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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 [...]

Byline: Bruce T. Anderson (1), Katharine Hayhoe (2), Xin-Zhong Liang (3) Abstract: Potential changes in summertime hydroclimatology over the northeastern (NE) region of the USA induced by increases in greenhouse gas (GHG) concentrations are investigated using a state-of-the-art regional climate modeling system. Results for a higher emissions scenario illustrate changes that may occur if dependence on fossil fuels continues over the coming century. Summertime precipitation is projected to decrease across much of the central NE, but increase over the southernmost and northernmost portions of the domain. Evaporation is expected to increase across the entire domain. The balance between these two results in a decrease in soil moisture content across most of the domain (by approximately 10 mm) and an increase in the summertime soil-moisture depletion rate (by approximately 10 mm/month). At the same time, an increase in both atmospheric near-surface specific and saturation specific humidity is projected, resulting in an increase in relative humidity across the southern portion of the domain, with slight decreases over the northern portion. Combined with an average increase in summer temperatures of 3.5degC, the projected increase in relative humidity results in a marked increase in the average daily maximum heat index for the region on the order of 3.9degC, as well as a 350--400% increase in the number of days with heat index values exceeding 32.2degC (90degF)--the level of 'extreme caution'. Taken together, these high-resolution, dynamically-generated projections confirm the potential for significant summertime climate change impacts on the NE over the coming century as suggested by previous studies. Author Affiliation: (1) Department of Geography and Environment, Boston University, Boston, MA, 02215, USA (2) Department of Geosciences, Texas Tech University, Lubbock, TX, 70409, USA (3) Illinois Department of Natural Resources, Illinois State Water Survey, University of Illinois, Urbana-Champaign, USA Article History: Registration Date: 05/08/2009 Received Date: 28/04/2008 Accepted Date: 19/06/2009 Online Date: 08/10/2009

We review twentieth century and projected twenty-first century changes in climatic and hydrologic conditions in the northeastern United States and the implications of these changes for forest ecosystems. Climate warming and increases in precipitation and associated changes in snow and hydrologic regimes have been observed over the last century, with the most pronounced changes occurring since 1970. Trends in specific climatic and hydrologic variables differ in their responses spatially (e.g., coastal vs. inland) and temporally (e.g., spring vs. summer). Trends can differ depending on the period of record analyzed, hinting at the role of detach)-scale climatic variation that is superimposed over the longer-term trend. Model predictions indicate that continued increases in temperature and precipitation across the northeastern United States can be expected over the next century. Ongoing increases in growing season length (earlier spring and later autumn) will most likely increase evapotranspiration and frequency of drought. In turn, an increase in the frequency of drought will likely increase the risk of fire and negatively impact forest productivity, maple syrup production, and the intensity of autumn foliage coloration. Climate and hydrologic changes could have profound effects on forest structure, composition, and ecological functioning in response to the changes discussed here and as described in related articles in this issue of the Journal. Nous avons passe en revue les changements dans les conditions climatiques et hydrologiques survenues pendant le 20e siecle et projetees au cours du 21e siecle dans le nord-est des Etats-Unis ainsi que les repercussions de ces changements sur les ecosystemes forestiers. Un rechauffement du climat et une augmentation des precipitations ainsi que des changements dans les regimes nivologique et hydrologique ont ete observes pendant le dernier siecle et les changements les plus prononces sont survenus depuis 1970. Les tendances de variables climatiques et hydrologiques specifiques different dans f espace (p. ex. zone cotiere vs interieur des terres) et dans le temps (p. ex. printemps vs ete) dans leur reponse. Les tendances different selon la periode couverte par les donnees qui sont analysees, un indice du role de la variation du climat a une echelle decennale qui se superpose a la tendance a long terme. Les predictions du modele indiquent qu'on peut s'attendre a des augmentations continues de la temperature et des precipitations partout dans la region au cours du prochain siecle. L'allongement en cours de la saison de croissance (printemps qui arrive plus tot et automne qui arrive plus tard) augmentera l'evapotranspiration et la frequence des secheresses. Des secheresses plus frequentes vont probablement augmenter les risques d'incendie et avoir un impact negatif sur la productivite de la foret, la production de sirop d'erable et l'intensite de la coloration automnale du feuillage. Les changements climatiques et hydrologiques pourraient avoir de serieux effets sur la structure, la composition et le fonctionnement ecologique de la foret en reaction aux hangements discutes dans cet article et decrits dans d'autres articles relies au meme sujet et publies dans ce numero. [Traduit par la Redaction], Introduction and overview of climate and hydrology in the region Introduction At a regional scale, interpretations of the effects of recent climate change and predictions of the potential effects of [...]

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.

