[Jweb_Guru]

The sun is also measurably brighter and hotter than it was when CO2 levels were significantly higher, meaning we should expect greenhouse effects to be more severe. CO2 levels aren't the only thing that contribute to climate. Additionally, while in the past the levels have been much higher, the rate of change in CO2 levels was never nearly this swift in the geological record--it's the sudden change that has many people worried.

[Pilfer]

I did not bring global temperatures into the discussion as I don't believe they pose as serious a threat as increased CO2 levels. Wikipedia has a nice graph showing estimated global average temperatures over time https://en.wikipedia.org/wiki/File:All_palaeotemps.svg . Please note the log-time scale.

The graph shows that between 10-250 million years ago, global temperatures were 1-12° C higher than they are today. Given that current projections, which include the 2% relative gain in solar irradiance, expect 3-4°C warming by 2100, the geologic temperature record suggests such increases are not detrimental to life on earth. Let us recall that life was thriving during the Jurassic and Cretaceous periods, when temperatures were 5-10°C higher and CO2 levels between 1000-2000ppm. Global temperatures today are actually lower than than average, on a geological timescale.

>the rate of change in CO2 levels was never nearly this swift

No, you can't conclude this. You cannot show there was no other a period in Earth's geologic history where CO2 levels rose by 150ppm (or 150%, whichever is not relevant) in less than 200 years. The geologic record is not detailed enough to make such a claim. It is unknowable if there was a series of major volcanic eruptions that caused CO2 levels to rise rapidly.

And sudden change has always been a part of climate and evolution. Rapid change can occur in just a few decades. For example, the Sahara wasn't even a desert 10,000 years ago. Also see punctuated equilibrium.

[zaarn]

> You cannot show there was no other a period in Earth's geologic history where CO2 levels rose by 150ppm (or 150%, whichever is not relevant) in less than 200 years

That assumes the CO2 is also gone just as quickly. But I doubt that would happen unless you can show a natural process that could not only increase the CO2 level by 150ppm but also decrease it by 150ppm just as quickly.

[Brockenstein]

Your argument seems to be that everything is OK because things always change and conditions have been much more extreme in the distant past. And other people are arguing that these sorts of extreme changes may have very real consequences that can dramatically effect human civilization and stability and we're doing very little to prepare or blunt the blows.