Showing papers by "Potsdam Institute for Climate Impact Research published in 2002"

Multifractal Detrended Fluctuation Analysis of Nonstationary Time Series

Abstract: We develop a method for the multifractal characterization of nonstationary time series, which is based on a generalization of the detrended fluctuation analysis (DFA). We relate our multifractal DFA method to the standard partition function-based multifractal formalism, and prove that both approaches are equivalent for stationary signals with compact support. By analyzing several examples we show that the new method can reliably determine the multifractal scaling behavior of time series. By comparing the multifractal DFA results for original series with those for shuffled series we can distinguish multifractality due to long-range correlations from multifractality due to a broad probability density function. We also compare our results with the wavelet transform modulus maxima method, and show that the results are equivalent.

Increasing river discharge to the Arctic Ocean

Abstract: Synthesis of river-monitoring data reveals that the average annual discharge of fresh water from the six largest Eurasian rivers to the Arctic Ocean increased by 7% from 1936 to 1999. The average annual rate of increase was 2.0 ± 0.7 cubic kilometers per year. Consequently, average annual discharge from the six rivers is now about 128 cubic kilometers per year greater than it was when routine measurements of discharge began. Discharge was correlated with changes in both the North Atlantic Oscillation and global mean surface air temperature. The observed large-scale change in freshwater flux has potentially important implications for ocean circulation and climate.

Ocean circulation and climate during the past 120,000 years

Abstract: Oceans cover more than two-thirds of our blue planet. The waters move in a global circulation system, driven by subtle density differences and transporting huge amounts of heat. Ocean circulation is thus an active and highly nonlinear player in the global climate game. Increasingly clear evidence implicates ocean circulation in abrupt and dramatic climate shifts, such as sudden temperature changes in Greenland on the order of 5-10 degrees C and massive surges of icebergs into the North Atlantic Ocean --events that have occurred repeatedly during the last glacial cycle.

Climatic control of the high-latitude vegetation greening trend and Pinatubo effect

Abstract: A biogeochemical model of vegetation using observed climate data predicts the high northern latitude greening trend over the past two decades observed by satellites and a marked setback in this trend after the Mount Pinatubo volcano eruption in 1991. The observed trend toward earlier spring budburst and increased maximum leaf area is produced by the model as a consequence of biogeochemical vegetation responses mainly to changes in temperature. The post-Pinatubo decline in vegetation in 1992-1993 is apparent as the effect of temporary cooling caused by the eruption. High-latitude CO(2) uptake during these years is predicted as a consequence of the differential response of heterotrophic respiration and net primary production.

Earth system models of intermediate complexity: closing the gap in the spectrum of climate system models

Abstract: We propose a new perspective on the hierarchy of climate models which goes beyond the ''classical'' climate modeling pyramid that is restricted mainly to atmospheric processes, Most notably, we introduce a new indicator, called ''integration'', which characterizes the number of interacting components of the climate system being explicitly described in a model. The location of several model types, from conceptual to comprehensive, is presented in a new spectrum of climate system models. In particular. the location of the Earth system Models of Intermediate Complexity (EMICs) in this spectrum is discussed in some detail and examples are given, which indicate that there is currently a broad range of EMICs in use. In some EMICs, the number of processes and/or the detail of description is reduced for the sake of simulating the feedbacks between as many components of the climate system as feasible. Others, with a lesser degree of interaction, or ''integration'', are used for long-term ensemble simulations to study specific aspects of climate variability. EMICs appear to be closer to comprehensive coupled models of atmospheric and oceanic circulation (CGCMs) than to ''conceptual'' or ''box'' models. We advocate that EMICs be considered as complementary to CGCMs and conceptual models, because we believe that there is an advantage of having a spectrum of climate system models which are designed to tackle specific aspects of climate and which together provide the proper tool for climate system modeling.

Effects of climate and land-use change on storm runoff generation: present knowledge and modelling capabilities

Abstract: There are several indications that changes in land cover have influenced the hydrological regime of various river basins. In addition, the effects of climate change on the hydrological cycle and on the runoff behaviour of river catchments have been discussed extensively in recent years. However, it is at present rather uncertain how, how much and at which spatial scale these environmental changes are likely to affect the generation of storm runoff, and consequently the flood discharges of rivers. Firstly, this paper gives an overview of the possible effects of climatic and land-use change on storm runoff generation. Secondly, it discusses models dealing with the hydrological response to climate and land-use variations, including both the downscaling of climate information from global circulation models and the way flood forecasting models represent land-use conditions. Finally, two modelling studies of meso-scale catchments in Germany are presented: the first shows the possible impacts of climate change on storm runoff production, and the second the impacts of land-use changes. Copyright  2002 John Wiley & Sons, Ltd.

