A paper published this week in the journal Earth Interactions counters alarmist claims that 'climate change' will cause the Amazon to dry up and shrink by 85%, finding instead, "Our results suggest that the core of the Amazon rainforest should remain largely stable as rainfall in the core of the basin is projected to increase."
Earth Interactions 2011 ; e-View doi: 10.1175/2011EI398.1
Will Amazonia dry out?: Magnitude and Causes of change from IPCC climate model projections
Brian Cook et al
Abstract: The Amazon rainforest may undergo significant change in response to future climate change. To determine the likelihood and causes of such changes, we analyzed the output of 24 models from the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC/AR4) and a dynamic vegetation model Vegetation-Global-Atmosphere-Soil (VEGAS) driven by these climate output. Our results suggest that the core of the Amazon rainforest should remain largely stable as rainfall in the core of the basin is projected to increase in nearly all models. However, the periphery, notably the southern edge of Amazonia and further south into central Brazil (SAB), are in danger of drying out, driven by two main processes. Firstly, a decline in precipitation of 11% during the southern Amazonia's dry season (May–September) reduces soil moisture. Two dynamical mechanisms may explain the forecast reduction in dry season rainfall: (1) a general subtropical drying under global warming when the dry season southern Amazon basin is under the control of subtropical high pressure; (2) a stronger north-south tropical Atlantic sea surface temperature gradient, and to a lesser degree, a warmer eastern equatorial Pacific. The drying corresponds to a lengthening of the “dry season” by approximately 10 days. The decline in soil moisture occurs despite an increase in precipitation during the wet season, due to nonlinear responses in hydrology associated with the decline in dry season precipitation, ecosystem dynamics and an increase in evaporative demand due to the general warming. In terms of ecosystem response, higher maintenance cost and reduced productivity under warming may also have additional adverse impact. While the IPCC models have substantial inter-model variation in precipitation change, these latter two hydroecological effects are highly robust because of the general warming simulated by all models. As a result, when forced by these climate projections, a dynamic vegetation model VEGAS projects an enhancement of fire risk by 20–30% in the SAB region. Fire danger reaches its peak in Amazonia during the dry season, and this danger is expected to increase primarily due to the reduction in soil moisture, and the decrease in dry season rainfall. VEGAS also projects a reduction of about 0.77 in Leaf Area Index (LAI) over the SAB region. The vegetation response may be partially mediated by the CO2 fertilization effect, as a sensitivity experiment without CO2 fertilization shows a higher 0.89 decrease in LAI. Southern Amazonia is currently under intense human influence as a result of deforestation and land use change. Should this direct human impact continue at present rates, added pressure to the region's ecosystems from climate change may subject the region to profound changes in the 21st century.