Responding to the CO2 Coalition's "Facts #19, #20, and #28" on Geologic Temperature Changes

CO2 Coalition seems to think it needs to say almost the same thing three times in slightly different ways. In "Fact #19," "Fact #20" and "Fact #28" they said in various ways that global temperatures have changed a lot across geologic history, and current temperatures are colder than most of geologic history. While these things are generally true, they are also completely irrelevant to whether humans are changing global temperatures through our carbon emissions and whether that is causing harm for human civilization and biodiversity. There are still several misleading things being said in these three "facts," beginning with the choice of graph.

CO2 Coalition shows some version of the above graph in all three of these "facts," claiming it comes from "Scotese (2002)." This gives the impression they got this from a scientific paper, but it's not. It's a schematic that Christopher Scotese put on a website over 20 years ago. He's updated his graph and published modified versions in journals since then, but CO2 Coalition appears to be unaware of this. They also apparently didn't bother to read what Scotese wrote about this graph. CO2 Coalition claims, "This figure shows more than 4 billion years’ temperature data as adapted from Scotese (2002)." But Scotese says on his website that the schematic covers only the last 2 billions years: "During the last 2 billion years the Earth's climate has alternated between a frigid 'Ice House', like today's world, and a steaming 'Hot House', like the world of the dinosaurs. This chart shows how global climate has changed through time." The Precambrian is not drawn to scale, and it's obviously oversimplified. Here's a more recent version of his graph.

Above is the most recent version of Scotese's graph that I've found, and it shows some clear differences from the oversimplified "ice house" vs "hot house" model he used in 2002. The end Ordovician glaciation is much colder, as are much of the Carboniferous and Permian. The global spike in temperatures following the eruption of the Siberian Traps at the end of the Permian is also more obvious. There were significant periods of time that were as cold or colder than the Quaternary, even though most of the Phanerozoic was indeed warmer than today.

But CO2 Coalition is mostly interested in finding ways to get you believe that the 1.2°C warming above the 1850-1900 mean is miniscule. For instance, in "fact #20," they say, "Viewed in the context of millions of years of Earth history, our recent increase of 0.8°C (1.4°F) appears minuscule. It barely registers as a blip on the chart." Here CO2 Coalition both lowered the amount of actual warming has occurred from 1.2°C to 0.8°C, and then they argue that this amount is "miniscule" because temperatures have been much warmer in the past. But this is clearly bad logic. The difference between the Last Glacial Maximum (LGM) and the Holocene Thermal Maximum (HTM) was only ~6°C. That's the difference between NYC being what it is today and NYC covered in an ice sheet a thousand feet thick.  Let's call this difference an Ice Age Unit (IAU). Current warming is about 1/5 of an IAU. With no mitigation strategies, we could easily see 3 C warming by 2100 or about 1/2 of an IAU. We can expect large changes in global climate with these changes in global temperature, whether or not you call them "miniscule." Just because it's been a lot warmer in the past doesn't mean that current warming is a good thing. The best evidence we have is that it's disrupting the relatively stable conditions civilization enjoyed throughout the Holocene, and that disruption will force expensive adaptations. We can mitigate to limit the harm expected from 3°C warming, but the problem still exists.