I recently read an article in Scientific American magazine that had a subhead which caught my attention: "Why can't we have a civil conversation about climate?" The article centered on climate scientist Judith Curry, who has enraged many of her colleagues by saying that some global warming critics - certainly not most of them - have legitimate concerns about the science that is being conducted. She says these real worries are too often ignored (or responded to as though they were merely political claptrap) by the mainstream science community.
I read it with interest as one who tends (like most of us) to consider sources over substance. But the article jumped back into my mind this afternoon as my lecture series moved to "terrestrial" planets like Venus and Mars. Venus, covered by clouds of sulfuric acid and with an atmosphere made up of 96 percent carbon dioxide and 4 percent nitrogen, exhibits the ultimate "runaway greenhouse effect." It's atmospheric pressure is 90 times that of Earth. The CO2 captures most heat radiated by the planet, leaving Venus baking in a temperature of 480 degrees Celsius night and day, everywhere. It's hot enough to melt lead.
Mars, on the other hand, had an "inverse greenhouse effect." The planet used to have surface water - the evidence is everywhere - but today its atmosphere - also mostly carbon dioxide - has decreased to only one percent of the atmospheric pressure of that of Earth. That makes it incapable of retaining surface water anymore. In addition, its temperature drops as low as -130 C. Not a nice place to live, either. What happened to the atmosphere? Maybe, if Mars experienced a cooling trend, being so small and far from the Sun, carbon dioxide began freezing out, decreasing greenhouse warming. That would cool the temperature, causing more air to freeze out, and so on.
At any rate, on both Venus and Mars the changes were permanent and devastating. Life that either planet might may have had never stood a chance.
Back on Earth, we fight about climate change, with each side unhappy about uncertainties in the data and in the computer models. But the uncomfortable fact is that as long as the uncertainties exist, things could turn out to much rosier than projections indicate (Whoopee!), but things also could turn out to be much worse. The deep history of the Earth shows that our planet has endured periods of both warming and cooling and has come out of them - although today either sort of change would have staggering social consequences. But when it comes to worst-case scenarios, we've got a couple of close planetary neighbors to give us pause ... in no uncertain terms.