Public Release: 

AGU journal highlights -- Jan. 27, 2011

American Geophysical Union

The following highlights summarize research papers that have been recently published in Geophysical Research Letters (GRL) and Water Resources Research (WRR).

In this release:

  1. China's lakes changed dramatically in recent decades
  2. Climate commitment in an uncertain world
  3. New satellite data improve estimates of ocean circulation
  4. Reversals in Saturn's polar radio emissions
  5. Modeling an asteroid's giant impact crater
  6. Tectonic plates relocked after 2005 earthquake in northeastern Japan
  7. Links to rivers affect Arctic lake habitat diversity in Canada
  8. More accurate measurements of solar energy reaching Earth

Anyone may read the scientific abstract for any already-published paper by clicking on the link provided at the end of each Highlight. You can also read the abstract by going to and inserting into the search engine the full doi (digital object identifier), e.g. 10.1029/2010GL045514. The doi is found at the end of each Highlight below.

Journalists and public information officers (PIOs) at educational or scientific institutions who are registered with AGU also may download papers cited in this release by clicking on the links below. Instructions for members of the news media, PIOs, and the public for downloading or ordering the full text of any research paper summarized below are available at

1. China's lakes changed dramatically in recent decades

Significant changes have taken place in the number and size of China's lakes over the past several decades. To create a comprehensive picture of these changes over all of China, Ma et al. combined data from a wide variety of sources, including satellite imagery and maps. They compare data from the 1960s to the 1980s, before China's rapid development and industrialization, with data from 2005 and 2006. The authors find that during that time period, the total surface area of Chinese lakes declined by 13 percent, and the total number of lakes with an area greater than 1 square kilometer decreased from 2928 to 2693. In addition, 243 lakes vanished, mainly in the northern provinces, while 60 new lakes appeared, mainly in the Tibetan Plateau region.

Both climate change and human activity can affect lakes. For instance, glacial melting on the Tibetan Plateau and neighboring areas, which may be due to global warming, may have led to the creation of new lakes and the expansion of existing lakes, while human development likely affected lakes in southern China. The authors note that more detailed regional studies are needed to confirm these preliminary, large-scale observations.

Title: A half-century of changes in China's lakes: Global warming or human influence?


Ronghua Ma and Hongtao Duan: State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing, China;

Chuanmin Hu: College of Marine Science, University of South Florida, St. Petersburg, Florida, USA;

Xuezhi Feng: Department of Geographic Information Science, Nanjing University, Nanjing, China;

Ainong Li: Institute of Mountain Hazards and Environment, Chinese Academy of Sciences, Chengdu, China;

Weimin Ju: International Institute for Earth System Science, Nanjing University, Nanjing, China;

Jiahu Jiang and Guishan Yang: State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences,Nanjing, China.

Source: Geophysical Research Letters, doi: 10.1029/2010GL045514, 2010

2. Climate commitment in an uncertain world

The planet will continue to warm, even if humans immediately cease all emission of carbon dioxide and other greenhouse gases, mostly due to thermal inertia of the world's oceans. This has been of interest recently for both scientists and policy-makers, because it provides a measure of the minimum climate change the planet will face due to human activity that has already occurred.

Several studies have looked at what would happen if human emissions of carbon dioxide were stopped, but these studies have overlooked the role of aerosols and other greenhouse gases such as methane and nitrous oxide, which Armour and Roe include in a new study. If human emissions stopped, atmospheric aerosols, which cool the planet by blocking light from the surface, would fall to preindustrial levels within weeks. However, other greenhouse gases would remain in the atmosphere for decades to centuries, and elevated levels of carbon dioxide would persist for millennia.

The authors find that the sudden loss of cooling from aerosols would cause abrupt warming of as much as 0.9 degrees Celsius (2 degrees Fahrenheit) above current temperatures in the decades following the cessation of greenhouse gas emissions. This could be followed by slight cooling or by further increased warming. Because there are large uncertainties in current observations of aerosol radiative forcing, there are large uncertainties in determining the amount of climate changes to which the Earth is already committed. The authors note that Intergovernmental Panel on Climate Change's models produce a narrower range of climate commitment than is allowed by constraints based on current observations.

Although there is large uncertainty, the authors find that fundamental constraints provide lower and upper bounds on climate commitment. Given these bounds, if all human emission stopped immediately, Earth would still not return to pre-industrial temperatures in the indefinite future, and we could already be committed to dangerous levels of warming.

