Publications (35)285.97 Total impact
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Article: Quasiresonant amplification of planetary waves and recent Northern Hemisphere weather extremes.
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ABSTRACT: In recent years, the Northern Hemisphere has suffered several devastating regional summer weather extremes, such as the European heat wave in 2003, the Russian heat wave and the Indus river flood in Pakistan in 2010, and the heat wave in the United States in 2011. Here, we propose a common mechanism for the generation of persistent longitudinal planetary-scale high-amplitude patterns of the atmospheric circulation in the Northern Hemisphere midlatitudes. Those patterns-with zonal wave numbers m = 6, 7, or 8-are characteristic of the above extremes. We show that these patterns might result from trapping within midlatitude waveguides of free synoptic waves with zonal wave numbers k ≈ m. Usually, the quasistationary dynamical response with the above wave numbers m to climatological mean thermal and orographic forcing is weak. Such midlatitude waveguides, however, may favor a strong magnification of that response through quasiresonance.Proceedings of the National Academy of Sciences 03/2013; · 9.68 Impact Factor -
Article: Global increase in record-breaking monthly-mean temperatures
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ABSTRACT: The last decade has produced record-breaking heat waves in many parts of the world. At the same time, it was globally the warmest since sufficient measurements started in the 19th century. Here we show that, worldwide, the number of local record-breaking monthly temperature extremes is now on average five times larger than expected in a climate with no long-term warming. This implies that on average there is an 80 % chance that a new monthly heat record is due to climatic change. Large regional differences exist in the number of observed records. Summertime records, which are associated with prolonged heat waves, increased by more than a factor of ten in some continental regions including parts of Europe, Africa, southern Asia and Amazonia. Overall, these high record numbers are quantitatively consistent with those expected for the observed climatic warming trend with added stationary white noise. In addition, we find that the observed records cluster both in space and in time. Strong El Niño years see additional records superimposed on the expected long-term rise. Under a medium global warming scenario, by the 2040s we predict the number of monthly heat records globally to be more than 12 times as high as in a climate with no long-term warming.Climatic Change 01/2013; · 3.38 Impact Factor -
Article: Expert judgements on the response of the Atlantic meridional overturning circulation to climate change
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ABSTRACT: We present results from detailed interviews with 12 leading climate scientists about the possible effects of global climate change on the Atlantic Meridional Overturning Circulation (AMOC). The elicitation sought to examine the range of opinions within the climatic research community about the physical processes that determine the current strength of the AMOC, its future evolution in a changing climate and the consequences of potential AMOC changes. Experts assign different relative importance to physical processes which determine the present-day strength of the AMOC as well as to forcing factors which determine its future evolution under climate change. Many processes and factors deemed important are assessed as poorly known and insufficiently represented in state-of-the-art climate models. All experts anticipate a weakening of the AMOC under scenarios of increase of greenhouse gas concentrations. Two experts expect a permanent collapse of the AMOC as the most likely response under a 4×CO2 scenario. Assuming a global mean temperature increase in the year 2100 of 4K, eight experts assess the probability of triggering an AMOC collapse as significantly different from zero, three of them as larger than 40%. Elicited consequences of AMOC reduction include strong changes in temperature, precipitation distribution and sea level in the North Atlantic area. It is expected that an appropriately designed research program, with emphasis on long-term observations and coupled climate modeling, would contribute to substantially reduce uncertainty about the future evolution of the AMOC.Climatic Change 04/2012; 82(3):235-265. · 3.38 Impact Factor -
Article: Climate sensitivity estimated from ensemble simulations of glacial climate
