Showing papers by "Potsdam Institute for Climate Impact Research published in 2003"

Evaluation of ecosystem dynamics, plant geography and terrestrial carbon cycling in the LPJ dynamic global vegetation model

Abstract: The Lund-Potsdam-Jena Dynamic Global Vegetation Model (LPJ) combines process-based, large-scale representations of terrestrial vegetation dynamics and land-atmosphere carbon and water exchanges in a modular framework. Features include feedback through canopy conductance between photosynthesis and transpiration and interactive coupling between these 'fast' processes and other ecosystem processes including resource competition, tissue turnover, population dynamics, soil organic matter and litter dynamics and fire disturbance. Ten plants functional types (PFTs) are differentiated by physiological, morphological, phenological, bioclimatic and fire-response attributes. Resource competition and differential responses to fire between PFTs influence their relative fractional cover from year to year. Photosynthesis, evapotranspiration and soil water dynamics are modelled on a daily time step, while vegetation structure and PFT population densities are updated annually. Simulations have been made over the industrial period both for specific sites where field measurements were available for model evaluation, and globally on a 0.5degrees x 0.5degrees grid. Modelled vegetation patterns are consistent with observations, including remotely sensed vegetation structure and phenology. Seasonal cycles of net ecosystem exchange and soil moisture compare well with local measurements. Global carbon exchange fields used as input to an atmospheric tracer transport model (TM2) provided a good fit to observed seasonal cycles of CO2 concentration at all latitudes. Simulated inter-annual variability of the global terrestrial carbon balance is in phase with and comparable in amplitude to observed variability in the growth rate of atmospheric CO2 . Global terrestrial carbon and water cycle parameters (pool sizes and fluxes) lie within their accepted ranges. The model is being used to study past, present and future terrestrial ecosystem dynamics, biochemical and biophysical interactions between ecosystems and the atmosphere, and as a component of coupled Earth system models.

Climate change and Arctic ecosystems: 2. Modeling, paleodata‐model comparisons, and future projections

Abstract: Large variations in the composition, structure, and function of Arctic ecosystems are determined by climatic gradients, especially of growing-season warmth, soil moisture, and snow cover. A unified circumpolar classification recognizing five types of tundra was developed. The geographic distributions of vegetation types north of 55degreesN, including the position of the forest limit and the distributions of the tundra types, could be predicted from climatology using a small set of plant functional types embedded in the biogeochemistry-biogeography model BIOME4. Several palaeoclimate simulations for the last glacial maximum (LGM) and mid-Holocene were used to explore the possibility of simulating past vegetation patterns, which are independently known based on pollen data. The broad outlines of observed changes in vegetation were captured. LGM simulations showed the major reduction of forest, the great extension of graminoid and forb tundra, and the restriction of low- and high-shrub tundra (although not all models produced sufficiently dry conditions to mimic the full observed change). Mid-Holocene simulations reproduced the contrast between northward forest extension in western and central Siberia and stability of the forest limit in Beringia. Projection of the effect of a continued exponential increase in atmospheric CO2 concentration, based on a transient ocean-atmosphere simulation including sulfate aerosol effects, suggests a potential for larger changes in Arctic ecosystems during the 21st century than have occurred between mid-Holocene and present. Simulated physiological effects of the CO2 increase (to >700 ppm) at high latitudes were slight compared with the effects of the change in climate.

A dynamic global vegetation model for use with climate models: concepts and description of simulated vegetation dynamics

