Media Contact: Jon Bashor
Technical contact: Michael Wehner

BERKELEY, CA — In the hotly debated arena of global climate change, using short-term trends that show little temperature change or even slight cooling to refute global warming is misleading, write two climate experts in a paper recently published by the American Geophysical Union — especially as the long-term pattern clearly shows human activities are causing the earth’s climate to heat up.

In their paper “Is the climate warming or cooling?” David R. Easterling of the National Oceanographic and Atmospheric Administration’s National Climatic Data Center and Michael Wehner of the Computational Research Division at the Department of Energy’s (DOE) Lawrence Berkeley National Laboratory note that a number of publications, websites and blogs often cite decade-long climate trends, such as that from 1998-2008, in which the earth’s average temperature actually dropped slightly, as evidence that the global climate is actually cooling.

However, Easterling and Wehner write, the reality of the climate system is that, due to natural climate variability, it is entirely possible, even likely, to have a period as long as a decade or two of “cooling” superimposed on the longer-term warming trend. The problem with citing such short-term cooling trends is that it can mislead decision-makers into thinking that climate change does not warrant immediate action. The article was published April 25 in Geophysical Research Letters.

“We wrote this paper, which was carefully reviewed by other researchers and is scientifically defensible, to clearly show that even though our climate is getting warmer, we can’t expect it to do so in a monotonic way – or that each year will be warmer than the preceding year,” said Wehner. “Even with the climate changes caused by human activity, we will continue to see natural variability including periods of cooler temperatures despite the fact that globally averaged temperatures show long-term global warming.”

“It is easy to ‘cherry pick’ a period to reinforce a point of view, but this notion begs the question, what would happen to the current concerns about climate change if we do have a sustained period where the climate appears to be cooling even when, in the end, the longer term trend is warming?” write Easterling and Wehner.

The research was funded by the DOE Office of Science’s Office of Biological and Environmental Research through its Climate Change Prediction Program.

Citing an accepted climate modeling scenario in which no efforts are made to reduce the amount of greenhouse gases released into the atmosphere, the earth’s climate is expected to warm by 4 degrees Celsius (7.2 degrees  Fahrenheit) by the end of the 21st century. The authors point out that this is consistent with other simulations contained in the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC 2007), which was recognized with the 2007 Nobel Peace Prize.

Measured changes in global temperature show ups and downs, with some periods of a decade or more defying the long-term trend. (Graphs provided by the authors, published by the AGU.)

“Climate scientists pay little attention to these short-term fluctuations as the short term ‘cooling trends’…are statistically insignificant and fitting trends to such short periods is not very meaningful in the context of long-term climate change,” the authors write. “On the other hand, segments of the general public do pay attention to these fluctuations and some critics cite the most recent period as evidence against anthropogenic-forced (human-induced) climate change.”

The authors used both observed climate data from 1901-2008 and a series of climate model simulations performed on supercomputers to study the occurrence of decade-long trends in globally averaged surface air temperature. They found that it is possible, and indeed likely, to see periods as long as a decade in the recent past which do not show a warming trend. The authors even found that running computer simulations for the 21st century with significant increases in greenhouse gas emissions showed some decades with lower or static average temperatures.

One such example can be found by looking at data from 1998 to 2008, which shows no real trend, even though global temperatures remain well above the long-term average. According to the authors, the unusually strong 1997-98 El Niño contributed to unusual warmth in the global temperature for 1998, so that without similar dramatic changes, the following decade does not appear to be warming. A similar interpretation can be made by looking at the short-term data from 1977-85 or 1981-89, “even though these periods are embedded in the 1975-2008 period showing a substantial overall warming,” Easterling and Wehner write. In the first example, dropping data from 1998 and looking at 1999-2008, the researchers found a strong warming trend.

Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California. Learn more at http://www.lbl.gov.