Eosbehavior of the heliospheric current sheet
during the winter of 1995, when NASA’s
Ulysses spacecraft traversed the heliosphere from the southern to northernhemispheres. The model’s high resolving
power allows the analysis of structuresthat span just 1° solar latitude by 1° solar
longitude. As a highlight of the model’s
power, the authors focus on the development and eventual destruction of a smallscale feature within the current sheet
called a fold, in which they suggest differences in the speed of the solar windcaused the sheet to buckle, pushing anisland of outward polarity into a sea of
inward polarity. Further, the authors say
the model saw hints of even smaller processes in and around the current sheet,
whose properties will be unlocked only
by further increases in model resolution.
(Geophysical Research Letters, doi:10.1029/​
2011GL047822, 2011) —CS

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Within the frosted peaks of the Swiss Alps,the cycle of winter precipitation accumulation and springtime melt provides a seasonal
flow of water to much of western Europe.
Research into the freshwater contributionsof Alpine glaciers has predominantly looked
to measure water released directly from glacier melt. A glacier’s overall water storage,
however, which takes into account the snowthat resides on the glacier’s surface, moreaccurately describes the role Swiss glaciers
play in feeding European streams. A new
analysis by Huss that investigated the effectsof changing Swiss glacier storage on theflows of four of Europe’s largest rivers suggests that glaciers may be more important
than previously realized.

Arctic air may become cleaner
as temperatures rise
The air in the Arctic is cleaner during
summer than during winter. Previous studies have shown that for light-scattering pollutants, this seasonal cycle is due mainly
to summer precipitation removing pollutants from the air during atmospherictransport from midlatitude industrial andagricultural sources.
With new measurements from Barrow,
Alaska, and Alert, Nunavut, Canada, Garrett et al. extended previous research toshow that light-absorbing aerosols such as
black carbon are also efficiently removed
by seasonal precipitation.
Precipitation removes these particles from the air most efficiently at highhumidities and relatively warm temperatures, suggesting that as the Arctic gets
warmer and wetter in the future, the airand snow might also become cleaner. If
Arctic aerosols have a net warming effect,
as is believed to be the case, precipitation removing these particles from the
air would represent a negative climate
feedback, mitigating anticipated Arcticwarming. (Geophysical Research Letters,
doi:10.1029/​2 011GL048221, 2011) —EB

Changes in glacier water storage for theRhône Glacier (pictured) provide an important source of freshwater for regions of Franceand Switzerland that lay along the Rhône
River. Similar processes affect the freshwater
availability for much of western Europe, acontribution that may be on the decline withglobal warming.
The author, drawing on glacier andstream gauge observations stretching backto 1908, found that the relative importanceof water derived from glaciers increases
throughout the summer, peaking inAugust when other sources evaporate. For
instance, during the 2003 European heatwave, the Danube drew 9% of its water fromglacier runoff despite the fact that only
0.06% of the river’s watershed is coveredby glaciers. Even during less extreme conditions, the Rhine, with 0.20% of its watershed being glacier covered, saw 6.6% of its
August flows over the past century being
composed of glacier runoff.
With climate projections estimating thatglacier coverage in the European Alps will
shrink to only 12% of its current value by
2100, the author suggests that the reduction may significantly affect Europe’s economy and agriculture. He notes that thecontributions of glaciers to Europe’s waterways are expected to decrease by 75%
by the end the century. (Water Resources
Research, doi:10.1029/​2 010WR010299, 2011)—CS

Examining nitrogen isotope ratios
as a paleoproxy
Lake Kivu, pictured here, contains large quantities of methane. Scientists are researching thesources and sinks of methane in the lake.

nitrogen isotope composition of diatoms.
Sedimentary nitrogen isotope ratios areoften used as a proxy for past conditions.
However, there are uncertainties associated with these nitrogen isotope proxies.
Bulk sediment nitrogen isotope ratios differ from fossil diatom-bound nitrogen isotope ratios. To learn more about these
uncertainties, Horn et al. measured nitrogen isotopes in five different species ofcultured diatoms. The results indicate that
there are significant interspecies variations in the offsets between isotope ratios
in the total biomass and isotope ratios inthe diatom-bound nitrogen. These differences should be considered when interpreting nitrogen isotope ratios. (Paleoceanography, doi:10.1029/​2 010PA002080,2011) —EB

The relative consumption of nutrients bydiatoms in the ocean, with regard to bothsupply and demand of nitrate, affects theSangeeta Sharma

Michael Schurter, Eawag

High-resolution model reproduces
heliospheric current sheet foldcorrelated with polynya extent than withsurface air temperature. (Journal of Geophysical Research-Oceans, doi:10.1029/​
2010JC006586, 2011) —EB

