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The IPCC third scientific assessment (IPCC 2001) shows that there is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities. Since the IPCC’s 1995 Report confidence in the ability of models to project future climate has increased. For example, there is now a longer and more closely scrutinized temperature record. Reconstructions of climate data for the past 1,000 years, as well as model estimates of natural climate variations, suggest that the observed warming over the past 100 years was unusual and is unlikely to be entirely natural in origin. In addition, detection and attribution studies consistently find evidence for an anthropogenic signal in the climate record of the last 35-50 years. However, there are still many remaining gaps in information and understanding about climate change.
An increasing body of observation gives a collective picture of a warming world. Globally it is very likely that the 1990s were the warmest decade, and 1998 the warmest year, in the instrumental record, since 1861. New analyses of data from tree rings, corals, ice cores and historical records for the Northern Hemisphere indicate that the increase in temperature in the 20th century is likely to have been the largest of any century during the past 1000 years, and it is likely that the 1990s were the warmest decade and 1998 was the warmest year (IPCC 2001).
In the mid- and high-latitudes of the northern hemisphere, it is very likely that snow cover has decreased by about 10% since the late 1960s, and the annual duration of lake- and river-ice cover has shortened by about two weeks over the 20th century. It is likely that there has been about a 40% decline in Arctic sea-ice thickness during late summer to early autumn in recent decades.
Since 1750, the atmospheric concentration of carbon dioxide has increased by 31% from 280 parts per million to about 367 ppm today. The present CO2 concentration has not been exceeded during the past 420,000 years and likely not during the past 20 million years.
The globally averaged surface temperature is projected to increase by 1.4 5.8°C from 1990 to 2100. This is higher than the 1995 Second Assessment Report’s projection of 1 - 3.5°C, largely because future sulphur dioxide emissions (which help to cool the Earth) are now expected to be lower. This future warming is on top of a 0.6°C increase since 1861 (IPCC 2001).
Global average water vapour concentration and precipitation are projected to increase. More intense precipitation events are likely over many northern hemisphere’s mid- to high-latitude land areas. The observed intensities and frequencies of tropical and extra-tropical cyclones and severe local storms, however, currently show no clear long-term trends, although data are often sparse and inadequate.
Sea-levels are projected to rise by 0.09 to 0.88 metres from 1990 to 2100. Despite higher temperature projections these sea level projections are slightly lower than the range projected in the Second Assessment Report (0.13 to 0.94 metres), primarily due to the use of improved models, which give a smaller contribution from glaciers and ice sheets (IPCC 2001).
The string of record warm years and other signs of climate disruption continues the trend already documented in the previous IPCC report and other studies (IPCC, 1995; Pearce, 1995c; Karl, 1998). The greatest temperature increase has been in the southern hemisphere (Salinger, et al. 1994; Bindoff and Church, 1992), and ice shelves are retreating significantly on the Antarctic Peninsula (Johannessen, et al., 1995; Vaughan and Doake, 1996). However, the effects of change are already and will probably continue to be seen in the northern hemisphere as well: according to a report published by the Department of the Environment in the UK, London will be as hot as the Loire Valley within 20 years (Parry et al., 1996). Sulphate aerosols have counteracted warming in industrialized areas of the northern hemisphere, at least on a short-term basis (IPCC, 1995; Pearce, 1995b), but as air pollution is controlled, this effect should diminish.
New evidence also shows that climate can change or oscillate more rapidly than expected, and that deep ocean currents may be part of the control mechanism. Worse yet, a process of ice formation off Greenland which drives one of these deep currents and helps to maintain the Gulf Stream (responsible for the mild western European climate), failed completely in 1994, probably as a result of global warming (Grootes et al., 1993; IPCC, 1995; Wadhams, 1996). The deep waters of the Atlantic, Pacific and Indian Oceans have also warmed in the last few decades (Salinger et al. 1994; Bindoff and Church, 1992; Parrilla et al., 1994). The role of oceans in absorbing carbon dioxide and stabilizing climate may also be affected by changes in plankton populations (Lovelock and Kump, 1994); ocean surface warming off California is associated with a decrease in zooplankton of 80 percent since 1951 (Roemmich and McGowan, 1995), and a significant change in intertidal marine life (Barry et al., 1995), with warm-water species moving up the coast and colder-water species retreating. It is these lateral shifts in temperature patterns that may be the most important signal of global warming. The sea surface has also warmed significantly in the western Pacific off Japan (Japanese Maritime Safety Agency, 1995). However, sea levels are now expected to rise more slowly than predicted earlier, perhaps by 45 cm by 2100 (EPA, 1995; IPCC, 1995).
