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Vol. 13 No. 3 - July 2007

Can climate change in the past serve as a basis for future indications?

By: Anjum Farooqui*

The response of the global dynamic system influenced by a range of forcing factors is well demonstrated by the past changes in the climatic scenario of the earth. Reliably, even the largest computers cannot simulate these changes. The past records, however, could provide instructive examples of the response of the planet to the past climatic changes.These examples may provide some clues as to how such changes may affect the globe in future.

During the last Glacial Maximum in the period 25,000 to 16,000 yrs. ago, the two largest continental areas of Asia and western pacific experienced the onset of expansion of major aridity. This was preceded by periods of heavy rainfall, and consequently, stream discharge and lake levels were considerably higher than today. By 10,000 yrs ago (Holocene epoch) there was an end of glaciations and the climate in both temperate parts of China and Australia returned to conditions somewhat wetter than what is today which has been well registered in lake levels and palaeo-vegetation patterns documented from the sediments. Conversely, interglacial warm periods saw a strengthening of the monsoon with increased rainfall.

Today, the pervasive influence of anthropogenic activities has so much changed the biological and physical condition of the earth that the use of past warm periods as analogues for future greenhouse changes presents a number of difficulties. Now, boundary conditions are significantly different from anything that existed before human intervention. The critical review from the evidence of the recent past is questionable. It is not clear whether the climatic changes are induced by greenhouse effect or due to the natural climatic variability. Although the evidence of human impact as an agency of change remains circumstantial, the rapid fall in water levels in lacustrine system is the largest for the past 2000 yrs., and that points firmly in the direction of a greenhouse component already affecting the hydrgeological system.

Evidence of past climates now forms the basis of a global programmes through the International Geosphere-Biosphere Program (IGBP). It is worth mentioning that the climate operates as a single, interconnected global system in which humanity has now become one of the lead players, and the atmospheric system and environment remain in a state of delicate balance.

About 15,000 years ago, the global climatic scenario was set for, as we know them today. At this time ice sheets of northern hemisphere retreated, temperature increased, sea level rose, until the ~7000 yrs. In India, the climatic scenario during the start of Holocene (10,000 yrs.) produced significant responses especially in vegetation patterns and in hydrologic changes as recorded in the levels of closed lakes on land, estuaries and lagoons in the coastal areas. However, this significant change reacted with the ecosystems depending on the local geomorphology and monsoonal variations. The increase in CO2 content to 75ppm in the atmosphere brings about the change from complete glacial to post-glacial environment. Many reconstructions of the palaeomonsoon over the World   have relied on sediment cores and palynological succession responding to global climatic pattern. It is since the Last Glacial Maximum that the monsoon winds have abruptly intensified. This occurred in two steps, firstly, relative maximum of winds during the Early and Mid-Holocene (~9000 to 4000 yrs. BP), and somewhat reduced winds during the Late Holocene (~3500 yrs. BP). In most palynological and isotopic records the summer monsoon wind reduction is expressed as a long-term declining trend, which is attributed to the decreasing insolation in the Northern Hemisphere and reduced warming of the Tibetan Plateau. Such an interpretation is broadly supported from different continents and sub-continents, although in many cases deviations from this trend can be discerned for specific time-intervals. But, it broadly shows consistency with climate models produced so far.

Three domains of contrasting monsoon evolution within the wider South Asian monsoon region have been identified so far: The Indian Peninsula, where summer monsoon rainfall appears to be higher at present than at any other time in the Holocene; the  mountaneous northern source of Ganga river- Brahmaputra river, where summer monsoon rainfall  has decreased steadily since the Mid-Holocene; and the Sindhu and makran region, which is in the reach of a westerly winter and spring source of rainfall. The synchroneity of Early Holocene moist climate or Mid-Holocene aridity reported from several sites of both the western Himalayas and Peninsular India may be due to the common influence of the SW monsoon.

The climate leaves impact on global sea level, which has already risen by around 10 to 25 centimeters during the last 100 years, at the rate of 1 to 2 millimeters per year. It is likely that most of this rise in sea level has been due to the increase in global temperature over the last 100 years. Global warming should, on average, cause the oceans to warm and expand thus increasing sea level. Climate models indicate that about 25% of the rise in sea level during the 20th century has been due to the thermal expansion of seawater. A second major cause of rising sea level is the melting of land-based ice caps. Presently, it is uncertain to what extent the melting of the Greenland and Antarctic ice caps has contributed to global sea level rise during the 20th century.

Forecasts of a rising sea level are based on climate model results, which indicate that the Earth's average surface temperature may increase by between 1.4 and 5.8oC during the 21st century. Global warming is expected to cause a further rise of between 9 and 88 centimeters in sea level by the year 2100, with a best estimate of 50 centimeters, if emissions of greenhouse gases remain uncontrolled. This expected rate of change (an average of 5 cm per decade) is significantly faster than that experienced over the last 100 years.

Forecasting sea level rise, however, involves many uncertainties. While most scientists believe that man-made greenhouse gas emissions are changing the climate, they are less sure about the details, and particularly the speed, of this change. Global warming is the main potential impact of greenhouse gas emissions, but other aspects of the climate besides temperature may also change. For example, some studies suggest that changes in precipitation will increase snow accumulation in Antarctica, which may help to moderate the net sea level rise. Another complication is that the sea level would not rise by the same amount all over the globe due to the effects of the Earth’s rotation, local coastline variations, variation in geomorphological patterns, changes in major ocean currents, regional land subsidence and emergence, and differences in tidal patterns or ecological changes induced by intrusion of sea water.

Nevertheless, some areas of Antarctica have warmed by 2.5oC during the past 50 years, a rate of warming 5 times faster than for the Earth as a whole. Whilst scientists believe this to reflect mostly regional changes in climate, the recent summertime disintegration of the Larsen Ice Shelf has renewed speculation that climatic changes in the polar regions have the potential to cause severe impacts via a rise in global sea level over the next 100 to 200 years.

The rate of observed sea level rise suggests that there has been a net positive contribution from the huge ice sheets of Greenland and Antarctica, but observations on the ice sheets do not yet allow meaningful quantitative estimates of their separate contributions. The ice sheets remain a major source of uncertainty in accounting for past changes in sea level because of insufficient data about these ice sheets over the last 100 years.

Monsoonal pattern is extricable linked to sea surface temperatures.  With so many influencing factors, the South Asian monsoon might appear to be a hopelessly complicated phenomenon. However, the advent of computer climate models has definitely increased our understanding and know-how of the relative roles of all the governing factors. The application of models thus prepared for palaeomonsoon and climatic changes has led to successful dynamic interpretations of proxy records generated from sediments cores world over. The simulated variability of palaeomonsoons is critically important to predict as to how the monsoon will change under future climate change scenarios and should aid in explaining recent observations that suggest a strengthening of the monsoon with global warming.

*Birbal Sahni Institute of Palaeobotany, 53, University Road, Lucknow, India
E-mail: [email protected]

This article has been reproduced from the archives of EnviroNews - Newsletter of ISEB India.

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