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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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...
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.