[lauraura]

Transportation CO2 emissions make up a tiny fraction of total greenhouse gases. Water vapour is by FAR the biggest greenhouse gas.

Water vapor is the largest greenhouse gas, but it's a relatively constant contribution; it's not part of anthropogenic contributions to greenhouse gases (which is usually what people are referring to in this sort of context).

[c1u]

Good point. I'm not trying to start an off-topic climate change debate, just trying to step back and look at the problem a little more broadly, but there's at least one big error in my calcs as has been pointed out.