|
Science 123
|
|
Spring 2014
|
|
|
Time Bomb Summary
|
|
-
Objective analysis of global warming requires quantitative knowledge in three areas
-
climate sensitivity
-
the current level of anthropogenic forcings
-
response time of the climate
-
Lessons of History
-
natural millennial climate swings are due to slow variations in the earth`s orbit
- leads to a re-distribution of radiational energy, not a great change in radiational energy
-
ice age climate swings provide an empirical measure of climate sensitivity
$\frac{\Delta T}{\Delta F} = 0.75 \pm 0.25$ degrees C per watt per meter squared
- more precise and reliable measure because all forcing factors are included
- climate models used to predict future changes exhibit the same climate sensitivity
-
Current Level of Climate Forcing
-
the net value of forcings added since 1850 is 1.6 $\pm$ 1.0 watts per meter squared (IPCC report - page 16)
- climate model temperatures using this amount of forcing match observed temperatures over the past several decades
- observed heat gained by the world ocean over the past 50 years is consistent with this level of forcing
-
If the current earth atmospheric composition does not change further, the earth`s surface will eventually warm another 0.4 to 0.7 degrees C
-
Dominant Issue: sea-level change and how fast ice sheets can disintegrate.
-
Once large-scale ice-sheet breakup begins, it will be impractical to stop
-
Mitigation versus Adaptation
-
Hansen believes we can, at most, allow another one degree C rise in temperature.
-
Carbon Dioxide Budget
-
Stabilizing atmospheric $\mbox{CO}_2$
-
IPCC Scenarios (page 14)