TABLE OF CONTENTS
WHAT IS CLIMATE CHANGE?
Life on earth as we know it is made possible by a natural greenhouse effect. As the sun's energy passes through the atmosphere to the planet's surface, it is absorbed and re-emitted as heat. Some of this heat energy escapes directly back into space. But certain gases in the atmosphere, known as greenhouse gases most importantly, carbon dioxide (
CO2), methane (CH4), nitrous oxide (N2O), and water vapour trap most of this heat and delay its escape. This greenhouse effect keeps the average temperature of the earth's surface at about 15 C; without it, the average would be -18 C.
Changes in the earth's climate have occurred naturally and over geologic time as the atmosphere has evolved. Only in the past decades, however, have we come to understand how human activities are significantly increasing the concentrations of greenhouse gases in the atmosphere at an unprecedented rate, and with unknown repercussions.
Fossil fuel consumption and deforestation since the start of the industrial revolution, for example, have led to a nearly 30% increase in CO2 levels in the atmosphere. Over the same period, CH4 levels more than doubled as a result of fossil fuel use, agricultural production and emissions from landfill sites. Humans have also added new greenhouse gases to the atmosphere, such as chlorofluorocarbons (CFCs).
The pace of change continues to be rapid, even as we begin to understand the problem. One-half of the CO2 added to the atmosphere in all of human history has been released in just the past 30 years. Present projections are that without control measures, the amount of CO2 emitted into the atmosphere over the next century will be seven times the amo unt released in all of human history to date.