and intensification of subtropical dry conditions occurs consistently
with global warming in our climate simulations
(Hansen et al., 2005a, 2007a). Held and Soden (2006) and
Lu et al. (2007) find agreement among a large number of
models in intensification of the pattern of precipitation minus
evaporation and its temporal variance, with poleward expansion
of subtropical conditions accompanying global warming.
Practical impacts may include increased drought and
fires in regions such as the Western United States, Mediterranean,
Australia and parts of Africa. Paleoclimate data
(Cook et al., 2004) provide evidence of strong drought in the
western United States accompanying global warming, and
the GISS model is able to reproduce this tendency for subtropical
drying in past warm climates as well as in modern
ones (Shindell et al, 2006). We cannot specify a threshold for
these effects, and there is already evidence of such tendencies
in the past decade. However, the simulated effects are proportional
to global warming (Held and Soden, 2006; Lu et
al., 2007), so end-of-century effects under BAU warming are
about three times greater than in the alternative scenario.
Let me repeat that last line: The simulated effects are proportional to global warming, so end-of-century effects under Business-As-Usual warming are about three times greater than the alternative scenario (in which the growth of carbon emissions is moderated). Three times! Holy cow.
This is why scientists and environmentalists alike are determined to bring down carbon emissions numbers. We’re going to crash, but will we survive the collision with the future? If we make no effort to change — probably not.
[Flickr photo of firefighters working the Lake Tahoe fire on Monday from Lorelei_29: all rights reserved]