Showing papers by "Potsdam Institute for Climate Impact Research published in 1999"

Comparing global models of terrestrial net primary productivity (NPP): overview and key results

Abstract: Seventeen global models of terrestrial biogeochemistry were compared with respect to annual and seasonal fluxes of net primary productivity (NPP) for the land biosphere. The comparison, sponsored by IGBP-GAIM/DIS/GCTE, used standardized input variables wherever possible and was carried out through two international workshops and over the Internet. The models differed widely in complexity and original purpose, but could be grouped in three major categories: satellite-based models that use data from the NOAA/AVHRR sensor as their major input stream (CASA, GLO-PEM, SDBM, SIB2 and TURC), models that simulate carbon fluxes using a prescribed vegetation structure (BIOME-BGC, CARAIB 2.1, CENTURY 4.0, FBM 2.2, HRBM 3.0, KGBM, PLAI 0.2, SILVAN 2.2 and TEM 4.0), and models that simulate both vegetation structure and carbon fluxes (BIOME3, DOLY and HYBRID 3.0). The simulations resulted in a range of total NPP values (44.4‐66.3 Pg C year ‐1 ), after removal of two outliers (which produced extreme results as artefacts due to the comparison). The broad global pattern of NPP and the relationship of annual NPP to the major climatic variables coincided in most areas. Differences could not be attributed to the fundamental modelling strategies, with the exception that nutrient constraints generally produced lower NPP. Regional and global NPP were sensitive to the simulation method for the water balance. Seasonal variation among models was high, both globally and locally, providing several indications for specific deficiencies in some models.

The science of adaptation: a framework for assessment

Abstract: This paper outlines what is meant by "adaptation" to climate change, and how it might be addressed in the IPCC Assessments. Two roles of adaptation in the climate change field are identified: adaptation as part of impact assessment (where the key question is: what adaptations are likely?), and adaptation as part of the policy response (where the central question is: what adaptations are recommended?). The concept of adaptation has been adopted in several fields including climate impact assessment and policy development, risk management, and natural hazards research. A framework for systematically defining adaptations is based on three questions: (i) adaptation to what? (ii) who or what adapts? and (iii) how does adaptation occur? The paper demonstrates that, for adaptation purposes, climate extremes and variability are integral parts of climate change, along with shifts in mean conditions. Attributes for differentiating adaptations include purposefulness, timing, temporal and spatial scope, effects, form and performance. The framework provides a guide for the treatment of adaptation in the IPCC assessments, both in the assessment of impacts and in the evaluation of adaptive policy options.

Simulation of an abrupt change in Saharan vegetation in the mid-Holocene

Abstract: Climate variability during the present inter- glacial, the Holocene, has been rather smooth in compar- ison with the last glacial. Nevertheless, there were some rather abrupt climate changes. One of these changes, the desertication of the Saharan and Arabian region some 4 - 6 thousand years ago, was presumably quite important for human society. It could have been the stimulus leading to the foundation of civilizations along the Nile, Euphrat and Tigris rivers. Here we argue that Saharan and Arabian de- sertication was triggered by subtle variations in the Earth's orbit which were strongly amplied by atmosphere- vegeta- tion feedbacks in the subtropics. The timing of this tran- sition, however, was mainly governed by a global interplay between atmosphere, ocean, sea ice, and vegetation.

‘Earth system’ analysis and the second Copernican revolution

Abstract: Optical magnification instruments once brought about the Copernican revolution that put the Earth in its correct astrophysical context. Sophisticated information-compression techniques including simulation modelling are now ushering in a second ‘Copernican’ revolution. The latter strives to understand the ‘Earth system’ as a whole and to develop, on this cognitive basis, concepts for global environmental management.

