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search.filters.applied.f.access: openAccess × End date: 2012 × Start date: 2012 × DDC: 500 ×

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Now showing 1 - 5 of 5
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    On the relation between Meridional Overturning Circulation and sea-level gradients in the Atlantic
    (München : European Geopyhsical Union, 2012) Kienert, H.; Rahmstorf, S.
    On the basis of model simulations, we examine what information on changes in the strength of the Atlantic Meridional Overturning Circulation (AMOC) can be extracted from associated changes in sea surface height (SSH), specifically from a broad Atlantic north–south gradient as has been suggested previously in the literature. Since a relation between AMOC and SSH changes can only be used as an AMOC diagnostic if it is valid independently of the specific forcing, we consider three different forcing types: increase of CO2 concentration, freshwater fluxes to the northern convection sites and the modification of Southern Ocean winds. We concentrate on a timescale of 100 yr. We find approximately linear and numerically similar relations between a sea-level difference within the Atlantic and the AMOC for freshwater as well as wind forcing. However, the relation is more complex in response to atmospheric CO2 increase, which precludes this sea-level difference as an AMOC diagnostic under climate change. Finally, we show qualitatively to what extent changes in SSH and AMOC strength, which are caused by simultaneous application of different forcings, correspond to the sum of the changes due to the individual forcings, a potential prerequisite for more complex SSH-based AMOC diagnostics
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    Extreme fire events are related to previous-year surface moisture conditions in permafrost-underlain larch forests of Siberia
    (Bristol : IOP Publishing, 2012) Forkel, Matthias; Thonicke, Kirsten; Beer, Christian; Cramer, Wolfgang; Bartalev, Sergey; Schmullius, Christiane
    Wildfires are a natural and important element in the functioning of boreal forests. However, in some years, fires with extreme spread and severity occur. Such severe fires can degrade the forest, affect human values, emit huge amounts of carbon and aerosols and alter the land surface albedo. Usually, wind, slope and dry air conditions have been recognized as factors determining fire spread. Here we identify surface moisture as an additional important driving factor for the evolution of extreme fire events in the Baikal region. An area of 127 000 km2 burned in this region in 2003, a large part of it in regions underlain by permafrost. Analyses of satellite data for 2002–2009 indicate that previous-summer surface moisture is a better predictor for burned area than precipitation anomalies or fire weather indices for larch forests with continuous permafrost. Our analysis advances the understanding of complex interactions between the atmosphere, vegetation and soil, and how coupled mechanisms can lead to extreme events. These findings emphasize the importance of a mechanistic coupling of soil thermodynamics, hydrology, vegetation functioning, and fire activity in Earth system models for projecting climate change impacts over the next century.
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    Comparing climate projections to observations up to 2011
    (Bristol : IOP Publishing, 2012) Rahmstorf, Stefan; Foster, Grant; Cazenave, Anny
    We analyse global temperature and sea-level data for the past few decades and compare them to projections published in the third and fourth assessment reports of the Intergovernmental Panel on Climate Change (IPCC). The results show that global temperature continues to increase in good agreement with the best estimates of the IPCC, especially if we account for the effects of short-term variability due to the El Niño/Southern Oscillation, volcanic activity and solar variability. The rate of sea-level rise of the past few decades, on the other hand, is greater than projected by the IPCC models. This suggests that IPCC sea-level projections for the future may also be biased low.
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    Land management and ecosystem services. How collaborative research programmes can support better policies
    (München : ÖKOM-Verlag, 2012) Eppink, Florian V.; Werntze, Andreas; Mäs, Stephan; Popp, Alexander; Seppelt, Ralf
    Land management, the organisation of the use and development of land, is an important instrument for addressing problems of rising greenhouse gas emissions and loss of natural resources. Yet, natural-social systems in which land management policies are implemented are poorly understood, thus decreasing the effectiveness of these policies. Local studies provide valuable insights, though only for the local conditions prevalent during the investigated period. Synthesising local studies in order to generalise results is impaired by the variety of local conditions. Collaborative research programmes may prevent some of these problems. They support the share of insights across temporal, ecological and spatial-economic contexts. On the basis of existing literature, we identify the challenges which face synthesis and demonstrate how a German research programme attempts to address some of them.
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    A statistically predictive model for future monsoon failure in India
    (Bristol : IOP Publishing, 2012) Schewe, Jacob; Levermann, Anders
    Indian monsoon rainfall is vital for a large share of the world's population. Both reliably projecting India's future precipitation and unraveling abrupt cessations of monsoon rainfall found in paleorecords require improved understanding of its stability properties. While details of monsoon circulations and the associated rainfall are complex, full-season failure is dominated by large-scale positive feedbacks within the region. Here we find that in a comprehensive climate model, monsoon failure is possible but very rare under pre-industrial conditions, while under future warming it becomes much more frequent. We identify the fundamental intraseasonal feedbacks that are responsible for monsoon failure in the climate model, relate these to observational data, and build a statistically predictive model for such failure. This model provides a simple dynamical explanation for future changes in the frequency distribution of seasonal mean all-Indian rainfall. Forced only by global mean temperature and the strength of the Pacific Walker circulation in spring, it reproduces the trend as well as the multidecadal variability in the mean and skewness of the distribution, as found in the climate model. The approach offers an alternative perspective on large-scale monsoon variability as the result of internal instabilities modulated by pre-seasonal ambient climate conditions.