AGU Journal highlights -- May 6, 2009

The following highlights summarize research papers that have been published or are "in press" (accepted, but not yet published) in Geophysical Research Letters (GRL) or the Journal of Geophysical Research – Atmospheres (JGR-D). 1. Cooling periods can occur within long-term global warming

Some Web sites, blogs, and media articles have pointed to the fact that average global temperature has not risen since 1998 as evidence against anthropogenic global warming. Although most climate scientists recognize the statistical insignificance of such short-term trends, Easterling and Wehner note that some segments of the public do pay attention to them. To quell such objections, they argue that such short periods are not meaningful in the context of long-term climate change. The authors analyze the observed globally averaged surface air temperatures for the period 1901� as well as several climate model simulations for the twentieth and 21st centuries, including anthropogenically forced models. They plot the probability distribution functions for decadal temperature trends and find that in the observed record and all models, decade-long periods of cooling can occur even within a strong overall warming trend. The authors expect that due to the natural variability of the real climate, in the 21st century there will probably be some multiyear periods of cooling or constant temperature within longer-term anthropogenic global warming.

Title: Is the Climate Warming or Cooling?

Authors: David R. Easterling: National Climatic Data Center, NOAA, Asheville, North Carolina, USA;

Michael F. Wehner: Lawrence Berkeley National Laboratory, Berkeley, California, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/22009GL037810, 2009; http://dx.doi.org/10.1029/2009GL037810

2. Scientists explore complexity of recent global dimming and brightening episodes

Several studies show evidence of a widespread decrease in solar radiation reaching the Earth's surface from the 1960s through about the early 1990s. Termed "global dimming," this decrease was followed by "brightening" during the late 1990s. Past efforts correlated brightening trends to decreases in atmospheric aerosol concentrations due to successful environmental regulations and recovery from the Mount Pinatubo eruption. To learn more about the mechanics of dimming and brightening, Long et al. study observations of shortwave radiation collected by the U.S. government at 12 surface sites across the United States. These data, aggregated into all-sky and clear-sky surveys, show that widespread brightening occurred over the continental United States during the past 12 years at rates higher than previously thought. The authors find that changes in aerosol concentrations and other direct effects cannot fully explain the observed changes in shortwave radiation. They show that decreases in cloudiness, which may be indirectly influenced by decreasing aerosol concentrations, have played a significant role in observed brightening patterns. Further, they suggest that the causes of global brightening are complex and are best studied locally or regionally, rather than on a global or continental scale.

Title: Significant decadal brightening of downwelling shortwave in the continental United States

Authors: C. N. Long, S. A. McFarlane, and C. J. Flynn: Climate Physics Group, Pacific Northwest National Laboratory, Richland, Washington, U.S.A.;

E. G. Dutton and J. A. Augustine: Global Monitoring Division, Earth System Research Laboratory, U.S. National Oceanic and Atmospheric Administration, Boulder, Colorado, U.S.A.;

W. Wiscombe: Brookhaven National Laboratory, Upton, New York, U.S.A.;

M. Wild: Institute for Atmospheric and Climate Science, Eidgenossische Technische Hochschule, Zurich, Switzerland.

Source: Journal of Geophysical Research-Atmospheres (JGR-D) paper 10.1029/2008JD011263, 2009; http://dx.doi.org/10.1029/2008JD011263

3. Analysis of unusual atmospheric circulation during the Dust Bowl may help predict Midwest droughts

The midwestern United States often experiences dry seasons, but the devastating drought during the 1930's Dust Bowl was extremely unusual, and scientists do not fully understand the mechanisms that caused it. Using historical upper air data, reconstructions, and model simulations, Brönnimann et al. analyze atmospheric circulation during the Dust Bowl, focusing on features thought to be critical to understanding Dust Bowl droughts. They find that during the drought, the Great Plains low level jet, which transports moisture from the Gulf of Mexico to the region, was weaker on its eastern side, shallower, and did not extend as far north as during wetter years. In addition, during the Dust Bowl years there was unusually strong ridging in the midtroposphere above the Great Plains and persistent flow anomalies in the upper troposphere across North America from the North Pacific to the Atlantic, which may have prevented moisture from reaching the region. The authors conclude that oceanic forcing triggered these changes. Models that included such forcing reproduced the atmospheric conditions associated with extreme droughts, which implies that droughts in the Midwest could potentially be predictable.

