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What is Climate Change?

Copied from: IPCC Working Group I Support Unit

Climate is generally defined as the average weather that a location experiences, from daily and seasonal temperatures to wind patterns and precipitation. Communities, regions and countries can experience different climates - differences which can be caused by factors ranging from geography and ocean currents to the direction of prevailing winds.

Science has shown that the earth’s climate has changed naturally over the course of its history. However, the ongoing discussions about climate change are focused much less on natural trends, but on changes that are most likely caused by human activity such as the combustion of fossil fuels and the destruction of forests which release significant volumes of greenhouse gases (GHGs) into the atmosphere. The latest science has found that human activity has caused the concentration of GHGs in the atmosphere to increase dramatically, and this in turn is the most likely cause of increased global average temperatures over the last century.

Carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), and water vapour are "greenhouse gases" that help insulate the Earth and maintain a temperature that can sustain life. The gases allow radiation from the Sun to pass through the atmosphere and heat the Earth. The gases also prevent this heat from escaping back into space. This "greenhouse effect" is natural but scientists believe that human activities have led to an increase in atmospheric greenhouse gases (GHG), upsetting the natural balance and is impacting the global climate.

Activities such as the burning of fossil fuels and deforestation have led to a 30 per cent rise in CO₂ levels since the Industrial Revolution. Carbon dioxide, as a product of combustion, is by far the most abundant greenhouse gas released by humans. But other gases, such as N₂O, have more heat-trapping potential and a longer lifespan in the atmosphere.

One of the major debates has been determining what would constitute a ‘dangerous’ change in temperature, but until recently there was no international agreement on what that level would be deemed dangerous. However, in July 2009 the Major Economies Forum (17 developed and advanced developing countries, including Canada and the United States, responsible for approximately 75% of global emissions) recognized the need to reduce emissions so that the average increase in global temperature does not exceed 2ºC over “pre-industrial levels”. The Forum said limiting temperature increase to a maximum of 2% over pre-industrial levels would require developed countries to reduce their emissions by 80% below 1990 levels by 2050, with global emissions reduced by 50% below 1990 levels by 2050. This agreement is generally consistent with the latest science which calls for substantive reductions in GHGs by 2050 from current emission levels.

As more heat gets trapped within the atmosphere, changes occur in the the global climate by impacting the Earth's water cycle, ocean temperatures, wind patterns and soil moisture levels.

The Intergovernmental Panel on Climate Change, established in 1988 by the United Nations and the World Meteorological Office, reported in 2007 that global average temperatures rose by 0.74°C between 1906 and 2005, though the warming rate over the past 50 years was approximately twice that for the past 100 years. The IPCC estimated that temperatures will most likely rise between 1.8 and 4 degrees from 1999 levels by the end of the century.