Short-Term Data Clouds the Climate Picture

Two established climate scientists have issued a warning about using short-term data in arguments over climate change. This is such a common point of confusion that I’ve published the news release from Lawrence Berkeley National Lab, in its entirety:

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.

“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.

Editors’ Note: Of course, this cuts both ways. Though it may be tempting to do so, it’s no more legitimate to point to the latest heat wave or a single fire season as proof of global warming. This is a conundrum that makes it difficult to find consensus on pubic policy. There are additional links posted with the full news release at the LBNL site.

  • Steve Bloom

    Craig, see:

    Warmer Weather Makes People More Sure About Global Warming

    I also wonder about the extent to which e.g. a cold winter (compared to recent ones) such as the one we just experienced affects attitudes, although to my knowledge this hasn’t been studied.

    The extent to which climate change tends to be seen in personal terms is illustrated by this interesting inverted pyramid graphic (worth reproducing here IMHO).

    From the same source comes the interesting speculation that all of the talk about fixing the climate may have led some people to think it’s really happening, noting that e.g this comment from Obama tend to give a false impression to the uninformed: “This was the moment when the rise of the oceans began to slow and our planet began to heal.”

    And of course this problem is ever with us:

    Students least informed about environmental science are most optimistic

    I can’t find a link at the moment, but I think there were poll results within the last year finding similarly that among U.S. adults the tendency to discount climate change is positively correlated with self-assessed (note not actual) knowledge of the problem.

    And last but not least:

    Expert Advice Shuts Your Brain Down

    This study was done relative to financial advice but the application to climate is obvious.

  • http://ncwatch.typepad.com/ Russell Steele

    You know the wonderful thing about computer models is the programer and analysis can get them to say want ever they want. You just keep adjusting the input parameters until the output you want appears on the print out. In science one puts forth a hypothesis and then sets about proving it, or disproving it. We have no way of proving or disproving a model, unless we can test it against real world data. So, far the models have failed to show the current cooling trend when tested against real world data from satellites, which are not influenced by urban heat islands, that have contaminated so much of the surface station data.

    While, I agree the earth is warming, it has been for 100s of years following the Little Ice Age and before that following the last Ice Age. What proof is provided that the CO2 rise it from human activity and not a natural result of the warming. We know from the ices cores that CO2 rises followed historical warming, not the other way around. Until the good professors come up with more than a computer models run on their super computer, it is just a dodgy model that runs faster than the last dodgy computer models they tried on a slower computer. Doing it faster does not make it any more accurate. Fast just produces more dodgy output faster.