Title: Exceptional atmospheric circulation during the "Dust Bowl"

Authors: S. Brönnimann, A. Stickler, T. Griesser, T. Ewen, A. N. Grant, A. M. Fischer, M. Schraner, and T. Peter: Institute for Atmospheric and Climate Science, ETH Zurich, Zurich, Switzerland; E. Rozanov: Institute for Atmospheric and Climate Science, ETH Zurich, Zurich, Switzerland, and PMOD, WRC, Davos, Switzerland; T. Ross: National Climatic Data Center, NOAA, Asheville, North Carolina, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL037612, 2009; http://dx.doi.org/10.1029/2009GL037612

4. Young and vast river valleys found on Mars

Most river valley systems on Mars existed only during the early history of the planet, before a major climate transition to colder, drier conditions. Dickson et al. report evidence of younger fluvial valley systems that formed during the middle to late Amazonian epoch, which extends from about 1.8 billion years ago to the present. The data suggests that these fluvial valley systems, located in the Lyot crater in the northern midlatitudes, are tens of kilometers in length and are among the youngest river valley systems of this size reported to date. Past research indicates that conditions overall on Mars during the middle to late Amazonian were too cold to support liquid water. However, the authors propose that in the microenvironment of the Lyot crater, high surface pressure due to low elevation, combined with local temperature conditions, at times made possible the melting of surface ice. They suggest that observed glacial deposits in the crater indicate that glaciers provided the continuous meltwater needed to form these fluvial valley systems.

Title: Amazonian-aged fluvial valley systems in a climatic microenvironment on Mars: Melting of ice deposits on the interior of Lyot Crater

Authors: J. L. Dickson, C. I. Fassett, J. W. Head: Department of Geological Sciences, Brown University, Providence, Rhode Island, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL037472, 2009; http://dx.doi.org/10.1029/2009GL037472

5. Models used in IPCC assessments underestimate global dimming and brightening

Although observations show that average temperatures across the Earth's surface substantially increased over the twentieth century, this increase has not been linear on decadal timescales. For example, from the 1950s to the 1980s, the Earth experienced "global dimming"—greenhouse warming trends were likely countered by aerosol pollutants, which scattered incoming solar radiation. Decreases in atmospheric aerosol concentrations from 1980 to 2000 due to successful environmental regulations led to widespread "brightening," which offset prior damping and revealed the full magnitude of greenhouse warming. Wild analyzes the decadal evolution of temperatures, in particular focusing on diurnal temperature ranges, which are highly correlated with surface solar radiation. He finds that the switch from global dimming to brightening was captured in observations by a marked reversal in diurnal temperature range trends. The author then compares observations with outputs from the latest generation of global climate models used in the fourth assessment report of the Intergovernmental Panel on Climate Change. These models tend to underestimate decadal variations in surface warming and diurnal temperature ranges, which suggests that they do not properly consider global dimming and brightening effects.

Title: How well do IPCC-AR4/CMIP3 climate models simulate global dimming/brightening and related effects on 20th century day- and night-time warming?

Author: Martin Wild: Institute for Atmospheric and Climate Science, ETH Zurich, Zurich, Switzerland.

