Natural Science Papers by Martin Claussen
Das Ziel des 2. Hamburger Klimaberichtes (2HKB) ist es, das wissenschaftliche Wissen über den ver... more Das Ziel des 2. Hamburger Klimaberichtes (2HKB) ist es, das wissenschaftliche Wissen über den vergangenen, den derzeitigen und den zukünftig möglichen Klimawandel und seine Wirkung in der Metropolregion Hamburg und Norddeutschland zu dokumentieren und zusammenzufassen. 2011 erschien der erste „Klimabericht für die Metropolregion Hamburg“ (1HKB) erstellt von einer Autorengruppe des KlimaCampus Hamburg und externen Partnern. Dieses Projekt wurde von Hans von Storch initiiert, vom Norddeutschen Küsten- und Klimabüro (HZG) organisiert und im Rahmen des Klima-Exzellenzclusters CliSAP (Integrated Climate System Analysis and Prediction) der Universität Hamburg und ihren außeruniversitären Partnern gefördert. Der Bericht sollte das Wissen über den regionalen Klimawandel in der Metropolregion Hamburg, mögliche Klimafolgen in der Region und mögliches Management, wie es in wissenschaftlichen Publikationen belegt ist, sichten und im Hinblick auf Konsens und Dissens bewerten.
Vorbilder dieses Wissensberichtes waren auf globaler Ebene der IPCC-Bericht und auf regionaler Ebene der BACC-Report „BALTEX Assessment of Climate Change in the Baltic Sea Basin“, der als erster regionaler Wissensbericht 2008 veröffentlicht wurde. Der 1. HKB spiegelt den Stand des Wissens im Sommer 2009 wider, da für den Bericht Material gesichtet wurde, das vor dem 1. August 2009 veröffentlicht worden war. Etwa 3 Jahre nach Erscheinen dieses Klimaberichtes wurde von den Partnern des KlimaCampus Hamburg im Herbst 2014 beschlossen, einen zweiten Bericht zu erarbeiten. Dieser 2. Hamburger Klimabericht – Wissen über Klima, Klimawandel und Auswirkungen in Hamburg und Norddeutschland – liegt nun vor. Das Material des ersten Berichtes wurde kritisch vor dem Hintergrund neuer Erkenntnisse analysiert. Neue Themen, die im ersten Bericht aufgrund fehlender Forschung keine Berücksichtigung fanden, wurden aufgegriffen. Das vorliegende Buch dokumentiert den Forschungsstand bis Oktober 2015.
Papers by Martin Claussen
Contributions to atmospheric physics, 1982
Journal of Advances in Modeling Earth Systems, 2019
• The 100-member MPI-GE is currently the largest publicly available ensemble of a comprehensive c... more • The 100-member MPI-GE is currently the largest publicly available ensemble of a comprehensive climate model • MPI-GE currently has the most forcing scenarios of all large ensemble projects; RCP2.6, RCP4.5, RCP8.5, 1%CO 2 • The power of MPI-GE is to estimate the forced response and internal variability, including changing variability, to unprecedented precision
Biogeosciences, 2018
We present how variations in plant functional diversity affect climate-vegetation interaction tow... more We present how variations in plant functional diversity affect climate-vegetation interaction towards the end of the African Humid Period (AHP) in coupled landatmosphere simulations using the Max Planck Institute Earth system model (MPI-ESM). In experiments with AHP boundary conditions, the extent of the "green" Sahara varies considerably with changes in plant functional diversity. Differences in vegetation cover extent and plant functional type (PFT) composition translate into significantly different land surface parameters, water cycling, and surface energy budgets. These changes have not only regional consequences but considerably alter large-scale atmospheric circulation patterns and the position of the tropical rain belt. Towards the end of the AHP, simulations with the standard PFT set in MPI-ESM depict a gradual decrease of precipitation and vegetation cover over time, while simulations with modified PFT composition show either a sharp decline of both variables or an even slower retreat. Thus, not the quantitative but the qualitative PFT composition determines climate-vegetation interaction and the climate-vegetation system response to external forcing. The sensitivity of simulated system states to changes in PFT composition raises the question how realistically Earth system models can actually represent climatevegetation interaction, considering the poor representation of plant diversity in the current generation of land surface models.
