Global Warming, Fact or Theory?
In a recent interview, I stated that a warming world is "scientific fact." Not surprisingly, this blunt characterization caught the attention of the [Tucson] Arizona Daily Star's ever-vigilant comment squad and triggered an email exchange with a particularly interested reader. It got me thinking: Is global warming fact or theory?
Dictionary.com says a scientific fact is "any observation that has been repeatedly confirmed and accepted as true; any scientific observation that has not been refuted." Webster's online concurs: "An observation that has been confirmed repeatedly and is accepted as true (although its truth is never final)."
So far so good: global temperature data from many different organizations agree that since the late 19th century, we've experienced a warming of about 1.5 degrees F (Figure 1). Each of the past three decades has been warmer than the preceding one. Unless thermometers don't work the way we think they do, the world is heating up.
Could thermometers be wrong about warming? No – many studies have explored, and refuted, potential biases related to urbanization, land use, and other factors. But if you really don't like thermometers, look at other indicators. A recent NOAA report highlights the breadth of observations that document a warming world. In addition to rising air temperatures, we’ve got melting snow and ice, warmer oceans, rising sea level, and increasing humidity – all consistent with a warming world. These data come from a variety of sources, not just thermometers but also satellites, buoys, tide gauges, and field surveys. And they confirm what we suspected: the planet is warming.
Last month, the National Academy of Sciences weighed in, calling a warming world "settled fact." In a report on Advancing the Science of Climate Change, the authors remind us that the philosophical basis of science does not allow for absolute proof. But stronger and stronger observational foundations for a warming world render the likelihood of opposing conclusions "vanishingly small."
Now comes the trickier question. Is a warming world a consequence of human-produced greenhouse gas emissions? Or is it part of Mother Nature's mysterious ways? Certainly many factors influence the world's temperature, but they basically come down to changes in the amount and distribution of energy in the system. A changing Sun can add or subtract energy; volcanic eruptions eject particles into the atmosphere that reflect the Sun's energy back to space. But for the warming of the past century, we can rule out the Sun and volcanoes as causes. Natural variations in climate (think El Niño or North Atlantic variability) redistribute energy among the parts of the climate system; these are important for year-to-year ups and downs, but do not track the long-term trend of the past century, as shown by climate variability indicators here.
The upward trend in greenhouse gases, a result of fossil fuel burning and land-use changes, closely tracks the general trend in global temperatures. Greenhouse gases (natural and otherwise) trap energy in the lower part of the atmosphere that would otherwise escape upwards. This trapping is readily measured in the Earth's atmosphere – in fact, without naturally occurring greenhouse gases, the Earth's average surface temperature would be below freezing.
So, where do we stand? We know greenhouse gas concentrations are rising over the same interval as warming is intensifying, and that the greenhouse effect works. No natural cause can explain the recent warming. In fact, the Academy report argues that "much of the warming is very likely due to human activities" – another "settled fact." To anyone who still wants to argue that humans are NOT responsible for recent warming, there's a heavy burden of proof to provide a better physical explanation.
Why does this subject continue to be so controversial? Maybe those who deny the reality of human-caused climate change recognize that, once we identify the problem as ours, we own it. But denying facts is ultimately a losing strategy, particularly when you are going up against Mother Nature (and the laws of physics). Instead, we need to get busy figuring out what to do about them. Scientific facts lead us to this point, but only human judgment and values can take us from here.