Date:    Fri, 18 Dec 1998 18:37:54 EST
From:    C 
Subject: GL:  NASA Analysis of 1998 Temps

http://www.giss.nasa.gov/research/observe/surftemp/sp/graham2.ram
[go to web site to see figures]

Global Temperature Trends
1998 Global Surface Temperature Smashes Record

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 Nio 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
(Fig. 2), in a pattern that commonly occurs in El Nio years. But
almost the entire world was warmer than normal in 1998. The El
Nio, 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
(Fig. 1). The increase of the 5-year mean temperature since 1975
is about 1/2C or almost 1F. 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 (Fig. 3). 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 1/2C or 1F, 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 (Fig. 5).

Fig. 6 (updated from Hansen 1997) shows the average frequency of
warm seasons for all meteorological stations located at middle
latitudes in the Northern Hemisphere and for all stations
globally. The result confirms that there is an increasing
tendency for warm seasons. However, perception of change by
people is a sociological matter, and it is not proven that this
amount of change, i.e., this degree of loading of the dice, is
sufficient yet to be noticeable to most people ( Hansen et al.
1998 ).

The temperatures analyzed by the NASA researchers are measured at
a height of about two meters over land areas, while the
temperatures refer to the sea surface in ocean regions sampled by
satellite measurements (Hansen et al. 1996). Temperature changes
are not necessarily the same at higher levels in the atmosphere
sampled by weather balloons and by satellite microwave
observations. However, the warming in 1998 is so large that the
NASA researchers believe that record warm temperatures should be
found by all observing systems measuring from the surface through
the troposphere (lowest 5-10 miles of atmosphere). According to
Dr. Hansen "There should no longer be an issue about whether
global warming is occurring, but what is the rate of warming,
what is its practical significance, and what should be done about
it."


References

Hansen, J. 1997. Public understanding of global climate change.
In Carl Sagan's Universe (Y. Terzian and E. Bilson, Eds.).
Cambridge Univ. Press, Cambridge.

Hansen, J., R. Ruedy, M. Sato, and R. Reynolds. 1996. Global
surface air temperature in 1995: Return to pre-Pinatubo level.
Geophys. Res. Lett. 23 , 1665-1668.

Hansen, J., M. Sato, J. Glascoe and R. Ruedy 1998. Common sense
climate index: Is climate changing noticeably? Proc. Natl. Acad.
Sci. .

Peterson, T.C., and R.S. Vose. 1997. An overview of the Global
Historical Climatology Network temperature database. Bull. Amer.
Meteorol. Soc. 78,2837-2849.