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AGU journal highlights -- August 6, 2008

1. Recent African drought heralds drier conditions to come

Human-induced climate change is projected to cause drier conditions in the midlatitudes. To assess whether the onset of drier conditions has already begun, Touchan et al. study newly developed multicentury tree ring records from Tunisia and Algeria for a longer-term perspective on drought in northwestern Africa. Using a new set of 13 chronologies from Atlas cedars (Cedrus atlantica) and Aleppo pines (Pinus halepensis), the authors analyze the widths of individual tree rings, following the basic principle that thinner bands indicate years when water was relatively scarce. Through this, they reconstruct the region's Palmer Drought Severity Index, an index of dryness based on precipitation and temperature, for the years between 1456 and 2002. The reconstruction reveals the magnitude of droughts from the historic record, despite there having been no instruments to record these droughts. Interestingly, the most recent drought (1999?) appears to be the worst since at least the middle of the fifteenth century. This drought is consistent with early signatures of a transition to more arid midlatitude conditions, as projected by several climate models.


Long term context for recent drought in northwestern Africa


Ramzi Touchan, David M. Melo, and Christopher Baisan: Laboratory of Tree-Ring Research, University of Arizona, Tucson, Arizona, U.S.A.

Kevin J. Anchukaitis: Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York, U.S.A.;

Said Attalah: Department of Agronomy, Faculty of Science, University of Ourgla, Ourgla, Algeria;

Ali Aloui: Institute of Sylvo-Pastoral of Tabarka, Tabarka, Tunisia.


Geophysical Research Letters (GRL) paper 10.1029/2008GL034264, 2008;

2. Is climate change reducing hail over China?

Hail, defined as precipitation of balls or irregular lumps of ice produced by storm clouds, forms when liquid drops freeze at altitudes above a threshold called the "freezing level height". If drops cannot be formed above the freezing level, precipitation will remain in a liquid state. To determine whether precipitation trends in hail have changed on regional scales, Xie et al. study an extensive data set of more than 753 stations over China, compiled by China's National Meteorological Information Center. To ensure a continuous data record, the authors choose 523 stations with complete observations from 1960 to 2005. For each year, the authors calculate the average "annual hail days" (AHD), which is the average amount of days each year when hail occurred at each station. Analysis reveals that while there is no trend in the mean AHD from 1960 to early 1980, China's AHD reduces thereafter from about 2 days to less than 1 day each year. The authors expect that this drop in hail precipitation is due to a rising trend in freezing level height.


Trends in hail in China during 1960?


Baoguo Xie and Qinghong Zhang: Department of Atmospheric Science, School of Physics, Beijing, China;

Yuqing Wang: Department of Meteorology and International Pacific Research Center, University of Hawai'i at Manoa, Honolulu, Hawaii, U.S.A.


Geophysical Research Letters (GRL) paper 10.1029/2008GL034067, 2008;

3. Mapping Venus's winds

Venus's lower atmosphere is dominated by cloud and haze layers that span 30?? kilometers (19-43 miles) in altitude. The rudimentary structure within these layers has been measured by numerous spacecraft and ground-based telescopes, which found strong flows directed westward along lines of latitude. Recently, more detailed observations have been obtained by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on board the European Space Agency's Venus Express spacecraft. Data from VIRTIS, consisting of images of Venus's cloud cover at three separated altitudes, allows Snchez-Lavega et al. to study the three-dimensional structure of Venus's winds. They find that at latitudes less than 55 degrees, wind speeds remain constant, with velocities of 60?? meters (200-230 feet) per second at the clouds' bases and about 105 meters (344 feet) per second at the clouds' tops. This creates strong vertical wind shear at low latitudes. Poleward of 55 degrees latitude, westward wind speeds are constant throughout the air column and decrease at a steady rate until the pole is reached. This creates a vertically coherent vortex structure of winds swirling about the poles.


Variable winds on Venus mapped in three dimensions


A. Snchez-Lavega, R. Hueso, J. Peralta, and S. Prez-Hoyos: Escuela Superior de Ingeniera, Universidad del Pas Vasco, Bilbao, Spain;

G. Piccioni: Istitoto Nazionale di Astrofsica Spaziale e Fisica Cosmica, Rome, Italy;

P. Drossart and S. Erard: Laboratoire d'Etudes Spatiales et d'Instrumentation en Astrophysique, Observatoire de Paris, CNRS, UPMC, Universit Paris-Diderot, Meudon, France;

C. F. Wilson and F. W. Taylor: Atmospheric, Oceanic and Planetary Physics, Department of Physics, University of Oxford, Oxford, U.K.;

K. H. Baines: Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, U.S.A.;

D. Luz: Laboratoire d'Etudes Spatiales et d'Instrumentation en Astrophysique, Observatoire de Paris, CNRS, UMPC, Universit Paris-Diderot, Meudon, France; also at Centro de Astronomia e Astrofsica da Universidade de Lisboa/Observatrio Astronmico de Lisboa, Lisbon, Portugal;