Abstract: Changes in vegetation structure and biogeography due to climate change feedback to alter climate by changing fluxes of energy, moisture, and momentum between land and atmosphere. While the current class of land process models used with climate models parameterizes these fluxes in detail, these models prescribe surface vegetation and leaf area from data sets. In this paper, we describe an approach in which ecological concepts from a global vegetation dynamics model are added to the land component of a climate model to grow plants interactively. The vegetation dynamics model is the Lund–Potsdam–Jena (LPJ) dynamic global vegetation model. The land model is the National Center for Atmospheric Research (NCAR) Land Surface Model (LSM). Vegetation is defined in terms of plant functional types. Each plant functional type is represented by an individual plant with the average biomass, crown area, height, and stem diameter (trees only) of its population, by the number of individuals in the population, and by the fractional cover in the grid cell. Three time-scales (minutes, days, and years) govern the processes. Energy fluxes, the hydrologic cycle, and carbon assimilation, core processes in LSM, occur at a 20 min time step. Instantaneous net assimilated carbon is accumulated annually to update vegetation once a year. This is carried out with the addition of establishment, resource competition, growth, mortality, and fire parameterizations from LPJ. The leaf area index is updated daily based on prevailing environmental conditions, but the maximum value depends on the annual vegetation dynamics. The coupling approach is successful. The model simulates global biogeography, net primary production, and dynamics of tundra, boreal forest, northern hardwood forest, tropical rainforest, and savanna ecosystems, which are consistent with observations. This suggests that the model can be used with a climate model to study biogeophysical feedbacks in the climate system related to vegetation dynamics.

Evaluation of the agreement between the first global remotely sensed soil moisture data with model and precipitation data

Abstract: [1] The lack of global soil moisture data has spurred research in the field of microwave remote sensing. Both passive (radiometers) and active (scatterometer) microwave data are very sensitive to the moisture content of the surface soil layer. To retrieve soil moisture, the effects of vegetation, surface roughness, and heterogeneous land cover must be taken into account. Field experiments have shown that passive microwave data at long wavelengths (L-band) are best suited for soil moisture retrieval. Nevertheless, the first global, multiannual soil moisture data set (1992–2000) has been derived from active microwave data acquired by the European Remote Sensing Satellites (ERS) ERS-1 and ERS-2 scatterometer (C-band). The retrieval algorithm is based on a change detection approach that naturally accounts for surface roughness and heterogeneous land cover. In this paper the scatterometer-derived soil moisture data are compared to gridded precipitation data and soil moisture modeled by a global vegetation and water balance model. The correlation between soil moisture and rainfall anomalies is observed to be best over areas with a dense rainfall gauge network. Also, the scatterometer-derived and modeled soil moisture agree reasonably well over tropical and temperate climates. The fact that the algorithm performs equally well for regions with summer rain and Mediterranean areas indicates that dynamic vegetation effects are correctly represented in the retrieval. More research is needed to better understand the backscattering behavior over dry (steppe, deserts) and cold (boreal zone, tundra) climatic regions. The scatterometer-derived soil moisture data are available to other research groups at http://www.ipf.tuwien.ac.at/radar/ers-scat/home.htm.

Climate change and Arctic ecosystems: 1. Vegetation changes north of 55°N between the last glacial maximum, mid‐Holocene, and present

Abstract: A unified scheme to assign pollen samples to vegetation types was used to reconstruct vegetation patterns north of 55°N at the last glacial maximum (LGM) and mid-Holocene (6000 years B.P.). The pollen data set assembled for this purpose represents a comprehensive compilation based on the work of many projects and research groups. Five tundra types (cushion forb tundra, graminoid and forb tundra, prostrate dwarf-shrub tundra, erect dwarf-shrub tundra, and low- and high-shrub tundra) were distinguished and mapped on the basis of modern pollen surface samples. The tundra-forest boundary and the distributions of boreal and temperate forest types today were realistically reconstructed. During the mid-Holocene the tundra-forest boundary was north of its present position in some regions, but the pattern of this shift was strongly asymmetrical around the pole, with the largest northward shift in central Siberia (∼200 km), little change in Beringia, and a southward shift in Keewatin and Labrador (∼200 km). Low- and high-shrub tundra extended farther north than today. At the LGM, forests were absent from high latitudes. Graminoid and forb tundra abutted on temperate steppe in northwestern Eurasia while prostrate dwarf-shrub, erect dwarf-shrub, and graminoid and forb tundra formed a mosaic in Beringia. Graminoid and forb tundra is restricted today and does not form a large continuous biome, but the pollen data show that it was far more extensive at the LGM, while low- and high-shrub tundra were greatly reduced, illustrating the potential for climate change to dramatically alter the relative areas occupied by different vegetation types.