Title: Climate commitment in an uncertain world

Authors: K. C. Armour: Department of Physics, University of Washington, Seattle, Washington, USA;

G. H. Roe: Department of Earth and Space Sciences, University of Washington, Seattle, Washington, USA.

Source: Geophysical Research Letters, doi:10.1029/2010GL045850, 2011

3. New satellite data improve estimates of ocean circulation

Ocean currents play a crucial role in regulating Earth's climate by redistributing heat and freshwater around the planet. Accurate knowledge of the subtle regional variations in Earth's gravity field is fundamental to the measurement of ocean currents. However, the challenge of mapping Earth's gravity in sufficient detail has previously limited scientists' ability to reliably determine the ocean's circulation. To address this problem, the European Space Agency launched the Gravity field and steady-state Ocean Circulation Explorer (GOCE) satellite in October 2009, with the aim of mapping Earth's gravity field with unprecedented spatial resolution.

An initial assessment of the performance of GOCE uses the first data from the satellite to estimate the circulation of the North Atlantic Ocean, and shows the satellite is already returning data superior to measurements from previous systems. The North Atlantic findings are particularly interesting because this region contains the Gulf Stream, the world's strongest current and a key component of the global overturning circulation, since it carries heat from the equator to high northerly latitudes. In many places, the current speeds estimated from the GOCE data compare well with those obtained from field instruments, providing strong validation of the GOCE mission design. Accumulation of data as the mission progresses could lead to a more accurate and detailed map of the ocean's currents.

Title: An initial estimate of the North Atlantic steady-state geostrophic circulation from GOCE

Authors: R. J. Bingham: School of Civil Engineering and Geosciences, Newcastle University, Newcastle Upon Tyne, UK;

P. Knudsen and O. Andersen: Department of Geodesy, Danish National Space Center, Copenhagen, Denmark;

R. Pail: Institute of Astronomical and Physical Geodesy, Technical University of Munich, Munich, Germany.

Source: Geophysical Research Letters, doi:10.1029/2010 GL045633, 2011

4. Reversals in Saturn's polar radio emissions

Saturn emits intense radio emissions at kilometer wavelengths from its auroral regions. Observations in recent years found that the Saturn kilometric radiation (SKR) emission from the northern auroral region has a clocklike modulation with a period of about 10.6 hours, while the SKR emission from the southern auroral region has a period of about 10.8 hours. Analyzing more recent observations from the Cassini spacecraft, the researchers now find that the rotational modulation rates of the southern and northern components reversed shortly after Saturn's equinox on 11 August 2009, so that the southern hemisphere SKR now has the shorter rotation period. They also analyze data from the Ulysses spacecraft to show that a similar reversal occurred during the previous equinox, in November 1995.

The authors suggest that these reversals are driven by seasonal changes in solar radiation to the two hemispheres as the planet orbits the Sun. The changing solar radiation could affect the planet's upper atmosphere and how its magnetosphere slips with respect to its internal rotation. Alterations in the magnetosphere could cause the observed changes in the rotational modulation of Saturn's radio emissions.

Changes in the magnetosphere could also cause the location of Saturn's northern and southern auroral ovals to oscillate, as observed in a related paper by Nichols et al. (Geophys. Res. Lett., 37, L24102, doi:10.1029/2010GL045818).

Title: The reversal of the rotational modulation rates of the north and south components of Saturn kilometric radiation near equinox

Authors: D. A. Gurnett, J. B. Groene, A. M. Persoon, J. D. Menietti, S.-Y. Ye, and W. S. Kurth: Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa, USA;

R. J. MacDowall: NASA Goddard Space Flight Center, Greenbelt, Maryland, USA;

A. Lecacheux: LEISA, UMR 8109, Observatoire de Paris, CNRS, Meudon, France.

Source: Geophysical Research Letters, doi:10.1029/2010GL045796, 2010

5. Modeling an asteroid's giant impact crater

This summer, NASA's Dawn mission will orbit Vesta. The asteroid 4 Vesta is one of the largest rocky bodies in the asteroid belt, with a mean diameter of about 530 kilometers (329.3 miles). An enormous impact crater, with a diameter of about 460 km (286 mi) and a depth of about 13 km (8 mi), covers almost half of the asteroid's southern hemisphere. To provide a framework for interpreting some of the expected observations, the researchers model the formation of the impact crater, which was probably created by a collision with an object approximately 50 km (31 mi) in diameter traveling at about 5 km per second.

Their high-resolution three-dimensional simulation shows that the collision exposed deep strata within the asteroid and changed its center of mass. Because the asteroid spins rapidly -- it completes a revolution every 5.3 hours -- debris ejected by the collision would be deposited in sequences with variable shapes, multiple folds, and abrupt terminations. These deposit sequences could be confused with other geologic forms such as thrusts and folds, so this model should help in interpreting images of Vesta obtained from the Dawn mission.

Title: Mega-ejecta on asteroid Vesta

Authors: Martin Jutzi and Erik Asphaug: Earth and Planetary Sciences Department, University of California, Santa Cruz, California, USA.

