First of all, it points out that unilateral carbon tariffs is likely to spark a trade war with countries like China and India, something which would harm America (or Europe) as much as these countries, and they therefore don't think it is a good idea.
Secondly, they also point out that unless carbon tariffs are imposed, efforts in America and Europe to reduce carbon emissions would only have a trivial effect on global carbon emissions, since these carbon emission reductions would lower the price of oil and coal and so likely increase its use in China and India, thus offsetting the reductions in America and Europe.
It could here be added that even carbon tariffs will not likely provide enough incentive since American and European goods will still be disadvantaged in markets outside of America and Europe.
And there is really little chance that China and India will really voluntarily agree to reduce its emissions. Here is what the Indian spokesman on the issue said:
"India will not accept any emission-reduction target -- period. This is a non-negotiable stand."
In case you wonder why they take that position and are unlikely to change their position, see this video from the Competitive Enterprise Institute about the suffering that carbon emission cuts would create: