Search

ABSTRACTS Findings from the most recent surveys of TV weathercasters--which are methodologically superior to prior surveys in a number of important ways--suggest that weathercasters' views of climate change may be [...]

This is the fourth in a series of four articles on historical and projected climate extremes in the United States. Here, we examine the results of historical and future climate model experiments from the phase 5 of the Coupled Model Intercomparison Project (CMIP5) based on work presented at the World Climate Research Programme (WCRP) Workshop on CMIP5 Climate Model Analyses held in March 2012. Our analyses assess the ability of CMIP5 models to capture observed trends, and we also evaluate the projected future changes in extreme events over the contiguous Unites States. Consistent with the previous articles, here we focus on model-simulated historical trends and projections for temperature extremes, heavy precipitation, large-scale drivers of precipitation variability and drought, and extratropical storms. Comparing new CMIP5 model results with earlier CMIP3 simulations shows that in general CMIP5 simulations give similar patterns and magnitudes of future temperature and precipitation extremes in the United States relative to the projections from the earlier phase 3 of the Coupled Model Intercomparison Project (CMIP3) models. Specifically, projections presented here show significant changes in hot and cold temperature extremes, heavy precipitation, droughts, atmospheric patterns such as the North American monsoon and the North Atlantic subtropical high that affect interannual precipitation, and in extratropical storms over the twenty-first century. Most of these trends are consistent with, although in some cases (such as heavy precipitation) underestimate, observed trends., This is the fourth in a series of BAMS articles on climate extremes in the United States (U.S.). These papers are based on workshops where leading scientists in the field [...]

Future projections of near-surface ozone concentrations depend on the climate/emissions scenario used to drive future simulations, the direct effects of the changing climate on the atmosphere, and the indirect effects of changing temperatures and C[O.sub.2] levels on biogenic ozone precursor emissions. The authors investigate the influence of these factors on potential future changes in summertime daily 8-h maximum ozone over the United States and China by comparing Model for Ozone and Related Chemical Tracers, version 2.4, (MOZART-2.4) simulations for the period 1996-2000 with 2095-99, using climate projections from NCAR--Department of Energy Parallel Climate Model simulations driven by the Intergovernmental Panel on Climate Change Special Report on Emissions Scenarios Alfi (higher) and B1 (lower) emission scenarios, with corresponding changes in biogenic emissions. The effect of projected climate changes alone on surface ozone is generally less than 3 ppb over most regions. Regional ozone increases and decreases are driven mainly by local warming and marine air dilution enhancement, respectively. Changes are approximately the same magnitude under both scenarios, although spatial patterns of responses differ. Projected increases in isoprene emissions (32%-94% over both countries), however, result in significantly greater changes in surface ozone. Increases of 1-15 ppb are found under Alfi and of 0-7 ppb are found under B1. These increases not only raise the frequency of 'high ozone days,' but are also projected to occur nearly uniformly across the distribution of daily ozone maxima. Thus, projected future ozone changes appear to be more sensitive to changes in biogenic emissions than to direct climate changes, and the spatial patterns and magnitude of future ozone changes depend strongly on the future emissions scenarios used.

Over the twenty-first century, the frequency of extreme-heat events for major cities in heavily air conditioned California is projected to increase rapidly, Extreme heat is defined here as the temperature threshold for the 90th-percentile excedence probability (T90) of the local warmest summer days under the current climate. Climate projections from three atmosphere-ocean general circulation models, with a range of low to midhigh temperature sensitivity forced by the Special Report on Emission Scenarios higher, middle, and lower emission scenarios, indicate that these increases in temperature extremes and variance are projected to exceed the rate of increase in mean temperature. Overall, projected increases in extreme heat under the higher Alfi emission scenario by 2070-99 tend to be 20%-30% higher than those projected under the lower B1 emission scenario. Increases range from approximately 2 times the present-day number of days for inland California cities (e.g., Sacramento and Fresno), up to 4 times for previously temperate coastal cities (e.g., Los Angeles and San Diego), implying that present-day 'heat wave' conditions may dominate summer months--and patterns of electricity demand in the future. When the projected extreme heat and observed relationships between high temperature and electricity demand for California are mapped onto current availability, maintaining technology and population constant for demand-side calculations, a potential for electricity deficits as high as 17% during T90 peak electricity demand periods is found. Similar increases in extreme-heat days are likely for other southwestern U.S. urban locations, as well as for large cities in developing nations with rapidly increasing electricity demands. In light of the electricity response to recent extreme-heat events, such as the July 2006 heat waves in California. Missouri. and New York, these results suggest that future increases in peak electricity demand will challenge current transmission and supply methods as well as future planned supply capacities when population and income growth are taken into account.