In this release:
Variability of North Atlantic heat transport observed from instrument data Methane exceeds nitrous oxide in rivers' contribution to warming Waste recycling primary source of energy in deep ocean Record Arctic ozone depletion could occur again Traveling supraglacial lakes observed on Antarctic ice shelf Lunar images alter understanding of impact history
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1. Variability of North Atlantic heat transport observed from instrument data
The Atlantic meridional overturning circulation (AMOC), which transports warm
water northward and cold water back southward, redistributes energy throughout
the North Atlantic Ocean. Some models predict that AMOC will slow down as
Earth's temperatures rise due to anthropogenic warming, which could have serious
climate consequences for the Northern Hemisphere. However, the response of
AMOC to global warming is uncertain-different models predict different rates of
slowdown-and there have been few continuous observations of AMOC heat
transport. Two research groups have analyzed almost 10 years of ocean instrument
data and compared it to the models' predictions.
Drawing on a decade of observations from the Meridional Overturning Variability
Experiment (MOVE), Send et al. report for the first time the detection of strong
interannual and decadal shifts in the strength of the AMOC. MOVE comprises
moored instruments at three sites spread east-west over a 1,000-kilometer (621-
mile) region near the Antilles Islands in the western tropical Atlantic that have
been collecting temperature, conductivity, and pressure measurements since 2000.
The information allowed the authors to calculate how the southbound stretch of the
AMOC varied from January 2000 to June 2009. They find that the cold-water flow
rate could vary substantially, ranging between 12 and 30 Sv (1 Sv = 1 million
cubic meters per second, or 35 million cubic feet per second) during the study
period. Further, they show that the AMOC's flow rate declined by 3 Sv over the
10-year period, corroborating recent modeling efforts. On the basis of the
intricacies of their observations, and on the agreement with circulation models, the
authors suggest that the decade-long drop in the strength of the AMOC is the
product of long-term natural variability and need not be a consequence of recent
climate change.
In a separate study, Hobbs and Willis used temperature, salinity, and displacement
data measured from floats in the Argo array, combined with sea surface heights
measured by satellites, to estimate a continuous time series of Atlantic meridional
heat transport from 2002 to 2010 at 41 degrees north latitude. They find the mean
heat transport is about 0.5 petawatts. The authors note that this estimate is
consistent with previous studies in similar latitudes based on atmospheric flux data
but is lower than most hydrographic estimates. Heat transport varies on an annual
cycle as well as on shorter time scales, with atmospheric variability explaining
most of the short-term variance. Hobbs and Willis note that the period of study was
too short to infer any long-term trends, and they emphasize the need for continued
monitoring of AMOC.
Send et al. Source:
Geophysical Research Letters, doi:10.1029/2011GL049801, 2011
http://dx.doi.org/10.1029/2011GL049801
Title:
Observation of decadal change in the Atlantic meridional overturning circulation
using 10 years of continuous transport data
Authors:
Uwe Send and Matthias Lankhorst: Scripps Institution of Oceanography, La Jolla,
California, USA;
Torsten Kanzow: Leibniz Institute of Marine Sciences, Kiel, Germany.
Hobbs and Willis Source:
Journal of Geophysical Research-Oceans, doi:10.1029/2011JC007039, 2012
http://dx.doi.org/10.1029/2011JC007039
Title:
Midlatitude North Atlantic heat transport: A time series based on satellite and
drifter data
Authors:
Will R. Hobbs and Joshua K. Willis: NASA Jet Propulsion Laboratory, California,
Institute of Technology, Pasadena, California, USA.
2. Methane exceeds nitrous oxide in rivers' contribution to warming
Nitrous oxide (N2O) emissions have been the leading area of concern for scientists
investigating the role of streams and rivers in global climate change for the past
decade. A potent greenhouse gas, nitrous oxide is produced in riverbed sediments
through nitrification and denitrification. Efforts to understand the rate at which
nitrous oxide diffuses through the water to the atmosphere have dominated the
field, yet diffusion is not the only relevant mechanism nor is nitrous oxide the only
relevant gas. Observations by Baulch et al. suggest that the global warming
potential of methane gas, which they measured bubbling up from several riverbeds,
exceeds that of nitrous oxide.
Gases produced in river sediments can travel to the atmosphere by diffusing
through the water column, escaping as bubbles, or through plant-facilitated
transport. The authors measured methane and nitrous oxide concentrations in the
water and in riverbed bubbles and measured bubble accumulation in surface
bubble traps for four Ontario streams to sort out whether diffusion or ebullition is
dominant for each gas. They find that 10 to 80 percent of methane emissions are in
the form of bubbles, while nitrous oxide emissions are almost completely through
diffusion.
