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One of the claims in "SuperFreakonomics" is, that reducing CO2 emissions produced by cars have very little practical impact on global warming. Their arguments are:

  • human activity is responsible for only 2% of CO2 emissions, with the reminder generated by natural processes;
  • greenhouse effect of methane is 70 times more than of CO2 (per molecule);
  • ruminants (mainly cows) are responsible for greenhouse gases affecting 50 times more than entire transport sector;
  • even if humanity would reduce CO2 to zero, it wouldn't significantly affect global warming within next few hundred years;

Does CO2 emitted by cars have significant impact on global warming?

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    I moved to a bigger city last year, and jogging here causes me to have breathing problems afterwards, which I didn't have where I come from. I can smell the gasses from cars when going to work as well. So it's not futile when you try to enjoy life.
    – Terry
    Commented Feb 21, 2012 at 15:22
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    @djerry: you can smell CO2? interesting...
    – vartec
    Commented Feb 21, 2012 at 15:30
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    @djerry what you smell is not CO2 but the soot, sulfer and other gasses. CO2 is odorless
    – ratchet freak
    Commented Feb 21, 2012 at 19:21
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    Transport is clearly about 30% of CO2 emissions and I don't recall Superfreakonomics claiming otherwise (or arguing based on the first two bullets at all). I think their argument depends more on the second two.
    – matt_black
    Commented Feb 21, 2012 at 23:54
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    The hoary old 'human acitivity is responsible for only 2% of CO2..." often serves as a springboard for erroneous arguments. Without human input, CO2 would be present in the atmosphere at some seady state level. The same holds for a constant human input of CO2. However, that steady state number is not obliged to be only 2% higher than the value w no human input. Absorption and emission are nonliner. Look at atmosphere CO2 for the past few decades. The effect of human additions is far more than 2%. We're up by more than 2% in the last 4 years alone: esrl.noaa.gov/gmd/ccgg/trends
    – user951
    Commented Feb 22, 2012 at 17:24

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tl;dr: No, reducing carbon emissions from cars is not futile.

See also Does a car with a hybrid engine and Lithium batteries pollute more than a car with conventional technology? which directly relates to the global-warming potential of fossil-fuel powered cars.

To determine the question of whether reducing carbon emissions from cars is futile or not, we can investigate three linked questions, each of which gets answered in whole volumes of text, and at the very least each could be a separate question here:

  • Are anthropogenic greenhouse gas emissions causing global warming?
  • Are cars a significant source of anthropogenic greenhouse gases?
  • Would reducing anthropogenic emissions reduce global warming?

If the answer to all those is yes, then it is not futile to reduce carbon emissions from cars. Let's look at each in turn.

Are anthropogenic greenhouse gas emissions causing global warming?

Yes.

Previous questions have covered some of the science; see: Is CO₂ the cause for Global Warming? , the accepted answer to which explicitly addresses the issue of anthropogenic greenhouse gases; and Do human activities contribute to climate change?

From that basis, we know that anthropogenic emissions of greenhouse gases are major contributors to global warming, and that cars are one source.

Are cars a significant source of anthropogenic greenhouse gases?

Yes.

According to the World Resources Institute, road transport accounts for 10% of global emissions.

In the UK, using the figures from the Government's advisory body, the Committee on Climate Change (CCC), surface transport accounts for 22% of national emissions, and of that, 60% is from cars: hence 13% (60% x 22%) of national emissions are from cars. The CCC identifies surface transport as a significant area for attention for two reasons: firstly, its non-negligible 22% contribution to the total; and secondly, that it is a sector where some rapid decarbonisation is possible. In their words, from the source linked at the start of this paragraph:

a 26% reduction in surface transport emissions from 2008 levels is possible by 2020, with a reduction of 44% by 2030

The figures for greenhouse gas emissions include different weightings to reflect the different global warming potential of, for example, methane relative to carbon dioxide. They also include the emissions from livestock.

Would reducing anthropogenic emissions reduce global warming?

Yes.

There are many different emissions scenarios available in the peer-reviewed literature. The IPCC4 summary in 2007 brought together the best available research at the time, and one summary of their different scenarios is available here. The gist reflects that basic underlying physics: higher anthropogenic emissions (scenario A2) means higher temperatures; measures that reduce anthropogenic emissions (scenario B1) will mitigate that, leading to lower temperatures than would otherwise be the case.

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    Also, skeptics.stackexchange.com/questions/3435/… does not address whether CO2 is anthopogenic, and references answer links to talk about "greenhouse gases" in general, not CO2 specifically.
    – vartec
    Commented Feb 23, 2012 at 12:31
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    @vartec Thanks for the comment. I've edited to cover your concern re anthropogenic - hope this helps. Sounds like you've got a lot of related questions here: perhaps you'd like to post them as separate questions: e.g. each of your four bullet points might be a candidate for its own question. See also skepticalscience.com/argument.php - you might find some of them already addressed, and well-referenced, there, to your satisfaction.
    – 410 gone
    Commented Feb 23, 2012 at 17:50
  • "anthropogenic greenhouse gas emissions causing global warming" AFAIK, scientist don't claim that anthropogenic are the only ones causing global warming. "Contributing to" isn't quite the same claim. Note, that the claim from the question is not of "no effect at all", but rather effect so limited, that not significant.
    – vartec
    Commented Feb 24, 2012 at 13:20
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    @vartec happy to take it over to chat to discuss further. I don't see anyone claiming that anthropogenic GHGs are the only ones. My answer covers, with referenced sources, the issue of the scale of the effect. "significant" might be subjective. My referenced sources show it's around 10-12%, and that substantial mitigation is possible. So yes, greenhouse gases emitted by cars have a measurable impact on global warming, and no, reducing their emissions is not futile. Whether you consider 10-12% to be significant or not is a subjective opinion that's outside the scope of this site.
    – 410 gone
    Commented Feb 24, 2012 at 15:24
  • @EnergyNumbers many in the political side of the AGW movement claim just that, or make claims so vague that their meaning is readily interpreted as such (playing on the general population's unfamiliarity with chemistry, thus not knowing that they themselves breathe out the stuff. See also DHMO...).
    – jwenting
    Commented Mar 1, 2012 at 6:40

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