JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105, NO. D15, PAGES 20,101-20,114,
AUGUST 16, 2000
Observed climate variability and change of relevance
to the biosphere
David R. Easterling, Thomas R. Karl, and Kevin P. Gallo
NOAA National Climatic Data Center, Asheville, North Carolina
David A. Robinson
Rutgers University, Department of Geography, New Brunswick, New Jersey
Kevin E. Trenberth and Aiguo Dai
National Center for Atmospheric Research, Boulder, Colorado
In this paper we review the current instrumental evidence regarding climate
variations and change during the 20th century emphasizing those changes that are likely
to have direct interactions with the biosphere. Three basic questions are addressed:
(1) Is the climate getting warmer, (2) is the hydrologic cycle changing, and (3) is the
climate becoming more extreme. Based on global near-surface temperature measurements for
the 20th century, it is clear that a warming of ;0.58C has occurred. More importantly
for biospheric systems, however, are the observed asymmetric changes in daily maximum
and minimum temperature, with the minimum temperatures increasing at a rate approximately
twice that of the maximum temperature. Other temperature-sensitive measures, such as
glacial and snow cover extent, reinforce the observed temperature trends. Examination of
the hydrologic cycle indicates that changes also appear to be occurring, although less
confidence can be placed on these analyses than those for temperature. Recent studies
suggest that precipitation has increased in higher latitudes, particularly in the Northern
Hemisphere. Increases in cloudiness, atmospheric water vapor, and changes in stream flow
also suggest that changes to a more vigorous hydrologic cycle are taking place. The final
question regarding climate extremes is much more difficult to assess due to a lack of high
temporal resolution climate databases. Of the few studies that have been performed,
however, there is evidence that precipitation extremes, particularly heavy rainfall events,
are increasing in the United States and Australia, also suggesting an enhanced hydrologic
cycle as the planet warms.
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Hongjun Zhang:
zhangho@ucar.edu