Global warming is the increase in the average temperature of the Earth's near-surface air and oceans in recent decades and its projected continuation. Global average air temperature rose 0.74 ± 0.18 °C (1.3 ± 0.32 °F) during the past century. The Intergovernmental Panel on Climate Change concludes, 'most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.'
The effects of global warming are expected to include sea level rise, floods, drought and changes in the frequency and intensity of extreme weather events. Other effects may include changes in agricultural yields, reduced summer streamflows, species extinctions and increases in the ranges of disease vectors.
Concentrations of greenhouse gases are now considerably higher than at any time during the last 650,000 years, the extent of the ice core record. It is believed that CO2 concentrations were last this high 20 million years ago. The primary international agreement on combating global warming is the Kyoto Protocol, which covers more than 160 countries and over 55% of global greenhouse gas emissions. The United States, the world's largest greenhouse gas emitter; Australia; and Kazakhstan have refused to ratify the treaty. China and India, two other large emitters, have ratified the treaty but are exempt from cutting emissions. International talks on a successor to the treaty, which ends in 2012, have begun.