Source: Journal of Geophysical Research-Atmospheres (JGR-D) paper 10.1029/2008JD011372, 2009; http://dx.doi.org/10.1029/2008JD011372

6. Jupiter's polar auroras display long-lived filaments

Jupiter's auroras include a relatively stable main auroral oval at high latitude as well as a highly variable and poorly understood auroral region poleward of the main oval. Striking new phenomena have been observed in this polar auroral region, Nichols et al. report. The features, called polar auroral filaments, are thin and long-lived (several days) and seem to be independent of solar wind conditions. The authors make the discovery using four sets of images taken by the Hubble Space Telescope over 6 days in 2007. The observed filaments have two components: one oriented toward the Sun that remains fixed in orientation, and one facing away from the Sun that rotates. The observation of these polar auroral filaments is significant, the authors state, because no coherent structures have previously been observed in Jupiter's highly variable high-latitude auroras. The authors suggest that the discovery could also shed light on the dynamics of Jupiter's magnetotail. Future missions such as NASA's planned Juno spacecraft could explore these features further.

Title: Observations of Jovian polar auroral filaments

Authors: J. D. Nichols: Department of Physics and Astronomy, University of Leicester, Leicester, UK and Center for Space Physics, Boston University, Boston, Massachusetts, USA;

J. T. Clarke: Center for Space Physics, Boston University, Boston, Massachusetts, USA;

J. C. Gérard and D. Grodent: LPAP, Université de Liège, Liège, Belgium.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL037578, 2009; http://dx.doi.org/10.1029/2009GL037578

7. Effect of aerosols on thin clouds

Increasing aerosol concentrations from industrialization are generally considered to offset global warming by reflecting incoming solar radiation. Indirect effects of aerosols include altering cloud lifetimes through suppressing precipitation. Lee et al. study this indirect effect by examining the liquid water path (LWP), a measure of liquid water present in an entire air column. Thin clouds, which have low LWPs, cover more than a quarter of the Earth's surface and have radiative fluxes that are very sensitive to LWP variations when the LWP becomes smaller than about 50 grams per square meters. Using a cloud system–resolving model coupled with cloud microphysics calculations, the authors find that rising aerosol concentrations increase the concentration of droplets within thin clouds, which indirectly leads to increased LWP in cases where precipitation reaches the Earth's surface. If precipitation is absent, LWP is seen to decrease due to preexisting conditions that favor evaporation just below the cloud base. Feedbacks are introduced when aerosol concentrations decrease, sustaining the cloud even though the cloud becomes more likely to shed precipitation.

Title: Aerosol effects on liquid-water path of thin stratocumulus clouds

Authors: Seoung Soo Lee and Joyce E. Penner: Department of Atmospheric, Oceanic, and Space Sciences, University of Michigan, Ann Arbor, Michigan, U.S.A.;

Stephen M. Saleeby: Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado, U.S.A.

Source: Journal of Geophysical Research-Atmospheres (JGR-D) paper 10.1029/2008JD010513, 2009; http://dx.doi.org/10.1029/2008JD010513

8. Differences in mineral dust flakes influence how solar radiation scatters

Mineral dust, an important aerosol type in the Earth's atmosphere, scatters incoming solar radiation. This scattering causes uncertainties to climate models and skews remote sensing observations. To learn more about exactly how mineral dust scatters solar radiation, Nousiainen et al. study the morphology and composition of particles obtained from the Sahara desert. Using electron microscope imaging, X-ray diffractometry, and energy-dispersive spectroscopy, the authors find that thin flakes composed of calcite and dolomite were abundant in the samples collected. These minerals are of particular interest due to their strong double refraction (birefringence). Through comparing their scattering properties with other particles, the authors find that such a strong birefringence has a substantial and systematic impact on the particles' ability to scatter solar radiation back to space. They also find that the commonly used models for mineral dust—spherical and spheroidal particles—were unsuccessful in predicting their scattering properties. The authors conclude that excluding the shape and the birefringence of dust particles could cause errors in assessing the direct aerosol effect in climate models.

