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The future of ice sheets and sea ice: Between reversible retreat and unstoppable loss
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We discuss the existence of cryospheric “tipping points” in the Earth’s climate system. Such critical thresholds have been sug- gested to exist for the disappearance of Arctic sea ice and the retreat of ice sheets: Once these ice masses have shrunk below an anticipated critical extent, the ice–albedo feedback might lead to the irreversible and unstoppable loss of the remaining ice. We here give an overview of our current understanding of such thresh- old behavior. By using conceptual arguments, we review the recent findings that such a tipping point probably does not exist for the loss of Arctic summer sea ice. Hence, in a cooler climate, sea ice could recover rapidly from the loss it has experienced in recent years. In addition, we discuss why this recent rapid retreat of Arc- tic summer sea ice might largely be a consequence of a slow shift in ice-thickness distribution, which will lead to strongly increased year-to-year variability of the Arctic summer sea-ice extent. This variability will render seasonal forecasts of the Arctic summer sea- ice extent increasingly difficult. We also discuss why, in contrast to Arctic summer sea ice, a tipping point is more likely to exist for the loss of the Greenland ice sheet and the West Antarctic ice sheet.
Greenland | West Antarctic | climate change | tipping point | Arctic
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The Genetic Architecture of Maize Flowering Time
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Flowering time is a complex trait that controls adaptation of plants to their local environment in
the outcrossing species Zea mays (maize). We dissected variation for flowering time with a set of
5000 recombinant inbred lines (maize Nested Association Mapping population, NAM). Nearly a
million plants were assayed in eight environments but showed no evidence for any single largeeffect
quantitative trait loci (QTLs). Instead, we identified evidence for numerous small-effect QTLs
shared among families; however, allelic effects differ across founder lines. We identified no
individual QTLs at which allelic effects are determined by geographic origin or large effects for
epistasis or environmental interactions. Thus, a simple additive model accurately predicts flowering
time for maize, in contrast to the genetic architecture observed in the selfing plant species rice
and Arabidopsis.
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The Global Extent and Determinants of Savanna and Forest as Alternative Biome States
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Theoretically, fire–tree cover feedbacks can maintain savanna and forest as alternative stable states.
However, the global extent of fire-driven discontinuities in tree cover is unknown, especially accounting
for seasonality and soils. We use tree cover, climate, fire, and soils data sets to show that tree cover is
globally discontinuous. Climate influences tree cover globally but, at intermediate rainfall (1000 to
2500 millimeters) with mild seasonality (less than 7 months), tree cover is bimodal, and only fire
differentiates between savanna and forest. These may be alternative states over large areas, including
parts of Amazonia and the Congo. Changes in biome distributions, whether at the cost of savanna (due to
fragmentation) or forest (due to climate), will be neither smooth nor easily reversible.
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The Global Plight of Pollinators
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Wild pollinators are in decline, and managed
honeybees cannot compensate for their loss.
29 MARCH 2013 VOL 339 SCIENCE
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The Greenhouse Is Making the Water-Poor Even Poorer
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How bad will global warming get? The question
has long been cast in terms of how hot
the world will get. But perhaps more important
to the planet’s inhabitants will be how
much rising greenhouse gases crank up the
water cycle. Theory and models predict that
a strengthening greenhouse will increase
precipitation where it is already relatively
high—tropical rain forests, for example—
and decrease it where it is already low, as in
the subtropics.
SCIENCE VOL 336 27 APRIL 2012
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The Greening of Synfuels
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An old, dirty technology to make transportation fuels from coal could
fight global warming, say proponents. The trick is using more biomass
and burying the carbon dioxide that’s generated
18 APRIL 2008 VOL 320 SCIENCE
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The Historical Dynamics of Socio-ecological Traps
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Environmental degradation is a typical unintended
outcome of collective human behavior. Hardin’s
metaphor of the ‘‘tragedy of the commons’’ has become a
conceived wisdom that captures the social dynamics leading
to environmental degradation. Recently, ‘‘traps’’ has gained
currency as an alternative concept to explain the rigidity of
social and ecological processes that produce environmental
degradation and livelihood impoverishment. The trap metaphor
is, however, a great deal more complex compared to
Hardin’s insight. This paper takes stock of studies using the
trap metaphor. It argues that the concept includes time and
history in the analysis, but only as background conditions and
not as a factor of causality. From a historical–sociological
perspective this is remarkable since social–ecological traps
are clearly path-dependent processes, which are causally
produced through a conjunction of events. To prove this point
the paper conceptualizes social–ecological traps as a process
instead of a condition, and systematically compares history
and timing in one classic and three recent studies of social–
ecological traps. Based on this comparison it concludes that
conjunction of social and environmental events contributes
profoundly to the production of trap processes. The paper
further discusses the implications of this conclusion for policy
intervention and outlines how future research might generalize
insights from historical–sociological studies of traps.
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The Holocene`
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Combining nine tree growth proxies from four sites, from the west coast of Norway to the Kola Peninsula of NW Russia, provides a well replicated
(> 100 annual measurements per year) mean index of tree growth over the last 1200 years that represents the growth of much of the northern pine
timberline forests of northern Fennoscandia. The simple mean of the nine series, z-scored over their common period, correlates strongly with mean
June to August temperature averaged over this region (r = 0.81), allowing reconstructions of summer temperature based on regression and variance
scaling. The reconstructions correlate significantly with gridded summer temperatures across the whole of Fennoscandia, extending north across Svalbard
and south into Denmark. Uncertainty in the reconstructions is estimated by combining the uncertainty in mean tree growth with the uncertainty in
the regression models. Over the last seven centuries the uncertainty is < 4.5% higher than in the 20th century, and reaches a maximum of 12% above
recent levels during the 10th century. The results suggest that the 20th century was the warmest of the last 1200 years, but that it was not significantly
different from the 11th century. The coldest century was the 17th. The impact of volcanic eruptions is clear, and a delayed recovery from pairs or multiple
eruptions suggests the presence of some positive feedback mechanism. There is no clear and consistent link between northern Fennoscandian summer
temperatures and solar forcing.
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The Hot Summer of 2010: Redrawing the Temperature Record Map of Europe
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The summer of 2010 was exceptionally warm in eastern
Europe and large parts of Russia. We provide evidence
that the anomalous 2010 warmth that caused adverse
impacts exceeded the amplitude and spatial extent of the
previous hottest summer of 2003. 'Mega-heatwaves' such
as the 2003 and 2010 events broke the 500-yr long
seasonal temperature records over approximately 50% of
Europe. According to regional multi-model experiments,
the probability of a summer experiencing 'megaheatwaves'
will increase by a factor of 5 to 10 within the
next 40 years. However, the magnitude of the 2010 event
was so extreme that despite this increase, the occurrence
of an analogue over the same region remains fairly
unlikely until the second half of the 21st century.
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The Impact of Boreal Forest Fire on Climate Warming
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We report measurements and analysis of a boreal forest fire, integrating the effects of greenhouse gases, aerosols, black carbon deposition on snow and sea ice, and postfire changes in surface albedo. The net effect of all agents was to increase radiative forcing during the first year (34 ± 31 Watts per square meter of burned area), but to decrease radiative forcing when averaged over an 80-year fire cycle (−2.3 ± 2.2 Watts per square meter) because multidecadal increases in surface albedo had a larger impact than fire-emitted greenhouse gases. This result implies that future increases in boreal fire may not accelerate climate warming.
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