WHAT IS CLIMATE CHANGE?

This section has been mainly adapted from the UNFCCC site, at ...

  Definition of Climate Change

The United Nations Framework Convention on Climate Change (UNFCCC) defines Climate Change as a change of climate which is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and which is in addition to natural climate variability observed over comparable time periods.

The Greenhouse Effect and the Carbon Cycle

Life on earth is made possible by energy from the sun, which arrives mainly in the form of visible light. About 30 per cent of sunlight is scattered back into space by the outer atmosphere, but the rest reaches the earth's surface, which reflects it in the form of infrared radiation. Infrared radiation is carried slowly aloft by air currents, and its eventual escape into space is delayed by greenhouse gases such as water vapour, carbon dioxide, ozone, and methane.

These greenhouse gases make up only about 1 per cent of the atmosphere, but they act like an insulator, trapping heat and keeping the planet some 15 - 30 degrees C warmer than it would be otherwise. The natural levels of these gases are being supplemented by emissions of carbon dioxide from the burning of coal, oil, and natural gas; by additional methane and nitrous oxide produced by farming activities and changes in land use; and by several long-lived industrial gases that do not occur naturally. 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, which 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.

The result is a warming of the earth's surface and lower atmosphere. The IPCC assesses with very high confidence that the globally averaged net effect of human activities since 1750 has been one of warming. The ‘best case' computer climate models estimate that the average global temperature will rise by 1.8° C to 4.0° C by the year 2100. A temperature increase of 0.74° C occurred last century and for the next two decades, a warming of about 0.2° C per decade is projected should greenhouse gas emissions continue to rise at their current pace and are allowed to double from their pre-industrial level.

A rise in temperature will be accompanied by changes in climate - in such things as cloud cover, precipitation, wind patterns, and the duration of seasons. Warmer temperatures mean greater evaporation, and a warmer atmosphere is able to hold more moisture - hence there is more water aloft that can fall as precipitation. Similarly, dry regions are apt to lose still more moisture if the weather is hotter ; this exacerbates droughts and desertification. Even the minimum predicted shifts in climate for the 21st century are likely to be significant and disruptive. Scientific understanding and computer models have improved recently and many projections can now be made with greater certainty. The matter is serious.   Predictions of future climate impacts show that the consequences could vary from disruptive to catastrophic. The minimum warming forecast for the next 100 years is more than twice the 0.6° C increase that has occurred since 1900. . . and that earlier increase is already having marked consequences. Extreme weather events are striking more often and sea levels have already risen by 10 to 20 cm over pre-industrial averages. Sea level rise will continue for centuries due to the time scales associated with climate processes and feedbacks.