Global warming is the observed increase in the average temperature of the Earths atmosphere and oceans in recent decades. They are released by the burning of fossil fuels, land clearing and agriculture, etc. The measure of the response to increased GHGs, and other anthropogenic and natural climate forcings is climate sensitivity. This sensitivity is usually expressed in terms of the temperature response expected from a doubling of CO2 in the atmosphere. The uncertainty in this range results from both the difficulty of estimating the volume of future greenhouse gas emissions and uncertainty about climate sensitivity. An increase in global temperatures can in turn cause other changes, including a rising sea level and changes in the amount and pattern of precipitation.

These changes may increase the frequency and intensity of extreme weather events, such as floods, droughts, heat waves, hurricanes, and tornados. Other consequences include higher or lower agricultural yields, glacier retreat, reduced summer streamflows, species extinctions and increases in the ranges of disease vectors.

Warming is expected to affect the number and magnitude of these events; however, it is difficult to connect particular events to global warming. Only a small minority of climate scientists discount the role that humanitys actions have played in recent warming. In principle, global warming is neutral as to the causes, but in common usage, global warming generally implies a human influence. Historical warming of the Earth See also: Temperature record of the past 1000 years Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale. 1979, according to satellite temperature measurements. 1850, world temperature is believed to have been relatively stable, with possibly regional fluctuations such as the Medieval Warm Period or the Little Ice Age. 1800s, exceeding the previous record set in 1998 by a few hundredths of a degree Celsius. Depending on the time frame, a number of temperature records are available.

These are based on different data sets, with different degrees of precision and reliability. An approximately global instrumental temperature record begins in about 1860; contamination from the urban heat island effect is believed to be small and well controlled for.

1000 years for a discussion of these records and their differences. The attribution of recent climate change is clearest for the most recent period of the last 50 years, for which the most detailed data are available. Satellite temperature measurements of the tropospheric temperature date from 1979. Climatologists agree that the earth has warmed recently. The detailed causes of this change remain an active field of research, but the scientific consensus identifies greenhouse gases as the primary cause of the recent warming. This conclusion can be controversial, especially outside the scientific community.

Earths atmosphere will, absent any mitigating actions or effects, result in warmer surface temperatures on Earth. Rather, the debate is about what the net effect of the addition of carbon dioxide and methane will be, when allowing for compounding or mitigating factors.