Klimabericht für die Metropolregion Hamburg, 2010
Geophysical Research Letters, 2009
Seven climate models were used to explore the biogeophysical impacts of human-induced land cover ... more Seven climate models were used to explore the biogeophysical impacts of human-induced land cover change (LCC) at regional and global scales. The imposed LCC led to statistically significant decreases in the northern hemisphere summer latent heat flux in three models, and increases in three models. Five models simulated statistically significant cooling in summer in near-surface temperature over regions of LCC and one simulated warming. There were few significant changes in precipitation. Our results show no common remote impacts of LCC. The lack of consistency among the seven models was due to: 1) the implementation of LCC despite agreed maps of agricultural land, 2) the representation of crop phenology, 3) the parameterisation of albedo, and 4) the representation of evapotranspiration for different land cover types. This study highlights a dilemma: LCC is regionally significant, but it is not feasible to impose a common LCC across multiple models for the next IPCC assessment.
Climatic Change, 2003
The stability of the climate-vegetation system in the northern high latitudes is analysed with th... more The stability of the climate-vegetation system in the northern high latitudes is analysed with three climate system models of different complexity: A comprehensive 3-dimensional model of the climate system, GENESIS-IBIS, and two Earth system models of intermediate complexity (EMICs), CLIMBER-2 and MoBidiC. The biogeophysical feedback in the latitudinal belt 60-70 • N, although positive, is not strong enough to support multiple steady states: A unique equilibrium in the climate-vegetation system is simulated by all the models on a zonal scale for present-day climate and doubled CO 2 climate. EMIC simulations with decreased insolation also reveal a unique steady state. However, the climate sensitivity to tree cover, T F , exhibits non-linear behaviour within the models. For GENESIS-IBIS and CLIMBER-2, T F is lower for doubled CO 2 climate than for present-day climate due to a shorter snow season and increased relative significance of the hydrological effect of forest cover. For the EMICs, T F is higher for low tree fraction than for high tree fraction, mainly due to a time shift in spring snow melt in response to changes in tree cover. The climate sensitivity to tree cover is reduced when thermohaline circulation feedbacks are accounted for in the EMIC simulations. Simpler parameterizations of oceanic processes have opposite effects on T F : T F is lower in simulations with fixed SSTs and higher in simulations with mixed layer oceans. Experiments with transient CO 2 forcing show climate and vegetation not in equilibrium in the northern high latitudes at the end of the 20th century. The delayed response of vegetation and accelerated global warming lead to rather abrupt changes in northern vegetation cover in the first half of the 21st century, when vegetation cover changes at double the present day rate.
Climatic Change, 2004
The Earth's climate system is highly nonlinear: inputs and outputs are not proportional, change i... more The Earth's climate system is highly nonlinear: inputs and outputs are not proportional, change is often episodic and abrupt, rather than slow and gradual, and multiple equilibria are the norm. While this is widely accepted, there is a relatively poor understanding of the different types of nonlinearities, how they manifest under various conditions, and whether they reflect a climate system driven by astronomical forcings, by internal feedbacks, or by a combination of both. In this paper, after a brief tutorial on the basics of climate nonlinearity, we provide a number of illustrative examples and highlight key mechanisms that give rise to nonlinear behavior, address scale and methodological issues, suggest a robust alternative to prediction that is based on using integrated assessments within the framework of vulnerability studies and, lastly, recommend a number of research priorities and the establishment of education programs in Earth Systems Science. It is imperative that the Earth's climate system research community embraces this nonlinear paradigm if we are to move forward in the assessment of the human influence on climate.
