How Does Drought Change With Climate Change

Tuesday, 16 December 2014: 8:02 AM
Kevin E Trenberth, NCAR, Boulder, CO, United States
Large disparities among published studies have led to considerable confusion over the question of how drought is changing and how it is expected to change with global warming. As a result the IPCC AR5 assessment has watered down statements, and failed to carry out an adequate assessment of the sources of the discrepancies. Quite aside from the different definitions of drought related to meteorological (absence of precipitation), hydrological (lack of water in lakes and rivers), and agricultural (lack of soil moisture) drought, there are many indices that measure drought. Good homogeneous datasets are essential to assess changes over time, but are often not available. Simpler indices may miss effects of certain physical processes, such as evapotranspiration (ET). The Palmer Drought Severity Index (PDSI) has been much maligned but has considerable merit because it can accommodate different ET formulations (e.g., Thornthwaite vs Penman-Monteith), it can be self calibrating to accommodate different regions, and it carries out a crude moisture balance. This is in contrast to simpler indices, such as the Standardized Precipitation Index, which provides only a measure of moisture supply, or the Standardized Precipitation Evapotranspiration Index, which also includes potential (but not actual) ET. The largest source of drought variations is ENSO: during La Niña more rain falls on land while during El Niño most precipitation is over the Pacific Ocean, exposing more land to drought conditions. It is essential to account for interannual and inter-decadal variability in assessing changes in drought with climate change. Yet drought is one time on land when effects accumulate, with huge consequences for wild fire risk. It is important to ask the right questions in dealing with drought.