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ABSTRACT: The concentration of greenhouse gases (GHGs) in the atmosphere continues to rise, hence estimating the climate system’s sensitivity to changes in GHG concentration is of vital importance. Uncertainty in climate sensitivity is a main source of uncertainty in projections of future climate change. Here we present a new approach for constraining this key uncertainty by combining ensemble simulations of the last glacial maximum (LGM) with paleo-data. For this purpose we used a climate model of intermediate complexity to perform a large set of equilibrium runs for (1) pre-industrial boundary conditions, (2) doubled CO2 concentrations, and (3) a complete set of glacial forcings (including dust and vegetation changes). Using proxy-data from the LGM at low and high latitudes we constrain the set of realistic model versions and thus climate sensitivity. We show that irrespective of uncertainties in model parameters and feedback strengths, in our model a close link exists between the simulated warming due to a doubling of CO2, and the cooling obtained for the LGM. Our results agree with recent studies that annual mean data-constraints from present day climate prove to not rule out climate sensitivities above the widely assumed sensitivity range of 1.5–4.5°C (Houghton et al. 2001). Based on our inferred close relationship between past and future temperature evolution, our study suggests that paleo-climatic data can help to reduce uncertainty in future climate projections. Our inferred uncertainty range for climate sensitivity, constrained by paleo-data, is 1.2–4.3°C and thus almost identical to the IPCC estimate. When additionally accounting for potential structural uncertainties inferred from other models the upper limit increases by about 1°C.Climate Dynamics 04/2012; 27(2):149-163. · 4.60 Impact Factor -
Article: Tropical versus high latitude freshwater influence on the Atlantic circulation
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ABSTRACT: We investigate the model sensitivity of the Atlantic meridional overturning circulation (AMOC) to anomalous freshwater flux in the tropical and northern Atlantic. Forcing in both locations leads to the same qualitative response: a positive freshwater anomaly induces a weakening of the AMOC and a negative freshwater anomaly strengthens the AMOC. Strong differences arise in the temporal characteristics and amplitude of the response. The advection of the tropical anomaly up to the deep water formation area leads to a time delayed response compared to a northern forcing. Thus, in its transient response, the AMOC is less sensitive to a constant anomalous freshwater flux in the tropics than in the north. This difference decreases with time and practically vanishes in equilibrium with constant freshwater forcing. The equilibrium response of the AMOC shows a non-linear dependence on freshwater forcing in both locations, with a stronger sensitivity to positive freshwater forcing. As a consequence, competitive forcing in both regions is balanced when the negative forcing is about 1.5 times larger than the positive forcing. The relaxation time of the AMOC after termination of a freshwater perturbation depends significantly on the AMOC strength itself. A strong overturning exhibits a faster relaxation to its unperturbed state. By means of a set of complementary experiments (pulse-perturbations, constant and stochastic forcing) we quantify these effects and discuss the corresponding time scales and physical processes.Climate Dynamics 04/2012; 27(7):715-725. · 4.60 Impact Factor -
Article: A decade of weather extremes
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ABSTRACT: 1 F or the United States, 2011 was a year of extreme weather, with 14 events that caused losses in excess of US$1 billion each 1 . The US National Oceanic and Atmospheric Administration spoke of "a year seemingly full of weather extremes" after July had set new monthly heat records for Texas, Oklahoma and Delaware 2 . The period from January to October was the wettest on record for several northeastern states, with wet soils contributing to the severe flooding when Hurricane Irene hit the region in August. During spring, the southern United States had been hit by the worst recorded tornado outbreak in history: April saw 753 tornadoes, beating the previous monthly record of 542 (from May 2003) by a large margin 3 . Other regions in the world were affected by extreme weather in 2011 as well: rainfall records were set in Australia, Japan and Korea, whereas the Yangtze Basin in China experienced record drought 1 . In western Europe, spring was exceptionally hot and dry, setting records in several countries (Table1) 1 . But 2011 was not unique: the past decade as a whole has seen an exceptional number of unprecedented extreme weather events, some causing major human suffering and economic damage 4 (Table 1 and Fig. 1). In August 2010, the World Meteorological Organization issued a statement on the "unprecedented sequence of extreme weather events", stating that it "matches Intergovernmental Panel on Climate Change (IPCC) projections of more frequent and more intense extreme weather events due to global warming" 5 . The Moscow heatwave and Pakistan flooding that year illustrated how destructive extreme weather can be to societies: the death toll in Moscow has been estimated at 11,000 and drought caused grain-harvest losses of 30%, leading the Russian government to ban wheat exports. At the same time Pakistan was hit by the worst flooding in its history, which affected approximately one-fiffth of its total land area and 20million people 6 . The unprecedented meteorological events listed in Table 1 occurred in a decade that was likely the warmest globally for at least a millennium 7 . But are these two observations linked? We focus our discussion on the