So what if CO2 was 2400 ppmv in the Mesozoic

This is a response to those who try to claim that global warming won't be so bad.  The gist of their argument is that since life thrived in the Mesozoic when CO2 was ~2400 ppmv and temperatures 8ºC warmer, climate change today isn't anything to be worried about.  Unfortunately, this argument ignores some very basic facts about biology and physics.  Here is some of what they're ignoring.

1) First, thanks to those individuals for accidentally confirming the relationship between CO2 and global temperature, as well as modern estimates of climate sensitivity.  At modern solar radiation levels and with climate sensitivity at 0.809 W/m2, the equilibrium climate model predicts that with CO2 at 2400 ppmv, global temperatures would rise by 9.3ºC above pre-industrial temperatures.  Factor in a weaker sun back in the Mesozoic and you get the 8ºC rise experienced from 2400 ppmv CO2 back then (Royer 2006).  Got to love it when those who dismiss science score an own goal and don't even realize it.

2) The species we have living on this Earth are not the same as the species that existed during the Mesozoic.  Then, the land was dominated by various species of dinosaurs, the air by pelicosaurs, and the seas by ithyosaurs, mosasaurs, and plesiosaurs.  The dominant plants for the Triassic and Jurassic was various species of gymnosperms while the Cretaceous saw the rise of the angiosperms.  But that is largely irrelevant for today's species.  Most of today's species evolved during the Pleistocene, when global average temperatures were usually 4.5ºC colder than today.  Species are highly sensitive to changes in the normal temperature regime to which they have evolved.  Even a shift of a few tenths of a degree C is enough to make species migrate toward the poles and change their phenology.  A temperature increase of 8ºC above today's levels would be catastrophic to today's species, many of which are already at the upper limits of their normal temperature range.

3) While the total amount of warming is important, the rate at which that warming occurs is even more important.  A slow rate would allow species to evolve adaptations to the change in temperatures.  Unfortunately, the current rate of temperature change is far faster than the  rate of evolutionary adaptation to changes in temperature.  Quintero and Wiens (2013) found that vertebrate species can adapt to at most 1ºC of temperature change per million years.  The current rate of temperature change over the past 30 years is 1.6ºC per century, over 10,000x faster.

I'm sure there's more that I've left out or just didn't think of while writing this.  The bottom line is that those who try to argue that increases in CO2 is no big deal are simply ignoring most of what we know about ecology, physiology, and evolution.

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