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A recent full-page newspaper ad from the libertarian Cato Institute takes issue with President Barack Obama's convictions about global warming.
The ad cites then-President-elect Obama's Nov. 19 statement: "Few challenges facing America and the world are more urgent than combating climate change. The science is beyond dispute and the facts are clear."
"With all due respect, Mr. President, that is not true," the ad states in bold letters. Below that is a statement they say was signed off on by more than 100 named scientists.
"We, the undersigned scientists, maintain that the case for alarm regarding climate change is grossly overstated. Surface temperature changes over the past century have been episodic and modest and there has been no net global warming for over a decade now. After controlling for population growth and property values, there has been no increase in damages from severe weather-related events. The computer models forecasting rapid temperature change abjectly fail to explain recent climate behavior. Mr. President, your characterization of the scientific facts regarding climate change and the degree of certainty informing the scientific debate is simply incorrect."
Global warming is a complicated subject, and we're not going to flesh it all out here. But we did want to address the underpinning of this ad, the claim that "there has been no net global warming for over a decade now."
First off, we should note that while there are some who disagree about the existence of global warming, the overwhelming consensus among scientists who study the climate is that it does, and that humans are worsening it.
The definitive statement on global warming comes from a 2007 study by the U.N. Intergovernmental Panel on Climate Change, a group of scientists from more than 130 nations. They concluded that the "warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice and rising global average sea level." The group also concluded that there is "very high confidence that the global average net effect of human activities since 1750 has been one of warming."
The problem with the assertion in the Cato statement is that it is impossible to make meaningful conclusions about climate trends based on looking at a 10-year window, said Richard Heim, a meteorologist at the NOAA National Climatic Data Center Climate Monitoring Branch.
People tend to think of global warming as a steady trend upward, Heim said, but that's not how it works. if you were to look at long-term trends, like a century, it looks more like steps. Temperatures wil rise for a few years, then level off or even go down a little bit, then go back up. That's why you've got to look at temperatures over many decades, he said.
And if you look at the trends over the last 100 years, Heim said, "the overall linear trend shows clear, unequivocal, unmistakable warming over that period."
Take a look for yourself at the NOAA graph of 100 years of global temperatures.
Now, over the last dozen years, he said, global temperatures have largely plateaued. That's consistent with the trends. Typically, he said, climates will go up, then plateau until they reach a tipping point, and then rise again.
"What you are seeing in the last eight or 10 years is kind of like one of those steps," Heim said. "The fact that it's not getting warmer doesn't mean we are not experiencing global warming. You can't talk about global warming over a 10 year period. The time scale is too short."
When you grab short time frames, say 10 years, it's easy to cherry-pick starting points that are particularly high or low to make your argument one way or the other. But even if you did select a 10-year frame — as cited by Cato — it would show a slight warming over time, Heim said. If you looked at just the last eight years, it would look flat. In fact, if you looked at the last four years, it would seem to be cooling.
"But this is crazy," Heim said. "You have to look at the big picture."
Gavin Schmidt, a NASA climate scientist who was a reviewer on the IPCC study, called the Cato argument that there has been no global warming over the last 10 years "fatuous and false."
"What if I said that there had been no global warming for an hour? You would rightly tell me that this was too short a period for it to be meaningful," Schmidt said. "The same is true for a 'decade'. But even so, it is false."
Schmidt directed us to a graph showing the 10-year climate trends using the four main temperature indices. The line bounces up and down like a lie detector graph, but the overall lines all trend upward .
"To be clear, the globe has warmed for the last decade," Schmidt said. "You can get different results if you pick out your start dates carefully, a practice known as cherry-picking since it is trying to use the data
to say something other than what it generally shows, but you are much better off looking at the longest time scales you have (such as these from NASA )."
Cato cited a study on the internal variability of the climate system to support its claim. But the paper, from Kyle Swanson and Anastasios Tsonis, doesn't dispute the long-term trend of climate change, Schmidt said. He cited this conclusion from the paper: "If the role of internal variability in the climate system is as large as this analysis would seem to suggest, warming over the 21st century may well be larger than that predicted by the current generation of models, given the propensity of those models to underestimate climate internal variability."
Said Schmidt: "Quoting this paper to support a claim that global warming has stopped is like quoting Ronald McDonald in support of vegetarianism."
In conclusion, most scientists who study climate say that cherry-picking a 10-year window is inappropriate. But in this case, cherry picking exactly 10 years (Cato said a decade) still doesn't support their claim. We rate this claim False.
Cato Institute, Climate Change ad
WoodforTrees.org, Interactive climate change graph, 1999-2009
NOAA National Climate Data Center, Climate of 2008 Annual Report , Jan. 14, 2009
Interview with Richard Heim, a meteorologist at the NOAA National Climatic Data Center Climate Monitoring Branch, April 1, 2009
Interview with Gavin Schmidt, a NASA climate scientis, April 1, 2009
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