An Intergovernmental Panel on Climate Change concludes that it is human activity that has essentially contributed to global warming in the last 50 years.
THERE is new and stronger evidence to suggest that most of the global warming observed over the last 50 years is attributable to human activity and that human influences will continue to change atmospheric composition throughout the century. This is the conclusion of the Third Assessment Report (TAR) of the Intergovernmental Panel on Climate Change (IPCC). Even if human influences are stabilised, the impact of the anthropogenic climate change that has been brought about already will persist for centurie s, the report, titled "Climate Change 2001", predicts.
The IPCC was established by the United Nations Environment Progra-mme (UNEP) and the World Meteorological Organisation (WMO) in 1988 to assess scientific, technical and socio-economic information that is relevant for the understanding of human-induced cl imate change, its potential impacts and options for mitigation and adaption. The volumes of the TAR dealing with various components of the problem were released during January and February.
The Second Assessment Report (SAR), which was released in 1996, had concluded: "The balance of evidence suggests a discernible human influence on global climate." It observed that the anthropogenic signal was still emerging from the background of natural climate variability. The TAR notes that since the release of SAR, "progress has been made in reducing uncertainty, particularly with respect to distinguishing and quantifying the magnitude of responses to different external influences." While complex Gl obal Climate Models (GCMs) cannot yet simulate all aspects of climate - for example, they still cannot fully account for the observed difference in the warming between the surface and the troposphere (the lowest part of the atmosphere) - confidence level s in the ability of these models to provide useful projections of future climate have improved, says the TAR.
The IPCC has three working groups (WGs). The report of WG1, "The Scientific Basis", was accepted and released in Shanghai on January 20; "Impacts, Adaptation and Vulnerability", the report of WG2, was released in Geneva on February 16; and "Mitigation", the report of WG3, was released in Accra on March 3. The first volume is the essence of the TAR and serves as the basis for evaluating impacts and vulnerabilities and arriving at strategies for adaptation and mitigation, which form the contents of the r eports of WG2 and WG3.
WG1's report builds upon the assessment made in the SAR and incorporates the findings of five years of research on climate change. A major contribution of the TAR is that it characterises better the effect of aerosols (microscopic airborne particles), li ke sulphates, particulate matter from biomass burning, fossil fuel organic carbon and fossil fuel soot as drivers of climate change. Improved simulation models have also contributed to a better assessment of climate change. The report describes the curre nt state of understanding of the climate system and provides estimates of its future evolution and their uncertainties.
According to the TAR, the globally averaged surface temperature, defined as the average of near surface air temperature over land and sea surface temperature, has increased by 0.6oC (plus or minus 0.2oC) during the 20th century. By 2100, however, the temperature is projected to increase dramatically by 1.4o to 5.8oC. The current increase of 0.6oC is about 0.15oC higher than what had been estimated by the SAR five years ago for the period up to 1994.
Similarly, the increase in the 21st century is higher than the SAR projection of 1.0o - 3.5oC. The TAR has attributed the increase of 0.6oC to the relatively high temperatures during 1995-2000 and improved methods of proc essing the data. The projection of greater increase up to 2100 has been attributed to the lower sulphur dioxide emissions in the revised emission scenarios considered by the TAR.
Interestingly, most of the warming in the 20th century occurred during two periods - 1910 to 1945 and 1976 to 2000. The report says that it is likely that the 1990s was the warmest decade and 1998 the warmest year in the instrumental record since 1861. N ew analyses of data for the Northern Hemisphere (NH) indicate that the increase in temperature in the last century was probably the largest during any century during the past 1,000 years. A similar statement, however, cannot be made about the Southern He misphere (SH) because of lack of reliable data.
On an average, the report says, between 1950 and 1993, night time daily minimum temperatures over land increased by 0.2oC a decade while the rate of increase in day time daily maximum temperature was only 0.1oC a decade. This implie s a longer freeze-free period in mid- and high-latitude regions. Using satellite data, the TAR has found that the extent of snow cover may have decreased by about 10 per cent since the late 1960s. On the other hand, ground-based observations indicate tha t a reduction of about two weeks in the annual duration of lake and river ice cover may have occurred in the mid- and high-latitudes of the NH during the 20th century.
