Public Release: 

AGU journal highlights -- April 5, 2011

American Geophysical Union

The following highlights summarize research papers that have been recently published or accepted for publication in Geophysical Research Letters (GRL) and Paleoceanography (PA).

In this release:

  1. New study says 2 degrees Celsius warming may be unavoidable by 2100
  2. Icelandic volcano exonerated for harsh winter of 1783-1784
  3. Droughts and floods becoming more common in northern Australia
  4. Improved model reproduces deadly European heat wave
  5. Tree ring record chronicles major pre-Hispanic droughts in Mesoamerica
  6. Antarctic and Greenland ice sheet melting accelerating

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. 10.1029/2010GL046270. 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. New study says 2 degrees Celsius warming may be unavoidable by 2100

When it comes to modeling climate change, researchers rely on the specification of plausible emissions scenarios to explore how climate will change over the coming century. Using a standardized set of carbon dioxide and other greenhouse gas scenarios allows researchers from different modeling centers to compare results and allows more methodical assessment of uncertainty in the Intergovernmental Panel on Climate Change (IPCC) reports. The set of emissions scenarios used in the past two IPCC reports were published in 2001 and need to be updated to take into account more recent socioeconomic modeling results.

In a new study, Arora et al. use a completely new set of scenarios, referred to as representative concentration pathways (RCPs). These will form the basis for new climate projections to be assessed in the IPCC's Fifth Assessment Report (due out in 2014). Using an upgraded Earth system model--which takes into account carbon dioxide and other greenhouse gases, aerosols, land use change, and the flow of carbon between the atmosphere and the underlying ocean and land surface--the researchers are able to calculate the carbon dioxide emissions compatible with each RCP and, in particular, the emissions reductions required to meet certain levels of global warming.

The authors find that even under the lowest concentration scenario, global average temperature increases exceed the 2 degrees Celsius (3.6 degrees Fahrenheit) limit agreed to by various governments in the Copenhagen accord. The researchers note that limiting warming to 2 degrees Celsius by 2100 will require global carbon dioxide emissions to be reduced to zero over the next 50 years, followed by measures to actively remove carbon dioxide from the atmosphere before the end of the century.

Source: Geophysical Research Letters, doi: 10.1029/2010GL046270, 2011

Title: Carbon emission limits required to satisfy future representative concentration pathways of greenhouse gases

Authors: V. K. Arora, J. F. Scinocca, G. J. Boer, G. M. Flato, V. V. Kharin, W. G. Lee, and W. J. Merryfield: Canadian Centre for Climate Modelling and Analysis, Environment Canada, University of Victoria, Victoria, British Columbia, Canada;

J. R. Christian: Canadian Centre for Climate Modelling and Analysis, Environment Canada, University of Victoria, Victoria, British Columbia, Canada and Fisheries; and Oceans Canada, Institute of Ocean Sciences, Sidney, British Columbia, Canada;

K. L. Denman: Canadian Centre for Climate Modelling and Analysis, Environment Canada, University of Victoria, Victoria, British Columbia, Canada; and VENUS, University of Victoria, Victoria, British Columbia, Canada.

2. Icelandic volcano exonerated for harsh winter of 1783-1784

In June 1783 the Laki volcano in Iceland began to erupt, and continued erupting for months, causing a major environmental disaster. The eruption spewed out toxic sulfuric acid aerosols, which spread over northern latitudes and caused thousands of deaths. That summer, there were heat waves, widespread famines, crop failures, and livestock losses. During the following winter, temperatures in Europe were about 2 degrees Celsius (3.6 degrees Fahrenheit) below average for the late 1700s; the winter was also one of the most severe of the past 500 years in eastern North America. The Laki eruption has been blamed for the anomalously cold winter of 1783-1784.

