2. Climate and global change issues
The estimated 1998 global mean temperature over land and marine areas combined (based on data collected from over 1000 land-based weather stations and approximately 7000 ships and 1000 ocean buoys) was +0.56°C above the 196190 base period mean (Fig. 1a), which made it the warmest year in the historical record dating back to 1860. This value far exceeded the previous estimated record departure of +0.43°C set in 1997. Estimated global mean temperatures have been above the 1961_90 average for the past twenty years, with the last below-average annual global temperature recorded in 1978 (Fig. 1a ). Also, the three warmest years in the record (1995, 1997, and 1998) have occurred in the past four years. The very strong 1997/98 Pacific warm episode (see section 3b) contributed substantially to the record-breaking temperatures observed in both 1997 and 1998.
Record annual temperatures were also set in both the Northern Hemisphere (Fig. 1b ) and Southern Hemisphere (Fig. 1c ) during 1998. The Southern Hemisphere anomaly of +0.47°C exceeded the previous record set in 1997 by 0.12°C, while the Northern Hemisphere anomaly of +0.64° surpassed the 1995 record anomaly by 0.11°C.
By partitioning the globe into the extratropics (30°_90°N, 30°_90°S) and Tropics (30°S_30°N), it becomes evident that record temperatures were also observed during 1998 in both the global Tropics and Northern Hemisphere extratropics. In the Tropics, the annual mean value of 0.60°C above the 1961_90 mean exceeded the previous record anomaly of 0.43°C set in 1997 (Fig. 2a). In the Northern Hemisphere extratropics, the 1998 value slightly surpassed the previous record anomaly set in 1995 (Fig. 2b). In the Southern Hemisphere extratropics the 1998 temperature anomaly was similar to that observed in 1990 but below the record value set in 1993 (Fig. 2c). In each of these three regions, temperatures have averaged well-above normal during most of the 1990s.
Annual land-only temperature (Fig. 3a) and ocean-only temperature (Fig. 3b) records were also set in 1998. The land-only temperature averaged more than 1.0°C above the 18801997 base period mean, and was approximately 0.3°C warmer than the previous record set in 1995. The ocean-only average temperature was 0.51°C above the 18801997 base period mean, which exceeded the previous record anomaly of 0.47°C set in 1997. In both of these time series annual mean temperatures have been above average every year since 1977.
During 1998 the pattern of annual temperature anomalies (Fig. 4) was dominated by above normal temperatures across most of the globe between 30°S and 60°N, with the strong El Niño (see section 3) contributing to above-average temperatures throughout the Tropics and across North America. Below-average temperatures were observed primarily over the Southern Hemisphere oceans south of 30°S, across the central North Pacific, over the central tropical Pacific, and over northern Russia. Over the central tropical Pacific Ocean (160°E and 130°W) the below normal sea surface temperatures (SSTs) reflected the rapid demise of El Niño conditions during May and the subsequent development of La Niña conditions (see section 3d). In contrast, annual temperatures in the eastern tropical Pacific averaged 1°2°C above normal, as SSTs were above average from JanuaryAugust. Over the tropical and subtropical Atlantic and Indian Oceans, SSTs were also above normal during most of 1998, with the largest anomalies observed during December 1997-February 1998 (DJF) (see section 5, Fig. 63) and March-May (MAM) (see section 5, Fig. 65).
Over the land areas, the most prominent regions of above average annual temperatures during 1998 included most of North America, all of southern Europe and southern Asia, and all of northern Africa (Fig. 4). Over North America, annual mean temperatures ranged from 1.0°3.0°C above normal across Canada, with the largest anomalies observed over the Northwest Territories. Overall, Canada recorded it warmest year in the 51-year record (Fig. 5a), as annual mean temperatures were 2.5°C above the 1951-80 base period mean and more than 0.5°C greater than the previous record set in 1981. National average seasonal temperature records were set in Canada during MAM, June-August (JJA), and September-November (SON) 1998, with DJF 1997-98 the second warmest such period on record. Individual monthly national temperature records were set during February, April, May, July, August, and September, with July being the warmest month in the historical record for that country.
Over the contiguous United States, annual mean 1998 temperatures averaged 1.2°C above the 1961_90 mean, which is the nation's second warmest year in the historical record (Fig. 5b ). Temperatures over the eastern half of the country averaged 1.0°2.0°C above normal for the year as a whole, while below-average temperatures were confined to the far West (Fig. 4). Eleven states located mainly in the Midwest and the Northeast set annual record temperatures during 1998, and 22 states recorded annual temperatures that ranked in the top five of the 104-year record dating back to 1895 (Fig. 6). California was the only state to record below-average annual mean temperatures during the year. It is interesting to note that the overall warmth observed over the United States during the 1980s and 1990s is comparable to that recorded during the 1920s and 1930s.
Annual mean temperatures across Europe and Asia were generally higher than average during 1998 in the region south of 60°N, and lower than average farther north (Fig. 4). Over northeastern Asia, temperatures averaged 0.5°2.0°C below normal, with negative anomalies recorded during DJF, MAM, and SON (see section 5, Figs.63, 65, 69 ). During SON temperatures averaged more than 4°C below normal across central Siberia (see section 5, Fig. 69). These conditions contrast with the much above normal temperatures that had previously prevailed throughout this region during much of the past two decades (e.g. Hurrell 1995).
Over southeastern Asia temperatures averaged 1°2°C above normal for the year, with positive temperature anomalies observed during all four seasons (see section 5, Figs. 63, 65, 67 , 69). In Japan, the annual mean temperature was the highest in the 101-year record (1.3°C above average), which broke the previous record anomaly of 1.2°C set in 1990 (Fig. 5c). Seasonal temperatures over most of Japan averaged more than 1.0°C above normal during all four individual seasons, with record warmth observed during MAM. More than 90% of the Japanese network of stations set springtime record high temperatures, and 75% of the country observed record high October temperatures.
In the Southern Hemisphere annual temperatures during 1998 averaged 0.5°2.0°C above normal across most of Australia, southern Africa, and South America, while below-average temperatures were confined to extreme south-central Australia and central South America (Fig. 4). These conditions were generally prominent during all four seasons. In both New Zealand (not shown) and Australia (Fig. 5d), annual mean high temperature records were set in 1998. Australia recorded its highest annual mean temperature (0.73°C above the 196190 base period mean) dating back to 1910, which surpassed the previous largest anomaly of 0.69°C set in 1988. Record high minimum temperatures played a major role in this record, as the mean maximum temperature (0.43° above average) was significantly below the previous highest maximum departure (0.85°C) observed in 1991. According to the Australian Bureau of Meteorology, the higher land temperatures were partly linked to above average SSTs around the country, which developed during the middle of the year in conjunction with the onset of Pacific cold episode conditions (see section 3d).