Climatic Change, 2003
By using a climate system model of intermediate complexity, we have simulated longterm natural cl... more By using a climate system model of intermediate complexity, we have simulated longterm natural climate changes occurring over the last 9000 years. The paleo-simulations in which the model is driven by orbital forcing only, i.e., by changes in insolation caused by changes in the Earth's orbit, are compared with sensitivity simulations in which various scenarios of increasing atmospheric CO 2 concentration are prescribed. Focussing on climate and vegetation change in northern Africa, we recapture the strong greening of the Sahara in the early and mid-Holocene (some 9000-6000 years ago), and we show that some expansion of grassland into the Sahara is theoretically possible, if the atmospheric CO 2 concentration increases well above pre-industrial values and if vegetation growth is not disturbed. Depending on the rate of CO 2 increase, vegetation migration into the Sahara can be rapid, up to 1/10th of the Saharan area per decade, but could not exceed a coverage of 45%. In our model, vegetation expansion into today's Sahara is triggered by an increase in summer precipitation which is amplified by a positive feedback between vegetation and precipitation. This is valid for simulations with orbital forcing and greenhouse-gas forcing. However, we argue that the mid-Holocene climate optimum some 9000 to 6000 years ago with its marked reduction of deserts in northern Africa is not a direct analogue for future greenhouse-gas induced climate change, as previously hypothesized. Not only does the global pattern of climate change differ between the mid-Holocene model experiments and the greenhouse-gas sensitivity experiments, but the relative role of mechanisms which lead to a reduction of the Sahara also changes. Moreover, the amplitude of simulated vegetation cover changes in northern Africa is less than is estimated for mid-Holocene climate.
A detailed description of the fourth-generation ECHAM model is presented. Compared to the previou... more A detailed description of the fourth-generation ECHAM model is presented. Compared to the previous version, ECHAM-3, a number of substantial changes have been introduced in both the numerics and physics of the model. These include a semi-Lagrangian transport scheme for water vapour, cloud water and trace substances, a new radiation scheme (ECMWF) with modifications concerning the water vapour continuum, cloud optical properties and greenhouse gases, a new formulation of the vertical diffusion coefficients as functions of turbulent kinetic energy, and a new closure for deep convection based on convective instability instead of moisture convergence. Minor changes concern the parameterizations of horizontal diffusion, stratiform clouds and land surface processes. Also, a new dataset of land surface parameters have been compiled for the new model. The climatology of the model, derived from two extended AMIP simulations at T42L19 resolution, is documented and compared with ECMWF operational analyses. Some of the biases noted for the previous model version remain virtually unchanged. For example, the polar upper troposphere and lower stratosphere is much too cold, and the zonal wind errors become very large above the 200 hPa level. Furthermore, the low-frequency variability is still too small but the errors are reduced by about 50% compared to ECHAM-3. The tropospheric temperature and zonal wind errors are generally smaller than in the previous model, except for the tropics, where the overestimation of Walker-type circulations in the equatorial plane is even more pronounced in the new model and the simulation of the Indian summer monsoon is less realistic. The most substantial improvements, compared to ECHAM-3, are found for the land surface climate. The temperature and precipitation errors are generally smaller than before, and the biome distributions derived from these parameters are more realistic in ECHAM-4. These improvements can be attributed to an improved represention of surface radiation fluxes via larger absorption of solar radiation in the atmosphere due to both water vapour and clouds.
We calculate the radiative forcing (RF) from surface albedo changes over the last millennium appl... more We calculate the radiative forcing (RF) from surface albedo changes over the last millennium applying a recently published, population-based reconstruction of anthropogenic land cover change (ALCC). This study thus allows for the first time to assess anthropogenic effects on climate during the pre-industrial era at high spatial and temporal detail. We find that the RF is small throughout the pre-industrial period on the global scale (negative with a magnitude less than 0.05 W/m 2) and not strong enough to explain the cooling reconstructed from climate proxies between A.D. 1000 and 1900. For the regional scale, however, our results suggest an early anthropogenic impact on climate: Already in A.D. 800, the surface energy balance was altered by ALCC at a strength comparable to presentday greenhouse gas forcing, e.g., À2.0 W/m 2 are derived for parts of India for that time. Several other regions exhibit a distinct variability of RF as a result of major epidemics and warfare, with RF changes in the order of 0.1 W/m 2 within just one century.
Abstract. By using a climate system model of intermediate complexity, we have simulated longterm ... more Abstract. By using a climate system model of intermediate complexity, we have simulated longterm natural climate changes occurring over the last 9000 years. The paleo-simulations in which the model is driven by orbital forcing only, i.e., by changes in insolation caused by changes in the Earth's orbit, are compared with sensitivity simulations in which various scenarios of increasing atmospheric CO 2 concentration are prescribed. Focussing on climate and vegetation change in northern Africa, we recapture the strong greening of the Sahara in the early and mid-Holocene (some 9000-6000 years ago), and we show that some expansion of grassland into the Sahara is theoretically possible, if the atmospheric CO 2 concentration increases well above pre-industrial values and if vegetation growth is not disturbed. Depending on the rate of CO 2 increase, vegetation migration into the Sahara can be rapid, up to 1/10th of the Saharan area per decade, but could not exceed a coverage of 45%. In...