There has also been a widespread retreat of mountain glaciers in non-polar regions during this period. The extent of summer and spring sea-ice in the NH may have decreased by about 10-15 per cent since the 1950s and there is likely to have been a 40 per cent decline in the Arctic sea-ice thickness during late summer to early autumn in recent decades and a considerably slower decline in the thickness of winter sea-ice. Tide gauge data show that the global average sea level rose between 0.1 and 0.2 m duri ng the last century.
There have been changes in other important aspects of climate as well.
Global heat content has increased since the 1950s, the period for which adequate data on sub-surface ocean temperatures are available. The report says that since the 1950s there has been a reduction in the frequency of extreme low temperatures with a sma ller increase in the frequency of extreme high temperatures.
Rainfall has probably increased by 0.2-0.3 per cent per decade over the tropical (10oN-10oS) land areas during the century gone by. However, data seem to suggest that increase in precipitation over the tropics is not evident over th e last few decades. During the same period, precipitation over the mid- and high-latitudes of the NH continents may have increased by 0.5-1.0 per cent a decade. Also, there was probably a 2-4 per cent increase in the frequency of heavy rainfall events in these areas. The reasons for this, according to the report, could be changes in atmospheric moisture content, thunderstorm activity and large-scale storm activity - all consequences of warming. However, over the sub-tropical NH (10oN-30o N), rainfall may have actually decreased by 0.3 per cent a decade.
Another globally significant change that has occurred as a result of warming is more frequent, persistent and intense warm episodes of the El Nino-Southern Oscillation (ENSO) phenomenon - which is linked to regional variations in precipitation and temper atures over much of the tropics (particularly monsoon behaviour), sub-tropics and some mid-latitude areas - since the mid-1970s as compared to the previous 100 years. However, between 1900 and 1995, there were relatively small increases in areas around t he world that have experienced severe drought or severe wet conditions. The changes in most regions have been dominated by inter-decadal and multi-decadal climate variability. The shift in ENSO towards more warm events is one such variation. But, the rep ort says, an overall trend for the entire century cannot be discerned. In some regions of Africa and Asia, the frequency of droughts has increased in recent decades.
There are also some important aspects of climate that appear not to have changed; at least not so far. For instance, a few areas of the world have not warmed in recent decades, mainly over some parts of the SH and parts of Antarctica. Indeed, no signific ant trends of Antarctic ice extent are apparent since 1978, the period of reliable satellite data. Changes in tropical and extra-tropical storm activity in terms of frequency and intensity too do not seem to show any trend for the entire century and are dominated by inter-decadal and multi-decadal variations. Analyses of storm activity based on different models seem to show different behaviour precluding any definitive conclusions, particularly in extra-tropics. Similarly, in the areas analysed, no syst ematic changes in the frequency of tornadoes, thunder days or hail events are discernible.
While these observations do indicate that global warming is a reality, it is still contested in some quarters whether the climate change is caused by human activity. The report dispels this notion by giving quantitative estimates of increases in the conc entrations of greenhouse gases (GHGs) and their 'radiative forcings'(RF) and concluding that warming in the last 50 years has been essentially owing to human activity. Radiative forcing (expressed in watts/sq.m) is a measure of the influence a factor has in altering the incoming-outgoing energy balance in the earth-atmosphere system and is an index of the importance of the factor as a potential driver of climate change. Changes in climate occur as a result of both variability within the climate system a s well as natural and anthropogenic external factors. A positive RF, such as the one caused by GHGs, tends to warm the surface. A negative RF, such as the one caused by some type of aerosol, tends to cool the surface. (Figure 2 shows a comparison of vari ous natural and anthropogenic factors.)
