An Inconvenient Truth - Scientific Basis

Scientific Basis

The film's thesis is that global warming is real, potentially catastrophic, and human-caused. Gore presents specific data that supports the thesis, including:

• The Keeling curve, measuring CO₂ from the Mauna Loa Observatory.
• The retreat of numerous glaciers is shown in before-and-after photographs.
• A study by researchers at the Physics Institute at the University of Bern and the European Project for Ice Coring in Antarctica presenting data from Antarctic ice cores showing carbon dioxide concentrations higher than at any time during the past 650,000 years.
• Temperature record since 1880 showing that the ten hottest years ever measured in this atmospheric record had all occurred in the previous fourteen years.
• A 2004 survey, by Naomi Oreskes of 928 peer-reviewed scientific articles on global climate change published between 1993 and 2003. The survey, published as an editorial in the journal Science, found that every article either supported the human-caused global warming consensus or did not comment on it. Gore also presented a 2004 study by Max and Jules Boykoff showing 53% of articles that appeared in major US newspapers over a fourteen year period gave roughly equal attention to scientists who expressed views that global warming was caused by humans as they did to global warming skeptics, creating a false balance.

The Associated Press contacted more than 100 climate researchers and questioned them about the film's veracity. All 19 climate scientists who had seen the movie said that Gore accurately conveyed the science, with few errors.

William H. Schlesinger, dean of the Nicholas School of Environment and Earth Sciences at Duke University said " got all the important material and got it right." Robert Corell, chairman of the Arctic Climate Impact Assessment was also impressed. "I sat there and I'm amazed at how thorough and accurate. After the presentation I said, 'Al, I'm absolutely blown away. There's a lot of details you could get wrong.'...I could find no error." Michael Shermer, scientific author and founder of The Skeptics Society, wrote in Scientific American that Gore's slide show "shocked me out of my doubting stance." Eric Steig, a climate scientist writing on RealClimate, lauded the film's science as "remarkably up to date, with reference to some of the very latest research." Ted Scambos, lead scientist from the National Snow and Ice Data Center, said the film "does an excellent job of outlining the science behind global warming and the challenges society faces in the coming century because of it."

One concern among scientists in the film was the connection between hurricanes and global warming, which remains contentious in the science community. Gore cited five recent scientific studies to support his view. "I thought the use of imagery from Hurricane Katrina was inappropriate and unnecessary in this regard, as there are plenty of disturbing impacts associated with global warming for which there is much greater scientific consensus," said Brian Soden, professor of meteorology and oceanography at the University of Miami. Gavin Schmidt, climate modeler for NASA, thought Gore appropriately addressed the issue. "Gore talked about 2005 and 2004 being very strong seasons, and if you weren't paying attention, you could be left with the impression that there was a direct cause and effect, but he was very careful to not say there's a direct correlation," Schmidt said. "There is a difference between saying 'we are confident that they will increase' and 'we are confident that they have increased due to this effect,'" added Steig. "Never in the movie does he say: 'This particular event is caused by global warming.'"

Gore's use of long ice core records of CO₂ and temperature (from oxygen isotope measurements) in Antarctic ice cores to illustrate the correlation between the two drew some scrutiny; Schmidt, Steig and Michael E. Mann back up Gore's data. "Gore stated that the greenhouse gas levels and temperature changes over ice age signals had a complex relationship but that they 'fit'. Both of these statements are true," said Schmidt and Mann. "The complexity though is actually quite fascinating...a full understanding of why CO₂ changes in precisely the pattern that it does during ice ages is elusive, but among the most plausible explanations is that increased received solar radiation in the southern hemisphere due to changes in Earth’s orbital geometry warms the southern ocean, releasing CO₂ into the atmosphere, which then leads to further warming through an enhanced greenhouse effect. Gore’s terse explanation of course does not mention such complexities, but the crux of his point–that the observed long-term relationship between CO₂ and temperature in Antarctica supports our understanding of the warming impact of increased CO₂ concentrations–is correct. Moreover, our knowledge of why CO₂ is changing now (fossil fuel burning) is solid. We also know that CO₂ is a greenhouse gas, and that the carbon cycle feedback is positive (increasing temperatures lead to increasing CO₂ and CH₄), implying that future changes in CO₂ will be larger than we might anticipate." "Gore is careful not to state what the temperature/CO₂ scaling is," said Steig. "He is making a qualitative point, which is entirely accurate. The fact is that it would be difficult or impossible to explain past changes in temperature during the ice age cycles without CO₂ changes. In that sense, the ice core CO₂-temperature correlation remains an appropriate demonstration of the influence of CO₂ on climate."

Steig disputed Gore's statement that you can visibly see the effect that the United States Clean Air Act has had on ice cores in Antarctica. "One can neither see, nor even detect using sensitive chemical methods any evidence in Antarctica of the Clean Air Act," he said, but did note that they are "clearly recorded in ice core records from Greenland." Despite these flaws, Steig said that the film got the fundamental science right and the minor factual errors did not undermine the main message of the film, adding "An Inconvenient Truth rests on a solid scientific foundation."

Lonnie Thompson, Earth Science professor at Ohio State University, whose work on retreating glaciers was featured in the film, was pleased with how his research was presented. "It's so hard given the breadth of this topic to be factually correct, and make sure you don't lose your audience," Thompson said. "As scientists, we publish our papers in Science and Nature, but very few people read those. Here's another way to get this message out. To me, it's an excellent overview for an introductory class at a university. What are the issues and what are the possible consequences of not doing anything about those changes? To me, it has tremendous value. It will reach people that scientists will never reach."

John Nielsen-Gammon from Texas A&M University said the "main scientific argument presented in the movie is for the most part consistent with the weight of scientific evidence, but with some of the main points needing updating, correction, or qualification." Nielsen-Gammon thought the film neglected information gained from computer models, and instead relied entirely on past and current observational evidence, "perhaps because such information would be difficult for a lay audience to grasp, believe, or connect with emotionally."

Steven Quiring, climatologist from Texas A&M University added that "whether scientists like it or not, An Inconvenient Truth has had a much greater impact on public opinion and public awareness of global climate change than any scientific paper or report."