Timing of abrupt climate change: A precise clock

Abstract: [1] Many paleoclimatic data reveal a ∼1,500 year cyclicity of unknown origin. A crucial question is how stable and regular this cycle is. An analysis of the GISP2 ice core record from Greenland reveals that abrupt climate events appear to be paced by a 1,470-year cycle with a period that is probably stable to within a few percent; with 95% confidence the period is maintained to better than 12% over at least 23 cycles. This highly precise clock points to an origin outside the Earth system; oscillatory modes within the Earth system can be expected to be far more irregular in period.

Consistency of MODIS surface bidirectional reflectance distribution function and albedo retrievals: 2. Validation

Abstract: [1] The evaluation of the first available satellite-based global albedo product at 1-km resolution is essential for its application in climate studies. We evaluate the accuracy of the Moderate-Resolution Imaging Spectroradiometer (MODIS) albedo product using available field measurements at Surface Radiation Budget Network (SURFRAD) and Cloud and Radiation Testbed–Southern Great Plains (CART/SGP) stations and examine the consistency between the MODIS surface albedos and the Clouds and Earth's Radiant Energy System (CERES) top-of-the-atmosphere albedos as well as historical global albedos from advanced very high resolution radiometer (AVHRR) and Earth Radiation Budget Experiment (ERBE) observations. A comparison with the field measurements shows that the MODIS surface albedo generally meets an absolute accuracy requirement of 0.02 for our study sites during April–September 2001, with the root mean square errors less than 0.018. Larger differences appear in the winter season probably due to the increased heterogeneity of surface reflectivity in the presence of snow. To examine the effect of spatial heterogeneity on the validation of the MODIS albedos using fine resolution field measurements, we derive an intermediate albedo product from four Landsat Enhanced Thematic Mapper Plus (ETM+) images at 30-m spatial resolution as a surrogate for the distributed field measurements. The surface albedo is relatively homogeneous over the study stations in growing seasons, and therefore the validation during April–September is supported. A case study over three SURFRAD stations reveals that the MODIS bidirectional reflectance distribution function model is able to capture the solar zenith angle dependence of surface albedo as shown by the field measurements. We also find that the MODIS surface shortwave albedo is consistent with the contemporary and collocated CERES top-of atmosphere albedos derived directly from broadband observations. The MODIS albedo is also well correlated with historical surface albedos derived from AVHRR and ERBE observations, and a high bias of 0.016 and a low bias of 0.034 compared to those of the latter albedos are reasonable considering the differences in instruments and retrieval algorithms as well as environmental changes.

Power-law persistence and trends in the atmosphere: A detailed study of long temperature records

Abstract: We use several variants of the detrended fluctuation analysis to study the appearance of long-term persistence in temperature records, obtained at 95 stations all over the globe. Our results basically confirm earlier studies. We find that the persistence, characterized by the correlation C(s) of temperature variations separated by s days, decays for large s as a power law, C(s);s 2g . For continental stations, including stations along the coastlines, we find thatg is always close to 0.7. For stations on islands, we find that g ranges between 0.3 and 0.7, with a maximum at g50.4. This is consistent with earlier studies of the persistence in sea surface temperature records where g is close to 0.4. In all cases, the exponent g does not depend on the distance of the stations to the continental coastlines. By varying the degree of detrending in the fluctuation analysis we obtain also information about trends in the temperature records.

Multifractality of river runoff and precipitation: Comparison of fluctuation analysis and wavelet methods

Abstract: We study the multifractal temporal scaling properties of river discharge and precipitation records. We compare the results for the multifractal detrended fluctuation analysis method with the results for the wavelet-transform modulus maxima technique and obtain agreement within the error margins. In contrast to previous studies, we find non-universal behaviour: on long time scales, above a crossover time scale of several weeks, the runoff records are described by fluctuation exponents varying from river to river in a wide range. Similar variations are observed for the precipitation records which exhibit weaker, but still significant multifractality. For all runoff records the type of multifractality is consistent with a modified version of the binomial multifractal model, while several precipitation records seem to require different models.

