The surface temperature of Venus is around 460 degree C. In the case of Mars, the temperature can dip to as low as – 140 degree C. The reason: apart from the distance factor, the amount of carbon dioxide present in the atmosphere plays a crucial role.The atmosphere of Venus is comprised mostly of CO{-2} that prevents the heat from escaping, thus turning the planet into an oven. In the case of Mars, it lacks a thick atmosphere that can retain heat.Earth has the right amount of CO{-2} in the atmosphere that traps the right amount of heat from escaping thus making our planet neither extremely hot nor cold.
Greenhouse effect
Carbon dioxide is called a greenhouse gas and the way heat is trapped in the atmosphere by CO{-2} and other greenhouse gases is called the greenhouse effect.The amount of CO{-2} in the atmosphere is crucial and any increase in its level will heat up the Earth. There is overwhelming evidence that the amount of CO{-2} emitted as a result of deforestation and burning fossil fuel (coal, oil and natural gas) has been increasing with the growth of industrialisation.There is overwhelming evidence, and there is growing consensus, that on an average the earth has warmed up by 0.6 degree C during the 20th Century. The warming of the earth, called global warming, has been pronounced during the last three decades.There is not much dispute that the earth is warming up. But what remains a contentious issue has been on predicting CO{-2} increase vis-À-vis climate change. An increase of CO{-2} level should, theoretically speaking, result in increased temperature.But the earth behaves in a more dynamic manner. Not all CO{-2} emitted ends up in the atmosphere. That is because there are several mechanisms that come into play to absorb the increased amounts of CO{-2}.
Carbon sinks
For instance, forests and oceans absorb more than 50 per cent of the CO{-2} humans produce. Hence they are called carbon sinks. Ice reflects sunlight thus reducing the impact. Yet things are beginning to look grim.
Warmer years
There has been a sudden increase in the atmospheric CO{-2} since 2001. The natural sinks may be losing their ability to absorb CO{-2}{-.} A study of ocean data between 2000 and 2007 showed that ocean’s ability to absorb CO{-2} reduced from 27 per cent to 24 per cent.The ice sheets of Antarctica and the Arctic are unable to reflect the same quantum of sunlight as they are themselves falling prey to global warming and shrinking in size (melting). East Antarctica, for instance has been losing at least 5 billion tonnes of ice every year since 2006. According to recent observations of ice loss, Antarctica could shrink by 33 per cent by 2100, leading to a sea-level rise of 1.4 metres.
Failing ocean sink
Algae and some marine organisms use CO{-2} dissolved in water and turn them into organic compounds, thus locking up the greenhouse gas. But the ocean’s ability to act as a sink depends on how well it is able to sustain life.As more and more CO{-2} gets dissolved, the ocean will turn acidic; an acidic ocean’s ability to absorb the greenhouse gas is compromised.Acidic ocean also makes it difficult for calcium-shelled marine organisms to build shells.In fact, acidic ocean can lead to leaching of shells and hence release of locked up carbon.
Plant growth myth
Increased CO{-2} levels can lead to faster plant growth (carbon fertilisation), and in turn absorb more of the greenhouse gas. Studies have however found that higher CO{-2} levels will help only certain plant species.For instance, while higher temperatures will boost plant growth in cooler regions, in the tropics they may actually impede growth.A two-decade-long study undertaken in the rainforest plots in Panama and Malaysia concluded that more than 1 degree C in temperature can actually reduce tree growth by half.But even the faster plant growth cannot remove the increasing levels of CO{-2}. And plant growth can get negatively affected when a warmer earth increases evaporation resulting in drier soils. Food crops depend on many factors like soil type, climate, and moisture content in soil. Any change in even one of them can affect a particular crop.
The consequences
Some trees are beginning to move towards higher and less hot latitudes and some fishes unable to bear the increasingly hot waters of the tropics are moving north. Glaciers are retreating for kilometres up valley, the tropic is expanding, hurricanes are becoming more frequent, drought and floods are increasingly seen.
The timeframe
While a few warmer winters cannot be attributed to global warming, more number of such warmer winters over an extended time period is more likely to be due to warming.It just depends on what timeframe one chooses to interpret the data. There is evidence from ice core samples, tree rings, and ancient corals that global temperature has been increasing.For instance, ice cores drilled from the Antarctic ice-sheet show a very close correlation between greenhouse gas levels and temperature over the past 800,000 years. A recent study has concluded that Antarctica warmed up by 0.5 degree C between 1957 and 2006, with particularly strong warming in west Antarctica.
Long predicted
But the strongest evidence of global warming comes not from actual records of past temperature but from physics and chemistry. That is the reason why scientists could predict global warming long before an increase in temperature over the 20th century became obvious.There is clear evidence that 1998 was the warmest year, and the three warmest years on record have all occurred since 1998; 19 of the warmest 20 have been after 1980. 2009 will become the fifth warmest year since 1850.
No uniformity
But a warmer globe does not produce a uniform change across the world. While some parts may see increased rainfall and floods, others may witness droughts. Even within a country, say India, average annual increase in temperature will not be uniform. There is uncertainty as to how the warming will change the rainfall pattern on a regional scale.While most of the continents experienced above-average temperature in 2009, North America actually cooled down. But there is a clear trend seen over large areas and over longer periods of time.One reason could be changes in sea surface temperature and circulation patterns. Increase in sea surface temperature can affect rainfall pattern, and reduction in ocean salinity due to ice-sheet melting can lead to changes in ocean circulation pattern.
The future scenario
It is estimated that if the current trend continues, atmospheric CO{-2} level will double pre-industrial levels by the end of this century. That will be enough to raise global temperature by around 3-5 degree C. Sea level rose by 3 mm per year in the 20th century. So how much will a 3-5 degree increase in temperature increase the sea-level rise?The Intergovernmental Panel on Climate Change has predicted sea-level rise of 0.2 to 0.6 metres by the end of this century. Scientists now think that this is a gross underestimation. They predict the sea level to rise much faster than predicted; it takes nearly 100 years for the CO{-2} emitted into the atmosphere to disperse.
The Hindu, December 10, 2009
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