unprecedented extremes of the past decade, that is, those setting new meteorological records in the observational data available, because these often have the greatest impacts on societies, they grab the headlines and their uniqueness simplififes statistical analysis (compared with analysing extreme events exceeding a given threshold value). A much broader assessment of extreme events by the IPCC 8 was published in March 2012. Unlike our Perspective, this has an emphasis on fifxed-threshold extremes, model results and projections of the future, societal impacts and possible policy strategies to deal with extremes. Here, we ask the simpler question of whether the unprecedented extremes observed during the past decade are related to climatic warming. We start with some method-ological remarks before discussing specififc types of extreme. The ostensibly large number of recent extreme weather events has triggered intensive discussions, both in-and outside the scientific community, on whether they are related to global warming. Here, we review the evidence and argue that for some types of extreme — notably heatwaves, but also precipitation extremes — there is now strong evidence linking specific events or an increase in their numbers to the human influence on climate. For other types of extreme, such as storms, the available evidence is less conclusive, but based on observed trends and basic physical concepts it is nevertheless plausible to expect an increase.Nature Climate Change 01/2012; -
Article: Increase of extreme events in a warming world.
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ABSTRACT: We develop a theoretical approach to quantify the effect of long-term trends on the expected number of extremes in generic time series, using analytical solutions and Monte Carlo simulations. We apply our method to study the effect of warming trends on heat records. We find that the number of record-breaking events increases approximately in proportion to the ratio of warming trend to short-term standard deviation. Short-term variability thus decreases the number of heat extremes, whereas a climatic warming increases it. For extremes exceeding a predefined threshold, the dependence on the warming trend is highly nonlinear. We further find that the sum of warm plus cold extremes increases with any climate change, whether warming or cooling. We estimate that climatic warming has increased the number of new global-mean temperature records expected in the last decade from 0.1 to 2.8. For July temperature in Moscow, we estimate that the local warming trend has increased the number of records expected in the past decade fivefold, which implies an approximate 80% probability that the 2010 July heat record would not have occurred without climate warming.Proceedings of the National Academy of Sciences 11/2011; 108(44):17905-9. · 9.68 Impact Factor -
Article: Reply to Grinsted et al.: Estimating land subsidence in North Carolina
Proceedings of the National Academy of Sciences 10/2011; 108(40):E783-E783. · 9.68 Impact Factor -
Article: Climate related sea-level variations over the past two millennia.
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ABSTRACT: We present new sea-level reconstructions for the past 2100 y based on salt-marsh sedimentary sequences from the US Atlantic coast. The data from North Carolina reveal four phases of persistent sea-level change after correction for glacial isostatic adjustment. Sea level was stable from at least BC 100 until AD 950. Sea level then increased for 400 y at a rate of 0.6 mm/y, followed by a further period of stable, or slightly falling, sea level that persisted until the late 19th century. Since then, sea level has risen at an average rate of 2.1 mm/y, representing the steepest century-scale increase of the past two millennia. This rate was initiated between AD 1865 and 1892. Using an extended semiempirical modeling approach, we show that these sea-level changes are consistent with global temperature for at least the past millennium.Proceedings of the National Academy of Sciences 06/2011; 108(27):11017-22. · 9.68 Impact Factor -
Article: Reply to Taboada and Anadon: Critique of sea-level rise study invalid.
Proceedings of the National Academy of Sciences 07/2010; · 9.68 Impact Factor -
Article: Global sea level linked to global temperature.
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ABSTRACT: We propose a simple relationship linking global sea-level variations on time scales of decades to centuries to global mean temperature. This relationship is tested on synthetic data from a global climate model for the past millennium and the next century. When applied to observed data of sea level and temperature for 1880-2000, and taking into account known anthropogenic hydrologic contributions to sea level, the correlation is >0.99, explaining 98% of the variance. For future global temperature scenarios of the Intergovernmental Panel on Climate Change's Fourth Assessment Report, the relationship projects a sea-level rise ranging from 75 to 190 cm for the period 1990-2100.Proceedings of the National Academy of Sciences 12/2009; 106(51):21527-32. · 9.68 Impact Factor -
Article: On the stability of the Atlantic meridional overturning circulation.
