AGU journal highlights -- Aug. 12, 2008

August 12, 2008

1. Fast rise of scorching days predicted

Assessing and predicting the frequency and strength of extreme climate events are critical to determining the consequences of future climate change. Motivated by western Europe's exceptionally hot summers of 2003 and 2006, Sterl et al. use an ensemble of climate models to investigate changes in extreme values of climate variables. Using a statistical method for determining return intervals for extreme events, the authors find that the recurrence time for extremely high temperatures will not only increase as average temperatures rise, but will also increase at a faster rate than rising average temperatures. After correcting for model biases, the authors also find that by the end of this century, extreme temperatures will far exceed 40 degrees Celsius (104 degrees Fahrenheit) in southern Europe and the U.S. Midwest and will even reach 50 degrees C (122 degrees F) in northeastern India and most of Australia. Because any point on land within roughly 40 degrees of the equator will have a 10 percent chance of exceeding 48 degrees C (118 degrees F) every decade by the end of this century, the authors urge that the risk to populations be taken very seriously.

When can we expect extremely high surface temperatures?

Andreas Sterl, Camiel Severijns, Wilco Hazeleger, Geert Jan van Oldenborgh, Gerrit Burgers, and Peter van Velthoven: Royal Netherlands Meteorological Institute, De Bilt, Netherlands;
Henk Dijkstra, Michiel van den Broeke, and Peter Jan van Leeuwen: Institute for Marine and Atmospheric Research Utrecht, Utrecht University, Utrecht, Netherlands;
Bart van den Hurk: Royal Netherlands Meteorological Institute, De Bilt, Netherlands; also at Institute for Marine and Atmospheric Research Utrecht, Utrecht University, Utrecht, Netherlands.

Geophysical Research Letters (GRL) paper 10.1029/2008GL034071, 2008;

2. Northeast U.S. lake yields 1000-year hurricane record

Recent studies suggest that hurricane activity may be increasing due to human-induced global warming. However, assessing recent trends in the context of natural hurricane variability is difficult because instrumental records extend back only about 130 years. Noting that natural archives such as lake sediments can preserve storm history, Besonen et al. examine a sediment record from the Lower Mystic Lake (near Boston, Massachusetts) that contains 1000 years of annual laminations. Some laminations include anomalous graded beds. Within the historical record, 10 out of 11 of the most prominent graded beds correspond to years in which hurricanes struck the Boston area. Assuming that these graded beds represent deposition related to precipitation and wind-driven disturbances from hurricanes, extrapolating back within the sediment record reveals an annually resolved history of category 2?? hurricane occurrence spanning the past 1000 years. This signal, unique to Boston, shows strong centennial-scale variations in hurricane frequency and includes a period of increased activity between the twelfth and sixteenth centuries. Decreased activity occurred during the eleventh as well as the seventeenth through nineteenth centuries.

A 1,000 year, annually-resolved record of hurricane activity from Boston, Massachusetts

Mark R. Besonen and Raymond S. Bradley: Department of Geosciences, University of Massachusetts, Amherst, Massachusetts, U.S.A.;
Manfred Mudelsee: Climate Risk Analysis, Hannover, Germany;
Mark B. Abbott: Department of Geology and Planetary Science, University of Pittsburgh, Pittsburgh, Pennsylvania, U.S.A.;
Pierre Francus: Institut National de la Recherche Scientifique, Québec, Canada; also at GEOTOP-UQAM-McGILL, Montréal, Québec, Canada.

Geophysical Research Letters (GRL) paper 10.1029/2008GL033950, 2008;

3. Australian marine climate zones shift south

With poleward flowing ocean currents along its eastern and western shores, Australia has significant coral communities, mangroves, estuaries, and sea grass beds along both coastlines. Noting that sea surface temperatures (SSTs) are significantly warming along Australia's northwest and northeast coasts, Lough analyzes the past 60 years of SSTs to calculate the magnitude and spatial distributions of observed warming. She finds that warming is comparable along the northeast and northwest coasts, although greater along the northeast coast south of 15 degrees South, greater at higher than lower latitudes, and greater for annual minimum than annual maximum SSTs. Average climate zones have shifted south more than 200 kilometers (124 miles) along the northeast coast and about 100 km (62 miles) along the northwest coast. If current trends continue, within the next 100 years annual average SSTs in northern parts could be about 0.5 degrees Celsius warmer and those in the southern parts could be about 2 degrees C warmer. Lough expects that given these projections, ecosystem responses in Australia to climate change will only intensify.