Carbon isotope fractionation during dark respiration and photorespiration in C3 plants

Abstract: Carbon isotope discrimination during photosynthetic CO2 assimilation has been extensively studied and rigorous models have been developed, while the fractionations during photorespiratory and dark respiratory processes have been less well investigated. Whilst models of discrimination have included specific factors for fractionation during respiration (e) and photorespiration (f), these effects have been considered to be very small, i.e. not significantly modifying the net discrimination expressed in organic material. On this paper we consider the fractionation effects associated with specific reactions set against the overall discrimination which occurs during source-product transformations. We review the studies which have recently shown that discrimination occurs during respiration at night in intact C3 leaves, leading to the production of CO2 enriched in 13C (i.e., e = −6‰ ), and modifying the signature of the remaining plant material. Under photorespiratory conditions (i.e. increased oxygen concentration and high temperature), the photorespiratory fractionation factor may be high (with f around +10‰ ), and significantly alters the observed net photosynthetic discrimination measured during gas exchange. Fractionation factors for both respiration and photorespiration have been shown to be variable among species and with environmental conditions, and we suggest that the term `apparent fractionation' be used to describe the net effect for each process. In this paper we review the fractionations during photorespiration and dark respiration and the metabolic origin of the CO2 released during these processes, and we discuss the ecological implications of such fractionations.

Physiology-based phenology models for forest tree species in Germany.

Abstract: Models of phenology are needed for the projection of effects of a changing climate on, for example, forest production, species competition, vegetation-atmosphere feedback and public health. A new phenology model for deciduous tree bud burst is developed and parameters are determined for a wide geographical range (Germany) and several forest tree species. The new model is based on considerations of simple interactions between inhibitory and promotory agents that are assumed to control the developmental status of a plant. Several alternative model structures were formulated emphasizing different hypothetical physiological processes. The new models fitted the observations better than classical models. The bias of the classical models, i.e. overestimation of early observations and underestimation of late observations, could be reduced but not completely removed. Differences in the best-fitting model equations for each species indicated that, for the late spring phases (bud burst of Fagus sylvatica and Quercus robur), the photoperiod played a more dominant role than for early spring phases (bud burst of Betula pendula and Aesculus hippocastanum). Chilling only plays a subordinate role for spring bud burst compared to temperatures preceding this event in our data. The presented modeling approach allowed for a species-specific weighting of the dominant processes. The model results are in accordance with experimental findings that indicate an important role of day length in late spring BB. Potentials for model improvement are discussed.

The potential role of spectral properties in detecting thresholds in the Earth system: application to the thermohaline circulation

Abstract: A simple two-box model of the hemispheric thermohaline circulation (THC) is considered. The model parameterizes fluctuations in the freshwater forcing by a stochastic process. The dependence of the power spectral density and the lifetime of quasistationary states of the THC on the distance to the bifurcation point, where the THC collapses, is calculated analytically. It is shown that power spectral properties change as the system is moved closer to the bifurcation point. These changes allow an estimate of the distance to the bifurcation point.

Using Specific Language to Describe Risk and Probability

Abstract: Good assessment of environmental issues, such as climate change, requires effective communication of the degree of uncertainty associated with numerous possible outcomes. One strategy that accomplishes this, while responding to people's difficulty understanding numeric probability estimates, is the use of specific language to describe probability ranges. This is the strategy adopted by the Intergovernmental Panel on Climate Change in their Third Assessment Report. There is a problem with this strategy, however, in that it uses words differently from the way lay readers of the assessment typically do. An experiment conducted with undergraduate science students confirms this. The IPCC strategy could result in miscommunication, leading readers to under-estimate the probability of high-magnitude possible outcomes.

