Scientific Symposium Presentation Abstracts
September 19
Dr. A. Barrie Pittock: Ten Reasons Why Climate Change May Be More Severe than Projected*
Uncertainties in climate change science are inevitably large, due both to inadequate scientific understanding and to uncertainties in future human behavior. Policies therefore must be based on risk management, that is, on consideration of the probability times the magnitude of any deleterious outcomes for different scenarios of human behavior. In this talk I want to focus on observations and modeling studies in the last year or two in ten areas of concern, which when taken together strongly suggest that the risk of more serious outcomes is greater than was understood previously.
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The climate sensitivity, or global warming after a doubling of the pre-industrial carbon dioxide concentration, is probably in the range of 2º–6°C rather than the 2001 IPCC estimate of 1.5º–4.5ºC. This suggests a more than 50% chance of that global warming by 2100 will be 3ºC or more, a level that many consider dangerous.
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Global dimming is large but decreasing. Reductions of sunlight at the Earth’s surface by atmospheric pollution particles are diminishing as particulate emissions are brought more under control, thus decreasing their cooling effect and making the warming effect of greenhouse gases more evident.
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Permafrost melting is widespread. Observations show rapid melting of permafrost (i.e., frozen ground). This tends to reduce the reflectivity of the surface to sunlight and, as vegetable matter in the soil starts to decay, leads to emissions of carbon dioxide and methane. These changes will enhance global warming.
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Biomass feedbacks are kicking in. Observations of soil and vegetation acting as sources rather than sinks of greenhouse gases suggest an earlier than expected positive feedback, or speeding up of climate change, via the terrestrial carbon cycle. This is partly due to increasing summer droughts and wild fires.
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Arctic sea ice is retreating rapidly. Rapid recession of Arctic sea ice has been observed, again leading to a speeding up of global warming as reduced reflection of sunlight increases surface heating.
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Changes in air and sea circulations in middle and high latitudes. Different rates of warming at low and high latitudes in both hemispheres have led to increasing sea level pressure in the middle latitudes and a movement poleward of the middle latitude westerlies. This has strengthened the major surface ocean circulations, including the Antarctic Circumpolar Current. These changes will significantly affect surface climate, including sea surface temperatures and storminess, and may already have accelerated melting in Antarctica.
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Rapid changes in Antarctica. Rapid disintegration of ice shelves around the Antarctic Peninsula and subsequent acceleration of outlet glaciers point to the role of ice shelves in retarding glacier outflow, and of surface meltwater in accelerating ice shelf disintegration and outlet glacier flow rates. Strengthening and warming of the Antarctic Circumpolar Current (point 6) may add to Antarctic ice sheet disintegration by enhancing local warming, preventing sea ice formation, and undercutting ice shelves. Some indirect observations suggest that Antarctic sea ice is already decreasing, while satellite radar observations and gravity surveys show Antarctica is already losing ice and adding to sea-level rise.
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Rapid melting and faster outlet glaciers in Greenland. Large increases in surface melting of the Greenland Ice Sheet have been observed, and acceleration of outlet glaciers, probably caused by meltwater lubrication of flow, is evident.
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Tropical cyclones may be more intense. Some observational analyses point to a rapid intensification of tropical cyclones. However, modeling of tropical cyclone behavior under enhanced global warming conditions suggests only a slow increase in intensity. While the observations have their limitations, as pointed out by some skeptics, it is also clear that the modeling to date of tropical cyclones has not been in sufficient detail to capture the finer points of tropical cyclone behavior, nor perhaps the effects of subsurface warming of the ocean that supplies the energy for cyclone formation.
- Changes are occurring in the North Atlantic Ocean. A significant slowing of the North Atlantic circulation that is in part powered by the sinking of dense, high salinity surface water has been reported. This slowdown could be related to an observed freshening of the surface waters due to increased rainfall, increased river inflow, and increased ice melting in the Arctic and North Atlantic region.
The above lines of evidence (supported by well over 100 recent scientific papers), while not definitive and in some cases controversial, suggest that the balance of evidence may be swinging toward a more extreme outcome. While some of the observations may be due merely to natural fluctuations, their conjunction and, in several cases, amplifying effects are causes for concern. They suggest that critical levels of global warming may occur at even lower greenhouse gas concentrations and emissions than was considered justified in the Third Assessment Report of the Intergovernmental Panel on Climate Change report issued in 2001. More rapid rise in sea level may be imminent (an acceleration has already been observed), and more rapid regional impacts may be expected. Taken together, the new observational evidence increases the urgency of further improving climate models, and of the importance of early actions to reduce greenhouse gas emissions if we are to lower the risk of unacceptable levels of climate change.
