[Pilfer]

Fair, but that doesn't detract from the point that CO2 levels maintained 1000-2000ppm for over 100 million years. This was during the Jurassic and Cretaceous periods, when life was thriving on earth.

Present CO2 levels are low on a geological timescale.

[gshubert17]

Due to stellar evolution, the solar constant is at its highest on a geological timescale.

https://archive.org/details/nasa_techdoc_19820009158

Back in the Mesozoic Era, the Sun was about 2% cooler than it is today. The extra CO2 back then added a helpful (for most life) greenhouse effect. Now, both the relatively high CO2 level and the rapid rate of increase are unhelpful.

[jessaustin]

That link performs a calculation based on some fairly back-of-the-envelope physics assumptions. Its conclusion:

Roughly one-half of the solar luminosity increase occurs during the last 2 billion years but there is no evidence for a parallel increase in the Earth's mean surface temperature. Indeed, isotopic studies of the Precambrian samples by Knauth and Epstein (ref. 12) indicate that the mean surface temperature has been decreasing during this time. Clearly, there is a need for further studies of the effects of crustal movements and volcanism, biological activity, etc. on the long-term evolution of the Earth's climate. At present, it appears that the effects of solar evolution are still buried in the "noise" due to other uncertainties in paleoclimatic models.

Is there anything since 1981 with more definitive conclusions? Is there any study that has attempted to measure sun output directly or indirectly, rather than simply calculating output based on the sun's age?

[Jweb_Guru]

Thank you. Nothing annoys me more than people who bring up the fact that CO2 used to be much higher without mentioning that the sun was cooler. It leads to a very misleading picture.

[graeme]

How well would that work for humans? 2000 ppm is well above the threshold when we start to find negative cognitive effects. And I imagine CO2 in buildings would be more like 3000-4000 ppm.

This is leaving aside sea level rise hurting infrastructure, crop failures due to shifting weather, and areas becoming regularly above 35 degrees wet bulb.

[nixpulvis]

That doesn't change the fact that there are adverse effects we are noticing, which CO2 is a contributor to.

Its also worth noting that a lot of what has scientists concerned is the rate at which we are seeing these changes.

[pharrington]

That is a true statement. Another true statement is that Earth's greenhouse gases are basically a trace compared with Venus's. Another true statement is that the current human causal rate of CO2 emissions will rapidly change our environment to one unsuitable for human life.

[eloff]

Source on that last one? Even the worst climate change predictions don't create an earth unsuitable to human life. Hostile changes, sure. Unsuitable? No.

[pharrington]

You're right, I shouldn't have said that. Unsuitable for human civilization is what I should have said.

[eloff]

That would only be in the worst case scenarios, and even then only a transitory condition. You can't keep humans down forever, sooner or later we'd rise again. It's debatable what it would take for civilization to collapse, and how long it would take to recover. But I think there's little doubt that recover it would.

[nixpulvis]

The sad truth is nobody actually knows what's going to happen. We can only read the signs.