By coupling an atmospheric general circulation model asynchronously with an equilibrium vegetatio... more By coupling an atmospheric general circulation model asynchronously with an equilibrium vegetation model, manifold equilibrium solutions of the atmosphere^biosphere system have been explored. It is found that under present-day conditions of the Earth's orbital parameters and sea-surface temperatures, two stable equilibria of vegetation patterns are possible: one corresponding to present-day sparse vegeta-tion in the Sahel, the second solution yielding savannah which extends far into the south-western part of the Sahara. A similar picture is obtained for conditions during the last glacial maximum (21 000 years before present (BP)). For the mid-Holocene (6000 years BP), however, the model ¢nds only one solution: the green Sahara.We suggest that this intransitive behaviour of the atmosphere^biosphere is related to a westward shift of the Hadley^Walker circulation. A conceptual model of atmosphere^vegetation dynamics is used to interpret the bifurcation as well as its change in ter...
In this overview two definitions of climate are presented, from the meteorological point of view ... more In this overview two definitions of climate are presented, from the meteorological point of view and from the climate system’s point of view. The origin of climate change is discussed, i.e., externally forced variability and free, or internal variability that is caused without external trigger by internal instabilities of the system. Both, forced and free variability can appear as periodic, randomly quasi-periodic, and abrupt climate change. Finally, various possibilities of climate forecast are considered.
Nature
A global coupled ocean-atmosphere model of intermediate complexity is used to simulate the equili... more A global coupled ocean-atmosphere model of intermediate complexity is used to simulate the equilibrium climate of both today and the Last Glacial Maximum, around 21,000 years ago. The model successfully predicts the atmospheric and oceanic circulations, temperature distribution, hydrological cycle and sea-ice cover of both periods without using 'flux adjustments'. Changes in oceanic circulation, particularly in the Atlantic Ocean, play an important role in glacial cooling.
Earth System Dynamics Discussions
A major link between climate and humans in Northern Africa, and the Sahel in particular, is land ... more A major link between climate and humans in Northern Africa, and the Sahel in particular, is land use and associated land cover change, mainly where subsistence farming prevails. Here we assess possible feedbacks between the type of land use and harvest intensity and climate by analyzing a series of idealized GCM experiments using the MPI-ESM. The base line for these experiments is a simulation forced by the RCP8.5 scenario which includes strong greenhouse gas emissions and anthropogenic land cover changes. The anthropogenic land cover changes in the RCP8.5 scenario include a mixture of pasture and agriculture. In subsequent simulations, we replace the entire area affected by anthropogenic land cover change in the region between the Sahara in the North and the Guinean Coast in the South (4 to 20° N) by either pasture or agriculture, respectively. In a second setup we vary the amount of harvest in case of agriculture. The RCP8.5 base line simulation reveals strong changes in mean ...
Climate of the Past Discussions
The recently proposed global monsoon hypothesis interprets monsoon systems as part of one global-... more The recently proposed global monsoon hypothesis interprets monsoon systems as part of one global-scale atmospheric overturning circulation, implying a connection between the regional monsoon systems and an in-phase behaviour of all northern hemispheric monsoons on annual timescales (Trenberth et al., 2000). Whether this concept can be applied to past climates and variability on longer timescales is still under debate, because the monsoon systems exhibit different regional characteristics such as different seasonality (i.e. onset, peak, and withdrawal). To investigate the interconnection of different monsoon systems during the pre-industrial Holocene, five transient global climate model simulations have been analysed with respect to the rainfall trend and variability in different sub-domains of the Afro-Asian monsoon region. Our analysis suggests that on millennial timescales with varying orbital forcing, the monsoons do not behave as a tightly connected global system. According to t...
