011) point out that if the present acceleration continues, the ice sheets alone could contribute up to 56 cm to sea-level rise by 2100.
The oceans play a major role as one of the Earth´s large CO2 sinks.
As atmospheric CO2 rises, the oceans absorb additional CO2 in an attempt to restore the balance between uptake and release at the oceans’ surface. They have taken up approximately 25 percent of
anthropogenic CO2 emissions in the period 2000–06 (Canadell et al. 2007).
In the geological past, such observed changes in pH have often been associated with large-scale extinction events (Honisch et al. 2012).
The rate of changes in overall ocean biogeochemistry currently observed and projected appears to
be unparalleled in Earth history (Caldeira and Wickett 2003; Honisch et al. 2012).
Critically, the reaction of CO2 with seawater reduces the availability of carbonate ions that are used by various marine biota for skeleton and shell formation in the form of calcium carbonate (CaCO3).
Apart from the ice covered area, ice thickness is a relevant indicator for the loss of Arctic sea ice. The area of thicker ice (that is, older than two years) is decreasing, making the entire ice cover more vulnerable to such weather events as the 2012 August storm, which broke the large area into smaller pieces that melted relatively rapidly.
Recent scientific studies consistently confirm that the observed degree of extreme Arctic sea ice loss can only be explained by anthropogenic climate change.
Apart from severe consequences for the Arctic ecosystem and human populations associated with them, among the potential impacts of the loss of Arctic sea ice are changes in the dominating air pressure systems. Since the heat exchange between ocean and atmosphere increases as the ice disappears, large-scale wind patterns can change and extreme winters in Europe may become more frequent (Francis and Vavrus 2012; Jaiser, Dethloff, Handorf, Rinke, and Cohen 2012; Petoukhov
and Semenov 2010).
The past decade has seen an exceptional number of extreme heat waves around the world that each caused severe societal impacts (Coumou and Rahmstorf 2012). Examples of such events include
the European heat wave of 2003 (Stott et al. 2004), the Greek heat wave of 2007 (Founda and Giannaopoulos 2009), the Australian heat wave of 2009 (Karoly 2009), the Russian heat wave of 2010 (Barriopedro et al. 2011), the Texas heat wave of 2011 (NOAA 2011; Rupp et al. 2012), and the U.S. heat wave of 2012 (NOAA 2012, 2012b). These heat waves often caused many heat-related deaths, forest fires, and harvest losses (for example, Coumou and Rahmstorf 2012).
The five hottest summers in Europe since 1500 all occurred after 2002, with 2003 and 2010 being exceptional outliers.
The death toll of the 2003 heat wave is estimated at 70,000 (Field et al. 2012), with daily excess mortality reaching up to 2,200 in France (Fouillet et al. 2006). The heatwave in Russia in 2010 resulted in an estimated death toll of 55,000, of which 11,000 deaths were in Moscow alone, and more than 1 million hectares of burned land (Barriopedro et al. 2011).
Aridity (that is, the degree to which a region lacks effective, life-promoting moisture) has increased since the 1970s by about 1.74 percent per decade.
The Russian heat wave and Pakistan flood in 2010 can serve as an example of synchronicity between extreme events. During these events, the Northern Hemisphere jet stream exhibited a strongly
meandering pattern, which remained blocked for several weeks.
Large negative effects of higher temperatures on the economic growth of poor countries have been shown, with a 1°C rise in regional temperature in a given year reducing economic growth in that year by about 1.3 percent.
The nonmitigation IPCC Special Report on Emissions Scenarios (SRES) (Nakicenovic and Swart 2000), assessed in the IPCC AR4, gave a warming range for 2100 of 1.6–6.9°C above preindustrial temperatures. In these projections, about half the uncertainty range is due to the uncertainties in the climate system response to greenhouse gas emissions.