Long-term global warming scenarios computed with an efficient coupled climate model

Abstract: We present global warming scenarios computed with an intermediate-complexity atmosphere-ocean-sea ice model which has been extensively validated for a range of past climates (e.g., the Last Glacial Maximum). Our simulations extend to the year 3000, beyond the expected peak of CO2 concentrations. The thermohaline ocean circulation declines strongly in all our scenarios over the next 50 years due to a thermal effect. Changes in the hydrological cycle determine whether the circulation recovers or collapses in the long run. Both outcomes are possible within present uncertainty limits. In case of a collapse, a substantial long-lasting cooling over the North Atlantic and a drying of Europe is simulated.

Tropical climates at the Last Glacial Maximum: a new synthesis of terrestrial palaeoclimate data. I. Vegetation, lake-levels and geochemistry

Abstract: Palaeodata in synthesis form are needed as benchmarks for the Palaeoclimate Modelling Intercomparison Project (PMIP). Advances since the last synthesis of terrestrial palaeodata from the last glacial maximum (LGM) call for a new evaluation, especially of data from the tropics. Here pollen, plant-macrofossil, lake-level, noble gas (from groundwater) and δ18O (from speleothems) data are compiled for 18±2 ka (14C), 32 °N–33 °S. The reliability of the data was evaluated using explicit criteria and some types of data were re-analysed using consistent methods in order to derive a set of mutually consistent palaeoclimate estimates of mean temperature of the coldest month (MTCO), mean annual temperature (MAT), plant available moisture (PAM) and runoff (P-E). Cold-month temperature (MAT) anomalies from plant data range from −1 to −2 K near sea level in Indonesia and the S Pacific, through −6 to −8 K at many high-elevation sites to −8 to −15 K in S China and the SE USA. MAT anomalies from groundwater or speleothems seem more uniform (−4 to −6 K), but the data are as yet sparse; a clear divergence between MAT and cold-month estimates from the same region is seen only in the SE USA, where cold-air advection is expected to have enhanced cooling in winter. Regression of all cold-month anomalies against site elevation yielded an estimated average cooling of −2.5 to −3 K at modern sea level, increasing to ≈−6 K by 3000 m. However, Neotropical sites showed larger than the average sea-level cooling (−5 to −6 K) and a non-significant elevation effect, whereas W and S Pacific sites showed much less sea-level cooling (−1 K) and a stronger elevation effect. These findings support the inference that tropical sea-surface temperatures (SSTs) were lower than the CLIMAP estimates, but they limit the plausible average tropical sea-surface cooling, and they support the existence of CLIMAP-like geographic patterns in SST anomalies. Trends of PAM and lake levels indicate wet LGM conditions in the W USA, and at the highest elevations, with generally dry conditions elsewhere. These results suggest a colder-than-present ocean surface producing a weaker hydrological cycle, more arid continents, and arguably steeper-than-present terrestrial lapse rates. Such linkages are supported by recent observations on freezing-level height and tropical SSTs; moreover, simulations of “greenhouse” and LGM climates point to several possible feedback processes by which low-level temperature anomalies might be amplified aloft.

Comparing global models of terrestrial net primary productivity (NPP): analysis of differences in light absorption and light‐use efficiency

Abstract: Twelve global net primary productivity (NPP) models were compared: BIOME3, CASA, CARAIB, FBM, GLO-PEM, HYBRID, KGBM, PLAI, SDBM, SIB2, SILVAN and TURC. These models all use solar radiation as an input, and compute either absorbed solar radiation directly, or the amount of leaves used to absorb solar radiation, represented by the leaf area index (LAI). For all models, we obtained or estimated photosynthetically active radiation absorbed by the canopy (APAR). We then computed the light use efficiency for NPP (LUE) on an annual basis as the ratio of NPP to APAR. We analysed the relative importance for NPP of APAR and LUE. The analyses consider the global values of these factors, their spatial patterns represented by latitudinal variations, and the overall grid cell by grid cell variability. Spatial variability in NPP within a model proved to be determined by APAR, and differences among models by LUE. There was a compensation between APAR and LUE, so that global NPP values fell within the range of ‘generally accepted values’. Overall, APAR was lower for satellite driven models than for the other models. Most computed values of LUE were within the range of published values, except for one model.