Byline: Daniel R. Cayan (1,2), Edwin P. Maurer (3), Michael D. Dettinger (1,2), Mary Tyree (1), Katharine Hayhoe (4) Abstract: To investigate possible future climate changes in California, a set of climate change model simulations was selected and evaluated. From the IPCC Fourth Assessment, simulations of twenty-first century climates under a B1 (low emissions) and an A2 (a medium-high emissions) emissions scenarios were evaluated, along with occasional comparisons to the A1fi (high emissions) scenario. The climate models whose simulations were the focus of the present study were from the Parallel Climate Model (PCM1) from NCAR and DOE, and the NOAA Geophysical Fluid Dynamics Laboratory CM2.1 model (GFDL). These emission scenarios and attendant climate simulations are not 'predictions,' but rather are a purposely diverse set of examples from among the many plausible climate sequences that might affect California in the next century. Temperatures over California warm significantly during the twenty-first century in each simulation, with end-of-century temperature increases from approximately +1.5degC under the lower emissions B1 scenario in the less responsive PCM1 to +4.5degC in the higher emissions A2 scenario within the more responsive GFDL model. Three of the simulations (all except the B1 scenario in PCM1) exhibit more warming in summer than in winter. In all of the simulations, most precipitation continues to occur in winter. Relatively small (less than ~10%) changes in overall precipitation are projected. The California landscape is complex and requires that model information be parsed out onto finer scales than GCMs presently offer. When downscaled to its mountainous terrain, warming has a profound influence on California snow accumulations, with snow losses that increase with warming. Consequently, snow losses are most severe in projections by the more responsive model in response to the highest emissions. Author Affiliation: (1) Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA, USA (2) U.S. Geological Survey, La Jolla, CA, USA (3) Santa Clara University, Santa Clara, CA, USA (4) Department of Geosciences, Texas Tech University, Lubbock, TX, USA Article History: Registration Date: 23/11/2007 Received Date: 02/08/2006 Accepted Date: 05/10/2007 Online Date: 26/01/2008

Byline: Daniel R. Cayan (1,2), Peter D. Bromirski (1), Katharine Hayhoe (3), Mary Tyree (1), Michael D. Dettinger (2,1), Reinhard E. Flick (4,1) Abstract: California's coastal observations and global model projections indicate that California's open coast and estuaries will experience rising sea levels over the next century. During the last several decades, the upward historical trends, quantified from a small set of California tide gages, have been approximately 20 cm/century, quite similar to that estimated for global mean sea level. In the next several decades, warming produced by climate model simulations indicates that sea level rise (SLR) could substantially exceed the rate experienced during modern human development along the California coast and estuaries. A range of future SLR is estimated from a set of climate simulations governed by lower (B1), middle--upper (A2), and higher (A1fi) GHG emission scenarios. Projecting SLR from the ocean warming in GCMs, observational evidence of SLR, and separate calculations using a simple climate model yields a range of potential sea level increases, from 11 to 72 cm, by the 2070--2099 period. The combination of predicted astronomical tides with projected weather forcing, El Nino related variability, and secular SLR, gives a series of hourly sea level projections for 2005--2100. Gradual sea level rise progressively worsens the impacts of high tides, surge and waves resulting from storms, and also freshwater floods from Sierra and coastal mountain catchments. The occurrence of extreme sea levels is pronounced when these factors coincide. The frequency and magnitude of extreme events, relative to current levels, follows a sharply escalating pattern as the magnitude of future sea level rise increases. Author Affiliation: (1) Scripps Institution of Oceanography, University of California, La Jolla, CA, USA (2) Water Resources Division, U. S. Geological Survey, La Jolla, CA, USA (3) Department of Geosciences, Texas Tech University, Lubbock, TX, USA (4) California Department of Boating and Waterways, La Jolla, CA, USA Article History: Registration Date: 23/11/2007 Received Date: 02/08/2006 Accepted Date: 05/10/2007 Online Date: 26/01/2008

Byline: Jeremy S. Fried (1), J. Keith Gilless (2), William J. Riley (3), Tadashi J. Moody (4), Clara Simon de Blas (5), Katharine Hayhoe (6), Max Moritz (4), Scott Stephens (4), Margaret Torn (3) Abstract: This study focused on how climate change-induced effects on weather will translate into changes in wildland fire severity and outcomes in California, particularly on the effectiveness of initial attack at limiting the number of fires that escape initial attack. The results indicate that subtle shifts in fire behavior of the sort that might be induced by the climate changes anticipated for the next century are of sufficient magnitude to generate an appreciable increase in the number of fires that escape initial attack. Such escapes are of considerable importance in wildland fire protection planning, given the high cost to society of a catastrophic escape like those experienced in recent decades in the Berkeley-Oakland, Santa Barbara, San Diego, or Los Angeles areas. However, at least for the three study areas considered, it would appear that relatively modest augmentations to existing firefighting resources might be sufficient to compensate for change-induced changes in wildland fire outcomes. Author Affiliation: (1) USDA Forest Service Pacific Northwest Research Station, Forest Inventory and Analysis, Portland, OR, USA (2) Berkeley, Agricultural and Resource Economics, University of California, Berkeley, CA, 94720-3100, USA (3) Lawrence Berkeley National Laboratory Center for Isotope Geochemistry, Berkeley, CA, USA (4) Berkeley Environmental Science, Policy, and Management, University of California, Berkeley, CA, USA (5) Universidad Rey Juan Carlos, Madrid, Spain (6) ATMOS Research and Consulting, Lubbock, TX, USA Article History: Registration Date: 29/10/2007 Received Date: 02/08/2006 Accepted Date: 05/10/2007 Online Date: 19/12/2007

