is it that the change is on such an epic and sudden scale that the temperatures of the past 20,000 years will all seem temperate...? Yes. That's what the scien

is it that the change is on such an epic and sudden scale that the temperatures of the past 20,000 years will all seem temperate...? Yes. That's what the scientists seem to be saying. I'm no climate scientist -- I just downloaded the first paper I've ever looked at on the subject. It's James Hansen's most recent preprint, which is conveniently available as a PDF here: http://www.columbia.edu/~jeh1/2008/TargetCO2_20080407.pdf http://www.columbia.edu/~jeh1/2008/TargetCO2_20080407.pdf It's a preprint, so it's still under review, some bits of it are more hypothetical than other bits, maybe the grad student dropped a decimal place, blah blah blah, caveats caveats. But presumably ice core temp data is pretty well documented in the literature by now, so if we stick to those figures we should be okay. Figure 2 is what we're looking for -- specifically, the red and the purple curves. The red curve is an estimate of global temperature going back 400k years, based on ice core data. The purple curve is measured global temperature since the late 1800s. My back-of-the-envelope summary here: -- Just by squinting at the purple curve, we see that the Earth has warmed something like 0.5C since 1900. -- The Earth, on average, is now about as warm as it has ever been since the end of the last glacial period, 10k years ago. Although, obviously, temperatures vary across the surface of the earth and over time by a lot more than 0.5C, and these curves are fuzzy estimates based on data at one or two points, so don't expect it to apply to your backyard, or to any specific winter's day. -- If the Earth warms another 1deg C, we'll have a global average temperature comparable to the highest seen in the last half million years. When will that happen? Considering (naively) only the slope of the purple curve in Figure 2, which is based on contemporary measurements by humans with thermometers, it might be a couple hundred years or less. -- Homo sapiens, according to genetic studies, is about 200,000 years old. So, yeah, the earth has been warmer than it is today "while humans were around", but your grandchildren might not be able to say that anymore. -- If you want to compare the current (global average) temps with the (global average) temperatures when your French walls were built, check out the inset graph at: http://commons.wikimedia.org/wiki/Image:Holocene_Temperature_Variations.png http://commons.wikimedia.org/wiki/Image:Holocene_Temperature... I could be wrong, but I'm pretty sure I can squint at this data and see the legendary Medieval Warm Period (peaking about 1000 years ago; that's when England had those nice vineyards) followed by the legendary Little Ice Age (the last 1000 years, up to about 150 years ago). And the current global temp is on there, too, on the left edge. If you can't see it at first glance: Look higher on the scale. ;) -- Yes, global temperatures have risen very quickly before. Check out the start of the Holocene interglacial, or the Eemian interglacial. But nobody was around to write about that -- it was before history was invented. So I think you can safely call this an "epic" event, since every recorded epic ever is newer than the last time this happened. It's hard to get your mind around. If only some older species than ours had kept records, maybe we'd have a better frame of reference. It's a pity those Homo erectus were such slackers.