Climate on a skid

Global warming is likely to be far greater and faster than previous estimates, says the latest report of the Hadley Centre for Climate Prediction and Research. Merely planting more trees won't help

Published: Sunday 31 December 2000

Simmertime <br>Projection show global warming over the next century could turn out to be much worse than previously estimated, says the latest report of the uk Met Office's Hadley Centre for Climate Prediction and Research. Even if greenhouse gas ( ghg ) emissions are stabilised (which would require an overnight cut of 60-70 per cent in global carbon dioxide emissions), there would be a 1 degree Celsius rise in atmospheric temperature and a seal level rise of about one metre. Based on the findings of the Intergovernmental Panel on Climate Change ( ipcc ), the centre projects that warming over land would be 80 per cent faster than over sea; the highest emission scenario could lead to a 6 degree Celsius rise in temperature over land by 2100, 2 degree Celsius C higher than previous estimates.

The Earth has a natural abalance of releasing the heat that it absorbs from sunlight.But in recent years there has been an enormous increase in ghg s like carbon dioxide, which trap heat, leading to alarming rise in temperatures. Global warming will also cause large-scale increase in rainfall over India's western coastline, says a joint study by Hadley Centre and the Indian Institute of Technology, New Delhi.

TREES: myths and reality If you think planting more trees to absorb carbon will solve the problem, read the report carefully. Planting trees will absorb carbon dioxide from the atmosphere in some parts of the world, but in others global warming may impede tree growth or even be responsible for their death. Therefore, afforestation may not be a complete solution. There is more for those who swear by forests as 'carbon sinks'. Forest cover is usually darker than the underlying surface, especially when the surface is snow, and absorbs more sunlight than areas with no trees. Hence they might accelerate warming as darker surfaces absorb more sunlight as compared to lighter ones. Different forests reflect differing amounts of sunlight; dark green forests absorb more solar radiation than land surface. Therefore, the beneficial effects of their carbon uptake could be reduced, and in some areas, be reversed by this 'darkening effect'. The findings undermine arguments put forth by industrialised countries like the us , which wants to fulfil a major part of its carbon reduction targets under the 1997 Kyoto Protocol through sinks.
The present scenario Hadley's scientists used the general circulation model ( gcm ) to make their predictions. gcm uses a set of mathematical relationships which represent the major processes in the climate system. They report that global average temperature in 1999 was lower than in the record-breaking year of 1998, but 1999 was still the fifth warmest year since global records began in 1860. The centre has discovered startling evidence that by 2050, the world's trees and soils will start perishing, thus reducing the rate of carbon absorption by the land.

Temperatures were lower in 1999 than in 1998 the tropical Pacific changed from the warm El Nio phase to the colder La Nia phase. The past decade has been the warmest period in the 20th century, about 0.6 degree Celsius warmer than the late 19th century. Proxy measurements, involving measuring the change in tree rings and coral, indicate that temperatures over the past decade are higher than in the last 1,000 years. This has been accompanied by an increase in the number of heat waves and a reduction in the frequency of frost in many parts of the world. There are also indications of more days with heavy rainfall, the latest to be a victim of this is Britain, which has experienced the worst flooding in the past 50 years.

A range of potential causes of climate change were assessed by the scientists. They conclude that human activities since 1850 have upset this balance. Global warming due to sunlight is likely to have been offset by volcanic aerosols.
The future at a glance ipcc has recently come out with predictions of future emissions. These predictions are based on the way in which the world might develop in terms of population and economies, among other things. The projections are based on the assumption that the future world would be more prosperous and technologically advanced. With global warming, global mean precipitation is expected to rise. During the 21st century, large scale reduction in the amount of rainfall can be expected in some areas such as southern Africa, Australia, Central America and the northern region of South America.The greatest increase in precipitation over land will occur over east Asia, central Africa, eastern South America and at high northern latitudes.