Earth System Dynamics Discussions
In Earth system model simulations we find different carbon cycle sensitivities for recent and gla... more In Earth system model simulations we find different carbon cycle sensitivities for recent and glacial climate. This result is obtained by comparing the transient response of the terrestrial carbon cycle to a fast and strong atmospheric CO<sub>2</sub> concentration increase (roughly 1000ppm) in C<sup>4</sup>MIP type simulations starting from climate conditions of the Last Glacial Maximum (<q>LGM</q>) and from Pre-Industrial times (<q>PI</q>). The sensitivity β to CO<sub>2</sub> fertilization is larger in the LGM experiment during most of the simulation time: The fertilization effect leads to a terrestrial carbon gain in the LGM experiment almost twice as large as in the PI experiment. The larger fertilization effect in the LGM experiment is caused by the stronger initial CO<sub>2</sub> limitation of photosynthesis, implying a stronger potential for its release upon CO<sub>2</sub> concentration increas...
Earth System Dynamics Discussions, 2012
Early warning signals (EWS) have become a popular statistical tool to infer stability properties ... more Early warning signals (EWS) have become a popular statistical tool to infer stability properties of the climate system. In Part 1 of this two-part paper we have presented a diagnostic method to find the hotspot of a sudden transition as opposed to regions that experience an externally induced tipping as a mere response. Here, we apply our method to the atmosphere-vegetation model PlanetSimulator (PlaSim)-VECODE using a regression model. For each of two vegetation collapses in PlaSim-VECODE, we identify a hotspot of one particular grid cell. We demonstrate with additional experiments that the detected hotspots are indeed a particularly sensitive region in the model and give a physical explanation for these results. The method can thus provide information on the causality of sudden transitions and may help to improve the knowledge on the vulnerability of certain subsystems in climate models.
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Natural Science Papers by Martin Claussen
Vorbilder dieses Wissensberichtes waren auf globaler Ebene der IPCC-Bericht und auf regionaler Ebene der BACC-Report „BALTEX Assessment of Climate Change in the Baltic Sea Basin“, der als erster regionaler Wissensbericht 2008 veröffentlicht wurde. Der 1. HKB spiegelt den Stand des Wissens im Sommer 2009 wider, da für den Bericht Material gesichtet wurde, das vor dem 1. August 2009 veröffentlicht worden war. Etwa 3 Jahre nach Erscheinen dieses Klimaberichtes wurde von den Partnern des KlimaCampus Hamburg im Herbst 2014 beschlossen, einen zweiten Bericht zu erarbeiten. Dieser 2. Hamburger Klimabericht – Wissen über Klima, Klimawandel und Auswirkungen in Hamburg und Norddeutschland – liegt nun vor. Das Material des ersten Berichtes wurde kritisch vor dem Hintergrund neuer Erkenntnisse analysiert. Neue Themen, die im ersten Bericht aufgrund fehlender Forschung keine Berücksichtigung fanden, wurden aufgegriffen. Das vorliegende Buch dokumentiert den Forschungsstand bis Oktober 2015.
Papers by Martin Claussen
Vorbilder dieses Wissensberichtes waren auf globaler Ebene der IPCC-Bericht und auf regionaler Ebene der BACC-Report „BALTEX Assessment of Climate Change in the Baltic Sea Basin“, der als erster regionaler Wissensbericht 2008 veröffentlicht wurde. Der 1. HKB spiegelt den Stand des Wissens im Sommer 2009 wider, da für den Bericht Material gesichtet wurde, das vor dem 1. August 2009 veröffentlicht worden war. Etwa 3 Jahre nach Erscheinen dieses Klimaberichtes wurde von den Partnern des KlimaCampus Hamburg im Herbst 2014 beschlossen, einen zweiten Bericht zu erarbeiten. Dieser 2. Hamburger Klimabericht – Wissen über Klima, Klimawandel und Auswirkungen in Hamburg und Norddeutschland – liegt nun vor. Das Material des ersten Berichtes wurde kritisch vor dem Hintergrund neuer Erkenntnisse analysiert. Neue Themen, die im ersten Bericht aufgrund fehlender Forschung keine Berücksichtigung fanden, wurden aufgegriffen. Das vorliegende Buch dokumentiert den Forschungsstand bis Oktober 2015.