The global average surface temperature has increased over the 20th century by about 0.6 degrees Celsius. This increase in temperature is likely to have been the largest for any century in the last 1000 years.
Evidence from tree ring records, used to reconstruct temperatures over this period, suggests that the 1990s was the warmest period in a millennium.
It is very likely that nearly all land areas will warm more rapidly than the global average, particularly those at high northern latitudes in the cold season. There are very likely to be more hot days; fewer cold days, cold waves, and frost days; and a reduced diurnal temperature range.
|presented on this graph indicates a strong correlation between carbon dioxide content in the atmosphere and temperature. A possible scenario is when anthropogenic emissions of greenhouse gases bring the climate to a state where it reverts to the highly unstable climate of the pre-ice age period. Rather than a linear evolution, the climate follows a non-linear path with sudden and dramatic surprises when greenhouse gas levels reach an as-yet unknown trigger point.
Natural versus man made
There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities. It is unlikely that the warming is to be entirely natural. Reconstructions of climate data from the last 1,000 years also indicate that this 20th century warming was unusual and unlikely to be the response to natural forcing alone. Volcanic eruptions and variation in solar irradiance do not explain the warming in the latter half of the 20th century, but they may have contributed to the observed warming in the first half.
As we can see from models of temperature changes caused by natural forcing, we should have observed a decrease in the global average temperature lately, but we have not. We have observed an increase.
A climate model can be used to simulate the temperature changes that occur from both natural and anthropogenic causes. The simulations in a) were done with only natural forcings: solar variation and volcanic activity. In b) only anthropogenic forcings are included: greenhouse gases and sulfate aerosols. In c) both natural and anthropogenic forcings are included. The best match is obtained when both forcings are combined, as in c.