GLOBAL SURFACE TEMPERATURE
Global surface temperatures in the 1998 meteorological year (Dec. 1, 1997, to Dec. 1, 1998) set a new record for the period of instrumental measurements, report researchers at the NASA Goddard Institute for Space Studies who analyze data collected from several thousand meteorological stations around the world. The global temperature (Fig. 1) exceeds that of the previous record year, 1995, by such a wide margin that the 1998 calendar year is certain to also set a new record. And unlike many recent years, the warmth is beginning to hit home; the United States this year is experiencing its warmest year in the past several decades.
Data for the meteorological year is averaged over the four seasons beginning with winter (December-January-February), and thus the analysis covers the period up to Dec. 1, 1998. The NASA scientists, James Hansen, Reto Ruedy, Jay Glascoe and Makiko Sato, use data that is collected at the NOAA National Climate Data Center in Asheville, North Carolina and made available to all researchers (see Hansen et al. 1996, Peterson and Vose 1997). In their analysis, the NASA scientists use data from rural stations to correct for urban influences on reported temperatures. They also use satellite measurements of ocean temperature to obtain a second measure of global temperature change that is more uniformly spaced over the world (Hansen et al. 1996). The results reported today will change slightly as late-reporting station data are included, but late data will not alter the conclusion that both analyses, with and without ocean data, easily set new records in 1998.
The global warming is believed by many researchers, including the NASA scientists, to be at least partly a consequence of increasing human-made gases in the atmosphere, especially carbon dioxide, which arises from the burning of coal, oil and gas. The 1998 warmth was associated partly with a strong El Niņo that warmed the air over the eastern tropical Pacific Ocean in the first half of the year and in turn affected weather around the world.
The largest temperature anomalies in 1998 were in North America, in a pattern that commonly occurs in El Niņo years. But almost the entire world was warmer than normal in 1998. The El Niņo, by itself, cannot account for either the observed long-term global warming trend or the extreme warmth of 1998. Because the Pacific Ocean temperature has returned to a more normal level, it is anticipated that the global temperature in 1999 will be less warm than 1998 but will remain well above the long-term mean for the period of climatology, 1951-1980.
The rapid global warming since the mid 1970s exceeds that of any previous period of equal length in the time of instrumental data. The increase of the 5-year mean temperature since 1975 is about 1/2°C or almost 1°F. This warming occurs during the time when greenhouse gases increased most rapidly, and thus, according to Dr. Hansen "the rapid warming of the past 25 years undercuts the argument of 'greenhouse skeptics', who have maintained that most of the global warming occurred early this century while greenhouse gases were increasing more slowly -- in fact, the fastest warming is occurring just when it is expected".
Temperatures in the United States were also warm in the meteorological year 1998. In this preliminary analysis the 1998 United States mean temperature is the warmest for at least 40 years. Exact comparison will depend on more complete station reporting, but it is clear that 1998 did not match the record warmth of 1934, which occurred during the Dust Bowl era. Fig. 3 also illustrates that the chaotic year-to-year fluctuations of temperature are much larger for the contiguous United States than for the global mean temperature. This is related to the fact that the United States covers only about 2 percent of the world area, and thus its temperature is influenced more by regional fluctuations.
Is the global warming of 1/2°C or 1°F, which is much smaller than day to day temperature changes, large enough to be noticeable to the lay person? Some insight into that question is provided by Fig. 4, which shows the temperature anomalies in the United States for each of the past four seasons. Even though 1998 was remarkably warm, in every season there was some part of the country that was cooler than "normal", that is, cooler than the 1951-1980 average. This is a result of the average warming being smaller than natural fluctuations of regional temperature.
Yet global warming may be approaching a level where the astute observer can notice its effects. It is apparent in Fig. 4 that the area with warm temperature anomalies exceeds that with cold anomalies. The issue of whether the more extensive warming is noticeable to people can be investigated using a "Common Sense Climate Index" defined earlier this year by the NASA researchers.
One of the elements in the "common sense" index is the frequency of unusually warm seasons, where the local temperature required to qualify as a warm season is chosen such that one-third of the seasons in the period 1951-80 were in this category. Climate models have predicted that the global warming would cause the frequency of such warm seasons to increase from 33% to 50-70% by the 1990s. In other words, although global warming is too small to prevent some seasons from being cooler than normal, the perceptive observer may notice that the "climate dice" are becoming loaded.
Written by: NASA Goddard Institute for Space Studies
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