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Earth's atmosphere is 0.038% carbon dioxide. Mars's atmosphere is 95.3% carbon dioxide. Venus's atmosphere is 96.5% carbon dioxide.

If Earth's climate is controlled by CO2, then why is Mars so cold in comparison to Venus? Mars is very cold (average about –60°C) and Venus is very hot (+460°C).

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    $\begingroup$ climate isn't controlled by CO2... it is just sensitive to it... just as it is sensitive to many other factors. $\endgroup$
    – f.thorpe
    Mar 8 '17 at 18:47
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Firstly, Mars is farther away from the sun than Venus or Earth, so it gets less heat from the sun. Secondly, Venus & Earth are volcanically active, whereas Mars is volcanically inert. Thirdly, the atmosphere on Mars is much thinner than those on Venus and Earth.

The density of the atmosphere on Venus is approximately $65\ \mathrm{kg/m}^3$, whereas, the density of Earth's atmosphere is $1.217\ \mathrm{kg/m}^3$ and the density of Mars's atmosphere is $0.02\ \mathrm{kg/m}^3$. Earth's atmosphere is 60 times denser than that of Mars; Venus's atmosphere is over 3000 times denser.

With such a thin atmosphere on Mars, there is little atmospheric mass to retain heat, despite the atmosphere being composed of 95.3 percent carbon dioxide.

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    $\begingroup$ So to rephrase a take home point: percentage is often a poor metric, because 98% of a sliver is still a sliver, and it's not enough to have a big impact. $\endgroup$ Mar 8 '17 at 8:01
  • $\begingroup$ Coincidentally, just in case you happen to know... any idea which of your three factors dominate the others Fred? If Mars had 1 of the 3, which would warm it the most? Its always nice to get a scale idea when possible on lists of synoptic factors. $\endgroup$ Mar 8 '17 at 8:05
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    $\begingroup$ iven if water is not mentioned in the question it do play a big role in combination whith co2 on the earths atmosphere .water on mars is too cold to stay liquid and end up in the atmosphere and venus is too hot for liquid water to form. $\endgroup$ Mar 8 '17 at 15:20
  • $\begingroup$ I think you should mention that Venus is not confirmed to be active it is believed but it has not been confirmed. $\endgroup$ Mar 9 '17 at 10:25
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    $\begingroup$ Just to elaborate the first point, the mean distances are 0.723 AU, 1 AU and 1.524 AU for Venus, Earth, and Mars, respectively. Radiation is proportional to the square of the distance, so the radiation received by Venus is 1.524^2 / 0.723^2 ~= 4.44 times as much as the radiation received by Mars. $\endgroup$ Jul 26 '18 at 16:03
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In addition to what has been said by @Fred, I'd like to add that CO2 doesn't actually generate heat, it is just very good at trapping heat. Mars doesn't get as much heat as Earth, and it also doesn't have enough CO2 to effectively trap heat that has been received, as a result, it's much colder than Earth.

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    $\begingroup$ And Venus gets lots more heat from the sun, and has lots more CO2 to trap that heat. Which is why the surface is hot enough to melt lead, instead of the steamy tropical jungle imagined by 1930s SF writers - which it probably would be if it had an Earthlike atmosphere (and water, of course). $\endgroup$
    – jamesqf
    Mar 8 '17 at 19:29
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    $\begingroup$ @jamesqf I was going to post an answer to that effect, but when I did the math, it turns out that isn't exactly true. Venus' albedo is very high (0.7) due to its clouds, while Earth (0.3) and Mars (0.16) are much lower. Therefore, in the climate model sense, it receives less heat than Earth and only a bit more than mars. The effective temperature of Venus is 232K, while Earth is 254K and Mars is 207K. $\endgroup$
    – kingledion
    Mar 9 '17 at 17:35
  • $\begingroup$ @kingledion: But effective temperature and surface temperature are two quite diffenent things, no? Perhaps I'll have a better answer after I work my way through "Principles of Planetary Climate", but this thread is likely to be a bit dated by then :-) $\endgroup$
    – jamesqf
    Mar 9 '17 at 23:45
  • $\begingroup$ Effective temperature is not very useful. It bears no relation to the thing that is really of interest and that is the temperature of the very tiny thin region centered about the surface where the biological entities may live, fly or swim. Surface temperature is "everything" pretty well, surface is where you land the probes, explore, build your colony etc. $\endgroup$
    – user7733
    Apr 14 '17 at 19:34
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First Venus has a very thick carbon dioxide atmosphere that traps heat. Wile mars also has a carbon dioxide atmosphere but unlike Venus it is very thin and mars is much further from the sun and there for can't really trap much heat.

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CO2 does NOT control Earth's Climate, it plays a role, but so do other factors., in it's long geologic and shorter term histories. It played a role when CO2 was 10x that of present. Even so we had an Ice Age during the Ordivician (4,400 ppm). Jurassic cooling period (1,700 ppm) The biggest influencer is the Sun. magnetic anomolies, sunspots, milankovitch cycles. Earth's orbit and axial tilt variation that permit More or less solar absorption.

For the last 800,000 years, Earth's ambient CO2 NEVER went up above 280 ppm average. In the last 10,000 years, we've had 20 major warming and cooling events and ambient co2 remained fairly constant.....

enter image description here

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  • $\begingroup$ Downvoted. Doesn't answer question and misrepresents the current significance of CO2 to climate. CO2 is the single strongest cause of current climate CHANGE. It IS "controlling" the change to Earth's climate now more than any other factor, irrespective of other periods of climate change not being because of changes to CO2. The graph is very misleading - deliberately? One region only, not Global. Missing the full range of relevant data available that together does show the significance of anthropogenic emissions. And with the largest part of recent warming - post 1970s - missing. $\endgroup$
    – Ken Fabian
    2 days ago
  • $\begingroup$ 1 degree C in the span of 150 years is not a detriment. Nor is it indicative of widespread shift. The little ice age lowered average Temps 1.5 degrees with same ambient co2. Compare CET central England temperature which goes back to the 1650s to CO2 by year. Note Temp curve failed to obey the Co2 curve. 80% of all greenhouse gas emissions originated in the last 50 years. Tempsvdid not follow the same geometric rate.. $\endgroup$
    – LazyReader
    2 days ago
  • $\begingroup$ Your answer and comment are not consistent with current science based understanding of our climate system, it's history and current climate change. $\endgroup$
    – Ken Fabian
    yesterday
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Hasn't anything to do with CO2. It's the atmospheric pressure. Venus receives very little sunlight but still very hot due to extreme pressure. Jupiter is extremely hot due the intense atmospheric pressure despite being much further from the sun. CO2 has very little effect trapping heat beyond 50ppm. The earth was in an ice age with 10 to 20 times the current CO2 levels

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    $\begingroup$ This is quite trivially incorrect - you would infer from that argument that if the sun was removed from the system, the planets would not freeze. For instance, Titan has an atmospheric surface pressure 1.5 times that of Earth, but is cold enough for liquid methane lakes to form. Jupiter is hot because it is still cooling down from it's formation. $\endgroup$ Jan 10 '18 at 14:30
  • $\begingroup$ @Ron Your answer is not consistent with current science based understanding of our climate system, it's history and current climate change. $\endgroup$
    – Ken Fabian
    6 hours ago
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The important difference between Mars and Venus is obviously not CO2 or water, or distance from the Sun for that matter, but the atmospheric pressure. You can easily determine the surface temperature if you know a few parameters like the molecular composition/weight and the pressure using the Boltzmann constant. Surface temperature is on no planet determined by only greenhouse gasses.

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