According to the report, the atmospheric concentration of carbon dioxide (CO2), a GHG, has increased by 31 per cent since 1750. The present concentration has not been exceeded during the past 420,000 years and probably not during the last 20 m illion years and the current rate of increase is unprecedented in the last 20,000 years. Seventy five per cent of CO2 emissions during the past 20 years is owing to fossil fuel burning; the rest is predominantly owing to changes in land use, e specially deforestation (green covers act as carbon sinks). The rate of increase of atmospheric CO2 has been 1.5 ppm (0.4 per cent) per year over the last two decades but there have been large year to year variations during the last decade (fr om 0.9 to 2.8 per cent) and this variation, according to the report, is owing to the climatic variability (like El Nino events) itself.
The atmospheric concentrations of methane (CH4), another GHG, has increased by 1050 ppb (151 per cent) since 1750 and continues to increase. Over 50 per cent of current CH4 emissions are anthropogenic - owing to fossil fuels, cattle , rice agriculture and landfills. In addition, the TAR has identified carbon monoxide (CO) emissions as a new cause that increases CH4 concentration. The atmospheric concentration of yet another GHG, nitrous oxide (N2O), has increas ed by 46 ppb (17 per cent) since 1750 and continues to increase. About a third of N2O emissions are anthropogenic, from agriculture soils, cattle feed lots and chemical industry.
According to the TAR, since 1995 regulations under the Montreal Protocol have resulted in either a slow increase or a decrease in the concentrations of halocarbons (such as CFC13 and CF2C12, both ozone-depleting GHGs). Th eir substitutes - the hydrofluorocarbons and other synthetic compounds (perfluorocarbons) and sulphur hexafluoride (SF6) - are GHGs and their concentrations are increasing. Stratospheric ozone depletion, owing to human activities involving hal ocarbons or chlorofluorocarbons (CFCs), between 1979 and 2000, had a negative RF of -0.15 w/sq m. However, the likely full compliance with the Montreal Protocol will have an interesting fallout: this negative forcing will be reduced as ozone builds up ag ain during the 21st century while positive RFs of CFCs will be replaced by positive RFs of the substitutes.
Build up of ozone in the troposphere owing to anthropogenic emissions of ozone-producing gases has a positive RF and this has increased by 36 per cent since 1750. However, unlike long-lived GHGs like CO2, these gases (and the resultant O3 build-up) respond more quickly to changes in emission patterns, notes the report.
The tar is gives a better understanding and a quantitative estimate of the effect of aerosols. The major sources of aerosols are fossil fuel and biomass burning. Since the SAR, significant progress in the aerosol mechanisms has provided a quantitative me asure of their forcings on the climate.
The direct RF of sulphate is estimated to be -0.4 w/sq.m. It is -0.2 w/sq.m for biomass aerosols, -0.1 w/sq m for fossil fuel carbon and +0.2 w/sq m for fossil fuel black carbon aerosols.
In general, aerosols mostly have a negative RF and thus produce a cooling effect, though one is far from being able to estimate accurately the total aerosol load in the atmosphere, its evolution over time and the total direct aerosol effect, the report s ays. Aerosols also have an indirect fallout of a negative RF through their effects on clouds but this mechanism needs to be studied better.
Like the short-lived GHGs, aerosols too, according to the report, respond quickly to changes in emissions.
According to the report, as against the net large positive RF of the above anthropogenic factors, natural factors, such as change in solar irradiance (due to solar oscillations and solar cycle) and stratospheric aerosols (owing to volcanic eruptions) hav e made only a small (net negative) contribution to the RF over the past century. The warming over the last 50 years owing to anthropogenic GHGs can thus be identified despite uncertainties in forcing due to anthropogenic sulphate aerosol and natural fact ors, it says.
Most of the studies, says the TAR, find that over the last 50 years the estimated rate and magnitude of warming owing to increasing concentrations of GHGs alone are comparable to, or larger than, the observed warming.
The best agreement between model simulations and observations over the last 140 years, according to the TAR, has been found when all the above anthropogenic and natural forcing factors are combined. While the forcings identified so far are sufficient to explain the observed climatic changes, the results do not preclude the possibility that other forcings may also have contributed, the report notes.
