Climate Change 2007 Update

1. What makes the climate change?

Greenhouse gases are produced mainly by the burning of fossil fuels

Greenhouse gas concentrations over the last 10 000 years.

The Earth’s climate is influenced by many factors, such as the amount of greenhouse gases and aerosols in the atmosphere, the amount of energy coming from the sun or the properties the Earth’s surface. Changes in those factors, through human-related or natural processes, have a warming or a cooling effect on the planet because they alter how much of this solar energy is retained or reflected back to space.

The concentrations in the atmosphere of greenhouse gases such as carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) have all increased markedly since 1750, and now exceed by far their pre-industrial levels.

Carbon dioxide is the most important anthropogenic greenhouse gas. Its concentration in the atmosphere (379 ppm in 2005) is now far higher than the natural range over the last 650 000 years (180 to 300 ppm) and is growing faster than ever since the beginning of its continuous direct measurement in 1960, mainly due to fossil fuel use and to a lesser extent to land use change. For instance, emissions of carbon dioxide from fossil fuel use increased from 6.4 Gt per year in the 90s to 7.2 Gt of carbon per year over the period 2000-2005. Concentrations of methane and nitrous oxide in the atmosphere have also greatly increased since pre-industrial times, and those increases are mostly due to human activities such as agriculture and fossil fuel use.

The effect on climate of each of the different drivers is expressed in terms of “radiative forcing”, with positive forcing causing a warming of the surface and negative forcing a cooling of it. The overall effect of human activities since 1750 is very likely (> 90% certainty) to be one of warming, with an estimated increase of energy, or radiative forcing, of 1.6 Watt per square meter over the whole planet. The relative contribution of various factors can be seen in figure 2. The main warming drivers are the various greenhouse gases and it is likely that the warming that they cause has been increasing during the industrial era at a higher rate than at any time over the last 10 000 years. The main cooling drivers are aerosols and the changes in cloud cover that they cause. More...