Assuming a “best guess” climate response, the warming response was projected at 2.3–4.5°C by 2100, the remaining uncertainty being due to different assumptions about how the world population, economy, and technology will develop during the 21st century.
The climate system is highly sensitive to concentrations of greenhouse gases in the atmosphere.
Ongoing ocean acidification is likely to have very severe consequences for coral reefs, various species of marine calcifying organisms, and ocean ecosystems generally (for example, Vézina & Hoegh-Guldberg 2008; Hofmann and Schellnhuber 2009).
If atmospheric CO2 reaches 450 ppm, coral reef growth around the world is expected to slow down considerably and at 550 ppm reefs are expected to start to dissolve (Cao and Caldeira 2008;
Silverman et al. 2009).
Thus, a CO2 level of below 350 ppm appears to be required for the long-term survival of coral reefs,
if multiple stressors, such as high ocean surface-water temperature events, sea-level rise, and deterioration in water quality, are included (Veron et al. 2009).
The most extreme droughts compared to local conditions are projected over the Amazon,
western United States, the Mediterranean, southern Africa, and southern Australia (Dai 2012).
According to models that bring together the biophysical impacts of climate change and economic indicators, food prices can be expected to rise sharply, regardless of the exact amount of warming (Nelson et al. 2010).
Process-based modeling considerations at the very high end of physically plausible ice-sheet melt, not used in this report, suggest that sea-level rise of as much as 2 m by 2100 might be possible at
maximum (Pfeffer et al. 2008).
For a 2°C warming by 2100 (2090–99), the median estimate of sea-level rise from the semi-empirical model is about 79 cm above 1980–99 levels.
The Greenland and Antarctic ice sheets themselves constitute a markedly different problem. Their potential contributions to future global mean sea-level rise is very large, namely 7 m and 57 m, respectively, for complete melting.
Past sea-level records indicate that it has varied by about 120 m between glacial periods and warmer interglacials most of which is due to ice-sheet melt and regrowth.
A more relevant period to look at is the last warm, or interglacial, period (120,000 years ago). The global mean temperature was then likely 1–2°C above current values, and sea level was 6.6–9.4 m above the present level (Kopp, Simons, Mitrovica, Maloof, and Oppenheimer 2009).
Despite the various caveats associated with the use of paleo-climatic data, a lesson from the past is that ice sheets may have been very sensitive to changes in climate conditions and did collapse
in the past.
The benefit of choosing a 2°C pathway rather than a 4°C pathway can be to limit up to about 20 cm of total global sea-level rise by the end of the century.
Climate change is projected to provoke a slowdown of the Gulf Stream during the 21st century and a corresponding flattening of the ocean surface. This effect alone would, in turn, cause sea level to rise in that area.
Main ice-melt sources (Greenland, Arctic Canada, Alaska, Patagonia and Antarctica).
Sea-level rise tends to be larger than the global mean at low latitudes, such as in vulnerable locations in the Indian Ocean or in the western Pacific, and less than the global mean at high latitudes, for example along the Dutch coast, because of the polar location of the ice sheets and their reduced gravitational pull after melting.
It should be noted that warming of 4°C above preindustrial temperatures by 2100 implies a commitment to further sea-level rise beyond this point, even if temperatures were stabilized.
The tourism industry, a major source of economic growth in these regions, was found to be very sensitive to sea-level rise.
Likewise, Stott et al. (2004) show that under unmitigated emission scenarios, the European summer of 2003 would be classed as an anomalously cold summer relative to the new climate by the end of the century.
Based on the same ensemble of simulations, Clark, Brown, and Murphy (2006) conclude that the intensity, duration, and frequency of summer heat waves are expected to be substantially greater over all continents, with the largest increases over Europe, North and South America, and East Asia.
In the Mediterranean and central United States, the warmest July in the period 2080–2100 will see temperatures close to 35°C, or up to 9°C above the warmest July for the present day. Note that temperatures presented here are monthly averages, which include night-time temperatures. Daytime temperatures can be expected to significantly exceed the monthly average.