Carbon cycle, vegetation, and climate dynamics in the Holocene: Experiments with the CLIMBER-2 model

Abstract: Holocene was accompanied by significant changes in vegetation cover and an increase inatmosphericCO2concentration.Theessentialquestioniswhetheritispossibletoexplain thesechangesinaconsistentway,accounting fortheorbitalparametersasthemainexternal forcing for the climate system. We investigate this problem using the computationally efficient model of climate system, CLIMBER-2, which includes models for oceanic and terrestrial biogeochemistry. We found that changes in climate and vegetation cover in the northern subtropical and circumpolar regions can be attributed to the changes in the orbital forcing. Explanation of the atmospheric CO2 record requires an additional assumption of excessive CaCO3sedimentation in the ocean. The modeled decrease in the carbonate ion concentration in the deep ocean is similar to that inferred from CaCO3 sediment data [Broecker et al., 1999]. For 8 kyr B.P., the model estimates the terrestrial carbon pool ca. 90 Pg higher than its preindustrial value. Simulated atmospheric d 13 C declines during the

Abrupt glacial climate changes due to stochastic resonance.

Abstract: Using an ocean-atmosphere climate model we demonstrate that stochastic resonance could be an important mechanism for millennial-scale climate variability during glacial times. We propose that the glacial ocean circulation, unlike today's, was an excitable system with a stable and a weakly unstable mode of operation, and that a combination of weak periodic forcing and plausible-amplitude stochastic fluctuations of the freshwater flux into the northern North Atlantic can produce glacial warm events similar in time evolution, amplitude, spatial pattern, and interspike intervals to those found in the observed climate records.

Simulating fire regimes in human‐dominated ecosystems: Iberian Peninsula case study

Abstract: A new fire model is proposed which estimates areas burnt on a macro-scale (10–100 km). It consists of three parts: evaluation of fire danger due to climatic conditions, estimation of the number of fires and the extent of the area burnt. The model can operate on three time steps, daily, monthly and yearly, and interacts with a Dynamic Global Vegetation Model (DGVM), thereby providing an important forcing for natural competition. Fire danger is related to number of dry days and amplitude of daily temperature during these days. The number of fires during fire days varies with human population density. Areas burnt are calculated based on average wind speed, available fuel and fire duration. The model has been incorporated into the Lund-Potsdam-Jena Dynamic Global Vegetation Model (LPJ-DGVM) and has been tested for peninsular Spain. LPJ-DGVM was modified to allow bi-directional feedback between fire disturbance and vegetation dynamics. The number of fires and areas burnt were simulated for the period 1974–94 and compared against observations. The model produced realistic results, which are well correlated, both spatially and temporally, with the fire statistics. Therefore, a relatively simple mechanistic fire model can be used to reproduce fire regime patterns in human- dominated ecosystems over a large region and a long time period.

Vegetation dynamics in central european forest ecosystems (near-natural as well as managed) after storm events

Abstract: All over the world forests and woodlands are damaged or reset to initial stages by fire, insect outbreaks or storms. In Central Europe storm events are the most important natural disturbances affecting stand structures of both natural and managed forests and yet only a few studies exist on long-term forest development following the destruction of the tree layer by a storm. This paper presents a permanent plot study established in 1988 in the Bavarian Forest National Park (SE Germany) on areas, where the tree layer had been destructed by a storm on August 1, 1983. The records concerning (1) floristic composition (spermatophytes, pteridophytes, bryophytes, lichens) and cover degree, (2) location and shape of each tree higher than 1 meter (height, diameter at breast height) including position of fallen trees and (3) number of seedlings and saplings were taken in 1988, 1993 and 1998. Two windfall areas, situated next to each other in the same broad and flat valley bottom on wet soils under local cold climate conditions (potential as well as recent vegetation:Calamagrostio villosae-Piceetum bazzanietosum) were analyzed, one of them with completely free development after the storm event (“untouched”), the other with dead wood cleared off after the event, but thereafter with free development (“cleared”). The vegetation analysis separated two major trends in vegetation dynamics: (1) On the cleared plots with intensive soil-surface disturbance (removal of the damaged wood) the species composition changed towards pioneer herb vegetation (Rubus sp.), and pioneer forest species (here: birch,Betula pendula and/orB. pubescens) established. Subsequently, vegetation dynamics leading towards clusters of forest ground-layer species composition took place. (2) In untouched stands, where soil-surface disturbances were restricted to pit-and-mound-system created by uprooted trees, the patchiness of forest vegetation increased and a regeneration of mainly terminal tree species (here: Norway spruce,Picea abies) started. Stand development for the next 100 years was simulated using the model FORSKA-M. The model is individual-based and includes competition for light, soil water, and nutrients. The simulations suggest that floristic structures of cleared and untouched plots, respectively, will remain different for several decades, but within one century, the floristic structure becomes rather similar. Major processes in forest ecosystems which can be used to improve forest management and nature conservation practices have been identified based on the results of the case study.

