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Our analysis, summarized in Figure 1 above, uses documented procedures for data over land (1), satellite measurements of sea surface temperature since 1982 (2), and a ship-based analysis for earlier years (3). Our estimated error (2σ, 95% confidence) in comparing nearby years, such as 1998 and 2005, increases from 0.05°C in recent years to 0.1°C at the beginning of the 20th century. Error sources include incomplete station coverage, quantified by sampling a model-generated data set with realistic variability at actual station locations, and partly subjective estimates of data quality problems (4).
Record warmth in 2005 is notable, because global temperature has not received any boost from a tropical El Niño this year. The 1998 meteorological year, on the contrary, was lifted 0.2°C above the trend line by the strongest El Niño of the past century.
Global warming is now 0.6°C in the past three decades and 0.8°C in the past century. It is no longer correct to say that "most global warming occurred before 1940". More accurately, there was slow global warming, with large fluctuations, over the century up to 1975 and subsequent rapid warming of almost 0.2°C per decade.
Recent warming coincides with rapid growth of human-made greenhouse gases. Climate models show that the rate of warming is consistent with expectations (5). The observed rapid warming thus gives urgency to discussions about how to slow greenhouse gas emissions (6).
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Figure 2 shows the temperature index at seasonal resolution for the globe and for low latitudes (23.6°N - 23.6°S). The low latitude temperature displays clearly the occurrence of substantial El Niños, especially the prominent 1969, 1972-3,1983 and 1998 El Niños. The quasi-regularity of recent El Niños at intervals of about 4 years (there was a weak El Niño in 2002) suggests the likelihood of an El Niño in 2006 or at latest 2007. In such a case the 2005 global temperature record will almost surely be broken.
Figure 3 shows the annual and seasonal temperature changes of the past 50 years. Largest warmings have occurred in Alaska, Siberia and the Antarctic Peninsula. Most ocean areas have warmed. The remote location of most warming makes it clear that the warming is not a product of local urban influence.
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