Instrumental Temperature Change in the Context of the Last 1000 Years

Abstract

The instrumental record of climate extends back in many regions of the world to the mid-nineteenth century. Over the period 1861–1999 average global surface temperatures have warmed by about 0.6°C. The warmest years of this record are in the 1990s with 1998 the warmest year. The warming has not occurred in a linear fashion, but in two phases, early 1920s to the mid-1940s and from the mid-1970s. Few regions exhibit local statistically significant warming, mainly because 20–25 year periods are too short to consider trends given the large interannual variability in some areas.

The warming since 1950, however, is highly significant in some areas. Recent advances in palaeoclimatology have enabled the last 150 years to be placed in the context of the whole millennium, although it is clear that errors of estimates involved in the pre-1850 period are considerably greater than for the instrumental era. The most striking feature of the millennial reconstructions is that the 20^th century was the warmest over the Northern Hemisphere, the 1990s was the warmest decade and 1998 the warmest individual year. Temperatures during the first five centuries of the millennium were warmer than those over the 1500 to 1900 period, but were at least 0.1°C below the 1961–90 average. The coldest decade, the 1600s, saw average hemispheric temperatures about 0.8°C below 1961–90 levels.