Additionally, the authors used streambed sediment samples to identify a
relationship between gas emissions and sediment properties. They find that high
levels of fine materials such as silt or clay are associated with increased emissions
of both nitrous oxide and methane. They suspect the fine sediments could limit the
availability of oxygen in the sediment. Depleted oxygen levels increase rates of
denitrification and methanogenesis, thus increasing gas production rates. The
authors also find that methane bubbles surpass diffused nitrous oxide in terms of
global warming potential, which they suggest could warrant a rethinking of the
importance of streams and rivers to global warming.
Source:
Journal of Geophysical Research-Biogeosciences, doi:10.1029/2011JG001656,
2011
http://dx.doi.org/10.1029/2011JG001656
Title:
Diffusive and ebullitive transport of methane and nitrous oxide from streams: Are
bubble-mediated fluxes important?
Authors:
Helen M. Baulch: School of Environment and Sustainability and the Global
Institute for Water Security, National Hydrology Research Centre, University of
Saskatchewan, Saskatoon, Saskatchewan, Canada;
Peter J. Dillon: Department of Environmental and Resource Studies, Trent
University, Peterborough, Ontario, Canada;
Roxane Maranger: Department of Biological Sciences, University of Montreal,
Montreal, Quebec, Canada;
Sherry L. Schiff: Department of Earth and Environmental Sciences, University of
Waterloo, Waterloo, Ontario, Canada.
3. Waste recycling primary source of energy in deep ocean
In the dark reaches of the deep ocean, far from the photosynthesizing plants and
plankton that fuel life in the surface waters, ecosystems survive on chemical
energy. Decades of research on the life that clusters around deep-sea hydrothermal
vents has hinted at the importance of light-free food webs, but a recent analysis by
Middelburg suggests that another system-waste recycling-could be the
dominant pillar of life on the abyssal plains.
The realization was a result of the author's attempt to calculate the importance of
chemoautotrophy to the carbon cycles of different ocean regions.
Chemoautotrophs are single-celled creatures that consume carbon dioxide and
other inorganic materials and convert them to forms that can then be used by other
organisms-a process known as carbon fixation. The author specifically focused
on chemoautotrophs that feed on biological waste, finding it to be, for the deep
ocean, the largest source of new organic carbon.
However, the author's investigation was not limited to the deep ocean. He finds
that for the global ocean, chemoautotrophy of inorganic waste could account for
the production of 0.77 petagrams (Pg) of carbon per year. The bulk of the carbon
fixation, 0.29 Pg carbon per year , occurred in the surface waters. A further 0.29
Pg carbon per year was produced in coastal and continental shelf sediments, while
chemoautotrophy in the dark ocean fixed 0.11 Pg carbon per year. While the role
of chemoautotrophy in supplying the ocean with organic carbon was nowhere near
that of photosynthesis, it did rival the contributions of other important sources, like
carbon carried in river effluent.
Source:
Geophysical Research Letters, doi:10.1029/2011GL049725, 2011
http://dx.doi.org/10.1029/2011GL049725
Title:
Chemoautotrophy in the ocean
Author:
Jack J. Middelburg: Department of Earth Sciences, Faculty of Geosciences,
Utrecht University, Utrecht, Netherlands.
4. Record Arctic ozone depletion could occur again
In the winter of 2010-2011, ozone levels above the Arctic declined to record lows,
creating the first Arctic ozone hole, similar to the well-known Antarctic ozone
hole. Scientists believe the ozone depletion was due partly to unusually cold
temperatures in the stratosphere above the Arctic, as colder stratospheric
temperatures make ozone-destroying chemicals such as chlorine more active. As
global climate change continues, the Arctic stratosphere is expected to get colder,
but levels of ozone-destroying chemicals should decline, as emissions of these
chemicals were banned by the Montreal Protocol.
To try to learn more about Arctic ozone dynamics and determine whether the
Arctic ozone hole is likely to recur, Sinnhuber et al. looked at satellite observations
of temperature, ozone, water vapor, and chemicals that affect ozone in the Arctic
atmosphere. They also used a model to determine how sensitive ozone levels are to
stratospheric temperatures and chemistry. They find that their model accurately
reproduced measured conditions. Their model suggests that stratospheric
temperatures 1 degree Celsius (1.8 degrees Fahrenheit) lower than in the 2010-11
winter would result in locally nearly complete ozone depletion in the Arctic lower
stratosphere with current levels of chemicals. A 10 percent reduction in ozone-
depleting chemicals would be offset by a 1 degree Celsius decrease in stratospheric
temperatures. The researchers conclude that although ozone-depleting substances
should decline in coming decades, temperature changes could offset those effects,
potentially leading to future severe Arctic ozone depletions similar to that during
the winter of 2010-11.