Systematic Multiscale Models for the Tropics

Abstract: Systematic multi-scale perturbation theory is utilized to develop self-consistent simplified model equations for the interaction across multiple spatial and/or temporal scales in the tropics. One of these models involves simplified equations for intraseasonal planetary equatorial synoptic scale dynamics (IPESD). This model includes the self-consistent quasi-linear interaction of synoptic scale generalized steady Matsuno-Webster-Gill models with planetary scale dynamics of equatorial long waves. These new models have the potential for providing self-consistent prognostic and diagnostic models for the intraseasonal tropical oscillation. Other applications of the systematic approach reveal three different balanced weak temperature gradient (WTG) approximations for the tropics with different regimes of validity in space and time: a synoptic equatorial scale WTG (SEWTG), a mesoscale equatorial WTG (MEWTG) which reduces to the classical models treated by others, and a new seasonal planetary equatorial WTG (SPEWTG). Both the SPEWTG and MEWTG model equations have solutions with general vertical structure yet have the linearized dispersion relation of barotropic Rossby waves; thus, these models can play an important role in theories for midlatitude connections with the tropics. The models are derived both from the equatorial shallow water equations in a simplified context and also as distinguished limits from the compressible primitive equations in general.

Consistency of MODIS surface bidirectional reflectance distribution function and albedo retrievals: 1. Algorithm performance

Abstract: [1] The first consistent year (November 2000 to November 2001) of global albedo product was produced at 1-km resolution every 16 days from the observations of the Moderate-Resolution Imaging Spectroradiometer (MODIS) instrument aboard NASA's Terra spacecraft. We evaluated the quality of the operational albedo retrievals in two ways: (1) by examining the algorithm performance using the product quality assurance (QA) fields (this paper) and (2) by comparing retrieved albedos with those observed at ground stations and by other satellite instruments (in a companion paper). The internal diagnostics of the retrieval algorithm adequately reflect the goodness of the model fit and the random noise amplification in the retrieved albedo. Global QA statistics show that the RossThick-LiSparse-Reciprocal model fits the atmospherically corrected surface reflectances very well, and the random noise amplification factors for white sky albedo and reflectance are generally less than 1.0. Cloud obscuration is the main reason for the activation of the backup magnitude retrieval algorithm. Over the 60°S to 60°N latitude band, 50% of the land pixels acquire more than six clear looks during 14–29 September 2001, and only 5% of these pixels are inverted with the backup algorithm. The latitude dependence and temporal distribution of the QA fields further demonstrate that the retrieval status mainly follows the pattern of angular sampling determined by cloud climatology and the instrument/orbit characteristics. A case study over the west coast of the United States shows that white sky shortwave albedos retrieved from magnitude inversions agree on average with those from full inversions to within 0.033 in reflectance units and have a slightly lower bias ranging from 0.014 to 0.023. We also explored the effect of residual cloud and aerosol contamination in the atmospherically corrected surface reflectance inputs in another case study over southern Africa. The quality assurance procedure of the operational MODIS bidirectional reflectance distribution function and albedo algorithm compensates for some of these residual effects and improves the albedo retrieval results by an order of 0.005 (10%) in the visible for more than 12% of pixels.

Assessing climate forcings of the Earth system for the past millennium

Abstract: [1] The effects of natural and anthropogenic forcings (solar activity, volcanism, atmospheric CO2 concentration, deforestation) on climate changes are estimated with the Earth system model of intermediate complexity, CLIMBER-2, for the past millennium. Simulated surface air temperatures for the Northern Hemisphere from the combined forcing correlate reasonably well with paleoclimatic data (r = 0.70). The largest negative anomalies occur when insolation minima coincide with volcanic eruptions. Anthropogenic forcings impose additional climate changes after 1850. The increasing warming from increasing CO2 concentrations is attenuated by the cooling effect from deforestation. Results from differently combined forcings suggest that the relatively cool climate in the second half of 19th century is largely attributable to cooling from deforestation.

Simulating past and future dynamics of natural ecosystems in the United States

Abstract: [1] Simulations of potential vegetation distribution, natural fire frequency, carbon pools, and fluxes are presented for two DGVMs (Dynamic Global Vegetation Models) from the second phase of the Vegetation/Ecosystem Modeling and Analysis Project. Results link vegetation dynamics to biogeochemical cycling for the conterminous United States. Two climate change scenarios were used: a moderately warm scenario from the Hadley Climate Centre and a warmer scenario from the Canadian Climate Center. Both include sulfate aerosols and assume a gradual CO2 increase. Both DGVMs simulate a reduction of southwestern desert areas, a westward expansion of eastern deciduous forests, and the expansion of forests in the western part of the Pacific Northwest and in north-central California. Both DGVMs predict an increase in total biomass burnt in the next century, with a more pronounced increase under the Canadian scenario. Under the Hadley scenario, both DGVMs simulate increases in total carbon stocks. Under the Canadian scenario, both DGVMs simulate a decrease in live vegetation carbon. We identify similarities in model behavior due to the climate forcing and explain differences by the different structure of the models and their different sensitivity to CO2. We compare model output with data to enhance our confidence in their ability to simulate potential vegetation distribution and ecosystem processes. We compare changes in the area of drought-induced decreases in vegetation density with a spatial index derived from the Palmer Drought Severity Index to illustrate the ability of the vegetation to cope with water limitations in the future and the role of the CO2 fertilization effect.

