NASA's Professor James Hansen has a paper [PDF] out analysing the effects of fossil fuel burning on the global climate and modelling several scenarios of future usage to get an idea of possible policy options. The short version is that it is basically all about coal - there just isn't enough conventional oil and natural gas to drive atmospheric CO2 concentrations above the (really) dangerous 450 parts per million level. If we constrain coal emissions, either by shutting down plants or using carbon capture and sequestration technology to freeze and then reduce emissions (first in developed countries, then in developing ones), and refrain from exploiting unconventional fossil fuel sources such as tar sands, shale oil and methane clathrates, then CO2 concentrations will peak at or below 450ppm and then decline. Given the effects we're already seeing, that's still probably not a safe level, but its a lot safer than the runaway levels we'll see if we keep burning coal.
Fortunately, New Zealand is already on the right path here: we've stuck a price on carbon (covering coal from 2010), and banned the construction of new coal-fired power plants. But we need to go further. Given its effects on the global climate, we really shouldn't be exporting coal to any country that does not have a Kyoto cap or equivalent policies to reduce emissions, and we should require an end-user certificate to ensure it is not transferred. Otherwise, we're simply undoing the progress we're making, and helping to perpetuate a problem we are struggling against
