The most recent IPCC report (AR6) gives the "very likely" (90% probability) range for "equilibrium climate sensitivity" - the eventual effect on temperature of doubling CO2 - as between 2C and 5C. Current CO2 levels are less than double pre-industrial levels, so the "baked in" warming could well be less than 2C. Also note that reaching the equilibrium temperature takes more like centuries than decades, making the concept somewhat academic, since lots of other things will change over that time period.
Also note that 2C warming over pre-industrial temperatures would just take us back to about what the temperature was 8000 years ago, when conditions were good enough for agriculture and civilization to start.
Ah, the ol' earth has been as warm or warmer before claim. This is true - absolutely true, but it misses one critical, I daresay crucial, point: earth's climate has never changed this rapidly in the past few million years. It's the rate of temperature change that's the problem, not necessarily the actual temperature itself since, as you've pointed out, earth has had these temperatures before. Given time, the biosphere will respond positively. If not given sufficient time, as measured in geological time units, the biosphere responds poorly. That fact is the crux of the problem. Also, with regards to your claim that earth was warmer 8,000 ago, that's a bit nuanced. You should check out this article for more details. https://reason.com/2013/03/07/earths-average-temperature-low...
How sure are we about "never" beyond recent geological history? The impact that finished of the dinosaurs must have brought some pretty rapid changes without killing off the entire biosphere. My understanding is that the stressed parts of the biosphere collapsed pretty quickly within a few generations. But I might be wrong.
The last time the Earth was above 400ppm during the Pliocene era, when sea level was roughly 25 meters higher than today. That in itself is going to be catastrophic once the climate and the planet catch up, but there are far worse consequences and emissions are still growing.
Have you ever read about the Russian wildfires of 2010 which had a massive impact to global food prices which then triggered the Arab spring revolutions? That was a single event in a world below 400ppm but these heat waves are starting to occur with more frequency now and will have serious consequences.
The IPCC is an organization that functions with consensus and their data is awfully outdated. Their assessments generally underreport the consequences of climate change. Here's a paper from 2010 which analyzed this:
The most recent IPCC report (AR6) gives the "very likely" (90% probability) range for "equilibrium climate sensitivity" - the eventual effect on temperature of doubling CO2 - as between 2C and 5C. Current CO2 levels are less than double pre-industrial levels, so the "baked in" warming could well be less than 2C. Also note that reaching the equilibrium temperature takes more like centuries than decades, making the concept somewhat academic, since lots of other things will change over that time period.
Also note that 2C warming over pre-industrial temperatures would just take us back to about what the temperature was 8000 years ago, when conditions were good enough for agriculture and civilization to start.
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