δ13C of CO2 respired in the dark in relation to δ13C of leaf carbohydrates in Phaseolus vulgaris L. under progressive drought

Abstract: A, leaf net CO2 assimilation a, fractionation against 13C for CO2 diffusion through air b, net fractionation against 13C during CO2 fixation by Rubisco and PEPc δ13C, carbon isotopic composition Δ, discrimination against 13C during CO2 assimilation d, the term including the fractionation due to CO2 dissolution, liquid phase diffusion and also discrimination during both respiration and photorespiration DW, leaf dry weight dδ13C, the difference between CO2 respired in the dark and plant material in their carbon isotope composition dΔ, variation in modelled discrimination at a given pi/pa relative to a reference value at pi/pa = 0·7 FW, leaf fresh weight gc, leaf conductance to CO2 diffusion HPLC, high-performance liquid chromatography LMA, leaf mass per area pa, ambient partial pressure of CO2 pi, intercellular partial pressure of CO2 PEPc, phosphoenolpyruvate carboxylase PPFD, photosynthetic photon flux density RPDB, 13C/12C ratio of standard PDB RS, 13C/12C ratio of sample Rubisco, ribulose 1,5 bisphosphate carboxylase-oxygenase RWC, leaf relative water content SW, leaf saturated weight VPD, vapour pressure deficit The variations in δ13C in both leaf carbohydrates (starch and sucrose) and CO2 respired in the dark from the cotyledonary leaves of Phaseolus vulgaris L. were investigated during a progressive drought. As expected, sucrose and starch became heavier (enriched in 13C) with decreasing stomatal conductance and decreasing pi/pa during the first half (15 d) of the dehydration cycle. Thereafter, when stomata remained closed and leaf net photosynthesis was near zero, the tendency was reversed: the carbohydrates became lighter (depleted in 13C). This may be explained by increased pi/pa but other possible explanations are also discussed. Interestingly, the variations in δ13C of CO2 respired in the dark were correlated with those of sucrose for both well-watered and dehydrated plants. A linear relationship was obtained between δ13C of CO2 respired in the dark and sucrose, respired CO2 always being enriched in 13C compared with sucrose by ≈ 6‰. The whole leaf organic matter was depleted in 13C compared with leaf carbohydrates by at least 1‰. These results suggest that: (i) a discrimination by ≈ 6‰ occurs during dark respiration processes releasing 13C-enriched CO2; and that (ii) this leads to 13C depletion in the remaining leaf material.

Modelling climate response to historical land cover change

Abstract: climate with other forcings, climate responses to the changing atmospheric CO2 concentration and In order to estimate the eVect of historical land solar irradiance are also analysed. When all three cover change (deforestation) on climate, we perform factors are taken into account, dynamics of northern a set of experiments with a climate system model hemisphere temperature during the last 300 years of intermediate complexity - CLIMBER-2. We within the model are generally in agreement with focus on the biophysical eVect of the land cover the observed (reconstructed) temperature trend. We change on climate and do not explicitly account for conclude that the impact of historical land cover the biogeochemical eVect. A dynamic scenario of changes on climate is comparable with the impact deforestation during the last millennium is of the other climate forcings and that land cover formulated based on the rates of land conversion forcing is important for reproducing historical to agriculture. The deforestation scenario causes a climate change. global cooling of 0.35 ∞C with a more notable cooling of the northern hemisphere (0.5 ∞C). The Key words. Modelling, climate change, climate forcing, land cover change, deforestation, solar cooling is most pronounced in the northern middle and high latitudes, especially during the spring irradiance, carbon dioxide, land-atmosphere interaction, climate-biosphere interactions. season. To compare the eVect of deforestation on

