Dr. Jim Hansen of GISS wrote a guest weblog on April 11 2009, and Climate Science will follow up with a few comments here. First, his willingness to engage in a constructive discussion of the science issues should be a message to other climate scientists, who have elected not to engage in such discussions. Thanks again to Jim for doing this.
With respect to the science, these are my comments:
- We both agree that land use change is a first-order climate forcing on the local scale;
- We both agree that in terms of the global average radiative imbalance, that land use change, so far, has been less than that caused by the anthropogenic addition of the well-mixed greenhouse gases and aerosols;
- We disagree, however, on what is the role of land use change in altering global climate patterns. Our research (e.g. see), and that of others (e.g. see) demonstrates that atmospheric circulations are significantly altered on the hemispheric and global scale due to human land management practices. These changes will, of course, result in long term effects of the frequency of such societally important climate events as droughts, tropical cyclones, and floods.
With respect to the last bullet, Climate Science urges the broadening of the climate metrics in order to more appropriately assess the human role in altering climate patterns. Our research (e.g. see) has proposed the assessment of the change using the spatial gradient of radiative heating (and other diabatic heating) due to all climate forcings, as one climate metric to complete such an evaluation. We have found in the tropics and subtropics, for example, (see) that the large spatial heterogeneity of aerosols has a 60X greater influence on the gradient of diabatic heating than is found with the spatial heterogeneity of the well-mixed greenhouse gases.
Existing research indicates that the multi-decadal changes in this gradient produce a greater influence on the climate change that society experiences than is communicated by a global average surface temperature trend, or any other global average.