So CO2 gets up to 7,000ppm like back in the Cambrian. What happens to global temperature?

Question

Does it take 800 years to move? If the Earth keeps warming up, the radiative surface in the atmosphere keeps moving out into space. What forces limit the distance between Earth's surface and the radiative surface? I read somewhere the "norm" is 5k meters (17k ft.).

Come to think of it -- what was the temperature of the Earth's surface back during the Cambrian when CO2 was 7,000ppm?

Answer 1

No, the '800 year lag' is the time it hisorically took atmospheric CO2 levels to increase after a warming event was initiated.  Once they increase, the warming is almost immediate.

During the Cambrian, temperatures were above 22°C (right now we're around 14.5°C).

http://www.geocraft.com/WVFossils/Carbon...

*edit* alm0st, you're neglecting the massive error bars on temperature data 500 million years ago.  Just because they're not shown on that graph doesn't mean they don't exist.

Answer 2

Using the forcing equation of WMO (1999) as reported in IPCC 2001 (6.3.5)

ΔF = 3.35(g(C)-g(C0)

where g(C)=ln(1+1.2C+.005C²+1.4e-6 C³), and using 280 ppmv as C0, the unperturbed value:

Then, for 7000 ppmv the radiative forcing from CO2 would be 23 W/m². With a climate sensitivity of 0.5 K W^-1 m^-2, that gives an increase of 11.5° C.

For 3500 ppmv, the same calculation gives an increase of 8.5° C. Although it must be emphasized that the climate sensitivity parameter is based on current CO2 levels and is based on existing feedback mechanisms in the current climate. There is no guarantee that those same feedbacks would still be present, and that the climate would have the same sensitivity, at much higher levels of CO2 and global temperature. In fact, that’s quite a doubtful proposition.

Answer 3

No one knows what the temperatures would be if co2 levels doubled or halved.  There is very little relationship between co2 levels and global temperatures.

Global temperatures have been declining since 2001 even while co2 levels continue to climb!

Answer 4

A reputable reference to your 7,000 ppm would be nice, as what I've read puts in more in the 4K range.

If all other factors were the same (e.g. solar output, orbital cycle, paleogeography, other greenhouse gases, etc.), which they aren't, then a ballpark calculation would go something like this:

- Assume a climate sensitivity of 2.0 C (that's how much the SST would increase for a doubling of atmospheric CO2, once stabilized)

- Assume CO2 levels were 4320 ppm (that's a 16-fold increase from modern pre-industrial levels).

Increase from 270 - 540 would cause 2.0 C warming

540 -> 1080 would cause another 2.0 C warming

1080 -> 2160 would cause another 2.0 C

2160 - 4320 could cause another 2.0 C.

So total would be about 8C warmer than pre-industrial levels. Of course, comparing current conditions to an era that different from our current geography isn't a trivial task and without addressing the other many factors this is a pretty meaningless exercise.

Here's a recent analysis of climate sensitivity published in Nature:

https://wesfiles.wesleyan.edu/home/droye...

"We conclude that a climate sensitivity greater than 1.5 C has probably been a robust feature of the Earth’s climate system over the past 420 million years, regardless of temporal scaling."

Edit:

How do you add 8C (my calculation) to the current average temperature (14C) and conclude that the Cambrian period should have thus been "well over 22 (C)"? Correct me if I'm wrong, but doesn't 14 + 8 = 22?

But again, it's meaningless because the paleogeological and paleoatmospheric factors are significantly different, so trying to use a 500 M year-old CO2/temperature ratio as proof for or against global warming (without a ton of caveats and large error bars) is not scientific in the least.