An analysis of combined land and ocean temperatures records indicates that during the last decade globally averaged surface temperatures have been higher than in any decade since the mid 18th century. In fact, when records of tree rings are used to reconstruct temperatures back a thousand years, the 1990s may be the warmest period in a millennium. During the 20th century, a global temperature increase of about 0.6oC years has been observed.
Temperatures however, have increased rather differently in the two Hemispheres. A rapid increase in the Northern Hemisphere temperature during the 1920s and 1930s contrasts with a more gradual increase in the Southern Hemisphere. Both Hemispheres had relatively stable temperatures from the 1940s to the 1970s, although there is some evidence of cooling in the Northern Hemisphere during the 1960s. Since the 1960s in the Southern Hemisphere but after 1975 in the Northern Hemisphere, temperatures have risen sharply.
Whilst globally averaged records of temperature offer a means of assessing climate change, it is important to recognise that they represent an over-simplification. Significant latitudinal and regional differences in the extent and timing of warming exist. In addition, winter temperatures and night-time minimums may have risen more than summer temperatures and daytime maximums.
Temperature changes higher up in the atmosphere are central to the problem of human-induced global warming, because climate models predict that temperature changes with elevated concentrations of greenhouse gases will have a characteristic profile through the layers of the atmosphere, with warming up to about 6 km, and cooling at higher altitudes within the stratosphere. The cooler stratospheric temperatures are an expected consequence of the increased trapping of heat from the Earth in the lowest part of the atmosphere. Records of stratospheric temperature confirm that cooling at higher altitudes has indeed occurred, particularly since 1980. The altitude at which warming changes to cooling however, is higher than that predicted by computer models.