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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:
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