New studies show that methane emissions from natural wetlands have serious implications for global warming. Methane, an important greenhouse gas, is produced in summer from wetlands in western Siberia, leading to high atmospheric concentrations. In eastern Siberia, methane is produced in the sediments of permafrost lakes, and large quantities of methane are stored in permafrost. Warming in permafrost areas of Siberia and Canada might increase methane emissions to the atmosphere significantly (Dallimore and Collett, 1995).
Climate inertia and the long life of gases mean that the full effects of past emissions will occur even if future emissions are reduced, slowing the effect of emissions reductions. Even if industrialized countries reduce emissions by 30-90 percent, global emissions would reach two to three times 1990 levels, so a slow start is difficult to correct later. There are large margins of error in calculating natural sources and sinks, such that an accurate calculation for terrestrial sources and sinks is not presently possible (ENB, 1997).
The IPCC has assessed regional vulnerability to climate change impacts in 10 regions, because the ability to predict impacts for specific places and times is limited. It concluded that ecosystems, especially forests and coral reefs, are highly sensitive to climate change. Billions of people could be affected by exacerbated problems in drinking water supply, sanitation, and drought. Food production could decrease in the tropics and subtropics, despite steady global production. Significant adverse effects on small island States and low-lying deltas such as in Bangladesh, Egypt and China could displace tens of millions of people with one meter of sea-level rise. Heat stress mortality and vector-borne diseases could increase. Most effects are negative for the most vulnerable developing countries (ENB, 1997).
Much of the controversy about proving the reality of climate change is because the wrong effects are being measured. The effects should appear not as global warming, since the tropics and the poles will show little temperature change, but as global heating expressed by increased variability and shifts in the latitude of biological and climatological features in temperate regions. The tropics will grow wider and the polar regions will shrink. These effects are already being demonstrated (Barry et al. 1995). An increased frequency of the El Niño/Southern Oscillation and other ocean/atmosphere oscillations, and more severe storms, could be a result of increased energy available from global heating. The biggest recent concern, based on coupled ocean/atmosphere models, is of major changes in the oceans, particularly the Southern Ocean, such as more stability in ocean temperature gradients and a reduction of nearly a quarter in ocean fertility over the next 75 years. This would reduce the capacity of the oceans to take up carbon dioxide, thus further accelerating the greenhouse effect (AtKisson, 1997).
In the meantime, while Europe as a whole may possibly stabilize its carbon emissions, this is clearly only the beginning of the effort needed to deal with climate change (EEA, 1995). Many other countries may have serious difficulties meeting their stabilization targets under the Framework Convention on Climate Change.
One positive result of the focus on climate change has been significant progress in methods of climate prediction and impact assessment, particularly with reference to inter-annual changes such as variations in rainfall associated with the El Niño-Southern Oscillation. This is now possible because of increased ocean observations from automatic buoys and satellites, new means of network communications, and the capability to monitor climate anomalies in near-real-time on a global basis. See for example the satellite data on the El Niño phenomenon at the Jet Propulsion Laboratory http://www.jpl.nasa.gov/elnino/. The first global assessment of the 1997-98 El Niño event was held in Guayaquil, Ecuador in November 1998 and adopted the Declaration of Guayaquil. If adequately supported and made operational through the Global Climate Observing System, mechanisms to make such predictions could provide significant economic benefits in many regions (Cane et al., 1994; WMO, 1995).
References and Sources
ENB. 1997. Report of the Meetings of the FCCC Subsidiary Bodies 20-31 October 1997. Earth Negotiations Bulletin, Vol 12, No. 66. http://www.iisd.ca/linkages/vol12/enb1266e.html
Johannessen, Ola, et al. 1995. Nansen Environmental and Remote Sensing Centre, Bergen, paper presented at International Astronautical Congress, Oslo, October 1995. Cited in MacKenzie, Debora. "Polar meltdown fulfils worst predictions". New Scientist, 12 August 1995, p. 4.
Pearce, Fred. 1995c. "Hottest year heralds global warming". New Scientist, 23 December 1995, p. 5; and Pearce, Fred. "Talks on global warming resume". Panos news release in Kenya Nation, 13 March 1996.
Salinger, Jim, Neville Nicholls, Nathan Bindoff, et al. 1994. Paper presented at Greenhouse '94 Conference, Wellington, NZ, (includes Bindoff Indian Ocean warming). Cited in Thwaites, Tim. "Are the Antipodes in hot water?" New Scientist, 12 November 1994, p. 21.
Wadhams, Peter. 1996. (Scott Polar Research Institute, and coordinator of European Sub-Polar Ocean Programme), on Odden feature, cited in Lean, Geoffrey. "Deep under the ice a chilling secret lies". The Independent, 18 February 1996.
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