Title: Single-scattering modeling of thin, birefringent mineral-dust flakes using the discrete-dipole approximation

Authors: Timo Nousiainen: Department of Physics, University of Helsinki, Helsinki, Finland;

Evgenij Zubko: Center for Atmospheric and Oceanic Studies, Tohoku University, Sendai, Japan; also at Astronomical Institute of Kharkov National University, Kharkov, Ukraine;

Jarkko V. Niemi: Department of Biological and environmental Sciences, University of Helsinki, Helsinki, Finland;

Kaarle Kupiainen: Finnish Environment Institute, Helsinki, Finland;

Martti Lehtinen: Geological Museum, University of Helsinki, Helsinki, Finland;

Karri Muinonen: Observatory, University of Helsinki, Helsinki, Finland;

Gorden Videen: Army Research Laboratory, Adelphi, Maryland, U.S.A.

Source: Journal of Geophysical Research-Atmospheres (JGR-D) paper 10.1029/2008JD011564, 2009; http://dx.doi.org/10.1029/2008JD011564

9. Global warming may turn forests into carbon sources

Global warming affects the carbon cycle, which in turn affects climate, making it critical to understand the mechanisms governing the effect of temperature change on ecosystem carbon sources and sinks. Previous studies had examined how mean annual temperature (MAT) affected net ecosystem productivity (NEP), a measure of the amount of carbon accumulated by an ecosystem. These studies had found NEP patterns only weakly correlated with current MAT. To investigate further, Piao et al. reanalyze temperature and carbon storage data from several temperate and boreal forests, applying a simple conceptual model. The authors show that while current mean annual temperatures do not correlate well with current NEP, temperature changes spanning the recent past (1980�) may be important factors that influence current carbon balance. In particular, changes in past springtime temperatures seem to have had the greatest effect on current annual NEP, the authors find. Their results also suggest that if global warming continues, forests will not continue to be carbon sinks in the future but may instead become carbon sources.

Title: Footprint of temperature changes in the temperate and boreal forest carbon balance

Authors: Shilong Piao: Department of Ecology, College of Urban and Environmental Science, Peking University, Beijing, China;

Pierre Friedlingstein and Philippe Ciais: LSCE, UMR1572, CEA, CNRS, Gif-sur-Yvette, France;

Philippe Peylin: Laboratoire de Biogéochimie Isotopique, Thiverval-Grignon, France;

Biao Zhu: Department of Environmental Studies, University of California, Santa Cruz, California, USA;

Markus Reichstein: Max Planck Institute for Biogeochemistry, Jena, Germany.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL037381, 2009; http://dx.doi.org/10.1029/2009GL037381

10. Mountains cause gravity waves in the middle atmosphere

Wind blowing over mountain ranges can generate gravity waves that propagate vertically upward in Earth's atmosphere. These waves, known as mountain waves, have previously been observed low in the atmosphere. Smith et al. report the first unambiguous images of mountain gravity waves higher up, in the upper mesosphere (80 -100 kilometers – 50 to 62.5 miles – in altitude). The authors observed the gravity waves using the all-sky imager at the El Leoncito Observatory in Argentina from 1 to 8 July 2008. They observed a large series of stationary waves in the nightglow emissions from hydroxyl (at 695-1050 nanometers), sodium (at 589.3 nm), and oxygen (at 557.7 nm), at altitudes spanning an 11-km (6.9 miles) range. The authors note that each night, the large band-like waves lasted several hours and did not propagate across the sky, features that distinguish mountain waves from other types of atmospheric disturbances. Further, these waves' north-south orientation, parallel to the Andes mountain range, suggests that they were generated by eastward winds flowing over the Andes.

Title: Evidence of mesospheric gravity-waves generated by orographic forcing in the troposphere

Authors: Steven Smith, Jeffrey Baumgardner, and Michael Mendillo: Center for Space Physics, Boston University, Boston, Massachusetts, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL036936, 2009; http://dx.doi.org/10.1029/2008GL036936

Source: American Geophysical Union