However, a new study by D'Arrigo et al. challenges that interpretation, suggesting instead that the cold winter was caused not by the Laki eruption but by an unusual combination of a negative phase of the North Atlantic Oscillation (NAO) and an El Niño-Southern Oscillation (ENSO) warm phase. The authors analyzed 600-year tree ring reconstructions to show that the NAO and ENSO indices were similar to their values during the 2009-2010 winter, which, like the 1783-1784 winter, was unusually cold and snowy across western Europe and eastern North America. The 2009� winter has been shown to be attributable to NAO and ENSO conditions (and their combined effect), not to greenhouse gas forcing or other causes. The authors add that other data and climate simulations support their hypothesis that this natural NAO/ENSO variability, not the Laki eruption, caused the cold winter of 1783-1784.

Source: Geophysical Research Letters, doi:10.1029/2011GL046696, 2011

Title: The anomalous winter of 1783-1784: Was the Laki eruption or an analog of the 2009-2010 winter to blame?

Authors: Rosanne D'Arrigo, Richard Seager, Jason E. Smerdon, and Edward R. Cook: Lamont-Doherty Earth Observatory, Earth Institute at Columbia University, Palisades, New York, USA;

Allegra N. LeGrande: NASA Goddard Institute for Space Studies, New York, New York, USA.

3. Droughts and floods becoming more common in northern Australia

Rainfall variability, including the frequency of extreme floods and droughts, is increasing in northeastern Australia, a new study shows. Rainfall in Queensland is very variable from year to year, partly due to El Niño-Southern Oscillation (ENSO) events. As massive corals grow, their skeletons create yearly growth patterns, similar to tree rings, and record a wealth of proxy information about the corals' environment over several centuries. Flood events show up in nearshore annually banded coral skeletons as luminescent lines.

Lough measures luminescence intensity in 20 coral cores from the Great Barrier Reef off the coast of Queensland and uses the measurements to reconstruct a record of northeastern Queensland rainfall back to the late seventeenth century. The author finds that since the late nineteenth century, average rainfall and variability have increased. Both droughts and floods are becoming more common. The study suggests that extreme events, such as the recent flooding in Queensland, which affected hundreds of thousands of people and caused billions of dollars in damage, may occur more frequently in the future.

See related press release:

Source: Paleoceanography, paper in press:

Title: Great Barrier Reef coral luminescence reveals rainfall variability over northeastern Australia since the 17th century

Author: Janice M. Lough: Australian Institute of Marine Science, Queensland, Australia

4. Improved model reproduces deadly European heat wave

In August 2003, record-breaking temperatures raged across much of Europe. In France, maximum temperatures of 37 degrees Celsius (99 degrees Fahrenheit) persisted for 9 days straight, the longest such stretch since 1873. In the end, 40,000 deaths (14,000 in France alone) were attributed to the extreme heat and low humidity. Various climate conditions must come into alignment to produce extreme weather like the 2003 heat wave, and despite a concerted effort, forecasting models have so far been unable to accurately reproduce the event, including the modern European Centre for Medium-Range Weather Forecasts (ECMWF) ensemble modeling system for seasonal forecasts, which went into operation in 2007.

In their new study, Weisheimer et al. find that an updated version of the ECMWF model, when fed with historical data, is able to predict the European heat wave. The model, CY33R1, has improved representations of atmospheric convection, soil moisture, and shortwave radiation. By systematically stripping the upgraded modules out of CY33R1, reverting them back to their previous ECMWF design, the authors find that although all physical improvements to the model contributed to the successful forecast, CY33R1's representation of surface water had the greatest impact on the accuracy of the forecast.

According to the researchers, the spring of 2003 was much drier than normal. When the summer heat kicked into full blast, energy that normally would have been used evaporating surface water instead helped to drive up the temperature. Models that did not take this interaction into account drastically underestimated the extent of the warming. The research shows that efforts to adapt and modify operational forecasting models could help next-generation systems better predict heat waves. Accurate predictions would allow people to prepare for and hopefully prevent such massive loss of life in future similar extreme events.

See related blog post:

Source: Geophysical Research Letters, doi:10.1029/2010GL046455, 2011

Title: On the predictability of the extreme summer 2003 over Europe

Authors: Antje Weisheimer and T. N. Palmer: ECMWF, Reading, UK and Atmospheric, Oceanic and Planetary Physics, University of Oxford, Oxford, UK;

Francisco J. Doblas-Reyes: ECMWF, Reading, UK; Institució Catalana de Recerca i Estudis Avançats, Barcelona, Spain; and Institut Català de Ciències del Clima, Barcelona, Spain;

Thomas Jung: ECMWF, Reading, UK; and AWI, Bremerhaven, Germany.