The tolerable windows approach : Theoretical and methodological foundations

Abstract: The tolerable windows (TW) approach is presented as a novel scheme for integrated assessment of climate change. The TW approach is based on the specification of a set of guardrails for climate evolution which refer to various climate-related attributes. These constraints, which define what we call tolerable windows, can be purely systemic in nature – like critical thresholds for the North Atlantic Deep Water formation – or of a normative type – like minimum standards for per-capita food production worldwide. Starting from this catalogue of knock-out criteria and using appropriate modeling techniques, those policy strategies which are compatible with all the constraints specified are sought to be identified. In addition to the discussion of the basic elements and the general theory of the TW approach, a modeling exercise is carried out, based on simple models and assumptions adopted from the German Advisory Council on Global Change (WBGU). The analysis shows that if the global mean temperature is restricted to 2°C beyond the preindustrial level, the cumulative emissions of CO2 are asymptotically limited to about 1550 Gt C. Yet the temporal distribution of these emissions is also determined by the climate and socio-economic constraints: using, for example, a maximal tolerable rate of temperature change of 0.2°C/dec and a smoothly varying emissions profile, we obtain the maximal cumulative emissions, amounting to 370 Gt C in 2050 and 585 Gt C in 2100.

Coastal Adaptation to Climate Change : Can the IPCC Technical Guidelines be Applied

Abstract: This paper evaluates the IPCC Technical Guidelines for Assessing Climate Change Impacts and Adaptations with respect to the guidance offered for coastal-adaptation assessment. It appears that the IPCC Technical Guidelines focus strongly on implementation. This paper uses both conceptual and empirical information is used in this paper to show that coastal adaptation embraces more than selecting one of the "technical" options to respond to sea-level rise (retreat, accommodate or protect). Coastal adaptation is a more complex and iterative process with a series of policy cycles. To be effective, an expanded adapta-tion framework involving four steps is suggested, including (i) information collection and awareness raising; (ii) planning and design; (iii) implementation; and (iv) monitoring and evaluation. The incom-plete coverage of these four steps in existing coastal-adaptation assessments constrains the development of adaptation strategies that are supported by the relevant actors and integrated into existing management. Researchers and policy-makers are recommended to work together to establish a framework for adaptation that is integrated within current coastal management processes and practices and takes a broader view on the subject.

Plant functional types and disturbance dynamics – Introduction

Abstract: . Plant functional traits and types are useful concepts in relation to disturbance responses of natural and managed ecosystems. To explore their applicability in greater depth, a set of 12 papers presents a broad range of issues from methodologies to the results of particular trait studies in the field, and modelling approaches. So far, empirical studies have only allowed us to identify a few functional traits that are consistently associated with disturbance. To determine the trait variations associated with climate, disturbance history and current disturbance regime as well as the interactions between these factors, global-scale comparisons of numerous individual studies are required. Significant advances toward this ambitious goal are presented in these papers, and include: (1) the articulation of experimental and analytical methodologies for individual studies that could usefully contribute to a global comparison; (2) the identification of core traits that can be used in the further search for disturbance-related traits common to a range of environments; (3) further information on vegetation response to disturbance in terms of trait representation, and the identification of attribute syndromes; (4) the identification of issues for modelling disturbance dynamics using functional types.

Multi‐criterial validation of TOPMODEL in a mountainous catchment

Abstract: The need for powerful validation methods for hydrological models including the evaluation of internal stages and spatially distributed simulations has often been emphasized. In this study a multi-criterial validation scheme was used for validation of TOPMODEL, a conceptual semi-distributed rainfall–runoff model. The objective was to test TOPMODEL's capability of adequately representing dominant hydrological processes by simple conceptual approaches. Validation methods differed in the type of data used, in their target and in mode. The model was applied in the humid and mountainous Brugga catchment (40 km2) in south-west Germany. It was calibrated by a Monte Carlo method based on hourly runoff data. Additional information for validation was derived from a recession analysis, hydrograph separation with environmental tracers and from field surveys, including the mapping of saturated areas. Although runoff simulations were satisfying, inadequacies of the model structure compared with the real situation with regard to hydrological processes in the study area were found. These belong mainly to the concept of variable contributing areas for saturation excess overland flow and their dynamics, which were overestimated by the model. The simple TOPMODEL approach of two flow components was found to be insufficient. The multi-criterial validation scheme enables not only to demonstrate limitations with regard to process representation, but also to specify where and why these limitations occur. It may serve as a valuable tool for the development of physically sound model modifications. Copyright © 1999 John Wiley & Sons, Ltd.

