In real terms it doesn't mean anything. I'm not sure how you could even come up with a number, becuase without commitments from others, we could go to zero and increases in other countries will take up what we stop. The best guess they can make is by using the dodgy climate models, and see what amount of atmospheric CO2 would limit the warming to 1.5°C. Given climate sensitivity is guessed at anywhere from 1 to 6°C per doubling of CO2, you could guess at the low end we need to stay be below 600ppm or at the high end maybe 450ppm. Given we are already over 1°C at 400ppm, I'd hazard a guess that even a 50% reduction in global emissions over 10 years isn't going to keep us below 1.5°C.
/warning:quick dodgy math used.
|