Interannual covariability in Northern Hemisphere air temperatures and greenness associated with El Niño‐Southern Oscillation and the Arctic Oscillation

Abstract: [1] In this paper, we estimate the year-to-year variations in northern vegetation greenness as they relate to the dominant modes of climate variability. In particular, we analyze spatial data of Northern Hemisphere satellite-sensed vegetation greenness, surface temperature, precipitation, and upper air data for the 1982–1998 period to isolate well correlated modes of variability between temperature and greenness and to assess their relationship to large-scale circulation anomalies. It is found that during spring, large-scale modes of interannual vegetation variability are strongly correlated with spatiotemporal modes of variability in the overlying temperature field. In addition, the results indicate that the two predominant hemispheric-scale modes of covariability are related to teleconnections associated with the El Nino-Southern Oscillation (ENSO) and the Arctic Oscillation (AO). The warm event ENSO signal is manifested as warmer and greener conditions in North America, Far East Asia, and to some extent central Europe, while the features of the positive phase AO signal include enhanced warm and green conditions over large regions in Europe and Asian Russia, with opposite anomalies in the eastern half of North America. Whether observed trends in vegetation activity over the past 20 years are also related to systematic changes in these two modes of variability is still unclear.

Modeling agriculture and land use in an integrated assessment framework

Abstract: The Agriculture and Land Use (AgLU) model is a top-downeconomic model with just enough structure to simulate globalland-use change and the resulting carbon emissions over one century.These simulations are done with and without a carbon policy representedby a positive carbon price. Increases in the carbon price createincentives for production of commercial biomass that affect thedistribution of other land types and, therefore, carbon emissionsfrom land-use change. Commercial biomass provides a link betweenthe agricultural and energy systems. The Integrated Assessmentof Climate Protection Strategies (ICLIPS) core model uses AgLUto provide estimates of carbon emissions from land-use changeas one component of total greenhouse gas emissions. Each majorland-use type is assigned an average carbon density used to calculatea total carbon stock; carbon emissions from land-use change arecalculated as the change in carbon stock between time periods.Significant carbon emissions from land-use change are presenteven in the reference scenario. An aggressive ICLIPS mitigationscenario results in carbon emissions from land-use change upto 800 million metric tons per year above the AgLU referencescenario.

Climate change in northern africa: the past is not the future

Abstract: By using a climate system model of intermediate complexity, we have simulated long- term 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 CO2 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 CO2 concentration increases well above pre-industrial values and if vegetation growth is not disturbed. Depending on the rate of CO2 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.

Thermohaline circulation: The current climate.

Abstract: Heat and freshwater fluxes at the ocean's surface play a key role in forming ocean currents, which in turn have a major effect on climate.