Projections of changes in climate extremes are critical to assessing the potential impacts of climate change on human and natural systems. Modeling advances now provide the opportunity of utilizing global general circulation models (GCMs) for projections of extreme temperature and precipitation indicators. We analyze historical and future simulations of ten such indicators as derived from an ensemble of 9 GCMs contributing to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC-AR4), under a range of emissions scenarios. Our focus is on the consensus from the GCM ensemble, in terms of direction and significance of the changes, at the global average and geographical scale. The climate extremes described by the ten indices range from heat-wave frequency to frost-day occurrence, from dry-spell length to heavy rainfall amounts. Historical trends generally agree with previous observational studies, providing a basic sense of reliability for the GCM simulations. Individual model projections for the 21st century across the three scenarios examined are in agreement in 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. Depictions of a wetter world and greater precipitation intensity emerge unequivocally in the global averages of most of the precipitation indices. However, consensus and significance are less strong when regional patterns are considered. This analysis provides a first overview of projected changes in climate extremes from the 1PCC-AR4 model ensemble, and has significant implications with regard to climate projections for impact assessments.

Substitution of natural gas for coal is one means of reducing carbon dioxide (C[O.sub.2]) emissions. However, natural gas and coal use also results in emissions of other radiatively active substances including methane (C[H.sub.4]), sulfur dioxide (S[O.sub.2]), a sulfate aerosol precursor, and black carbon (BC) particles. Will switching from coal to gas reduce the net impact of fossil fuel use on global climate? Using the electric utility sector as an example, changes in emissions of C[O.sub.2], C[H.sub.4], S[O.sub.2] and BC resulting from the replacement of coal by natural gas are evaluated, and their modeled net effect on global mean-annual temperature calculated. Coal-to-gas substitution initially produces higher temperatures relative to continued coal use. This warming is due to reduced S[O.sub.2] emissions and possible increases in C[H.sub.4] emissions, and can last from 1 to 30 years, depending on the sulfur controls assumed. This is followed by a net decrease in temperature relative to continued coal use, resulting from lower emissions of C[O.sub.2] and BC. The length of this period and the extent of the warming or cooling expected from coal-to-gas substitution is found to depend on key uncertainties and characteristics of the substitutions, especially those related to: (1) S[O.sub.2] emissions and consequent sulphate aerosol forcing; and (2) the relative efficiencies of the power plants involved in the switch.

Methane (C[H.sub.4]) is the most abundant organic trace gas in the atmosphere. In the distant past, variations in natural sources o methane were responsible for trends in atmospheric methane levels recorded in ice cores. Since the 1700s, rapidly growing human activities, particularly in the areas of agriculture, fossil fuel use, and waste disposal, have more than doubled methane emissions. Atmospheric methane concentrations have increased by a factor of 2-3 in response to this increase, and continue to rise. These increasing concentrations have raised concern due to their potential effects on atmospheric chemistry and climate Methane is important to both tropospheric and stratospheric chemistry, significantly affecting levels of ozone, water vapor, the hydroxyl radical, and numerous other compounds. In addition, methane is currently the second most important greenhouse gas emitted from human activities. On a per molecule basis, it is much more effective a greenhouse gas than additional C[O.sub.2]. In this review, we examine past trends in the concentration of methane in the atmosphere, the sources and sinks that determine it growth rate, and the factors that will affect its growth rate in the future. We also present current understanding of the effects of methane on atmospheric chemistry, and examine the direct and indirect impacts of atmospheric methane on climate. Keywords: methane; greenhouse gases; atmospheric chemistry; global change

Byline: Claudia Tebaldi (1), Katharine Hayhoe (2,3), Julie M. Arblaster (4,5), Gerald A. Meehl (4) Author Affiliation: (1) Institute for the Study of Society and Environment, National Center for Atmospheric Research (NCAR), PO BOX 3000, Boulder, CO, 80301, USA (2) Department of Atmospheric Sciences, University of Illinois at Urbana-Champaign, Urbana, IL, USA (3) Department of Geosciences, Texas Tech University, Lubbock, TX, USA (4) Climate and Global Dynamics Division, NCAR, Boulder, CO, USA (5) Bureau of Meteorology Research Centre, Melbourne, Australia Article History: Registration Date: 19/02/2007 Online Date: 15/03/2007 Article note: The online version of the original article can be found at: doi: 10.1007/s10584-006-9051-4.