Why do we believe the climate is changing?
Observations of surface temperature averaged across the globe show a linear warming trend of 0.6°C&plusmin;0.2°C since the beginning of the 20th Century. Rates of temperature rise are greater in recent decades: since 1979, global surface temperatures have increased more than 0.4°C. Effects of urbanization and land-use change on the land-based temperature record are negligible as far as continental- and hemispheric-space averages are concerned, because the very real but local effects are accounted for in analyses of the temperature records. Recent warming is strongly evident at all latitudes over each of the ocean basins and, averaged over the globe, sea surface temperatures have warmed 0.35°C since 1979. The trends over the past 25 years have been fairly linear; however the global temperature changes over the entire instrumental record are best described by relatively steady temperatures from 1861-1920, a warming of about 0.3°C to 1950, a cooling of about 0.1°C until the mid-1970s, and a warming of about 0.55°C since then. Thus, global surface temperatures today are about 0.75°C warmer than at the beginning of the 20th Century.
The warmest year in the 145-year global instrumental record remains 1998, since the major 1997-98 El Niño enhanced it. The year 2005 is the second warmest on record, followed by the years 2002-2004. Based on reconstructions of temperature from proxy data, like tree rings and ice cores, several studies have also concluded that NH surface temperatures are warmer now than at any time in at least the last 1,000 years.
Global warming does not mean that temperature increases are spatially uniform or monotonic: some places warm more than the average and some places cool because of accompanying changes in the circulation of the atmosphere and oceans. Land regions have warmed the most (0.7°C since 1979), with the greatest warming in winter and spring over the Northern Hemisphere continents.