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New Climate
Science Findings
October 1999
Since the Second IPCC
Assessment in 1995, the scientific evidence concerning human influences on the
global climate system has continued to accumulate. In particular, recent data
and analyses strengthen earlier findings that the surface temperature of the
Earth is increasing and that this increase can be attributed, in large part, to
human-caused increases of greenhouse gases in the atmosphere. The continuation
of these trends is likely to be associated with climatic changes capable of
adversely affecting ecosystems, wildlife, and large numbers of people.
Northern
Hemisphere Temperatures Warmest in 1,000 years
Recent studies published in Nature,
Science, and Geophysical Research Letters have provided additional evidence of
20th century warming. Analysis of tree rings, corals, ice cores, and lake
sediments showed this century's surface temperatures for the Northern
Hemisphere to be the warmest since at least 1400 a.d.
A newer study of Northern Hemisphere
temperatures found it highly likely that the 20th century has been the warmest
century of the millennium; the 1990s have been the warmest decade; and 1998
has been the warmest year. A study of temperature data from 600-1,800-foot
deep boreholes in North America, Europe, Africa, and Australia found that the
Earth's average surface temperature has increased by about 1.8¡ Fahrenheit
(F) over the last five centuries, and that half of this total warming occurred
in this century. (Mann et al., Nature, Vol. 392 (1998), p.
779; Mann et al., Geophysical Research Letters, Vol. 26, No. 3 (1999), p. 759;
Pollack et al., Science, Vol. 282 (1998), p. 279.)
The 11 Warmest Years Have All Occurred
Since 1983 Of the 120 to 140 years for which thermometer records are
sufficiently complete to define a global average temperature, the 11 warmest
years have all occurred since 1983. The three warmest years on record were
1998, 1997, and 1995, in that order. In addition, 1998 was also 1.2¡ F above
the long-term average temperature-the 20th consecutive year in which this
benchmark has been exceeded. (NOAA National Climatic Data
Center, UK Meteorological Office.)
Greenhouse Gases from Human
Activities Dominant Driver of 20th Century Warming
The 20th century temperature records are
consistent with what is expected under human-induced climate change scenarios
and cannot be explained solely by other hypothesis, such as solar variability,
volcanic eruptions, and El Ni–o cycles. The case for anthropogenic climate
change is further strengthened by the record of regional patterns of
temperature changes across the surface of the Earth and the vertical patterns
of temperature changes as one ascends through the atmosphere. (Mann
op cit.; Wigley et al., Science, Vol. 282 (1998), p. 1676; Peterson et al.,
Geophysical Research Letters, Vol. 26 (1999), p. 329; Wentz and Schabel,
Nature, Vol. 394 (1998), p. 661.)
Emerging Scientific Evidence
Suggests We May Already Be Seeing Impacts of Climate Change
Changes in species distributions and
ecosystem dynamics are likely attributable to multiple causes and stresses,
but scientists have deduced that climate change is playing a role in the
demise of several frog and toad species in Monteverde, shifts in the growing
season in Europe, shifts in growth of grass and forb species in Colorado,
changes in bird ranges in western Europe, and alterations in patterns of ocean
biodiversity. (Menzel and Fabian, Nature, Vol. 397 (1999), p.
659; Pounds et al., Nature, Vol. 398 (1999), p. 611; Parmesan et al., Nature,
Vol. 399 (1999), p. 579; Alward et al., Science, Vol. 283 (1999), p. 229.)
New Study Shows Role of
CO2 in Ice Age Warming
A study of Antarctic ice cores published
in the March 12, 1999, issue of Science compared changes in temperature and
levels of atmospheric carbon dioxide during several ice ages. The study showed
that as the Earth began to warm due to minor changes in its orbit, carbon was
released from the ocean into the air, raising atmospheric CO2
levels that then greatly magnified and accelerated the warming process. The
study confirms that small changes in the factors that affect climate can have
large effects on global temperatures. (Fisher et al., Science,
Vol. 283, p. 1712.)
U.S. Environmental Protection
Agency
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