The principal reason for the mounting thermometer is a century and a half of industrialization: the burning of ever-greater quantities of oil, gasoline, and coal, the cutting of forests, and certain farming methods.
These activities have increased the amount of "greenhouse gases" in the atmosphere, especially carbon dioxide, methane and nitrous oxide. Such gases occur naturally and are critical for life on earth; they keep some of the sun's warmth from reflecting back into space, and without them the world would be a cold and barren place.

Greenhouse gases make up only about 1 per cent of the atmosphere, but they act like a blanket around the earth, or like the glass roof of a greenhouse. They trap heat and keep the planet some 15 degrees C warmer than it would be otherwise.

But in augmented and increasing quantities, these gases are pushing the global temperature to artificially high levels and altering the climate. Our problem today is that the volume of greenhouse gases, especially CO2, has increased too much. Emissions of these gases from industry, private households and traffic are constantly increasing and our atmosphere is heating up to an unnaturally high level. The industrialised countries in particular are responsible for this additional, unnatural greenhouse effect. While a citizen of India causes around 1 tonne of CO2 per year, a European causes 9 tonnes, and a US citizen as much as 20 tonnes per year. There will be far-reaching global consequences should it not be possible to keep this warming under control.

Carbon dioxide is responsible for over 60 per cent of the "enhanced greenhouse effect." Humans are burning coal, oil, and natural gas at a rate that is much, much faster than the speed at which these fossil fuels were created. This is releasing the carbon stored in the fuels into the atmosphere and upsetting the carbon cycle, the millennia-old, precisely balanced system by which carbon is exchanged between the air, the oceans, and land vegetation. Currently, atmospheric levels of carbon dioxide are rising by over 10 per cent every 20 years.

According to the Third Assessment Report of the Intergovernmental Panel on Climate Change (IPPC) that brings together the world's leading experts in this field, the globally averaged surface temperature is projected to increase by between 1.4 and 5.8°C from 1990 to 2100 under business-as-usual, and sea levels are expected to rise by between 9 and 88 centimetres over the same period. If nothing is done to reduce these changes, they will have major consequences for the ecosystem and our economies.

Climate change is one of the greatest environmental, social and economic threats facing the planet.

These consequences will include geographic shifts in the occurrence of different species and/or the extinction of species. Changes in rainfall patterns will put pressure on water resources in many regions, which will in turn affect both drinking water supplies and irrigation. Extreme weather events and floods will become more frequent with their well-known economic costs and human suffering. Warm seasons will become dryer in most mid-latitude continental interiors, increasing the frequency of droughts and land degradation. This will be particularly serious for areas where land degradation, desertification and droughts are already severe. Developing countries will suffer particularly, and tropical diseases will extend their geographic range. The 1990s appear to have been the warmest decade of the last Millennium, and 1998 the warmest year.

Climate change is already one of the main causes of natural disasters such as floods and droughts. The negative effects of climate change will be felt all over the world, and actually the consequences are expected to be most severe in least-developed nations which have produced few emissions.

The industrialized countries of North America and Western Europe, along with a few other states, such as Japan, are responsible for the vast bulk of past and current greenhouse-gas emissions. These emissions are a debt unwittingly incurred for the high standards of living enjoyed by a minority of the world's population. Yet those to suffer most from climate change will be in the developing world. They have fewer resources for coping with storms, with floods, with droughts, with disease outbreaks, and with disruptions to food and water supplies. They are eager for economic development themselves, but may find that this already difficult process has become more difficult because of climate change. The poorer nations of the world have done almost nothing to cause global warming yet are most exposed to its effects.