Hot and Cold

The planet used to be a lot hotter than it is now. There used to be a lot more CO2 in the air than there is now. It was hotter in the Eemian interglacial than it is now, and the polar bears survived. All arguments used by our deniolator friends.

And all true.

Of course, the further we go back, the harder it is to read the record, especially of CO2, but we have reasonable data about both planetary temperatures and CO2 for 450 kiloyears or so. The temperature record we can tell a lot about for a long time, and the fact is that it's now colder than for the vast majority of the last 500 Megayears. The last five million years have been a pretty cold period, thanks to the series of glaciations probably kicked off by the formation of the Isthmus of Panama.

However, it is worth noting that we, our crops, our domestic animals, and our geographic distribution all did evolve during this really cold period. If we look at the last 450,000 years, we are now very near the peak temperature that has occurred in that period - only a few brief interglacials (like the Eemian) got perhaps a degree or so warmer than the present. In those periods, the oceans rose by several meters, making Scandinavia an island and flooding big chunks of land.

More significantly, the amount of CO2 in the atmosphere during this entire period was always much less than at present. Present evidence is that the CO2 levels we would be left with even if all fossil emissions stopped immediately would bost our current temperatures to or past those interglacial peaks.

Of course, that still might leave us well short of the temperatures and CO2 concentrations of 5 million years ago, but those temnperatures would plunge us into a really different world.


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