Impacts of land use and a global biofuels policy on climate, Part 2: Hydroclimatological response

The hydro-climatological impact of future land-use and energy policy scenarios as well as the implications for the future photosynthetic productivity, and by implication the future agricultural capacity is explored. The study considers two economically-modeled land-use frameworks: (i) Pure Cost Conversion Response (PCCR), or 'deforestation', where the price of land constrains agricultural conversion, including growing biofuels, and; (ii) Observed Land Supply Response (OLSR), or 'intensification', where legal, environmental and other constraints encourage more intense use of existing agricultural land (i.e. less forest clearing). These two land-use frameworks were used to explore how widespread plantation of cellulosic biofuels to meet global energy demand impacts the future hydrological conditions globally, as well as the potential for drought across the contiguous United States in 2050. The land cover of the Community Atmospheric Model Version 3.0 (CAM3.0) was manipulated to reflect these four different land-use and energy scenarios (i.e. PCCR and OLSR with and without biofuels). CAM3.0 was run to equilibrium, under 1990 and 2050 climate conditions, in order to assess the impact these land cover changes have on the atmospheric and hydrological states. In order to analyze how drought risk might change in response to our four policy scenarios, a simple water stress index, P-E, was calculated for the entire globe. Our results show that in the most extreme land cover change (LCC) scenario, P-E will increase in a small area of the Amazon basin (by up to 1.0 mm/d) and western central Africa, but will decrease in other areas in northern South America (by up to 0.6 mm/d) and eastern Africa (and small areas of SE Asia). However, in the more realistic LCC scenario (OLSR), there is much less impact, and only small patches of decreasing P-E in South America, and a few small, scattered patches of statistically significant change of up to +/-0.6 mm/d in P-E in Africa. There is also a weak decrease (0.2 mm/d or less) across the entire Sahara region. Our study indicates that these changes in P-E, in all scenarios, are driven by changes in total precipitation marginally more than total evaporation, and that while all three scenarios show similar patterns of change, Changes to P-E are greatest in the PCCR scenario, with less severe and less extensive changes seen in the OLSR and PCCR-without-biofuels scenarios, in that order. This indicates that it is the replacement of vegetated areas with cropland which occurs due to the biofuel policy, more than the deforestation which occurs to a larger extent in the PCCR scenario, which is responsible for the most dramatic changes in P-E. Further analysis is conducted on patterns of photosynthesis, as a measure of productivity, across all scenarios. In the OLSR and PCCR-without-biofuels scenarios, there is a noticeable decrease in photosynthesis throughout the tropics, albeit far less severe and extensive than the changes seen in PCCR, particularly in central Amazon basin, China and SE Asia. The patterns of change in transpiration show a much closer correspondence to patterns of change in photosynthesis than the patterns of P-E. These results suggest that the greater the LCC, the greater the risk of water stress and potential for drought. Current work involves further assessment of drought risk in these LCC scenarios using more sophisticated indices, including the SPI and the Palmer Drought Severity Index.