The global average surface temperature has increased from the 1860s to the year 2000, the period of instrumental record. Over the 20th century this increase was 0.6°C with a very likely (see Box 2-1) confidence range of 0.4-0.8°C (see Figure 2-3).It is very likely that the 1990s was the warmest decade, and 1998 the warmest year, of the instrumental record. Extending the instrumental record with proxy data for the Northern Hemisphere indicates that over the past 1,000 years the 20th century increase in temperature is likely to have been the largest of any century, and the 1990s was likely the warmest decade (see Figure 2-3). Insufficient data are available in the Southern Hemisphere prior to the year 1860 to compare the recent warming with changes over the last 1,000 years. Since the year 1950, the increase in sea surface temperature is about half that of the mean land surface air temperature. During this period the nighttime daily minimum temperatures over land have increased on average by about 0.2°C per decade, about twice the corresponding rate of increase in daytime maximum air temperatures. These climate changes have lengthened the frost-free season in many mid- and high-latitude regions.