Source:
Geophysical Research Letters, doi:10.1029/2011GL049784, 2011
http://dx.doi.org/10.1029/2011GL049784
Title:
Arctic winter 2010/2011 at the brink of an ozone hole
Authors:
B.-M. Sinnhuber, G. Stiller, R. Ruhnke, T. von Clarmann, and S. Kellmann:
Institute for Meteorology and Climate Research, Karlsruhe Institute of
Technology, Karlsruhe, Germany;
J. Aschmann: Institute of Environmental Physics, University of Bremen,
Bremen, Germany.
5. Traveling supraglacial lakes observed on Antarctic ice shelf
A sequence of lakes on top of the George VI ice shelf in Antarctica has been
observed to move along the boundary of the ice shelf with Alexander
Island. LaBarbera and MacAyeal analyzed satellite images of the George VI ice
shelf in Antarctica taken from 2001 through 2010. They find that the supraglacial
lakes move in a direction and with a speed that differs from ice shelf flow: parallel
to the grounding line of the ice shelf, in the manner of a traveling wave. The
authors present a simple model to show how the lakes form in the depressions
associated with compressions of the ice sheet. The study could help researchers
better understand the dynamics of the ice shelf.
Source:
Geophysical Research Letters, doi:10.1029/2011GL049970, 2011
http://dx.doi.org/10.1029/2011GL049970
Title:
Traveling supraglacial lakes on George VI Ice Shelf, Antarctica
Authors:
C. H. LaBarbera: Department of Geology, Cornell College, Mount Vernon, Iowa,
USA.
D. R. MacAyeal: Department of Geophysical Sciences, University of Chicago,
Chicago, Illinois, USA.
6. Lunar images alter understanding of impact history
New images from the Lunar Reconnaissance Orbiter could change our view of the
history of impacts on the Moon. In 1972 the Apollo 17 mission took samples from
the region of the Serenitatis impact basin. Scientists believed that the impact that
created the Serenitatis basin was responsible for the formation of the Apollo 17
impact melt samples, which were dated to 3.8 billion years ago. A geological
feature known as the Sculptured Hills, which surround the Apollo 17 landing site,
was also believed to have been created from ejecta from the Serenitatis impact.
The new images show that the Sculptured Hills lie on top of the rims of several
craters known to be created after the Serenitatis basin. In addition, the Sculptured
Hills have similar morphology to features created by ejecta from the impact that
formed the nearby Imbrium crater. Thus, Spudis et al. argue that the Sculptured
Hills, and possibly also the Apollo 17 samples, may have been formed from the
Imbrium impact, not the Serenitatis. The Serenitatis basin could be older than
scientists had thought, the authors suggest.
If the Apollo 17 samples are indeed formed from the Imbrium impact, not the
Serenitatis, scientists will have to revise their understanding of the impact melting
process of large body impacts because it was based in part on the assumption that
those samples were formed from the Serenitatis impact. Alternatively, if the
Apollo 17 samples are indeed from the Serenitatis basin-forming impact and the
Serenitatis basin is 3.8 billion years old, then many of the impact basins and large
craters on the Moon all must have been created within a short 30-million-year time
window around 3.8 billion years ago-definitely a global impact "cataclysm"-the
authors suggest. Either of these interpretations changes scientists' view of the
impact process and the history of the Moon.
Source:
Journal of Geophysical Research-Planets, doi:10.1029/2011JE003903, 2011
http://dx.doi.org/10.1029/2011JE003903
Title:
The Sculptured Hills of the Taurus Highlands: Implications for the relative age of
Serenitatis, basin chronologies and the cratering history of the Moon
Authors:
Paul D. Spudis: Lunar and Planetary Institute, Houston, Texas, USA;
Don E. Wilhelms: U. S. Geological Survey (retired), San Francisco, California,
USA;
Mark S. Robinson: ASU School of Earth and Space Exploration, Tempe, Arizona,
USA.
INFORMATION:
Contact:
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Phone (direct): +1 (202) 777 7530
Phone (toll free in North America): +1 (800) 966 2481 x530
Email: mcadams@agu.org
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