Source: Geophysical Research Letters, doi:10.1029/2010GL045517, 2011

6. Tectonic plates relocked after 2005 earthquake in northeastern Japan

In August 2005 a magnitude 7.2 earthquake shook northeastern Japan. The quake occurred on the boundary of the North American plate and the Pacific plate off Miyagi prefecture. Earthquakes with a magnitude of about 7.5 have been occurring in this area approximately every 37 years. Sato and his colleagues use seafloor geodetic observations from a combination of Global Positioning System (GPS) and acoustic measurements to detect seafloor movements during and after the earthquake. They focus on the movement of a seafloor reference point located about 10 kilometers (6.2 miles) east of the epicenter of the 2005 quake. Their measurements indicate that some built-up strain was released by the 2005 earthquake. Then for about 1 to 2 years following the quake, there was erratic movement of the reference point, and at the end of 2006 constant strain accumulation began again. Since the end of 2006 the reference point has been moving about 5.7 cm (2.2 inches) per year toward the west-northwest. The results are consistent with other measurements and support the interpretation that the tectonic plates have been relocked since late 2006.

Title: Restoration of interplate locking after the 2005 Off-Miyagi Prefecture earthquake, detected by GPS/acoustic seafloor geodetic observation

Authors: Mariko Sato, Hiroaki Saito, Tadashi Ishikawa, Yoshihiro Matsumoto, and Masayuki Fujita: Hydrographic and Oceanographic Department, Japan Coast Guard, Tokyo, Japan;

Masashi Mochizuki and Akira Asada: Institute of Industrial Science, University of Tokyo, Tokyo, Japan.

Source: Geophysical Research Letters, doi:10.1029/2010GL045689, 2011

7. Links to rivers affect Arctic lake habitat diversity in Canada

The 45,000 lakes in the Mackenzie River delta, Canada, represent an especially diverse aquatic habitat compared to lakes in surrounding Arctic areas. River-to-lake connectivity could play a role in this diversity, as river water entering lakes replenishes nutrients and affects water transparency. Lesack and Marsh seek to better understand how river-to-lake connectivity affects biodiversity in the region. Using 40 years of data on water levels in the delta combined with a floodplain geometry model, they estimate the volumes of river water added to lake water during the annual flood peak for nine lakes at different elevations. They also estimate the connection time between the lakes and the river by counting the number of days each year when the river water level exceeded the highest elevation along the channel connecting the lake and the river. In addition, they consider interannual variability of the river-to-lake connectivity for each of the nine lakes. The results showed a broad range of connectivities. Lakes with short and variable connection times had greater individuality, whereas lakes with longer and less varying connection times were more similar to each other. The researchers suggest that the wide variation in river-to-lake connectivity could be a key to the distinctive habitat and biodiversity in the delta.

Title: River-to-lake connectivities, water renewal, and aquatic habitat diversity in the Mackenzie River Delta

Authors: Lance F. W. Lesack: Department of Geography and Department of Biological Sciences, Simon Fraser University, Burnaby, British Columbia, Canada;

Philip Marsh: National Water Research Institute, Saskatoon, Saskatchewan, Canada.

Source: Water Resources Research, doi:10.1029/2010WR009607, 2010

8. More accurate measurements of solar energy reaching Earth

Radiation from the Sun is the dominant source of energy input to the Earth's climate system; even small variations in solar irradiance can produce noticeable climate changes on global and regional scales. Determining how much of the observed global change can be attributed to variations in the Sun's output and how much can be attributed to human or other influences requires an accurate record of solar irradiance.

Measurements of solar irradiance made by the Total Irradiance Monitor (TIM) on NASA's Solar Radiation and Climate Experiment (SORCE) satellite give a value of total solar irradiance that is significantly lower than previously accepted values. Kopp and Lean show that this new, lower value is more accurate than measurements made using older instruments. They use laboratory studies and satellite calibrations to diagnose and quantify error sources on the TIM and other space-based solar radiometers and found that earlier radiometers measured higher values of solar irradiance because they included scattered instrument light in their signals, while the different optical design of the TIM radiometer reduces this spurious signal and acquires more accurate measurements. They also show that the high stability of the TIM gives improved agreement with models estimating solar variability, concluding that this new instrument provides the most accurate value of solar irradiance and helps improve estimates of the Sun's influence on climate.

An AGU press release about this paper that is available online at

Title: A new, lower value of total solar irradiance: Evidence and climate significance

Authors: Greg Kopp: Laboratory for Atmospheric and Space Physics, Boulder, Colorado, USA;

Judith L. Lean: Space Science Division, Naval Research Laboratory, Washington, D. C., USA.

Source: Geophysical Research Letters, doi:10.1029/2010GL045777, 2011


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