Terrestrial npp: toward a consistent data set forglobal model evaluation

Abstract: Progress in modeling the global carbon cycle is inhibited by the lack of a high-quality data set based upon field observations of net primary productivity (NPP) with which to calibrate, parameterize, and evaluate terrestrial biosphere models. Under the auspices of the Global Primary Production Data Initiative (GPPDI), an activity endorsed by the International Geosphere–Biosphere Program’s Data and Information System, a small international workshop was held in Cincinnati, Ohio, USA, in December 1996 to address the problem of extrapolating sparse field observations of NPP to produce a consistent database representative of major biomes. We report the conclusions of this workshop and the goals of GPPDI—to further expand the existing data compilation, to agree upon consistent standards for cross-site comparisons and allometric relationships for various biome types, and to document methodologies for spatial extrapolation from point measurements to grid cells. The resulting NPP database will also have intrinsic ...

Syndromes of Global Change: a qualitative modelling approach to assist global environmental management

Abstract: A novel transdisciplinary approach to investigate Global Change (GC) is presented. The approach rests on the decomposition of the intrigue dynamics of GC into patterns of civilization–nature interactions (“syndromes”) by an iterative scientific process of observations, data and system theoretical analyses, and modelling attempts. We illustrate the approach by a detailed analysis of the Sahel Syndrome, which describes the rural poverty driven overuse of natural resources. The investigation is performed by (i) identifying relevant “symptoms” and interlinkages which are characteristics for this pattern, and (ii) a qualitative model representing the internal dynamics of the essential flywheel. The geographical patchwork of the regions affected by the syndrome which is obtained by global data analysis, proves the high global relevance of this pattern. The qualitative model is employed for an evaluation of basic policy strategies debated in the context of rural poverty driven environmental degradation. It turns out that a mixed policy of combating poverty and introducing soil preserving agricultural techniques and practices is most promising to tackle the syndrome dynamics.

Shifting seas in the greenhouse

Abstract: Models of the Earth's possible responses to global warming are continually being improved. The latest simulation of changes in deep flow in the Atlantic operates without several of the fudge factors previously required.

Oral progestogen-only contraceptives and cardiovascular risk: results from the Transnational Study on Oral Contraceptives and the Health of Young Women

Abstract: Background The risk of cardiovascular disease associated with progestogen-only pills has rarely been studied so far.Methods In the Transnational case-control study we were looking for a potential cardiovascular disease risk with oral progestogen-only pills in women aged 16-44 years. A total of 1058 cases of myocardial infarction, thromboembolic cerebrovascular accident or venous thromboembolism, and 3808 controls unaffected by these diseases, were enrolled. The group of women who had either used oral progestogen-only pills or no oral contraceptives included 394 cardiovascular disease cases (123 cases of myocardial infarction, 90 cases of thromboembolic cerebrovascular accident and 181 cases of venous thromboembolism) and 2366 controls.Results The adjusted (matched) odds ratio (OR) for all cardiovascular diseases combined for women using progestogen-only pills compared with non-users of oral contraceptives was 0.84 (95% confidence interval (CI), 0.45-1.58). The adjusted ORs for myocardial infarction, throm...

