Cooler North American temperatures in 2008 resulted from a strong
natural effect, and the overall warming trend that has been observed
since 1970 is likely to resume, according to university and NOAA
“Our work shows that there can be cold periods,
but that does not mean the end of global warming. The recent coolness
was caused by transitory natural factors that temporarily masked the
human-caused signal,” said Judith Perlwitz, lead author of the study
and a researcher with the Cooperative Institute for Research
Environmental Sciences, and NOAA Earth System Research Laboratory, both in Boulder, Colo. The paper will be published Dec. 8 in Geophysical Research Letters.
computer-generated models as well as observations, the team analyzed
causes for climate variations in the recent decades. Special emphasis
was given to the reasons for North American coolness in 2008. The
research is an exercise in climate attribution, a scientific process
for identifying the sources of observed climate and weather patterns.
Climate attribution is a vital part of NOAA’s climate services.
found that North American coolness resulted from a strong bout of
naturally caused cooling in the tropical and northeastern Pacific sea
surface temperatures,” said Martin Hoerling, a NOAA meteorologist and
co-author. “This illustrates how regional patterns can vary independent
of the overall global average. In 2008, global land temperatures were
the sixth warmest on record, whereas it was the coldest year in North
America since 1996.”
The analysis included historical data and
climate model simulations that were conducted in the U.S. and
internationally. The science team discerned both natural and
human-caused influences for 2008.
“North American temperatures
would have been considerably colder in 2008 had there been no
human-induced warming influence present,” Perlwitz said.
scientists conclude that the North American temperatures are likely to
resume increasing again, and do not see the recent coolness as an
emerging downward trend.
“Our work shows the importance of
the role of natural climate variability in temporarily masking or
enhancing human-induced climate change. Through diagnosis, we ensure
that natural changes, when occurring, are not misunderstood to mean
that climate change is either not happening or is happening more
intensely than the expected human influence,” said Arun Kumar, a NOAA
meteorologist and co-author.
Authors of A strong bout of natural cooling in 2008
are Judith Perlwitz, Cooperative Institute for Research in
Environmental Sciences, Boulder, Colo., and NOAA Earth System Research
Laboratory, Boulder, Colo.; Martin Hoerling, NOAA Earth System Research
Laboratory, Boulder, Colo.; Jon Eischeid and Taiyi Xu, both of the
Cooperative Institute for Research in Environmental Sciences, Boulder,
Colo., and NOAA Earth System Research Laboratory, Boulder, Colo.; and
Arun Kumar, NOAA Climate Prediction Center, Camp Springs, Md.
The work was funded by the NOAA Climate Program Office.