Regional climate change, ecosystem responses, and climate feedbacks.

摘要

I use empirical/statistical models and physically based general circulation models to assess the capacity for the Arctic Oscillation (AO) and the El Nino Southern Oscillation (ENSO) to influence terrestrial ecosystems, and the potential for those ecosystems to feedback to the climate system. AO warming leads to modest reductions in Eurasian carbon stocks; ∼17 Pg carbon are lost to the atmosphere, primarily from increased soil decomposition. Precipitation reductions in southern Africa associated with increased frequency of El Nino events lead to a reduction in tree cover and expansion of grasslands in the north and a reduction in grass cover in drier areas. Here half the carbon cycle changes are driven by the loss of tree cover, leading to a net loss of ∼5 Pg of carbon to the atmosphere. Over southern Africa, positive soil moisture anomalies lead to reduced precipitation through enhanced subsidence and reduced moisture convergence. Higher snow cover alone in Eurasia leads to minor albedo increases and moderate localized cooling (3°-5°C), mostly at very high latitudes (>70°N) and during the spring season. When vegetation is allowed to interact, increased snow cover leads to southward retreat of boreal vegetation, widespread cooling, and persistent snow cover over much of the boreal region during the boreal summer, with cold anomalies of up to 15°C. In southern Africa, the feedback experiments suggest a negative feedback between soil moisture and precipitation over the same area, implying this region may be resistant to externally forced changes in precipitation. In Eurasia, a persistent high phase of the AO leads to winter warming, but the feedback response is complicated. Warming during this season has been associated with increased snowfall, which could increase snow cover and albedo, countering the AO warming. Conversely, increased temperatures could lead to increased snow melting and decreased albedo, amplifying the AO warming.