Climate change is defined as long-term fluctuations in temperature, precipitation, wind and other elements of the earth’s climate system (CPCB, 2002). It is basically the average change of climatic events over at least 30 years. Various palaeo-ecological, palaeo-botanical, palaeo-climatic studies and instrumental measurements reveal that climate has always been changing in every geological era.
However, it is only in the recent past that the concerns have been raised regarding such change. It is because of this fact the rate of climate change has been accelerated due to various anthropogenic activities especially after industrial revolution, which led to depletion of many ecological and environmental resources. Therefore, various biological communities including human beings are now unable to adapt these accelerated changes and are facing various threats.
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Changes in various elements of climate are often measured using instrumental records, e.g., direct measurements of surface temperature, precipitation and winds and proxy records, e.g., tree ring analysis (Palaeo-ecology and Palaeo-botany). After late 1970s, earth-observation satellites have also successfully been used to assess various components of the climate system. This section stresses on present information of past changes in climate variables, i.e., temperature, precipitation, snow cover, extent of land and sea ice, extreme weather and climate events, etc., derived by instrumental records, proxy data and satellite observations.
1. Changes in Temperature:
Change in global temperature recorded by instrumental records reveals that global average surface temperature has increased by 0.6 ± 0.2° C since the 19th century. The temperature is projected to further increase by 1.4° C ± 5.8° C by the end of 21st century relative to 1990. Altogether, it is estimated to increase by about 2° C till 2030 and about 4° C till 2090 relative to pre-industrial levels (IPCC, 2001).
Records show that 1990s was the warmest decade and 1998 the warmest year since 1861. Records also indicate that most of the increase in global temperature 20th century has occurred in two distinct periods, i.e., 1910 to 1945 and since 1976. The rate of increase of temperature during both periods is about 0.15°C/decade.
The changes of temperature experienced are not uniform over the globe, rather there are regional differences. Also, there are differences in the experiences of climate change between northern hemisphere and southern experiences and over land and oceans. Warming is more over northern hemisphere than southern hemisphere.
Also, warming has been greater over land compared to oceans in recent past. The rise of temperature on sea surface during the period of 1950 to 1993 is about half that of the mean land-surface air temperature. Also, regional patterns of the warming in the later half of 20th century are different than that occurred in the early part of the century. Recent period of warming, i.e., 1976 till date has been almost global, but the largest increases of temperature occurred over the mid and high latitudes on the land part in the northern hemisphere .
Satellite and balloon records of temperature, which are mostly available from 1979, show that the earth’s surface has warmed, while stratosphere has cooled. Analysis also shows warming in middle-to-lower troposphere at a rate of approximately 0.05 ± 0.10°C per decade. The global average surface temperature has increased significantly by 0.15 ± 0.05°C/decade.
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Palaeo-ecological and palaeo-botanical studies observe that the rate and duration of rising temperature in 20th century is greater than any other time during the last millennium. The 1990s was warmest decade of the millennium in the northern hemisphere and 1998 was warmest year. The proxy data show a relatively warm period during 11th to 14th centuries and a relatively cool period during 15th to 19th centuries in the northern hemisphere (IPCC, 2001). Very interestingly, study does not support ‘Medieval Warm Period’ and ‘Little Ice Age’ periods.
According to projections of IPCC (2001), global average surface temperature is projected to increase by 1.4° C to 5.8° C during 1990 to 2100. Estimates also suggest that current regional patterns and trends of warming would continue in same manner. Almost all land areas will warm more rapidly than the global average, particularly at northern high latitudes in the cold season as has been the case in later half of last century.
Few regions of North America and northern and central Asia will experience more warming than global mean warming. On the other hand, the warming will be less than the global mean change in South and South East Asia in summer and in southern South America in winter (Convention on Biological Diversity (CBD), 2003; 2006).
2. Changes in Precipitation and Atmospheric Moisture:
IPCC suggested that there is not a similar experience of precipitation change over all land areas. A few areas have experienced increased precipitation and a few decreased precipitations. Annual precipitation on land has increased at a rate of 0.5 to 1.0 per cent/decade in middle and high latitudes of the northern hemisphere except over Eastern Asia.
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Rainfall has decreased on an average of about 0.3 per cent/decade over the sub-tropics (10°N to 30°N) land surface. Tropical land-surface has experienced (about 0.2 to 0.3%/decade during the 20th century) increased precipitation, but such trends are not evident over the past few decades. However, in northern hemisphere no systematic trend of precipitation has been detected.
It is likely that concentration of atmospheric water vapour has increased by about 10 per cent per decade over many regions of the northern hemisphere. In situ surface observations and lower- tropospheric measurements from satellites and weather balloons of about last 25 years show a pattern that surface and lower- tropospheric water vapours have increased.
Precipitation and water vapour concentration is projected to increase in the decades to come. However, no trend has been estimated from available records of precipitation. There are going to be regional variations in precipitation changes pattern and trend.
3. Changes in Snow and Land-Sea-Ice Extent:
Glaciological, geodetic studies and satellite image interpretation suggest that world’s glacial extent has reduced significantly during last century, which can be positively correlated with rising temperature. Satellite data show that about 10 per cent of ice cover has been lost since late 1960s.
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There are sufficient evidences to support major retreat of alpine and continental glaciers in response to the 20th century warming. In a few maritime regions, increases in precipitation due to regional atmospheric circulation changes have overshadowed increases in temperature in the past two decades, thus glaciers have re-advanced.
Ground-based observations show that there has been a reduction of about two weeks in the annual duration of lake and river ice in the mid-to-high latitudes of the northern hemisphere over the past 100 to 150 years. Northern hemisphere sea-ice is decreasing, but no significant trends in Antarctic sea-ice extent are apparent.