In order to make projections of atmospheric concentrations of GHGs and aerosols, and hence of the climate, for the 21st century, the TAR used six different emission scenarios (differing in the assumptions regarding the underlying population and economic growth patterns and social and environmental structures) which are an update on the IS92 series used in the SAR. These projections show that emissions of CO2 owing to fossil fuel burning are "virtually certain" to be the dominant influence on the trends in the atmospheric CO2 concentrations in the 21st century. More significantly, as CO2 concentrations increase the capacit y of oceans and land as feedback sinks of CO2 will diminish. By 2100, the carbon cycle models project atmospheric CO2 concentrations of 540 to 970 ppm for the TAR scenarios (which is 90-250 per cent above the 280 ppm of 1750 levels) . If one took into account the uncertainties in the feedback mechanisms of the biosphere sinks, the range is 490 to 1260 ppm (75-350 per cent above 1750 levels).
The scenarios also project that, relative to the year 2000, the global mean RF owing to GHGs will continue to increase, with the fraction due to CO2 increasing from the present value of slightly over half to nearly three-quarters. The effect o f direct and indirect aerosol RF will continue to be smaller in magnitude than that of CO2.
As a result, based on the new emission scenarios considered, the TAR has projected the globally averaged surface temperature to increase by 1.4o - 5.8oC. GCM simulations suggest that as a consequence nearly all land areas will warm more rapidly than the global average, particularly those at the northern high latitudes in the cold season.
The most notable, the report says, is the level of warming in the northern regions of North America, and northern and central Asia, which exceeds global mean warming by over 40 per cent. In contrast, the warming is less than the global mean change in sou th and southeast Asia in summer and in southern South America in winter.
Based on GCM simulations, water vapour concentration and rainfall are projected to increase in the 21st century. By the second half of the century, precipitation is likely to increase over northern mid- and high-latitudes and Antarctica in winter. At low latitudes, there would be both regional and temporal variations over land areas. Larger year to year variations are projected in areas where an increased mean precipitation is projected.
More significantly, from the Indian perspective, even with little or no change in the magnitude of the El Nino warming, global warming is likely to result in greater extremes of drying and heavy rainfall and increase the consequent risk of droughts and f loods, says the report. Referring specifically to the monsoon, the report says that there will be an increase in the variability of monsoon precipitation. Changes in monsoon mean duration and strength would, however, depend on the emission scenario, it a dds. However, given the limited success of GCMs in simulating monsoon behaviour, the confidence in such projections is limited, it says.
Warming over the century will reduce the extent of snow cover and sea-ice in the northern hemisphere further. Similarly, glaciers and ice caps are projected to continue their widespread retreat during the 21st century. The Antarctic ice sheet is likely t o gain mass as a result of increased precipitation while the Greenland ice sheet is likely to decrease because increased runoff will exceed increased precipitation. As a result of thermal expansion caused by warming and loss of mass from glaciers and ice caps, global mean sea level is projected to rise by 0.09 to 0.88 m between 1990 and 2100 for the full range of TAR scenarios. The SAR scenarios had given a projection of 0.13 to 0.94 m.
The lower projections, in spite of higher rates of temperature change, are attributed to the use of improved models which give a smaller contribution from glaciers and ice sheets.
Even if emissions stabilise sometime in the century, the anthropogenic climate change will persist for many centuries, the TAR observes. This is chiefly owing to the fact that GHGs (CO2, N2O, PFCs, SF6) have a lasting eff ect on atmospheric composition, radiative forcing and climate. After GHG concentrations have stabilised, the global average surface temperatures would rise at the rate of a few tenths of a degree per century rather than several degrees as would happen in the 21st century if stable values are not attained. Ice-sheets will continue to react to climate warming and contribute to sea level rise for thousands of years after the climate stabilises. Owing to the long time-scales on which deep oceans adjust to c limate change, global mean surface temperature increases and rising sea level from thermal expansion of oceans will continue for hundreds of years after stabilisation of GHG concentrations (even at present levels), says the report.