Turn Do wn t he H e at: W h y a 4 ° C War m e r Wor ld Mu st B e Avoided
Latitude
60 30 0 −30 −60 60
120 180 240 300 Longitude
70 65 60 55 50 45 40 35 30 25 20 15 10 5 0
The difference in regional sea-level rise patterns between 4°C and 2°C warming above preindustrial temperatures is indicated in Figure A1.2 for both ice-sheet scenarios by the end of the century. In both ice-sheet scenarios, the spatially variable component of the difference is closely related to ocean dynamics (see Figure A1.3). The benefit of choosing a 2°C pathway, rather than a 4°C pathway can be to limit more than 20 cm of local sea-level rise (Figure A1.2). Note that the authors do not exclude higher benefits of mitigation:
High ice−sheet scenario [96 cm]
cm
Low ice−sheet scenario [47 cm]
60
120 180 240 300 Longitude
140 130 120 110 100 90 80 70 60 50 40 30 20 10 0
cm
Figure A1.1: Regional sea-level projection for the lower ice-sheet scenario (left) and the higher ice sheet scenario (right). The numbers in brackets denote the corresponding global mean value for sea-level rise, of 47 cm and 96 cm, respectively.
in particular, potential (but uncertain) crossing of tipping points with respect to ice-sheet collapse could increase the impact of a 4°C world compared to a 2°C world. The regional projections presented here incorporate the uncertainties from the methods that were applied to estimate global mean sea-level rise. In order to reduce these uncertainties, further research on the dynamic changes in the ice sheets is needed, using reconstruction of past responses to climate and observations of
Figure A1.2:
Low ice−sheet scenario [13 cm] High ice−sheet scenario [17 cm]
Latitude
60 30 0 −30 −60 60 120 180 240 300 Longitude 68
60 120 180 240 300 Longitude
40 35 30 25 20 15 10 5 0
cm
Difference in sea-level rise between a 4°C world and a 2°C world for the lower (left) and higher (right) ice-sheet scenario. The numbers in brackets indicate the difference in global mean sea-level rise. Grey shaded areas indicate regions where sea-level is higher in a 2°C world: they correspond to regions where sea level is actually projected to drop in the coming century because of land uplift and gravitational effects.