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I know this is a very simple or naive question but am just being curious.

How would the global temperature change if all vehicles stopped for a whole year?

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    $\begingroup$ I think this is actually a great question. Such questions (that can be answered with models) serve to provide understanding in how the climate system works. Not sure why this question is downvoted. The question may be simple, the answer might not be. $\endgroup$ – gerrit Oct 6 '14 at 14:47
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    $\begingroup$ You should send this question to Randall... $\endgroup$ – R.. Oct 6 '14 at 23:36
  • $\begingroup$ +1 on this question, this kind of question are important for the variables that affect climate models. $\endgroup$ – user889 Oct 7 '14 at 9:59
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There are two factors to be aware of: First, cars (and all road vehicles) account for only a fraction of human CO2 output. Off-hand I've a feeling it is about a quarter or a third although I don't have the exact figure handy. That is, stop all cars, and atmospheric CO2 would continue to rise, albeit it at a lower rate.

Second, the atmosphere/ocean temperatures lag the atmospheric CO2 content. Therefore even if humans stopped all CO2 emissions, temperatures would continue to rise (although they would eventually stop, and the rate of increase would not get any worse). There was an article in Geoscientist about 5 years ago that compared total anthropogenic CO2 emissions to those of the PETM (Paleogene-Eocene Thermal Maximum) which were of a similar level. The Earth recovered from that little incident but it took over 100ka, and not before there were tropical plants at the North Pole (admittedly the starting temperature was higher than today).

Ongoing research has started to draw in other earlier events - even the Permian-Triassic. It is beginning to appear that the level of CO2 itself isn't so important, but the rate of increase is. Temperatures rise too quickly (and ocean acidification occurs too quickly) for the various feedback systems to correct. Hence mass extinctions. Research is definitely ongoing, but this is where the scary (to a geologist) scenarios start to appear...

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    $\begingroup$ All transportion sources put together (road, rail, air, marine) account for 13% of anthropologic greenhouse gas emissions: epa.gov/climatechange/ghgemissions/global.html $\endgroup$ – DavePhD Oct 6 '14 at 16:38
  • $\begingroup$ This is probably quite minor in comparison, but what about the heat output from running the vehicles. With supposedly ~1 billion vehicles in use as of 2010 it seems like this might also have some impact. $\endgroup$ – Michael Mior Oct 6 '14 at 17:05
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    $\begingroup$ Re The Earth recovered from that little incident ... That paints the PETM in a bad light. The PETM was a minor spike before the much longer and slightly warmer Eocene Optimum. For most of life's history, the Earth has been in a hothouse Earth phase. A hothouse Earth provides much better conditions for life overall than does a icehouse Earth (what we have right now). Life will eventually prosper if AGW kicks the Earth out of this ice age. The question is, will humans prosper? $\endgroup$ – David Hammen Oct 6 '14 at 18:50
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    $\begingroup$ @David, Yes warm conditions are the norm (and this is why I was skeptical of ocean acidification for so long), but research is suggesting the rate of change is what causes extinctions (eg. the cooking of sediments by the emplacement of a Large Igneous Province) - not the final temperature. As I say ongoing research, but Anthropological CO2 is growing fast, and appears to be similar rates to the P/T - hence my "scary" comment. $\endgroup$ – winwaed Oct 6 '14 at 19:50
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    $\begingroup$ Interesting question - here is an article that studied surface temperature of the earth before, during and after 911. Although this does not involve cars, it does involve airplanes and also all ship in shipping lanes into and out of the US. Read the article: atmos.washington.edu/~rennert/etc/courses/pcc587/ref/… $\endgroup$ – Rick Oct 7 '14 at 15:43
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Not much at first. The damage has already been done. Humanity has added at least 100 gigatonnes of carbon to the atmosphere since the start of the industrial age. Natural sequestration of that excess carbon is a slow process. Suppose all human production of carbon ceases (and note that vehicular production of CO2 is but a fraction of the total). It will take centuries to return to pre-industrial levels, and temperatures will continue to rise for decades to come.

Humanity will not countenance a decree to stop using vehicles. You would somehow need to magically dispense the laws of physics, at least those that pertain to internal combustion engines. Food shipments would stop. Riots would ensue within a week of your use of your magic wand. Your magic want is a death sentence for a huge portion of humanity. People will die in droves. Killing off 90% of humanity is of course one long term solution to the anthropogenic warming problem. It's not a very nice solution.

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    $\begingroup$ Although the comment in the second paragraph is probably accurate, it does not pertain to the question and belongs more at a site like what-if.xkcd.com than on Earth Science SE. $\endgroup$ – gerrit Oct 6 '14 at 14:56
  • $\begingroup$ @gerrit - If that's the case then this whole question belongs at what-if.xkcd.com rather than here. We cannot stop using all vehicles for even a week, let alone for a whole year. $\endgroup$ – David Hammen Oct 6 '14 at 18:36
  • $\begingroup$ the hypothetical question how the climate system would respond if we instantly stopped emitting anything anthropogenic, is a valid question that provides insight to the climate system. $\endgroup$ – gerrit Oct 6 '14 at 19:29
  • $\begingroup$ @gerrit - That's your opinion. Mine obviously differs from yours. To me, I see no difference conceptually between this kind of question and questions of the sort "Suppose I violate the laws of physics here and here. What do the laws of physics say will happen when I do that?" $\endgroup$ – David Hammen Oct 6 '14 at 19:45
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    $\begingroup$ It is my opinion. A common exercise in climate modelling is to test the model's sensitivity to a prescribed, possibly instantaneous (and therefore non-physical) doubling of CO₂. For example, see Manabe and Wetherald, 1975 or Wu et al. (2012). So I'm not the only one to believe this is a valuable exercise. $\endgroup$ – gerrit Oct 6 '14 at 20:29

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