Overview of Annual Extremes Analysis

Overview of Annual Extremes Analysis

How the climate system changes in response to increases in greenhouse gases (GHG) is one of the foremost questions for the scientific community, policy makers and the general public. Nearly all instrument measurements, proxy records, such as tree rings and bore holes, and climate model simulations indicate that temperatures have warmed over the past century. While many centers around the world have conducted simulations of GHG-induced climate change, until recently most of the analyses of these simulations have considered seasonally or annually averaged quantities. Yet some of the most important consequences for both humans and the environment will occur through changes in extreme events that occur on sub-monthly time scales, such as changes in the frequency, intensity, and duration of floods, heat waves, high wind events, etc. (e.g. Table 3 Summary for Policymakers of the IPCC AR4 Synthesis Report).   

As detailed in the Weather and Climate Extremes in a Changing Climate CCSP Synthesis and Assessment product (SAP) 3.3 (2008) observational analyses indicate that changes in daily temperature and precipitation have already begun to occur. Frich et al. (2002) have developed a set of 10 indices to examine whether the frequency and/or severity of observed temperature and precipitation extremes changed over the continents during the second half of the 20th century. They found coherent spatial patterns of significant changes, including an increase in warm summer nights, a decrease in the number of frost days and a decrease in intra-annual extreme temperature range. Indicators based on daily precipitation had mixed patterns of change but significant increases have been seen in the extreme dry spells and number of heavy rainfall events.

Tibaldi et al. (2006) analyzed historical and future simulations of these 10 indicators as derived from an ensemble of 9 GCMs from the IPCC-AR4, under a range of emissions scenarios. The simulated trends of the 20th century generally agreed with previous observational studies. Individual model projections for the 21st century across the three greenhouse gas scenarios (A, A1B, and B1) were in agreement, showing greater temperature extremes consistent with a warmer climate. For any specific temperature index, minor differences appear in the spatial distribution of the changes across models and across scenarios, while substantial differences appear in the relative magnitude of the trends under different emissions rates. The models simulated more precipitation during more intense events in the global averages of most of the precipitation indices, but the signals were not as strong (statistically significant) and the spatial patterns more variable when compared with the temperature changes.

While both of these papers have been invaluable to the climate community, they also have some inherent short comings from a practical stand point, including that the maps are global but their physical dimensions are quite small, so it is difficult to see the regional structure of the changes, and the model results are presented as the multi-model ensemble, so the range of model responses is difficult to determine. To address these issues we have developed this web based display system for extremes and the standard deviation of the extremes from the coupled models developed as part of the IPCC AR4. To date, we have put maps of the 10 Frich indices on-line that have been archived at the Program for Climate Model Diagnosis and Intercomparison as part of the Coupled Model Intercomparison Project (CMIP; Meehl et al. 2007) for different regions of the globe.

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