Gj. Boer et al., A transient climate change simulation with greenhouse gas and aerosol forcing: projected climate to the twenty-first century, CLIM DYNAM, 16(6), 2000, pp. 427-450
The potential climatic consequences of increasing atmospheric greenhouse ga
s (GHG) concentration and sulfate aerosol loading are investigated for the
years 1900 to 2100 based on five simulations with the CCCma coupled climate
model. The five simulations comprise a control experiment without change i
n GHG or aerosol amount, three independent simulations with increasing GHG
and aerosol forcing, and a simulation with increasing GHG forcing only. Cli
mate warming accelerates from the present with global mean temperatures sim
ulated to increase by 1.7 degrees C to the year 2050 and by a further 2.7 d
egrees C by the year 2100. The warming is non-uniform as to hemisphere, sea
son, and underlying surface. Changes in interannual variability of temperat
ure show considerable structure and seasonal dependence. The effect of the
comparatively localized negative radiative forcing associated with the aero
sol is to retard and reduce the warming by about 0.9 degrees C at 2050 and
1.2 degrees C at 2100. Its primary effect on temperature is to counteract t
he global pattern of GHG-induced warming and only secondarily to affect loc
al temperatures suggesting that the first order transient climate response
of the system is determined by feedback processes and only secondarily by t
he local pattern of radiative forcing. The warming is accompanied by a more
active hydrological cycle with increases in precipitation and evaporation
rates that are delayed by comparison with temperature increases. There is a
n "El Nino-like" shift in precipitation and an overall increase in the inte
rannual variability of precipitation. The effect of the aerosol forcing is
again primarily to delay and counteract the GHG-induced increase. Decreases
in soil moisture are common but regionally dependent and interannual varia
bility changes show considerable structure. Snow cover and sea-ice retreat.
A PNA-like anomaly in mean sea-level pressure with an enhanced Aleutian lo
w in northern winter is associated with the tropical shift in precipitation
regime. The interannual variability of mean sea-level pressure generally d
ecreases with largest decreases in the tropical Indian ocean region. Change
s to the ocean thermal structure are associated with a spin-down of the Atl
antic thermohaline circulation together with a decrease in its variability.
The effect of aerosol forcing, although modest, differs from that for most
other quantities in that it does not act primarily to counteract the GHG f
orcing effect. The barotropic stream function in the ocean exhibits modest
change in the north Pacific but accelerating changes in much of the Souther
n Ocean and particularly in the north Atlantic where the gyre spins down in
conjunction with the decrease in the thermohaline circulation. The results
differ in non-trivial ways from earlier equilibrium 2 x CO2 results with t
he CCCma model as a consequence of the coupling to a fully three-dimensiona
l ocean model and the evolving nature of the forcing.