C and N mineralisation in the decomposer food webs of a European forest transect

Abstract: Schro¨ter, D., Wolters, V. and De Ruiter, P. C. 2003. C and N mineralisation in thedecomposer food webs of a European forest transect. – Oikos 102: 294–308.Belowground processes are essential for the overall carbon and nitrogen fluxes inforests. Neither the functioning of the soil food web mediating these fluxes, nor itsmodulation by environmental factors is sufficiently understood. In this study thebelowground carbon and nitrogen mineralisation of four European coniferous forestsites (northern Sweden to north-east France) with different climate and N deposi-tional inputs was analysed by investigating the soil food webs using field observationsand modelling. The soil fauna directly contributed 7–13% to C mineralisation,among which the testate amoebae (Protozoa) made the largest contribution. Micro-bial grazing was suggested to have an important indirect effect by stimulatingbacterial turnover. Due to relatively high C:N ratios of their substrate, bacteriaimmobilized N, while the fauna i.e. testate amoebae, nematodes, microarthropodsand enchytraeids, counteracted this N immobilisation.Despite similar food web biomass, the sites differed with respect to food webstructure and C and N flows. Model calculations suggested a significant influence offood web structure on soil ecosystem processes in addition to environmental factorsand resource quality. Mineralisation rates were lowest at the low N input boreal sitewith a food web dominated by fungal pathways. Further south, as N availabilityincreased, bacterial pathways became more important and the cycling of C and Nwas faster. The bioavailability of degradable C sources is suggested to be a limitingfactor for microbial activity and overall mineralisation rates. In this respect, above-and belowground interactions e.g. transfers of labile C sources from the vegetation tothe decomposer system deserve further attention.Our study revealed the combined effects of climate and nutrient inputs to ecosys-tems and the subsequent changes in the structure and functioning of the systems. Ifdecomposition, and therefore carbon loss, is stimulated as a consequence of struc-tural and/or nutritional changes, resulting for example from continuous industrial Nemission, the storage capacity of forest ecosystems could be altered.

Constraining temperature variations over the last millennium by comparing simulated and observed atmospheric CO2

Abstract: The response of atmospheric CO2 and climate to the reconstructed variability in solar irradiance and radiative forcing by volcanoes over the last millennium is examined by applying a coupled physical–biogeochemical climate model that includes the Lund-Potsdam-Jena dynamic global vegetation model (LPJ-DGVM) and a simplified analogue of a coupled atmosphere–ocean general circulation model. The modeled variations of atmospheric CO2 and Northern Hemisphere (NH) mean surface temperature are compatible with reconstructions from different Antarctic ice cores and temperature proxy data. Simulations where the magnitude of solar irradiance changes is increased yield a mismatch between model results and CO2 data, providing evidence for modest changes in solar irradiance and global mean temperatures over the past millennium and arguing against a significant amplification of the response of global or hemispheric annual mean temperature to solar forcing. Linear regression (r = 0.97) between modeled changes in atmospheric CO2 and NH mean surface temperature yields a CO2 increase of about 12 ppm for a temperature increase of 1 °C and associated precipitation and cloud cover changes. Then, the CO2 data range of 12 ppm implies that multi-decadal NH temperature changes between 1100 and 1700 AD had to be within 1 °C. Modeled preindustrial variations in atmospheric δ13C are small compared to the uncertainties in ice core δ13C data. Simulations with natural forcings only suggest that atmospheric CO2 would have remained around the preindustrial concentration of 280 ppm without anthropogenic emissions. Sensitivity experiments show that atmospheric CO2 closely follows decadal-mean temperature changes when changes in ocean circulation and ocean-sediment interactions are not important. The response in terrestrial carbon storage to factorial changes in temperature, the seasonality of temperature, precipitation, and atmospheric CO2 has been determined.

Analyzing the Ecosystem Carbon Dynamics of Four European Coniferous Forests Using a Biogeochemistry Model

Abstract: This paper provides the first steps toward a regional-scale analysis of carbon (C) budgets. We explore the ability of the ecosystem model BIOME-BGC to estimate the daily and annual C dynamics of four European coniferous forests and shifts in these dynamics in response to changing environmental conditions. We estimate uncertainties in the model results that arise from incomplete knowledge of site management history (for example, successional stage of forest). These uncertainties are especially relevant in regional-scale simulations, because this type of information is difficult to obtain. Although the model predicted daily C and water fluxes reasonably well at all sites, it seemed to have a better predictive capacity for the photosynthesis-related processes than for respiration. Leaf area index (LAI) was modeled accurately at two sites but overestimated at two others (as a result of poor long-term climate drivers and uncertainties in model parameterization). The overestimation of LAI (and consequently gross photosynthetic production (GPP)), in combination with reasonable estimates of the daily net ecosystem productivity (NEP) of those forests, also illustrates the problem with modeled respiration. The model results suggest that all four European forests have been net sinks of C at the rate of 100 ‐300 gC/m 2 /y and that this C sequestration capacity would be 30%‐70% lower without increasing nitrogen (N) deposition and carbon dioxide (CO 2 ) concentrations. The magnitude of the forest responses was dependent not only on the rate of changes in environmental factors, but also on sitespecific conditions such as climate and soil depth. We estimated that the modeled C exchange at the study sites was reduced by 50%‐100% when model simulations were performed for climax forests rather than regrowing forests. The estimates of water fluxes were less sensitive to different initializations of state variables or environmental change scenarios than C fluxes.

