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The heat is on

Print edition : Mar 09, 2007



The Intergovernmental Panel on Climate Change makes its strongest statement yet on global warming and its dangerous consequences.


THERE can be no room any more for scepticism on global warming. "The warming of the climate system is unequivocal, as is now evident from increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global mean sea level," says a report of the Intergovernmental Panel on Climate Change (IPCC) released on February 2. The report is the Summary for Policymakers of "Climate Change 2007: The Physical Science Basis", which is the first volume of the Fourth Assessment Report (AR4) on climate change. The Third Assessment Report (TAR) was issued in 2001.

The report summarises the progress made in understanding human and natural drivers of climate change, observed climate change, climate processes and causes, and projections on climate change. The first volume itself, which is on the scientific basis of climate change and is the contribution of the IPCC's Working Group I (WGI) of about 600 authors from 40 countries, will be released in early March. The work received over 30,000 comments after reviews by more than 600 experts. Two other reports, one on the impacts of climate change and another on the options for mitigation, prepared by Working Groups II (WGII) and III (WGIII) respectively, will be released in early April and May. In addition, a Synthesis Report, combining the essentials of all the three volumes, will be released in November.

The WGI report builds on past IPCC assessments and incorporates new findings since 2001 to provide, on the basis of climate science, the strongest statement so far on the extent and causes of climate change. It relies on large amounts of new and more comprehensive data, more sophisticated analysis, improved understanding of the physics and chemistry of atmospheric processes, tighter limits on uncertainties, and new projections of climate change using simulations by 19 climate models, to state with very high confidence that the net effect of human activities since industrialisation began (in 1750) has been one of warming.

Significantly, the report notes that the rates of warming accelerated in the 20th century. "Most of the observed increase in globally averaged temperatures since the mid-20th century," says the report in particular, "is very likely [greater than 90 per cent probability] due to the observed increase in anthropogenic greenhouse gas (GHG) concentrations." This should be compared with the conclusion of the TAR in 2001 that "most of the observed warming in the last 50 years is likely [greater than 66 per cent probability] to have been due to the increase in GHG concentrations". The report also concludes, on the basis of climate models, that it is very unlikely (less than 10 per cent probability) that climate changes of at least seven centuries prior to 1950 were due to variability generated within the climate system alone, such as aerosol load from volcanic eruptions and changes in solar irradiance, and not due to anthropogenic forcing, which became evident only in the early 20th century.

The report adds that the impact of human activities now extends to other aspects of climate, including ocean warming and sea level rise, continental average temperatures, temperature extremes and wind patterns. An important new finding is that sea level rise accelerated in the 20th century. The report describes other important changes that have become pronounced in recent years, such as more intense precipitation, an increase in precipitation in higher latitudes and its decrease in lower latitudes, the increased frequency of droughts across the world and probably tropical cyclones too, more warm nights and fewer cold nights, the increased retreat of glaciers since the 1990s and its contribution to sea level rise, and greater warming of the Arctic than the rest of the world.

On the basis of analyses of ice cores spanning thousands of years, the report concludes that anthropogenic emissions have resulted in a marked increase in atmospheric concentrations of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) since 1750 and now far exceed `pre-industrialisation' values. The global increase in CO2 concentration, points out the report, is primarily because of fossil fuel use and land use change, while the increases in CH4 and N2O concentrations are because of agriculture. In particular, the increase in CO2 concentration from 280 parts per million (ppm) to 379 ppm in 2005 is far greater than the natural increase (from 180 ppm to 300 ppm) over the last 650,000 years.

The 1.9 ppm growth rate of CO2 concentration during 1995-2005 was, notwithstanding inter-annual variability, larger than the 1.4 ppm recorded from 1960, when continuous direct atmospheric measurement began, up to 2005, says the report. "The U.N. Framework Convention on Climate Change (UNFCCC) was in 1992. But CO2 radiative forcing increased by 20 per cent during 1995-2005, the largest change for any decade in at least the last 200 years," points out R.K. Pachauri, Chairman of the IPCC and Director-General of The Energy and Resources Institute (TERI) based in New Delhi.

While the pre-industrial concentration of CH4 was 715 parts per billion (ppb), its value in the 1990s was 1,732 ppb and in 2005 it was 1,774 ppb. This, again, is above the natural range over the last 650,000 years of 320 ppb to 790 ppb. However, the growth rate of methane, the report points out, has declined since the 1990s with total emissions (natural + anthropogenic) remaining nearly constant. Similarly, the N2O concentration increased from 270 ppb in 1750 to 319 ppb in 2005, with the growth rate more or less constant since 1980.

The report notes that numerous long-term changes in climate have been observed at continental, regional and ocean basin scales. These include changes in Arctic temperatures and ice, widespread changes in precipitation amounts and wind patterns, and extreme weather conditions such as droughts, heavy precipitation, heat waves and greater intensity of tropical cyclones. Eleven of the last 12 years (1995-2006) rank among the 12 warmest years since the global surface temperature began to be recorded in 1895 (see graph). The 0.76 oC increase in temperature over the past 100 years is higher than the TAR figure of 0.6 oC for the period 1901-2000. The warming trend over the past 50 years, according to the report, has been twice that for the past 100 years.

This finds corroboration in the increased values of water vapour content over both land and ocean since 1980, consistent with what warmer air can hold. Observations since 1961 show that the ocean has warmed to a depth of at least 3,000 metres and has been absorbing more than 80 per cent of the heat added to the climate system. Such warming causes water to expand, resulting in a rise in sea level. Mountain glaciers and snow cover have declined in both hemispheres, which also contributes to the rise in sea level.