Given the humanitarian impacts of recent extreme heat waves, the strong increase in the number of extreme heat waves in a 4°C world as reported here would pose enormous adaptation challenges for societies.
For example, flooding of agricultural land is also expected to severely impact crop yields in the future: 10.7 percent of South Asia´s agricultural land is projected to be exposed to inundation, accompanied by a 10 percent intensification of storm surges, with 1 m sea-level rise (Lange et al. 2010).
As agriculture is the primary water consumer globally, potential future water scarcity would put
at risk many societies’ capacity to feed their growing populations.
Finally, one major outcome of the above studies is that it is primarily the combination of climate change, population change, and changes in patterns of demand for water resources that will determine future water stress around the world, rather than climate change alone.
Ecosystems will be affected by the increased occurrence of extremes such as forest loss resulting from droughts and wildfire exacerbated by land use and agricultural expansion (Fischlin et al., 2007).
Salazar and Nobre (2010) estimates a transition from tropical forests to seasonal forest or savanna in the eastern Amazon could occur at warming at warming of 2.5–3.5°C when CO2 fertilization is not considered and 4.5–5.5°C when it is considered.
The Great Barrier Reef, for example, has been estimated to have lost 50 percent of live coral cover since 1985, which is attributed in part to coral bleaching because of increasing water temperatures
(De’ath et al., 2012).
Loss of those species presently classified as ‘critically endangered’ would lead to mass extinction on a scale that has happened only five times before in the last 540 million years.
Thus, there is a growing risk that climate change, combined with other human activities, will cause the irreversible transition of the Earth´s ecosystems into a state unknown in human experience (Barnosky et al., 2012).
It can be expected that warmer temperatures and exposure to extreme weather events
will have negative effects on psychological and mental health, as well as increase the occurrence of conflict and violence.
The scale and rapidity of climate change will not be occurring in a vacuum. It will occur in the context of economic growth and population increases that will place increasing stresses and
demands on a planetary ecosystem already approaching, or even exceeding, important limits and boundaries (Barnosky et al. 2012; Rockström et al. 2009).
With global warming exceeding 2°C, the risk of crossing activation thresholds for nonlinear tipping elements in the Earth System and irreversible climate change impacts increases (Lenton et al. 2008), as does the likelihood of transitions to unprecedented climate regimes.
There is a significant risk that the rain forest covering large areas of the Amazon basin will be lost as a result of an abrupt transition in climate toward much drier conditions and a related change in
the vegetation system. Once the collapse occurs, conditions would likely prevent rain forest from re-establishing. The tipping point for this simulation is estimated to be near 3–5°C global warming
(Lenton et al. 2008; Malhi et al. 2009; Salazar and Nobre 2010). A collapse would have devastating consequences for biodiversity, the livelihoods of indigenous people, Amazon basin hydrology and water security, nutrient cycling, and other ecosystem services.
Loss of coral reef systems would have far-reaching consequences for the human societies that depend on them.
Loss of oceanic food production could have very negative consequences for international food security as well as lead to substantial economic costs.
New estimates for crossing a threshold for irreversible decay of the Greenland ice sheet (which holds ice equivalent to 6 to 7 m of sea level) indicate this could occur when the global average temperature increase exceed roughly 1.5°C above preindustrial (range of 0.8 to 3.2°C) (Robinson et al. 2012).
While the risk of more rapid ice sheet response appears to be growing, there remains an open question as to whether risk planning should be oriented assuming 1 meter rise by 2100 or a substantially larger number, such as, 2 meters.
A 4°C world will pose unprecedented challenges to humanity.
Given that it remains uncertain whether adaptation and further progress toward development goals will be possible at this level of climate change, the projected 4°C warming simply must not be allowed to occur—the heat must be turned down.
Only early, cooperative, international actions can make that happen.