Flood Risk in Central Europe

Abstract: Several extreme floods have occurred in Central European rivers (including the Rhine, Danube, Odra, and Wisla), culminating in the disastrous August 2002 flood in the Elbe River basin and parts of the Danube basin. Rough cost estimates for the Elbe 2002 flood alone are ~$3 billion in the Czech Republic and >$9 billion in Germany--the worst flood ever in Europe. In this [Policy Forum][1], the Elbe 2002 flood is described in its historical context. Future flood protection and prevention planning must also consider the potential role of global warming in severe flooding of this kind. [1]: http://www.sciencemag.org/cgi/content/full/300/5622/1099

The challenge of long-term climate change.

Abstract: Climate policy needs to address the multidecadal to centennial time scale of climate change. Although the realization of short-term targets is an important first step, to be effective climate policies need to be conceived as long-term programs that will achieve a gradual transition to an essentially emission-free economy on the time scale of a century. This requires a considerably broader spectrum of policy measures than the primarily market-based instruments invoked for shorter term mitigation policies. A successful climate policy must consist of a dual approach focusing on both short-term targets and long-term goals.

The changing role of nation states in international environmental assessments—the case of the IPCC

Abstract: In 1988, the incoming head of one of the most prominent scientific assessment bodies, the Intergovernmental Panel on Climate Change (IPCC), explained: “Right now, many countries, especially developing countries, simply do not trust assessments in which their scientists and policymakers have not participated.” Since then, the international community gathered significant experience in designing and organising international assessments that allow for broad participation by representatives of national governments and influence domestic and international policy making. By analysing the case study of the IPCC, the paper focuses on the role of individual nation states played in international assessments and how this role changed over time and why. It is shown that there is a tendency of increasing internationalisation in the field of climate research and climate policy that limits the specific influence of individual nation states. The study of the internal processes of the IPCC reveals a growth of internal dynamics that built up boundaries for the behaviour of individual actors (such as government representatives and scientists). However, over the years the actors involved also learned how to use the IPCC mechanisms more effectively.

Stability analysis of the climate-vegetation system in the northern high latitudes

Abstract: The stability of the climate-vegetation system in the northern high latitudesis 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 andMoBidiC. The biogeophysical feedback in the latitudinal belt 60–70° N, although positive, is not strong enough to support multiple steady states: A unique equilibriumin the climate-vegetation system is simulated by all the models on a zonal scale for present-day climate and doubled CO2 climate.EMIC simulations with decreased insolation also reveal a unique steady state. However, the climate sensitivity to tree cover,Δ TF, exhibits non-linear behaviour within the models. For GENESIS-IBIS and CLIMBER-2, Δ TF islower for doubled CO2 climate than for present-day climate due to a shorter snow season and increased relative significance ofthe hydrological effect of forest cover. For the EMICs, Δ TF is higher for low tree fraction than for high treefraction, mainly due to a time shift in spring snow melt in response to changes in tree cover. The climate sensitivity to tree coveris reduced when thermohaline circulation feedbacks are accounted for in the EMIC simulations. Simpler parameterizations of oceanic processes have opposite effects onΔ TF: Δ TF is lower in simulations with fixed SSTs and higher in simulations with mixed layer oceans. Experiments with transient CO2 forcing show climate and vegetation not in equilibrium in the northern high latitudes at the end of the 20thcentury. The delayed response of vegetation and accelerated global warming lead to rather abrupt changes in northern vegetation cover in the first halfof the 21st century, when vegetation cover changes at double the present day rate.