S. Lebonnois: Laboratoire de Mtorologie Dynamique/IPSL, CNRS/UPMC, Paris, France.


Geophysical Research Letters (GRL) paper 10.1029/2008GL033817, 2008;

4. Deep evidence shows past and present warming

Reconstructing past climate provides a useful context for discussions of current and future climate changes. One way to assess past temperatures is by using temperature measurements in deep boreholes, which are narrow vertical shafts drilled several hundred meters into the ground. Because variations in the ground surface temperature over time affect the distribution of temperature in the subsurface, scientists can carefully measure the temperature at depth within these holes and then use mathematical formulas to infer past temperatures at the surface. By integrating a global database of terrestrial heat flux measurements with another database of temperature versus depth within boreholes and with the twentieth-century instrumental record of surface temperature, Huang et al. reconstruct the surface temperature history over the past 20,000 years. The authors clearly identify a long-term warming from the Last Glacial Maximum, a mid-Holocene warm episode, the Medieval Warm Period, the Little Ice Age, and the rapid warming of the twentieth century.


A late Quaternary climate reconstruction based on borehole heat flux data, borehole temperature data, and the instrumental record


S. P. Huang and H. N. Pollack: Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan, U.S.A.;

P.-Y. Shen: Department of Earth Sciences, University of Western Ontario, London, Ontario, Canada.


Geophysical Research Letters (GRL) paper 10.1029/2008GL034187, 2008;

5. Climate models may underestimate heat stored by ground

General circulation models (GCMs), the primary tool for estimating the magnitude of future climate change, rely on realistic inputs to generate accurate predictions. New studies have revealed that over the past 60 years, the continents absorbed an amount of heat comparable to that absorbed by the whole atmosphere during the same time interval. However, bottom boundary conditions in soil components of GCMs, particularly those used by the Intergovernmental Panel on Climate Change, only extend 10 meters (33 feet) below ground, perhaps leading to underestimations of continental heat absorption. To better estimate this, MacDougall et al. calculate the subsurface heat content from two future climate simulations and compare the results with those obtained from running a separate model of soil heat content. They find that if boundary conditions for the soil extend down to 600 meters (1970 feet), then the subsurface can absorb 6 times more heat than if the soil boundary conditions were at 10 meters (33 feet). This suggests that current models underestimate the heat stored by the ground, particularly for the northern high latitudes.


Quantification of subsurface heat storage in a GCM simulation


Andrew H. MacDougall, M. Bruce Stevens, and Hugo Beltrami: Environmental Sciences Research Centre, St. Francis Xavier University, Antigonish, Nova Scotia, Canada;

J. Fidel Gonzlez-Rouco: Departamento de Astrofsica y Ciencias de al Atmsfera, Universitas Complutense de Madrid, Madrid, Spain.


Geophysical Research Letters (GRL) paper 10.1029/2008GL034639, 2008;

6. Soot from ships worse than expected

Produced during combustion of fossil fuels and biofuels, light-absorbing carbon (soot) creates haze and absorbs light with an efficiency nearly one third that of carbon dioxide. This is particularly hazardous for places such as the Arctic, where the presence of soot can drastically increase the amount of light absorbed by snow and ice surfaces. To learn more about sources of soot, Lack et al. study emissions from commercial shipping, which is expected to increase by 2?? percent each year. Using data taken from a survey in the Gulf of Mexico, the authors find that the highest soot emitters (per unit fuel combusted) are tugboats. Their study reveals as well that emissions of soot from cargo and noncargo vessels are double the most recent estimates and also independent of engine load. In total, the authors find that soot emitted by the shipping industry contributes to about 2 percent of the total soot present in the atmosphere. Although relatively small, this 2 percent can increase soot burdens in coastal areas close to ports by nearly 40 percent, significantly worsening local air quality.

See 9 July 2008 press release about this paper at


Light absorbing carbon emissions from commercial shipping


Daniel Lack, Brian Lerner, Paola Massoli, and Eric Williams: NOAA Earth System Research Laboratory, Boulder, Colorado, U.S.A.; also at Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, U.S.A.;

Claire Granier: NOAA Earth System Research Laboratory, Boulder, Colorado, U.S.A.; also at Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, U.S.A.; also at Service d'Aronomie, CNRS, Universit Pierre et Marie Curie, Paris, France;

Tahllee Banyard: NOAA Earth System Research Laboratory, Boulder, Colorado, U.S.A.; also at Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, U.S.A.; now at Lockheed Martin Coherent Technologies, Longmont, Colorado, U.S.A.;

Edward Lovejoy and A. R. Ravishankara: NOAA Earth System Research Laboratory, Boulder, Colorado, U.S.A.


Geophysical Research Letters (GRL) paper 10.1029/2008GL033906, 2008;


Contact: Peter Weiss
American Geophysical Union

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