The IPCC noted in 2007 that:
— changes in atmospheric concentrations of greenhouse gases and aerosols, land cover and solar radiation alter the energy balance of the climate system;

— global greenhouse gas emissions due to human activities have grown since pre-industrial times, with an increase of 70% between 1970 and 2004;

— carbon dioxide (CO2) is the most important anthropogenic greenhouse gas. Its annual emissions grew by about 80% between 1970 and 2004.

The IPCC concluded that “most of the observed increase in globally averaged temperatures since the mid-20th century is very likely (over 90% probability) due to the observed increase in anthropogenic (human-made) greenhouse gas concentrations”.

The IPCC noted that global atmospheric
concentrations of CO2, methane (CH4) and nitrous oxide (N2O) have increased markedly as a result of human activities since 1750 and now far exceed pre-industrial values determined from ice cores spanning many thousands of years. Global increases in CO2 concentrations are due primarily to fossil fuel use, with land use change providing another significant, but smaller, contribution. It is very likely that the observed increase in CH4 concentration is due predominantly to agriculture and fossil fuel use. Methane growth rates have declined since the early 1990s, consistent with the total emission (the sum of anthropogenic and natural sources) being nearly constant during this period. The increase in N2O concentration is primarily due to agriculture.

The natural greenhouse effect which has maintained global temperatures within the paleo-climatologically observed range is mainly due to water vapour, with other influences coming from a wide range of variables, such as surface albedo and clouds, that can change according to circumstances. The natural greenhouse effect is much larger than that due to CO2 and the other greenhouse gases whose concentration has a direct anthropogenic influence. Much of the uncertainty in global climate change is due to how these two components of the potential greenhouse effect interact. Climate models all show positive feedback from the water vapour, which is mainly due to a warmer climate increasing water vapour content in the atmosphere.

Assessments by the IPCC indicate that human influences extend beyond increases in global average temperature to other aspects of climate.

They have:

— very probably contributed to sea level rise during the latter half of the 20th century;

— probably contributed to changes in wind patterns, affecting extra-tropical storm tracks and temperature patterns;

— probably increased temperatures of extreme hot nights, cold nights and cold days;

— more likely than not increased risk of heat waves, areas affected by drought since the 1970s and frequency of heavy precipitation events.