(12/21 UPDATE: Roy Spencer and John Christy of the University of Alabama at Huntsville have posted a reply to this post, which can be read here)
An interesting press release marking the 33rd anniversary of the satellite temperature record makes questionable claims dismissive of the human role in global warming. These claims are not supported by the scientific literature.
The release, from the University of Alabama at Huntsville (UAH), reports that the warming trend in the lower atmosphere is lower than at the surface, and states that the satellite data points to flaws in climate models used to project how the planet’s climate system may respond to increasing amounts of greenhouse gases in the air.
“Globally averaged, Earth’s atmosphere has warmed about 0.45 Celsius (about 0.82° F) during the almost one-third of a century that sensors aboard NOAA and NASA satellites have measured the temperature of oxygen molecules in the air,” the release states. “This is at the lower end of computer model projections of how much the atmosphere should have warmed due to the effects of extra greenhouse gases since the first Microwave Sounding Unit (MSU) went into service in Earth orbit in late November 1978.”
“While 0.45 degrees C of warming is noticeable in climate terms, it isn’t obvious that it represents an impending disaster,” says John Christy of UAH, who, along with Roy Spencer, has maintained one of the satellite-derived temperature records.
The press release may seem straightforward at first, but it makes claims that have long been debunked by mainstream climate scientists.
For example, Christy claims that computer models are flawed, since there are discrepancies between their projections for surface warming compared to warming in the lower layers of the atmosphere, known as the troposphere. “The climate models produce some aspects of the weather reasonably well, but they have yet to demonstrate an ability to confidently predict climate change in upper air temperatures,” Christy states in the press release.
“The satellites should have shown more deep-atmosphere warming than the surface, not less,” Spencer says. “Whatever warming or cooling there is should be magnified with height. We believe this is telling us something significant about exactly why the climate system has not warmed as much as expected in recent decades.”
Interestingly, these statements were contradicted by a 2006 U.S. Climate Change Science Program report that Christy and Spencer helped write, which examined the discrepancy between surface temperature trends and warming in the lower atmosphere. That Federally-sponsored report concluded: “Given the range of model results and the overlap between them and the available observations, there is no conflict between observed changes and the results from climate models.” The report did emphasize, however, lingering differences between the pattern of warming in the tropics and what computer models project for these areas, and it is generally recognized that surface warming is outpacing atmospheric warming, but not by as much as Christy and Spencer claim.
Moreover, a 2010 review of more recent research on this topic concluded: “There is no reasonable evidence of a fundamental disagreement between tropospheric temperature trends from models and observations when uncertainties in both are treated comprehensively”
A second misleading claim the press release makes is that it’s simply not possible to identify the human contribution to global warming, despite the publication of studies that have done just that. “While many scientists believe it [warming] is almost entirely due to humans, that view cannot be proved scientifically,” Spencer states.
Ben Santer, a climate researcher at Lawrence Livermore National Laboratory in California, said Spencer and Christy are mistaken. “People who claim (like Roy Spencer did) that it is “impossible” to separate human from natural influences on climate are seriously misinformed,” he wrote via email. “They are ignoring several decades of relevant research and literature. They are embracing ignorance.”
“Many dozens of scientific studies have identified a human “fingerprint” in observations of surface and lower tropospheric temperature change,” Santer stated.
Santer, along with several coauthors, recently published a study in the Journal of Geophysical Research, Atmospheres that found there are many independent observations showing that temperatures at the surface and lower atmosphere are warming in a way that is consistent with expectations, although surface warming has outpaced atmospheric warming. The paper found the lower troposphere has warmed by 0.5 degrees C, or 0.9 degrees F, during the past 32 years.
Santer’s recent study found that the warming seen in the UAH dataset is unlikely to be the result of natural variability alone.
He and other researchers contacted for this column noted that there have been several instances when Christy and Spencer have had to correct their datasets for factors such as changes in satellite orbits over time, and with each correction the data has come into better alignment with surface warming and model projections.
For this reason and others, Andrew Dessler, a climate researcher at Texas A&M University, says he is skeptical of the satellite data’s reliability. “As far as the data go, I don’t really trust the satellite data. While satellites clearly have some advantages over the surface thermometer record, such as better sampling, measuring temperature from a satellite is actually an incredibly difficult problem. That’s why, every few years, another big problem in the UAH temperature calculation is discovered. And, when these problems are fixed, the trend always goes up,” he said via email.
“It’s also worth noting that there have not been any similar revisions to the surface temperature data, despite the fact that people have looked at it very, very carefully.”