Large-scale instabilities of the Laurentide ice sheet simulated in a fully coupled climate-system model

Abstract: [1] Heinrich events, related to large-scale surges of the Laurentide ice sheet, represent one of the most dramatic types of abrupt climate change occurring during the last glacial. Here, using a coupled atmosphere-ocean-biosphereice sheet model, we simulate quasi-periodic large-scale surges from the Laurentide ice sheet. The average time between simulated events is about 7,000 yrs, while the surging phase of each event lasts only several hundred years, with a total ice volume discharge corresponding to 5–10 m of sea level rise. In our model the simulated ice surges represent internal oscillations of the ice sheet. At the same time, our results suggest the possibility of a synchronization between instabilities of different ice sheets, as indicated in paleoclimate records. INDEX TERMS: 1827 Hydrology: Glaciology (1863); 1620 Global Change: Climate dynamics (3309); 3344 Meteorology and Atmospheric Dynamics: Paleoclimatology; 5416 Planetology: Solid Surface Planets: Glaciation. Citation: Calov, R., A. Ganopolski, V. Petoukhov, M. Claussen, and R. Greve, Largescale instabilities of the Laurentide ice sheet simulated in a fully coupled climate-system model, Geophys. Res. Lett., 29(24), 2216, doi:10.1029/2002GL016078, 2002.

From Participation to Partnership in Biodiversity Protection: Experience from Germany and South Africa

Abstract: Losses in biodiversity are becoming alarming. To safeguard what is left means placing priority on the protection of species and habitats. Ideally, this should be a goal in its own right. In practice, people have to see a direct gain. However, participation is by no means guaranteed to ensure adequate safeguard. However, participation as a prelude to partnerships that bind government, agencies, and communities through coordinated funding is much more likely to be successful. In the Cape Floral Kingdom of South Africa, one of the world's 25 "hot-spots," public-private partnerships are regarded as vital. In the Uckermark Lakes Nature Park in northeast Germany the partnership approach is proving the only uncontested way to biodiversity management. This article discusses why partnerships are so essential for the future of biodiversity. It also assesses what difficulties remain in order to ensure cooperative linkages between community gain and ecological enhancement.

Impact of land use changes on water dynamics––a case study in temperate meso and macroscale river basins

Abstract: The paper illustrates impact studies of land use changes on the water balance in meso and macroscale river basins Problems of impact studies in large river basins result mainly from a huge heterogeneity inside and the relative rough input database Simple approaches are needed to setup possible land use changes on the basis of easily available spatial data The investigations are carried out for the river basins of the Saale (24 000 km 2 ) and the Havel (19 000 km 2 ), two main tributaries of the Elbe River The hydrological simulations were done with the modelling system ArcEGMO (Becker et al, in: Mathematical Models of Watershed Hydrology, Water Resources Publications, Littleton, CO, 2002)

Stochastic resonance in glacial climate

Abstract: It seems like magic. In an attempt to make it audible, you feed a faint signal, a wave of a particular frequency, through an amplifier—a strange black box that you bought at a junk yard, because your proposal to buy a proper one was turned down once again. Of course you hear nothing. The darn thing doesn't seem to work. Out of sheer destructive frustration, you add some random noise to your signal, and voila! Suddenly you hear it. The noise, rather than blurring your signal, makes it come through loud and clear. Take the noise away and the signal vanishes again.

Bulk contribution from isotropic media in surface sum-frequency generation

Abstract: We present systematic conceptual and experimental investigations of the bulk contribution to surface sum-frequency generation (SFG) from an isotropic medium. A practical method for the analysis of spectra obtained in a transmission geometry is presented which allows to estimate the bulk contribution in any future application. A systematic degeneracy due to the molecular nature of the material is found which makes a subtle interpretation of transmission experiments necessary. Practical consequences for the analysis of typical SFG spectra are discussed.

Climate-vegetation interaction

Abstract: The climate exerts the dominant control on the spatial distribution of the major vegetation types on a global scale. In turn, vegetation cover affects climate via alteration of the physical characteristics of the land surface like albedo, roughness, water conductivity (biogeophysical mechanisms) and atmospheric gas composition, for example, CO 2 and CH 4 (biogeochemical effects). The chapter covers biogeophysical interactions between the land surface and the atmosphere. Recent progress in vegetation and land surface modelling is briefly discussed. Research on climate-vegetation interaction is mostly concentrated on hot spots where the interaction is the most significant: boreal forests, North Africa, and Amazon forest. Boreal forests, even deciduous ones, significantly reduce the albedo of snow-covered surfaces. Simulations with different climate models reveal that positive feedback between forest and surface air temperature in the boreal region is not strong enough to establish multiple steady states. Nonetheless, the simulations show a significant cooling trend due to historical land cover changes, mainly as a result of temperate and boreal deforestation. In general, the climate models agree that tropical deforestation exerts a net regional warming while an effect on extratropical regions is more uncertain. In the Sahel/Sahara region, several models are able to simulate green Sahara phenomenon during the mid-Holocene. Some models reveal multiple steady states in the region due to a strong interaction between vegetation and monsoon precipitation. Sensitivity simulations show that some expansion of vegetation cover into the Sahara is possible under CO 2 -induced climate changes.