5. Tree ring record chronicles major pre-Hispanic droughts in Mesoamerica

A new tree ring record chronicles major Mesoamerican droughts in the past millennium that may have contributed to the decline of some pre-Hispanic civilizations. Although there is other evidence of droughts during the past millennium, the paleoclimate record had gaps. Stahle et al. use core samples from Montezuma bald cypress trees found in Barranca de Amealco, Querétaro, Mexico, to develop a 1238-year tree ring chronology. The authors reconstruct the soil moisture record from the tree ring growth patterns. The new record provides the first dated, annually resolved climate record for Mexico and Central America spanning this time period.

The record shows that a drought known as the Terminal Classic occurred from 897 to 922 and extended farther than previously thought into central Mexico; this drought has been associated with the decline of the Mayan civilization. The researchers also document prolonged droughts that occurred during the decline of the Toltec state (1149-1167) and during the Spanish conquest of the Aztecs (1514-1539). This is the first record to provide accurate dates for these droughts.

Although other factors clearly contributed to the decline of these civilizations, the tree ring record helps nail down exactly when droughts occurred, and the improved chronology could help researchers better understand the factors involved in the cultural changes these civilizations underwent.

See related press release:

Source: Geophysical Research Letters, doi:10.1029/2010GL046472, 2010

Title: Major Mesoamerican droughts of the past millennium

Authors: D. W. Stahle, D. J. Burnette, F. K. Fye, M. K. Cleaveland and D. K. Stahle: Department of Geosciences, University of Arkansas, Fayetteville, Arkansas, USA;

J. Villanueva Diaz, J. Cerano Paredes: Laboratorio de Dendrocronologia, Instituto Nacional de Investigaciones Forestales, Agricolas, y Pecuarias, Gomez Palacio, Mexico;

R. R. Heim Jr.: Climate Services and Monitoring Division, NOAA National Climatic Data Center, Asheville, North Carolina, USA;

R. Acuna Soto: Departamento Microbiologia y Parasitologia, UNAM, Mexico City, Mexico;

M. D. Therrell: Department of Geography, Southern Illinois University, Carbondale, Illinois, USA.

6. Antarctic and Greenland ice sheet melting accelerating

The Antarctic and Greenland ice sheets are melting at an accelerating pace, a new study shows. Rignot et al. present a record of the mass balance of these polar ice sheets using two methods. One method measures ice sheet mass using gravity data from the Gravity Recovery and Climate Experiment (GRACE) satellite; the other method calculates ice lost at the ice sheet perimeter using atmospheric climate model data and measurements of ice thickness and ice motion, measured with airborne radio echo sounding and interferometric synthetic aperture radar (InSAR) data, respectively, from several satellites. The authors reconcile the two methods and find good agreement.

The researchers find that in 2006, these ice sheets were losing mass at a combined rate of about 475 gigatonnes per year, which is equivalent to about 1.3 millimeters per year (0.05 inches per year) of sea level rise. (A gigatonne is one billion metric tons, or more than 2.2 trillion pounds.) Furthermore, the rate of ice loss is accelerating, with a combined total acceleration of about 36 gigatonnes per year. They note that this rate of acceleration is 3 times greater than the acceleration of ice mass loss for mountain glaciers and ice caps. If the trend continues, the "ice sheets will be the dominant contributor to sea level rise in the 21st century," the authors report.

See related press release:

Source: Geophysical Research Letters, doi:10.1029/2011GL046583, 2011

Title: Acceleration of the contribution of the Greenland and Antarctic ice sheets to sea level rise

Authors: E. Rignot and I. Velicogna: Earth System Science, University of California, Irvine, California, USA; and Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA;

M. R. van den Broeke, J. T. M. Lenaerts: Institute for Marine and Atmospheric Research, Utrecht University, Utrecht, Netherlands;

A. Monaghan: National Center for Atmospheric Research, Boulder, Colorado, USA.


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