New data since the TAR seem to indicate that losses from the ice sheets of Greenland and Antarctica contributed to the sea level rise during 1993-2003. Losses due to warming alone suffice to explain almost the entire net mass loss from Antarctica and about half the net mass loss from Greenland, with the remaining loss from Greenland being accounted for by inadequate compensation by snowfall accumulation. Arctic temperatures increased at almost twice the global average in the past 100 years. Satellite data since 1978 show that average Arctic sea ice extent decreased about 8 per cent, while the summertime minimum extent shows a decline of about 22 per cent.

According to findings based on improved satellite and in situ tide gauge measurements, the global average sea level rose 1.8 mm a year during 1961-2003 (see graph). The rate, however, was about 3.1 mm a year during 1993-2003, though it cannot be concluded yet whether this reflects a long-term trend or a decadal variability. But the report is able to say with high confidence that from the 19th to the 20th century the rate of sea level increase accelerated. The total increase in the 20th century was estimated at 0.17 m. Interestingly, however, like the TAR for the period 1910-1990, the present report too finds a discrepancy in that the total climate contributions cannot account fully for the observed sea level increase for the period 1961-2003.

On a regional scale, a significant regional report is the long-term trend in precipitation from 1900 to 2005 over many large regions. While there is increased precipitation in the upper latitudes, there is a decrease in the lower latitudes, including parts of South Asia, an aspect of concern to India. This is also consistent with the finding that while mid- and high-latitude ocean waters have freshened, low-latitude waters have become more saline, leading to changes in evaporation and precipitation. In addition, a high degree of spatial and temporal variability has been found. More intense and longer droughts have been observed over wider areas since the 1970s, particularly in the tropics and the subtropics, according to the report. The causative links to these changes include, besides higher prevalent temperatures and decreased precipitation, changes in sea-surface temperature (SST), wind patterns and decreased snow pack and snow cover.

Further, consistent with the observed warming and increased atmospheric water vapour, the frequency of intense precipitation events has increased over most land areas, notes the report. Widespread changes in extreme temperatures have also been noticed over the last 50 years. Cold days, cold nights and frost have become less frequent, while hot days, hot nights and heat waves have become more frequent. While there is observational evidence of intense tropical cyclone activity, including the North American hurricanes, there is no clear long-term trend in the annual number of tropical cyclones, the report points out.

Some aspects of climate have, however, not been seen to change. For example, the diurnal temperature range (the temperature change during the day) does not seem to have changed from 1979 to 2004, though the TAR indicated a decrease on the basis of limited data. As against the Arctic, the Antarctic sea ice extent, while showing inter-annual variability, does not seem to indicate any long-term trend. Similarly, there is insufficient evidence to suggest any long-term trend in meridional circulation of global ocean or the flow rate of the Gulf Stream or small-scale phenomena such as tornadoes, lightning and hail and dust storms.

A palaeoclimatic perspective, which has gained increased confidence from additional data since the TAR, also supports the interpretation that the warmth of the last half-century is unusual in at least the previous 1,300 years. The last time the polar regions were significantly warmer than the present for an extended period was about 125,000 years ago. Polar temperatures were 3-5 oC higher than they are at present mainly because of differences in the earth's orbit. Reductions in polar ice volume led to a 4 to 6 m rise in sea level.

On the basis of model studies, the report projects a warming of 0.2 oC a decade for the range of emission scenarios considered. Significantly, it says that even if the concentrations of all GHGs and aerosols had been kept constant at year 2000 levels, a further warming of about 0.1oC a decade would be expected. Continued GHG emissions at or above current rates, notes the report, would cause further warming and induce in the global climate system during the 21st century many changes that would very likely (greater than 90 per cent probability) be larger than those observed during the 20th century. The report expresses confidence in model predictions of patterns of warming as well as global and regional features, including changes in wind patterns, precipitation and some aspects of extreme weather and of ice.

The report points out that anthropogenic warming and sea level rise would continue for centuries owing to the timescales associated with climate processes, the response times and feedbacks, even if GHG concentrations were to be stabilised today. Simulations with a range of models suggest that it is very likely (greater than 90 per cent probability) that hot extremes, heat waves and heavy precipitation events will become more frequent. It is likely that (greater than 66 per cent probability) future tropical cyclones (typhoons and hurricanes) will become more intense, with larger peak wind speeds and heavier precipitation caused by the increase in tropical SSTs.

As regards projected precipitation, the confidence now is greater than in the TAR, says the report. The trend of increased precipitation in the upper latitudes and decreased precipitation in the lower latitudes is likely to continue into the long term as well. According to the report, if global average temperatures were to go up by 1.9 - 4.6 C compared with pre-industrial temperatures, the Greenland ice sheet would lose matter faster than it gained, producing a net contribution to sea level rise. If sustained for thousands of years, the loss of ice would eventually lead to complete elimination of the Greenland ice sheet and contribute an additional 7-8 m of sea level rise.

Clearly, no nation can afford to be complacent on the issue any more. There is pressure mounting on all countries to act decisively, including developing countries that have no targets to meet up to 2012 under Phase-I of the Kyoto Protocol. At the 12th Conference of the Parties of UNFCCC (COP 12) held in November 2006, besides the reviewing of progress and the laying down of commitments beyond 2012 by developed countries, there were proposals mooted (for instance, by Russia) to encourage voluntary commitments by developing countries as well. As emissions continue to increase at a faster rate from large developing countries such as China and India, the findings of the IPCC report are bound to have a political effect on countries like India as well.



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