# How hot the earth will be if we have 20% CO2 in the atmosphere instead of ~0.04? and how cold would it be if doesn't have any CO2?

I know Earth once had this amount in the past, but if we got the former atmosphere with around 20% of CO2, how much the temperature will rise? assuming the atmosphere is still dominated by 78% nitrogen and 2% of other gasses.

Would it go runaway greenhouse effect?

And what about if Earth has the same atmosphere that we have today but with 0% CO2, how cold it will be? assuming other greenhouse gasses aren't changed and still affecting the planet.

And I know that neither the temperature nor the atmosphere will be friendly, at least not to us.

I don't want to know what will happen to life or if the Earth will still livable because it's obvious. (Edited)

Without CO2, Earth would be as cold as Mars or the outer planets between 200-250 degrees Kelvin (-80 degrees below zero fahrenheit) As for more, a 20% gain in current CO2 would be 504 Parts per million in Atmosphere. That's impossible without a century timeframe. But temperature predictions have failed to coalesce. Namely computer models that fail to predict actual temp increases.

Google John Tyndall. https://en.wikipedia.org/wiki/John_Tyndall He measured the radiative properties of CO2 and other “greenhouse gases” in the late 1850s if you want a longer time-frame for human CO2 emissions this should do. Note temperature curve failed to obey CO2 curve. Also note, humanity built it's first civilizations where it's constantly warm....Mesopotamia, Egypt, Minoans....

Overall CO2 input's have to be logarithmic. IN other words you want another degree of warming you need to double CO2 rates. 800 ppm 1600 ppm 3200 ppm

You say 20%, that's 200,000 ppm, Impossible amount and a partially toxic atmosphere. CO2 begins to become toxic at around 10,000 ppm.

• How bad/hot 1% of CO2 will be? – Khalid Feb 7 at 15:01
• 1% co2, that's 10,000 parts per million in atmosphere. An atmosphere not seen, ever. Highest observed co2 average was 7000-8000 ppm, during the Cambrian era. As for temp I don't know, about 5-6 degrees, MAYBE more. 9-10. – LazyReader Feb 7 at 15:03
• The graph is inadequate (single location, not global), misleading, lacks sources, includes a 'climate model prediction' that appears unrelated to any climate model prediction. Claims are made "you want another degree of warming you need to double CO2 rates." that are not supported by current science. – Ken Fabian Feb 17 at 0:17

There are other greenhouse gasses besides carbon dioxide (in particular water vapor) but in order to give a definite answer, let's pretend the atmosphere is completely free of them. In this case, it's easy to calculate the average temperature of the earth by holding the incoming shortwave heat flux at its present value and assuming that it is balanced by outgoing blackbody radiation from the surface.

Similarly, rather than deal with your 20% carbon dioxide atmosphere explicitly, let's just say the atmosphere provides a "perfect" single-layer greenhouse blanket: an isothermal one that allows shortwave radiation through in the same way the existent one does, but absorbs all the outgoing longwave radiation (I don't think anywhere close to 20% concentration of carbon dioxide would be required to approach this limit). This greenhouse blanket reaches a temperature such that it radiates the same amount of heat to space as is coming in from the sun. Since it radiates equally upward and downward (and we assume all of the downward longwave radiation is absorbed), this means the total energy absorbed (and therefore, in equilibrium, radiated) at the surface is doubled relative to what it would be with no greenhouse blanket. Note that a larger warming effect is possible if the “blanket” is allowed to stratify so that the bottom surface is hotter than the top.

So, with no greenhouse gasses in the atmosphere: $$F_{sun}=\sigma T^4,$$ where $$F_{sun}$$ is the average, over the whole surface of the earth, of the heat flux absorbed from the sun (roughly $$240 W/m^2$$), $$\sigma$$ is the Stefan-Boltzmann constant, $$5.67 \times 10^{-8}Wm^{-2}K^{-4}$$, and T is the average temperature of the earth's surface. This results in $$T=255K=-18^{\circ}C$$. Pretty chilly.

With a "perfect" greenhouse atmosphere, $$2F_{sun}=\sigma T^4,$$ which results in $$T=303K=30^{\circ}C$$, about 15 degrees warmer than the present average. You could reach a slightly higher average surface temperature--perhaps a couple of degrees--by assuming that the ice caps melt, lowering the albedo.

• Thanks but Is the "perfect" greenhouse atmosphere have more than 20% of CO2? – Khalid Feb 7 at 4:47
• As I mentioned, I think essentially all outgoing longwave would be absorbed even with substantially less than 20%. Once you’ve reached that point, adding more greenhouse gas doesn’t add more warming. – Ben51 Feb 7 at 4:51
• What about the runaway greenhouse effect? – Khalid Feb 7 at 4:52
• Note that this perfect greenhouse is only the upper bound for a single layer (i.e., isothermal) model of the atmosphere. Stronger greenhouse gas concentrations effectively add more absorption layers to this radiative model, which progressively increase the surface temperature at equilibrium, maintained by decreasing atmospheric temperature with height. – Deditos Feb 7 at 17:17
• @Deditos yes, you’re right. I was wrong to imply that this model represented the strongest effect possible. – Ben51 Feb 7 at 17:19

The concentration of CO2 in the Earth's atmosphere has remained less than 1% in the past 470 million years when land plants first appeared--long before mammals, and long before humans graced the earth.

If we are talking about having a biosphere comparable to the one we know, we need to be talking about CO2 and global average temperatures that are comparable to levels during the relatively stable Holocene epoch of the past 11,000+ years. Those are the conditions that support life, with the current overshoot beyond 350 ppm being brief (in terms of geological time).

As for 0% CO2 concentration in the Earth's atmosphere, that would remove warming that essential for life on the planet. The goal is to have CO2 levels that are stable and also at a level which provides the warming for which life in the biosphere is accustomed to--which is less than 350 and probably even less than 325 ppm. An accepted scientific paper is Target Atmospheric CO2: Where should humanity aim? [https://arxiv.org/pdf/0804.1126.pdf].

This discussion should also switch from the concentration of CO2 in the atmosphere to global emissions from a few human activities (roughly 90% being combustion of fossil fuels). The question to ask is how much do those human emissions need to change for the current annual increases in CO2 concentrations to stop rising? They need to return to emission levels that were essentially zero as they were prior to the Industrial Revolution. That is when natural global emissions and absorption (aka sinks) cancelled one another out to keep levels stable. However, cutting global human CO2 emissions by half (or let's say 60% to be safe) will stop the rise in CO2 levels for a few years. If cuts do not go further within the decade toward 100%, CO2 levels in the atmosphere will start to rise again. Reference: IPCC AR4 FAQ 10.3 [https://archive.ipcc.ch/publications_and_data/ar4/wg1/en/faq-10-3.html].

• Thank you for the answer, my question was out of curiosity, so I don't want to know what will happen to life or if it was habitable, just a close number of temperature. (This is my fault for not writing that in the question) – Khalid Feb 6 at 9:57

Looking at other planets in the solar system is a good way to get an indication of the role extreme variations of CO2 has. Mercury is closest to the sun, and has a high maximum temperature (449 centigrade) but a very low minimum (-170 centigrade). It has practically no atmosphere, and virtually no CO2. Venus, on the other hand - the planet between the Earth and Mercury - has a minimum temperature that is higher than Mercury's maximum (!) at 465 centigrade.... and also the same temperature for the maximum. It has 96% CO2 in its thick atmosphere, keeping the planet very hot all of the time.