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ABSTRACT: One of the most important large-scale ocean current systems for Earth's climate is the Atlantic meridional overturning circulation (AMOC). Here we review its stability properties and present new model simulations to study the AMOC's hysteresis response to freshwater perturbations. We employ seven different versions of an Ocean General Circulation Model by using a highly accurate tracer advection scheme, which minimizes the problem of numerical diffusion. We find that a characteristic freshwater hysteresis also exists in the predominantly wind-driven, low-diffusion limit of the AMOC. However, the shape of the hysteresis changes, indicating that a convective instability rather than the advective Stommel feedback plays a dominant role. We show that model errors in the mean climate can make the hysteresis disappear, and we investigate how model innovations over the past two decades, like new parameterizations and mixing schemes, affect the AMOC stability. Finally, we discuss evidence that current climate models systematically overestimate the stability of the AMOC.Proceedings of the National Academy of Sciences 11/2009; 106(49):20584-9. · 9.68 Impact Factor -
Article: Recent climate observations compared to projections.
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ABSTRACT: We present recent observed climate trends for carbon dioxide concentration, global mean air temperature, and global sea level, and we compare these trends to previous model projections as summarized in the 2001 assessment report of the Intergovernmental Panel on Climate Change (IPCC). The IPCC scenarios and projections start in the year 1990, which is also the base year of the Kyoto protocol, in which almost all industrialized nations accepted a binding commitment to reduce their greenhouse gas emissions. The data available for the period since 1990 raise concerns that the climate system, in particular sea level, may be responding more quickly to climate change than our current generation of models indicates.Science 06/2007; 316(5825):709. · 31.20 Impact Factor -
Article: A semi-empirical approach to projecting future sea-level rise.
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ABSTRACT: A semi-empirical relation is presented that connects global sea-level rise to global mean surface temperature. It is proposed that, for time scales relevant to anthropogenic warming, the rate of sea-level rise is roughly proportional to the magnitude of warming above the temperatures of the pre-Industrial Age. This holds to good approximation for temperature and sea-level changes during the 20th century, with a proportionality constant of 3.4 millimeters/year per degrees C. When applied to future warming scenarios of the Intergovernmental Panel on Climate Change, this relationship results in a projected sea-level rise in 2100 of 0.5 to 1.4 meters above the 1990 level.Science 02/2007; 315(5810):368-70. · 31.20 Impact Factor -
Article: How cold was the Last Glacial Maximum?
Geophysical Research Letters 01/2006; · 3.79 Impact Factor -
Article: Possible solar origin of the 1,470-year glacial climate cycle demonstrated in a coupled model.
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ABSTRACT: Many palaeoclimate records from the North Atlantic region show a pattern of rapid climate oscillations, the so-called Dansgaard-Oeschger events, with a quasi-periodicity of approximately 1,470 years for the late glacial period. Various hypotheses have been suggested to explain these rapid temperature shifts, including internal oscillations in the climate system and external forcing, possibly from the Sun. But whereas pronounced solar cycles of approximately 87 and approximately 210 years are well known, a approximately 1,470-year solar cycle has not been detected. Here we show that an intermediate-complexity climate model with glacial climate conditions simulates rapid climate shifts similar to the Dansgaard-Oeschger events with a spacing of 1,470 years when forced by periodic freshwater input into the North Atlantic Ocean in cycles of approximately 87 and approximately 210 years. We attribute the robust 1,470-year response time to the superposition of the two shorter cycles, together with strongly nonlinear dynamics and the long characteristic timescale of the thermohaline circulation. For Holocene conditions, similar events do not occur. We conclude that the glacial 1,470-year climate cycles could have been triggered by solar forcing despite the absence of a 1,470-year solar cycle.Nature 12/2005; 438(7065):208-11. · 36.28 Impact Factor -
Article: The earth system model of intermediate complexity CLIMBER-3α. Part I: description and performance for present-day conditions
