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?
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?
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?
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.