From catastrophic sea level rise to jarring changes in local weather, humanity faces a potentially dangerous threat from the changes our own pollution has wrought on Earth’s climate. But since nothing in science can ever be proven with 100 percent certainty, how is it that scientists can be so sure that we are the cause of global warming?
For years, there has been clear scientific consensus that Earth’s climate is heating up and that humans are the culprits behind the trend, says Naomi Oreskes, a historian of science at the University of California, San Diego.
A few years ago, she evaluated 928 scientific papers that dealt with global climate change and found that none disagreed about human-generated global warming. The results of her analysis were published in a 2004 essay in the journal Science.
And the Intergovernmental Panel on Climate Change (IPCC), the National Academy of Sciences and numerous other noted scientific organizations have issued statements that unequivocally endorse the idea of global warming and attribute it to human activities.
“We’re confident about what’s going on,” said climate scientist Gavin Schmidt of NASA’s Goddard Institute of Space Science in New York.
But even if there is a consensus, how can scientists be so confident about a trend playing out over dozens of years in the grand scheme of the Earth's existence? How do they know they didn’t miss something, or that there is not some other explanation for the world’s warming? After all, there was once a scientific consensus that the Earth was flat. How can scientists prove their position?
Best predictor wins
Contrary to popular parlance, science can never truly “prove” a theory. Science simply arrives at the best explanation of how the world works. Global warming can no more be “proven” than the theory of continental drift, the theory of evolution or the concept that germs carry diseases.
“All science is fallible,” Oreskes told LiveScience. “Climate science shouldn’t be expected to stand up to some fantasy standard that no science can live up to.”
Instead, a variety of methods and standards are used to evaluate the viability of different scientific explanations and theories. One such standard is how well a theory predicts the outcome of an event, and climate change theory has proven to be a strong predictor.
The effects of putting massive amounts of carbon dioxide in the air were predicted as long ago as the early 20th century by Swedish chemist Svante Arrhenius.
Noted oceanographer Roger Revelle’s 1957 predictions that carbon dioxide would build up in the atmosphere and cause noticeable changes by the year 2000 have been borne out by numerous studies, as has Princeton climatologist Suki Manabe’s 1980 prediction that the Earth’s poles would be first to see the effects of global warming.
Also in the 1980s, NASA climatologist James Hansen predicted with high accuracy what the global average temperature would be in 30 years time (now the present day).
Hansen's model predictions are “a shining example of a successful prediction in climate science,” said climatologist Michael Mann of Pennsylvania State University.
Schmidt says that predictions by those who doubted global warming have failed to come true.
“Why don’t you trust a psychic? Because their predictions are wrong,” he told LiveScience. “The credibility goes to the side that gets these predictions right.”
Besides their successful predictions, climate scientists have been assembling a “body of evidence that has been growing significantly with each year,” Mann said.
Data from tree rings, ice cores and coral reefs taken with instrumental observations of air and ocean temperatures, sea ice melt and greenhouse gas concentrations have all emerged in support of climate change theory.
“There are 20 different lines of evidence that the planet is warming,” and the same goes for evidence that greenhouse gases are increasing in the atmosphere, Schmidt said. “All of these things are very incontrovertible.”
But skeptics have often raised the question of whether these observations and effects attributed to global warming may in fact be explained by natural variation or changes in solar radiation hitting the Earth.
Hurricane expert William Gray, of Colorado State University, told Discover magazine in a 2005 interview, "I'm not disputing that there has been global warming. There was a lot of global warming in the 1930s and '40s, and then there was a slight global cooling from the middle '40s to the early '70s. And there has been warming since the middle '70s, especially in the last 10 years. But this is natural, due to ocean circulation changes and other factors. It is not human induced.”
Isaac Newton had something to say about all this: In his seminal “Principia Mathematica,” he noted that if separate data sets are best explained by one theory or idea, that explanation is most likely the true explanation.
And studies have overwhelmingly shown that climate change scenarios in which greenhouse gases emitted from human activities cause global warming best explain the observed changes in Earth’s climate, Mann said—models that use only natural variation can’t account for the significant warming that has occurred in the last few decades.
Mythic ice age
One argument commonly used to cast doubt on the idea of global warming is the supposed predictions of an impending ice age by scientists in the 1970s. One might say: First the Earth was supposed to be getting colder; now scientists say it’s getting hotter—how can we trust scientists if they’re predictions are so wishy-washy?
Because the first prediction was never actually made. Rather, it’s something of an urban climate myth.
Mann says that this myth started from a “tiny grain of truth around which so much distortion and misinformation has been placed.”
Scientists were well aware of the warming that could be caused by increasing greenhouse gases, both Mann and Schmidt explained, but in the decades preceding the 1970s, aerosols, or air pollution, had been steadily increasing. These tiny particles tended to have a cooling effect in the atmosphere, and at the time, scientists were unsure who would win the climate-changing battle, aerosols or greenhouse gases.
“It was unclear what direction the climate was going,” Mann said.
But several popular media, such as Newsweek, ran articles that exaggerated what scientists had said about the potential of aerosols to cool the Earth.
But the battle is now over, and greenhouse gases have won.
“Human society has made a clear decision as to which direction [the climate] is going to go,” Mann said.
One of the remaining skeptics, is MIT meteorologist Richard Lindzen. While he acknowledges the trends of rising temperatures and greenhouse gases, Lindzen expressed his doubt on man’s culpability in the case and casts doubt on the dire predictions made by some climate models, in an April 2006 editorial for The Wall Street Journal.
“What the public fails to grasp is that the claims neither constitute support for alarm nor establish man's responsibility for the small amount of warming that has occurred,” Lindzen wrote.
To be sure, there is a certain degree of uncertainty involved in modeling and predicting future changes in the climate, but “you don’t need to have a climate model to know that climate change is a problem,” Oreskes said.
Climate scientists have clearly met the burden of proof with the mounting evidence they’ve assembled and the strong predictive power of global warming theory, Oreskes said-- global warming is something to pay attention to.
Schmidt agrees. “All of these little things just reinforce the big picture,” he said. “And the big picture is very worrying.”
- Video: Goldilocks and the Greenhouse
- Top 10 Surprising Results of Global Warming
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Andrea Thompson is an associate editor at Scientific American, where she covers sustainability, energy and the environment. Prior to that, she was a senior writer covering climate science at Climate Central and a reporter and editor at Live Science, where she primarily covered Earth science and the environment. She holds a graduate degree in science health and environmental reporting from New York University, as well as a bachelor of science and and masters of science in atmospheric chemistry from the Georgia Institute of Technology.