One thing that is worth mentioning is that Hansen suggests 350 ppm because palaeoclimate data suggests that climate sensitivity is 6 degrees C when taking into account ice sheet effects. A scenario that might stabilise at 350 ppm at a sensitivity of 3 degrees C will have more carbon cycle feedback effects with a higher sensitivity, and stabilise at a higher concentration. MAGICC seems to suggest that at a sensitivity of 6 C, the 350 scenario will stabilise at 370-380 ppm.

This effect is much more serious for higher stabilisation targets of course.

http://www.cgd.ucar.edu/cas/wigley/magicc/

If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted, paleoclimate evidence and ongoing climate change suggest that CO2 will need to be reduced from its current 385 ppm to at most 350 ppm.

and later states

We suggest an initial objective of reducing atmospheric CO2 to 350 ppm, with the target to be adjusted as scientific understanding and empirical evidence of climate effects accumulate.

What Hansen appears to be saying is that 350 ppm is an upper bound, above which we risk changing the planet quite dramatically. One of the main reasons for this feedbacks involving loss of albedo because of loss of ice. Obviously any target below 350 ppm is safer (and harder to reach).

What sort of emissions reductions are required to reach 350 ppm? The following papers are a good place to start:

* Meinshausen et al. (2006) Multi-gas Emissions Pathways to Meet Climate Targets Climatic Change 75: 151–194
* Hare and Meinshausen (2006) How Much Warming are We Committed to and How Much can be Avoided? Climatic Change 75: 111–149
* Wigley et al. (1996) Economic and environmental choices in the stabilization of atmospheric CO2 concentrations Nature 379, 240 – 243

The paper by Meinshausen et al suggests that we should reduce CO2 emissions by slightly more than 5% per year to get to 350 ppm. Some trajectories in other papers have less reductions by 2020, but more dependence later on technologies that remove carbon from the atmosphere (e.g. biomass burning with carbon capture and storage).

Right. “Almost”. Says an air travel user? Sure. Dream on, and feel good about participating in the most efficient way in the destruction of the climate.

“Aviation already accounts for 13% of the UK’s total climate impact, and is now the fastest rising cause of greenhouse gas emissions in the UK. The Tyndale Centre in Norwich is now predicting that aviation emissions on their current trajectory will actually account for up to 100% of the government’s carbon budget by 2050. In other words, even if we retired every car from the road, unplugged every electronic device and closed every factory, we would still not meet our climate change targets because of aviation.”

A flying leap, Michael Meacher (former UK environment minister), The Guardian, Nov. 12th, 2008.

I would like to lower the CO2 to 350ppm, but Steve Chu is realistic unless people are ready for much more drastic and severe action than 350.org has suggested. Removing CO2 from the air would require an enormous energy and money expenditure. Consider devoting NASA’s budget or the Defense budget to the project. It would be money well spent, but it will never get thruough congress. I am waiting for a realistic plan from 350.org. Until they have one, why should anybody listen to them?

PS: Air travel is almost irrelevant. The #1 source of CO2 is coal fired power plants. The #2 source is industrial processes. The source of energy that produces the least CO2 per kilowatt hour is nuclear. The cleanest and safest is nuclear. Nuclear reactors can be made in factories. Nuclear fuel can be recycled. There is not enough wind. Solar never works at night.
PPS: Coal contains uranium.
What will actually happen: The rainfall bands in the midwest will move, disrupting agriculture. Food production will stop. Civilization will fall.