the tipping point for the presence, or absence, of any substantial
ice-sheets on Earth is around 450 ppm (plus or minus 100 ppm) of CO2.
This means that the CO2 levels often associated with a 2?C rise may just
be the tipping point for the total loss of all ice sheets on the planet
and a huge sea-level rise.
To achieve the return of the Arctic sea-ice, which plays a key role in
the global climate system, Hansen says we need a target in the range
300–325ppm CO2, well below the current level of 387 ppm.
That means we need to halt emissions growth before it gets to 450 ppm,
then extract CO2 from the atmosphere (via forest growth, biochar
sequestration, etc) to a level where we see the Arctic ice cover at a
level where it's albedo effect helps stabilise global temperatures.