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ABSTRACT: We herein present the CLIMBER-3α Earth System Model of Intermediate Complexity (EMIC), which has evolved from the CLIMBER-2 EMIC. The main difference with respect to CLIMBER-2 is its oceanic component, which has been replaced by a state-of-the-art ocean model, which includes an ocean general circulation model (GCM), a biogeochemistry module, and a state-of-the-art sea-ice model. Thus, CLIMBER-3α includes modules describing the atmosphere, land-surface scheme, terrestrial vegetation, ocean, sea ice, and ocean biogeochemistry. Owing to its relatively simple atmospheric component, it is approximately two orders of magnitude faster than coupled GCMs, allowing the performance of a much larger number of integrations and sensitivity studies as well as longer ones. At the same time its oceanic component confers on it a larger degree of realism compared to those EMICs which include simpler oceanic components. The coupling does not include heat or freshwater flux corrections. The comparison against the climatologies shows that CLIMBER-3α satisfactorily describes the large-scale characteristics of the atmosphere, ocean and sea ice on seasonal timescales. As a result of the tracer advection scheme employed, the ocean component satisfactorily simulates the large-scale oceanic circulation with very little numerical and explicit vertical diffusion. The model is thus suited for the study of the large-scale climate and large-scale ocean dynamics. We herein describe its performance for present-day boundary conditions. In a companion paper (Part II), the sensitivity of the model to variations in the external forcing, as well as the role of certain model parameterisations and internal parameters, will be analysed.Climate Dynamics 07/2005; 25(2):237-263. · 4.60 Impact Factor -
Article: Thermohaline Circulation Changes: A Question of Risk Assessment
Climatic Change 12/2004; 68(1):241-247. · 3.38 Impact Factor -
Article: Dynamic sea level changes following changes in the thermohaline circulation
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ABSTRACT: Using the coupled climate model CLIMBER-3a, we investigate changes in sea surface elevation due to a weakening of the thermohaline circulation (THC). In addition to a global sea level rise due to a warming of the deep sea, this leads to a regional dynamic sea level change which follows quasi-instantaneously any change in the ocean circulation. We show that the magnitude of this dynamic effect can locally reach up to ~1m, depending on the initial THC strength. In some regions the rate of change can be up to 20-25 mm/yr. The emerging patterns are discussed with respect to the oceanic circulation changes. Most prominent is a south-north gradient reflecting the changes in geostrophic surface currents. Our results suggest that an analysis of observed sea level change patterns could be useful for monitoring the THC strength. Comment: Climate Dynamics (2004), submitted. See also http://www.pik-potsdam.de/~anders08/2004; -
Article: A low-order model for the response of the Atlantic thermohaline circulation to climate change
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ABSTRACT: Concern has been expressed that anthropogenic climate change may lead to a slowdown or even collapse of the Atlantic thermohaline circulation (THC). Because of the possibly severe consequences that such an event could have on the northern North Atlantic and northwestern Europe, integrated assessment models (IAMs) are needed to explore the associated political and socioeconomic implications. State-of-the-art climate models representing the THC are, however, often too complex to be incorporated into an integrated assessment framework. In this paper we present a low-order model of the Atlantic THC which meets the main requirements of IAMs: it (1) is physically based, (2) is computationally highly efficient, (3) allows for comprehensive uncertainty analysis and (4) can be linked to globally aggregated climate models that are mostly used in IAMs. The model is an interhemispheric extension of the seminal Stommel model. Its parameters are determined by a least-squares fit to the output of a coupled climate model of intermediate complexity. Results of a number of transient global warming simulations indicate that the model is able to reproduce many features of the behaviour of coupled ocean–atmosphere circulation models such as the sensitivity of the THC to the amount, regional distribution and rate of climate change.Ocean Dynamics 01/2004; 54(1):8-26. · 1.77 Impact Factor
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Institutions
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1999–2012
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Potsdam-Institut für Klimafolgenforschung
Potsdam, Brandenburg, Germany
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2011
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University of Pennsylvania
- Department of Earth and Environmental Science
Philadelphia, PA, USA
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2001
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Universität Potsdam
- Institute of Physics and Astronomy
Potsdam, Brandenburg, Germany
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