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climatologist, is a professor in the Department of Biological Sciences
Climate Change: Warming Up To The Evidence

In public appearances about global warming, even these days, I often hear: "I don't believe in global warming" and I then typically get asked why I do "when all the evidence is not in". "Global warming is not about beliefs", I typically retort, "but an accumulation of evidence over decades so that we can now say the vast preponderance of evidence — and its consistency with basic climate theory — supports global warming as well established, not that all aspects are fully known, an impossibility in any complex systems science".

But it hasn't always been that way, especially for me at the outset of my career in 1971, when I co-authored a controversial paper calculating that cooling effects from a shroud of atmospheric dust and smoke — aerosols — from human emissions at a global scale appeared to dominate the opposing warming effect of the growing atmospheric concentrations of the greenhouse gas carbon dioxide. Measurements at the time showed both warming and cooling emissions were on the rise, so a calculation of the net balance was essential — though controlling the aerosols made sense with or without climate side effects since they posed — and still pose — serious health effects on vulnerable populations.  In fact for the latter reason laws to clean up the air in most rich countries were just getting negotiated about that time.

When I traveled the globe in the early 1970s to explain our calculations, what I slowly learned from those out there making measurements was that two facts had only recently come to light, and together they appeared to make me consider flipping sign from cooling to warming as the most likely climatic change direction from humans using the atmosphere as a free sewer to dump some of our volatile industrial and agricultural wastes.  These facts were that human-injected aerosols, which we assumed were global in scale in our cooling calculation — were in fact concentrated primarily in industrial regions and bio-mass burning areas of the globe — about 20% of the Earth's surface, whereas we already knew that CO2 emissions are global in extent and about half of the emitted CO2 lasts for a century or more in the air.

But there were new facts that were even more convincing: not only is CO2 an important human-emitted greenhouse gas, but so too were methane, nitrous oxide and chlorofluorocarbons (many of the latter gases now banned because they also deplete stratospheric ozone) , and that together with CO2, these other greenhouse gasses were an enhanced global set of warming factors. On the other hand, aerosols were primarily regional in extent and could not thus overcome the warming effects of the combined global scale greenhouse gases.

I was very proud to have published in the mid-1970s what was wrong with my early calculations well before the so-called "contrarians" — climate change deniers still all too prevalent even today — understood the issues, let alone incorporated these new facts into updated models to make more credible projections. Of course, today the dominance of warming over cooling agents is now well established in the climatology community, but our remaining inability to be very precise over how much warming the planet can expect to have to deal with is in large part still an uncertainty over the partially counteracting cooling effects of aerosols — enough to offset a significant, even if largely unknown, amount of the warming. So although we are very confident in the existence of human-caused warming in the past several decades from greenhouse gases, we are still are working hard to pin down much more precisely how much aerosols offset this warming. Facts on that offset still lag the critical need to estimate better our impacts on climate before they become potentially irreversible.

The sad part of this story is not about science, but the misinterpretation of it in the political world. I still have to endure polemical blogs from contrarian columnists and others about how, as one put it in a grand polemic: "Schneider is an environmentalist for all temperatures" — citing my early calculations. This famous columnist somehow forgot to bring up the later-corrected (by me) faulty assumptions, nor mention that the 1971 calculation was based on not-yet-gathered facts. Simply getting the sign wrong was cited, ipso facto in this blog, as somehow damning of my current credibility. 

Ironically, inside the scientific world, this switch of sign of projected effects is viewed as precisely what responsible scientists must do when the facts change. Not only did I change my mind, but published almost immediately what had changed and how that played out over time. Scientists have no crystal ball, but we do have modeling methods that are the closest approximation available. They can't give us truth, but they can tell us the logical consequences of explicit assumptions. Those who update their conclusions explicitly as facts evolve are much more likely to be a credible source than those who stick to old stories for political consistency. Two cheers for the scientific method!