Relating MODIS-derived surface albedo to soils and rock types over Northern Africa and the Arabian peninsula

Abstract: [1] We use the MODerate resolution Imaging Spectroradiometer (MODIS) aboard the Terra spacecraft to derive surface albedo for the arid areas of Northern Africa and the Arabian peninsula. Albedo in seven MODIS spectral bands for land and three broad bands (for shortwave, near infrared, and visible portions of the spectrum) is produced. Surface albedo is derived from MODIS observations during a sixteen-day period and is analyzed at 1 km spatial resolution. MODIS data show considerable spatial variability of surface albedo in the study region that is related to soil and geological characteristics of the surface. For example, solar shortwave white-sky albedo varies by a factor of about 2.5 from the darkest volcanic terrains to the brightest sand sheets. Vegetation contribution to surface reflectance is essentially negligible since we only considered pixels with under 10 percent fractional canopy cover. Few, if any, coupled land-atmosphere global or regional models capture this observed spatial variability in surface reflectance or albedo. Here we suggest a scheme that relates soil groups (based on the United Nations Food and Agriculture Organization, FAO, soil classification) and rock types (based on the United States Geological Survey, USGS, geological maps) to MODIS derived surface albedo statistics. This approach is a first step towards the incorporation of the observed spatial variability in surface reflective properties into climate models.

Evaluation of terrestrial carbon cycle models with atmospheric CO2 measurements: Results from transient simulations considering increasing CO2, climate, and land-use effects

Abstract: An atmospheric transport model and observations of atmospheric CO2 are used to evaluate the performance of four Terrestrial Carbon Models (TCMs) in simulating the seasonal dynamics and interannual variability of atmospheric CO2 between 1980 and 1991. The TCMs were forced with time varying atmospheric CO2 concentrations, climate, and land use to simulate the net exchange of carbon between the terrestrial biosphere and the atmosphere. The monthly surface CO2 fluxes from the TCMs were used to drive the Model of Atmospheric Transport and Chemistry and the simulated seasonal cycles and concentration anomalies are compared with observations from several stations in the CMDL network. The TCMs underestimate the amplitude of the seasonal cycle and tend to simulate too early an uptake of CO2 during the spring by approximately one to two months. The model fluxes show an increase in amplitude as a result of land-use change, but that pattern is not so evident in the simulated atmospheric amplitudes, and the different models suggest different causes for the amplitude increase (i.e., CO2 fertilization, climate variability or land use change). The comparison of the modeled concentration anomalies with the observed anomalies indicates that either the TCMs underestimate interannual variability in the exchange of CO2 between the terrestrial biosphere and the atmosphere, or that either the variability in the ocean fluxes or the atmospheric transport may be key factors in the atmospheric interannual variability.

Evaluation of methods for the combination of phenological time series and outlier detection

Abstract: There are several applications of combined phenological time series; e.g., trend analysis with long continuous time series, obtaining a compound and representative time series around weather stations for model fitting, data gap filling and outlier detection. Various methods to combine phenological time series have been proposed. We show that all of these methods can be analyzed within the theory of linear models. This has the advantage that the underlying assumptions for each model become transparent providing a theoretical basis for selecting a model for a particular situation. Moreover, the common theoretical background provides a means of comparing methods by Monte-Carlo simulation and with real data. Additionally, we explored the influences of two outlier detection methods. We show that the error called the month-mistake, whose origin is known and which is one of the few mistakes that can be detected in phenological data because of its large deviation, is best detected by the distribution-free 30-day residual rule in combination with a robust estimation procedure based on the minimization of the sum of absolute residuals (L1-norm).

Coping with variability and change: Floods and droughts

Abstract: Floods and droughts are natural phenomena for which the risks of occurrence are likely to continue to grow. Increasing levels of exposure and insufficient adaptive capacity are among the factors responsible for the rising vulnerability. The former is conditioned by anthropopressure (e.g., economic development of flood-prone areas) and adverse effects of climate change; scenarios for future climates indicate the possibility of amplified water-related extremes. This article presents the current situation of coping with extreme hydrological events within the pressure-state-response framework. Among promising response strategies, the role of forecast and warning, and of watershed management are reviewed. Sample success stories and lessons learnt related to hydrological extremes are given and policy implications discussed.