Global and regional climate in 2004

The average temperature near the surface of the earth in 2004 was 0.44 degC (with a ± 2 standard errors range of ± 0.06 degC) above the 1961–90 average (Fig. 1(a)). The year 2004 was the fourth warmest in the 144-year series: the warmest year so far has been 1998 at 0.54 ± 0.06 degC above average. All the 10 warmest years globally have occurred since 1990, including each year since 1997. Over the twentieth century, the global surface temperature increased by more than 0.6 degC. The rate of change for the period since 1976 is roughly three times that for the past 100 years as a whole. Globally, the landsurface air temperature anomalies for October and November 2004 were the warmest on record for these months. The annual anomaly of 0.62 ± 0.07 degC for the Northern Hemisphere in 2004 was fourth warmest (Fig. 1(b)) and the sea surface temperature anomaly for the Northern Hemisphere for 2004 was the warmest on record at 0.49 degC. In the Northern Hemisphere as a whole, the surface temperature anomaly averaged over the most recent five years (2000–2004) was 0.58 degC. In the Southern Hemisphere the annual anomaly for 2004, 0.25 ± 0.06 degC, was the sixth highest on record (Fig. 1(c)). The tropical belt 20°N to 20°S had its sixth warmest year (Fig. 1(d)); the extratropical Northern Hemisphere had its second warmest year (Fig. 1(e)), and the extratropical Southern Hemisphere had its third warmest year, equal with 1997 and 2002 (Fig.1(f )). For our analyses of 2004 and our time series in Figs. 1(a)-(f ), we have continued to use the ‘HadCRUT2v’ dataset introduced for the analysis of 2003 (Parker et al. 2004; see also Jones and Moberg (2003)). The global and hemispheric series are optimal averages, whereas the tropical and extratropical regional temperature anomaly series are area-weighted averages. Observations since 1950 over a worldwide network of land stations, representative of those used in the global and hemispheric series, have shown the same trends of nighttime minimum temperature on relatively still nights as on windy nights (Fig. 1(g)). This suggests that the land surface air temperatures used in Figs. 1(a)–(c), being selected to try to avoid urbanization problems, have indeed not been significantly impacted by the growth of urban communities (Parker 2004). Nevertheless, the uncertainties calculated in Figs. 1(a)–1(c) do still include some allowance for possible urbanization problems (Folland et al. 2001a). Significant positive annual regional temperature anomalies, notably across much of central Asia, China, Alaska and parts of the western United States, as well as across major portions of the North Atlantic Ocean and part of the subtropical South Pacific and Indian Ocean, contributed to the high global mean surface temperature ranking in 2004 (Figs. 2 (a, b)). In northern winter (December 2003 to February 2004) much of Siberia and Canada were several degrees warmer than the 1961–90 average (Fig. 3 (a), upper left) but in percentile terms the most remarkable warmth was in the eastern North Atlantic, western tropical Pacific and a few areas of Asia (Fig. 3 (b), upper left). The most unusual warmth in March to May 2004 was in a zone extending from the Caspian Sea to China (Figs. 3 (a, b), upper right). The Benguela Current upwelling region of the South Atlantic off south-western Africa was the only exceptionally cold region; other small regions of cold anomalies in the Canadian and Siberian Arctic were not below the 1961–90 10th percentile. Warmth in June to August 2004 was most notable in the eastern North Atlantic and Alaska. By contrast, central North America was exceptionally cold (Figs. 3(a, b), lower left), the only such region seen. In northern autumn (Figs. 3(a, b), lower right) warmth was most marked in the tropical North Atlantic, central North America (which had been very cool in the summer) and parts of central and eastern Asia. The North Atlantic hurricane season was more than usually active in 2004 with 15 named storms, 9 hurricanes and 6 major hurricanes (Bell et al. 2004a). The averages for these are 10.6 named storms, 5.9 hurricanes and 2.0 major hurricanes respectively. A number of years – most recently 1996 and 1961 – have experienced six major hurricanes, but only 1950, with seven, has experienced more. In the 2004 season, the Accumulated Cyclone Energy (ACE, see Bell et al. 2004b), which accounts for the combined strength and duration of tropical storms and hurricanes, was 257% of the 1951–2000 median. This is the third largest in the 54-year record, exceeded only by 1950 (the first year in the record) and 1995. Hurricane Ivan alone produced an ACE value of 70.4 × 10 kt, the largest single-storm total in the reliable record. Ekman drift and vertical mixing in the ocean associated with the active hurricane season are likely to have contributed to the cold anomaly south of Bermuda in northern autumn (Figs. 3 (a, b): lower right panels). Despite this active season, there is no evidence of long-term trends in North Atlantic hurricane activity, but there is evidence of multi-decadal variability (Goldenberg et al. 2001).