Sea level rise is directly related to climate change. As the temperatures rises, there will be an increase in the frequency of short-lived extreme high water events such as storms. Bigger waves and more storms could destroy coastal defences and increase flooding. These events that will present the greatest threat. According to the scientists, the global mean sea level will rise due to thermal expansion of the ocean, the melting of glaciers and ice sheets of Greenland and Antarctica. While sea level is predicted to rise almost everywhere, there is considerable spatial variation. In some regions the sea level rise may only be close to nil, while others might experience a rise of as much as twice the global average. The predicted patterns show large increases in sea level in parts of the north Pacific and to the west of Greenland.
Melting of the Arctic The Hadley Centre has also predicted that Arctic ice will have all but melted away by 2080 unless urgent action is taken to cut down atmospheric concentration of greenhouse gases. The ice is melting due to the warming of the waters of the north Atlantic as well as rising air temperatures in the region. The thinning of the ice in the north is less worrying as most of the Arctic ice sheet floats in the ocean; its melting is not expected to significantly affect sea levels. But if ice melts on Antarctica, it is a different story as this ice is on land. Melting ice from the frozen continent will increase the volume of water in the oceans. The rise in sea level, ipcc experts say, would be 15-95 centimetres by 2100.
Disturbed ocean currents Normally, a large amount of heat is transported poleward in the north Atlantic by large ocean currents like theGulf Stream, which is a broad band of water moving slowly across the Atlantic in the tropics. The Gulf Stream has a lot of significance for Britain as it warms up its port to a considerable extent. Hadley scientists predict a possible reduction in the Gulf Stream, leading to a considerable decrease in the warm waters transported to the poles from the tropics. While this might cool the northern latitudes, the cooling will be insignificant as compared to the greenhouse effect, and both the oceans and northwestern Europe will warm up. Melting of sea ice can be expected to accelerate global warming as the ocean surface would not reflect as much sunlight as ice sheets.
Effect on the carbon cycle The Hadley report mentions that it is the first to assess the complex interrelation between climate change and the carbon cycle. Carbon reservoirs in the oceans, land and in the atmosphere continuously exchange carbon. The oceans absorb carbon dioxide at the colder higher latitudes, releasing it near the tropics. Plants, too, absorb carbon dioxide from the atmosphere during photosynthesis. Animals release carbon dioxide through respiration. This is known as the carbon cycle.

While ocean and land ecosystems absorb about half the carbon emissions at present, this absorption is sensitive to changes in climate as well as to atmospheric carbon dioxide concentrations. Though a warmer climate would increase photosynthesis, leading to more absorption of carbon dioxide, it isn't that simple. A warmer atmosphere would also increase plant and soil respiration rates, leading to release of more carbon into the atmosphere. In some regions, the changes in climate can also reduce the rate of photosynthesis, thus reducing the ability of vegetation to absorb carbon. But the Hadley Centre claims it is the first to study how global warming might impact upon the carbon cycle itself. Therefore, it questions earlier predictions of climate change - even those that take the carbon cycle into account. The scientists say additional carbon resulting from human activity has raised levels of atmospheric carbon dioxide by 30 per cent over the past 150 years. Climate change will alter the much larger natural carbon cycle.
Plants and animals With global warming, vegetation is predicted to move poleward in the Northern Hemisphere. The Hadley report also says soils and vegetation will stop absorbing carbon and start emitting more of it by 2050. The main causes are greater respiration by plants in warmer soils and a collapse of the Amazonian rainforest due to drier conditions.

The World Meteorological Organisation says deaths from heat waves in big cities are expected to double worldwide over the next two decades if nothing is done to curb global warming. In the biggest cities of the us, an average of 1,500 deaths occur due to heat waves each year. This is anticipate to increase to 3,000-4,000 deaths by 2020. Other cities vulnerable to similar changes are Toronto, Shanghai, Athens and Madrid. The problem is expected to be more acute in cities of developing countries.
The way out The media has been linking recent natural disasters, such floods in Britain, to global warming. But many have been debating such claims, going to the extent of questioning the science of climate change. There have been serious disagreements among various countries and scientific groups about the provisions of international climate treaties, which are based more on political and economic considerations than the science of ecology (see pp 30-39: Hold up! ).

There may never be complete agreement among scientists on climate change, given the wide field of scientific enquiry. Yet it is likely to become a reality worse than a nightmare if no preventive action is taken. The reports from ipcc and the Hadley Centre reinforce this arguments.

Subscribe to Daily Newsletter :

Comments are moderated and will be published only after the site moderator’s approval. Please use a genuine email ID and provide your name. Selected comments may also be used in the ‘Letters’ section of the Down To Earth print edition.