Retreat of sea-ice extent of 10 to 15 per cent in Arctic during spring and summer is consistent with an increase in spring temperatures since the 1950s and, to a lesser extent, summer temperatures in the high latitudes. There is little indication of reduced Arctic sea-ice extent during winter when temperatures have increased in the surrounding regions. By contrast, there is no readily apparent relationship between decadal changes of Antarctic temperatures and sea-ice extent since 1973.
After an initial decrease in the mid-1970s, Antarctic sea-ice extent has remained stable, or even slightly increased. New data indicate that there has been an approximately 40 per cent decline in Arctic sea-ice thickness in late summer to early autumn between the period of 1958 to 1976 and the mid-1990s, and a substantially smaller decline in winter.
It is estimated that snow and sea-ice extent will decrease further in 21st century over northern hemisphere. The Antarctic ice sheet is projected to gain mass because of greater precipitation, while the Greenland ice sheet will lose mass because the increase in runoff will exceed the precipitation increase (CBD, 2003; 2006).
4. Climate Variability and Extreme Weather Events:
Analyses of various climatic elements show that regions where precipitation has increased have experienced increasing extreme precipitation events (Roy and Singh, 2002). In some regions, extreme events have increased despite the fact that total precipitation has decreased or remained constant. This is attributed to a decrease in the frequency of precipitation events.
Overall, significant increase has occurred in precipitation derived from extreme precipitation events for many mid and high latitude areas in northern hemisphere. Also, there has been a 2 to 4 per cent increase in the frequency of heavy precipitation events over the later half of the 20th century.
Over the 20th century, there was little increase in global land areas experiencing severe drought or severe floods. In some regions, e.g., parts of Asia and Africa, the frequency and intensity of droughts have been observed to increase in recent decades. In many regions, these changes are dominated by inter-decadal and multi-decadal climate variability. Inter-daily temperature variability has decreased in many regions.
Increase in daily minimum temperature has reduced the freeze period in most mid and high latitude regions. Since 1950, there has been a significant reduction in the frequency of much-below-normal seasonal mean temperatures across the globe. There is no forceful evidence to indicate that the characteristics of tropical and extra-tropical storms have changed.
Due to incomplete, limited and conflicting analyses, it is uncertain as to whether there have been any long-term and large-scale increases in the intensity and frequency of tropical and extra-tropical cyclones in the northern hemisphere. Increasing cyclones have been identified in the North Pacific, parts of North America, and Europe over the past several decades.
In the southern hemisphere, fewer studies have been completed, but they suggest a decrease in extra-tropical cyclone activity since the 1970s. Recent analyses of changes in severe local weather (e.g., tornadoes, thunderstorm days, and hail) in a few selected regions do not provide compelling evidence to suggest long-term changes. In general, trends in severe weather events are notoriously difficult to detect because of their relatively rare occurrence and large spatial variability (CBC, 2003; 2006).
5. Changes in Atmospheric Concentration of Greenhouse Gases:
Greenhouse gases include carbon dioxide, carbon monoxide, methane, etc. Concentration of carbon dioxide (CO2) in atmosphere has increased by about 3 per cent since 1750. The present CO2 concentration has been estimated as maximum in geological history. About 75 per cent of CO2 emission to the atmosphere during the past 20 years is due to fossil fuel burning.
The rest is predominantly caused by land-use change, especially deforestation. The rate of increasing CO2 concentration has been about 1.5 ppm, which is about 0.4 per cent per year over the past two decades. During the 1990s, increase of CO2 varied from 0.9 ppm (0.2%) to 2.8 ppm (0.8%). The concentration of methane (CH4) has increased by 1060 ppb (151%) since 1750. The present CH4 concentration is highest during the past 4, 20,000 years.
The annual growth in CH4 concentration slowed and became more variable during 1990s. About half of present CH4 emissions are caused by anthropogenic activities, e.g., burning of fossil fuels, cattle, rice agriculture and landfills. The concentration of nitrous oxide (N2O) has increased by 46 ppb (17%) since 1750. About a third of present N2O emissions are of anthropogenic origin, e.g., agricultural activities, soils, cattle feed lots and chemical industry (IPCC, 2001 and CBD, 2003).
The ozone in the troposphere is estimated to increase by 36 per cent since 1750, primarily due to anthropogenic emissions of ozone-forming gases. Since 1995, the atmospheric concentrations of many halocarbon gases that are both ozone-depleting and greenhouse gases are either increasing more slowly or decreasing, both in response to reduced emissions under the regulations of the Montreal Protocol.
Studies done by various researchers in different parts of country confirm that India is facing varied climate changes. Overall, average temperature, mean monthly maximum and mean monthly minimum temperatures have increased. Duration of winter seasons has been reduced significantly, while on the other hand duration of summers has lengthened (Kaul, 1979).
Rainfall is estimated to increase almost in all the parts of country (Gupta and Singh, 1981; Singh and Pandey, 1984; Mooley, 1977) but duration of season has shortened drastically. According to Ramachandaran and Rao (1992), the onset of monsoon has been delayed by two to three weeks and the withdrawal advanced by as much as four weeks over the years. The occurrence of dry spells has also increased in the country.
Extreme weather events such as floods and droughts confirm no general trend of occurrence. However, some generalization can be made in different parts of the country. North-western part of country has experienced increasing trend of severe droughts incidences, while eastern part confirms increasing intensity of moderate drought. This can be attributed to shortened rainfall seasons.
All the rainfall is received in very short duration due to which most of the water escapes towards sea and there is not enough time for percolation. On the other hand, increased rainfall in very short duration increases the frequency of floods. Northern Bihar, West Bengal and Assam have been facing increasing floods.