Matthias Huss

Africa’s Lake Kivu, which borders
Rwanda and the Republic of the Congo,
contains huge amounts of methane (anestimated 60 cubic kilometers) and carbon
dioxide (an estimated 300 cubic kilometers). Although the methane could provide
a source of energy if it could be tapped,
the high concentrations of gases pose arisk of dangerous gas eruptions, threatening the local population. Furthermore, it
has been reported that concentrations ofmethane have risen by as much as 15% in
the past 30 years. Scientists need to learn
more about the sources and sinks of methane in Lake Kivu to better assess the riskof catastrophic outgassing and sustainability of methane harvesting.
Pasche et al. studied present-day formation and oxidation of methane. By analyzing isotopes of carbon in samples fromthe lake, they determined that an unusually high proportion of methane (65%) in
the deep parts of the lake is derived fromgeologic sources or produced biologicallyfrom geogenic carbon dioxide entering thelake through subaquatic springs. A significant amount of methane also comes fromdegrading organic material. Accumulation of such material has been increasing
in recent decades as a result of such factors as the introduction of nonnative fish,
enhanced internal recycling of nutrients
due to increased discharges of subaquatic
springs, and increased levels of nutrients
caused by rising human populations. In
addition, researchers found that oxidationof methane by aerobic bacteria is themain process preventing methane fromescaping into the atmosphere, with anaerobic methane oxidation playing a minorrole. (Journal of Geophysical ResearchBiogeosciences, doi:10.1029/​2 011JG001690,2011) —EB

Mapping of annual cumulative sea ice production averaged over 1992–2007.

Shrinking Alpine glaciers
spell trouble for Europe’s rivers

Sources and sinks of methane
in Lake Kivu

Scanning electron microscope images oftropical foraminifera show the signs of posthumous alteration and the target site of a newtechnique that hopes to improve paleoclimate
ocean temperature reconstructions.

Spewing from the Sun, magnetic fieldlines are carried by the solar wind to theouter reaches of the solar system. Crossing over the extended reach of the Sun’sequator, these magnetic field lines turninward toward the Sun’s opposite hemisphere. Particularly during solar minimum,
the path traced by these magnetic fields
creates an equatorially partitioned solar
system, with distinct halves of inward andoutward polarity. The heliospheric current
sheet, a thin region of space with no magnetic polarity and a large electrical current,
divides the inward polarity and outward
polarity regions.
Investigations into the behavior of thecurrent sheet are hampered by its small
size, which, at roughly 10,000 kilometers
thick, is well below the resolving power ofmost computer models.
Using a modified version of the highresolution Lyon-Fedder-Mobarry magnetospheric model, Merkin et al. studied theIn Arctic coastal polynyas, persistentareas of thin ice, a large amount of newice forms during winters. Significant heatis lost through these regions of thin ice.
Knowing how much ice is produced inpolynyas is important for determining
overall cold saline water formation in
the Arctic Ocean. However, because of
the difficulty of making in situ measurements, there have been few studies of ice
production in coastal Arctic polynyas.
Tamura and Ohshima developed analgorithm for detecting polynyas andestimating sea ice thickness using datafrom satellite microwave sensors. They
used their method to create the first mapof sea ice production in coastal polynyas over the entire Arctic Ocean. They
also investigated the interannual variability of sea ice production in 10 Arctic
coastal polynyas from 1992 to 2007. They
found that sea ice production was better

Using a numerical model of the inner heliosphere, researchers track the development of aheliospheric current sheet fold as it propagates
outward from the Sun.The heliospheric current
sheet divides the (red) outward polarity and(blue) inward polarity regions of the Sun’smagnetic field.

Reinhard Kozdon

The “cool tropics paradox” has been athorn in the side of paleoclimate researchers, who draw on diverse techniques toimprove their estimates of ancient climates.
For the early Paleogene, between approximately 56 million and 49 million years
ago, atmospheric reconstructions hint ata world much warmer than present, yetestimates of tropical sea surface temperatures determined by looking at the shells offoramanifera­— a type of marine plankton—­skew low, falling below present values.
Throughout their lives, foramanifera form acalcium carbonate shell that records chemical parameters of the ambient seawater,
and analyzing the oxygen isotope ratios
in preserved shells is one of the dominant
methods of estimating ancient ocean temperatures. Recently, however, researchers
have found that posthumous alteration candisplace the original biological structures,changing the oxygen isotope ratio andpresenting a potential explanation for thesource of the cool tropics paradox.
To minimize the unwanted effects ofchemical alteration, Kozdon et al. used anion microprobe to analyze the oxygen isotope ratios of small pustular outgrowths
on chambers of foramanifera belonging
to the genus Morozovella, which research
has suggested are resistant to posthumousalteration. The ion microprobe detects
the isotopic composition of the shell over
10-micron increments while the classical technique consumes the whole shell,
including the chemically altered parts.
Analyzing shells from the early Paleogene, the authors found that the oxygenisotope ratios of the outgrowths were significantly lower than the whole-shell datareported in previous studies. The updated
isotope analysis represents a tropical sea
surface that is 4°–8°C warmer than thatinferred from previous studies, providing a potential solution to the cool tropics
paradox. (Paleoceanography, doi:10.1029/​
2010PA002056, 2011) —CS

First map of sea ice production in Arctic coastal polynyas

Viacheslav Merkin

Potential solution to the cool tropics
paradoxVOLUME 92 NUMBER 41 11 OCTOBER 2011

Measurements made at Alert, Canada (thelocation of this photo), along with data fromBarrow, Alaska, show that precipitation fromclouds cleans the Arctic by scavenging black
carbon from midlatitudes. A sooty haze layer
can be seen in this photo.

—Ernie Balcerak and Colin Schultz, StaffWriters