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 scientists at National Oceanic and Atmospheric Administration (NOAA) and CIRES (Cooperative Institute for Research Environmental Sciences), a cooperative institute of the University of Colorado Boulder and NOAA.
A paper on the study will be published 8 December in the journal Geophysical Research Letters.
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.—Judith Perlwitz, lead author of the study and a researcher with the Cooperative Institute for Research Environmental Sciences, and NOAA Earth System Research Laboratory
|Left side: 1970-2007 trend in annual surface air temperature. Right side: 2008 annual surface air temperature, shown as a departure from the 1971-2000 climatology. Credit: NOAA. Click to enlarge.|
Using 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.
We found that North American coolness resulted from a strong bout of naturally caused cooling in the tropical and northeastern Pacific sea surface temperatures. 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.—Martin Hoerling, a NOAA meteorologist and co-author
The analysis included historical data and climate model simulations that were conducted in the US 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.—Judith Perlwitz
The 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.—Arun Kumar, a NOAA meteorologist and co-author
Perlwitz, J., M. Hoerling, J. Eischeid, T. Xu, and A. Kumar (2009) A strong bout of natural cooling in 2008. Geophys. Res. Lett., in press doi: 10.1029/2009GL041188