Shifting climate zones for Australia's tropical marine ecosystems

J. M. Lough: Australian Institute of Marine Science, Townsville, Queensland, Australia.

Geophysical Research Letters (GRL) paper 10.1029/2008GL034634, 2008;

4. A new approach to hydrological prediction

In the American West, deep spring snowpacks reliably explain most variations in subsequent spring and summer runoff. At other times of year, forecasters must depend more heavily on soil moisture conditions and the predictability of future precipitation. Wood and Lettenmaier describe a framework for quantifying the relative importance of initial conditions versus future climate predictions for seasonal hydrologic forecast skill. Working with a retrospective forecast data set, they contrast results from Ensemble Streamflow Prediction (ESP), in which an ensemble of hydrological outcomes is generated by combining a perfect initial condition with a range of climate prediction, with results from an approach that they term "reverse-ESP," in which the ensemble is created by combining a range of possible initial conditions with a perfect climate forecast. Using examples from northern California and southern Colorado, the authors show that knowing initial conditions enables accurate streamflow prediction during the transition from the wet to the dry season, whereas in the reverse transition, climate forecasts are more critical. A key implication is that the emphasis placed on improving estimates of initial land surface moisture conditions versus future climate predictions should vary depending on season and location.

An ensemble approach for attribution of hydrologic prediction uncertainty

Andrew W. Wood: Department of Civil and Environmental Engineering, University of Washington, Seattle, Washington, U.S.A.; now at 3TIER, Inc., Seattle, Washington, U.S.A.;
Dennis P. Lettenmaier: Department of Civil and Environmental Engineering, University of Washington, Seattle, Washington, U.S.A.

Geophysical Research Letters (GRL) paper 10.1029/2008GL034648, 2008;

5. Gauging a volcanic gas in the stratosphere

Carbonyl sulfide (OCS), the most abundant sulfur-containing gas in the atmosphere, is important to biological nutrient cycles. Emitted by volcanoes and deep-sea vents, OCS is transported to the stratosphere where it can react with atomic oxygen or hydroxyl ions to ultimately form sulfate aerosol particles, which in the stratosphere scatter incoming solar radiation away from Earth and thus are important to regulating the Earth's energy budget. Recent research also suggests that knowledge of OCS concentrations can help constrain calculations of gross primary production, which is important to calculating carbon budgets. However, despite a wealth of global tropospheric measurements of OCS, stratospheric observations are sparse, restricting scientists' understanding of the full role of OCS in Earth systems. Using OCS vertical profiles retrieved by Canada's Atmospheric Chemistry Experiment (ACE) satellite, Barkley et al. examine the seasonality of global OCS distributions in the upper troposphere and stratosphere. Because tracer gases in the stratosphere are linearly correlated, the authors use concurrent ACE measurements of certain chlorofluorocarbons to derive a robust estimate of the stratospheric lifetime of OCS.

Global distributions of carbonyl sulfide in the upper troposphere and stratosphere

Michael P. Barkley and Paul I. Palmer: School of GeoSciences, University of Edinburgh, Edinburgh, U.K.;
Chris D. Boone: Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada;
Peter F. Bernath: Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada; also at Department of Chemistry, University of York, York, U.K.;
Parvadha Suntharalingam: Department of Environmental Sciences, University of East Anglia, Norwich, U.K.; also at School of Engineering and Applied Science, Harvard University, Cambridge, Massachusetts, U.S.A.

Geophysical Research Letters (GRL) paper 10.1029/2008GL034270, 2008;

6. Ice melt speeds mountain temperature rise

Forecasting how mountains will respond to climate change requires not only model estimations of large-scale regional warming trends, but also estimates of if and how warming rates vary with elevation. To study this, Pepin and Lundquist examine temperature records from more than 1000 high-elevation stations across the globe, ranging in elevation from 500 meters (1640 feet) up to 4700 meters (15400 feet). Through analysis of topography, ice, and free atmospheric change, the authors find that twentieth-century temperature trends are most rapidly seen at elevations where temperatures approach the melting point of ice. This is because melting exposes darker ground cover, which absorbs more sunlight and enhances further warming. Because local factors heavily influence temperatures at different mountain sites, the authors find no simplistic elevational increase in warming rates. However, they note that water resources and ecosystems near elevations where temperatures approach the melting point of ice are at increased risk from accelerated global warming. Also, exposed mountain summits, away from the effects of urbanization and topographic sheltering, may provide a relatively unbiased record of the planet's climate.

Temperature trends at high elevations: Patterns across the globe

N. C. Pepin: Department of Geography, University of Portsmouth, Portsmouth, U.K.;
J. D. Lundquist: Department of Civil and Environmental Engineering, University of Washington, Seattle, Washington, U.S.A.

Geophysical Research Letters (GRL) paper 10.1029/2008GL034026, 2008;
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American Geophysical Union

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