Comparing global models of terrestrial net primary productivity (NPP): global pattern and differentiation by major biomes

Abstract: Annual and seasonal net primary productivity estimates (NPP) of 15 global models across latitudinal zones and biomes are compared. The models simulated NPP for contemporary climate using common, spatially explicit data sets for climate, soil texture, and normalized difference vegetation index (NDVI). Differences among NPP estimates varied over space and time. The largest differences occur during the summer months in boreal forests (50∞ to 60∞N) and during the dry seasons of tropical evergreen forests. Differences in NPP estimates are related to model assumptions about vegetation structure, model parameterizations, and input data sets.

Climate change decision-support and the tolerable windows approach

Abstract: The Tolerable Windows Approach (TWA) to Integrated Assessments (IA) of global warming is based on external normative specifications of tolerable sets of climate impacts as well as proposed emission quotas and policy instruments for implementation. In a subsequent step, the complete set of admissible climate protection strategies which are compatible with these normative inputs is determined by scientific analysis. In doing so, minimum requirements concerning global and national greenhouse gas emission paths can be determined. In this paper we present the basic methodological elements of TWA, discuss its relation to more conventional approaches to IA like cost–benefit analyses, and present some preliminary results obtained by a reduced-form climate model.

Comparing global models of terrestrial net primary productivity (NPP): importance of vegetation structure on seasonal NPP estimates

Abstract: This paper was published as Global Change Biology, 1999, 5 (S1), pp. 35-45. It is available from http://onlinelibrary.wiley.com/doi/10.1046/j.1365-2486.1999.00005.x/abstract. Doi: 10.1046/j.1365-2486.1999.00005.x

The role of spatial variability of soil moisture for modelling surface runoff generation at the small catchment scale

Abstract: . A Bayesian approach is described for dealing with the problem of infilling and generating stochastic flow sequences using rainfall data to guide the flow generation process, and including bounded (censored) observed flow and rainfall data to provide additional information. Solutions are obtained using a Gibbs sampling procedure. Particular problems discussed include developing new procedures for fitting transformations when bounded values are available, coping with additional information in the form of values, or bounds, for totals of flows across several sites, and developing relationships between annual flow and rainfall data. Examples are shown of both infilled values of unknown past river flows, with assessment of uncertainty, and realisations of flows representative of what might occur in the future. Several procedures for validating the model output are described and the central estimates of flows, taken as a surrogate for historical observed flows, are compared with long term regional flow and rainfall data.

Modelling river discharge for large drainage basins: from lumped to distributed approach

Abstract: The paper presents an upscaled application of the HBV model to the German part of the Elbe drainage basin, and intercomparison of lumped and distributed versions of the model. The objectives of the work were (a) to check the model performance for large-scale basins, and (b) to compare the lumped and distributed versions of the model. Three versions of the HBV model, one lumped and two distributed, were applied first to a number of sub-basins of the Elbe with different hydrological regimes (area > 1000 km2), and then to the whole German part of the basin (area 80 657 km2). The model performed well in all cases. The distributed model versions are more data intensive but enabled better results to be achieved. The perspectives for using the model for large-scale water quality assessment, for climate change impact studies and for coupled land-atmosphere modelling are discussed.

Capabilities and limitations of detailed hillslope hydrological modelling

Abstract: Hillslope hydrological modelling is considered to be of great importance for the understanding and quantification of hydrological processes in hilly or mountainous landscapes. In recent years a few comprehensive hydrological models have been developed at the hillslope scale which have resulted in an advanced representation of hillslope hydrological processes (including their interactions), and in some operational applications, such as in runoff and erosion studies at the field scale or lateral flow simulation in environmental and geotechnical engineering. An overview of the objectives of hillslope hydrological modelling is given, followed by a brief introduction of an exemplary comprehensive hillslope model, which stimulates a series of hydrological processes such as interception, evapotranspiration, infiltration into the soil matrix and into macropores, lateral and vertical subsurface soil water flow both in the matrix and preferential flow paths, surface runoff and channel discharge. Several examples of this model are presented and discussed in order to determine the model's capabilities and limitations. Finally, conclusions about the limitations of detailed hillslope modelling are drawn and an outlook on the future prospects of hydrological models on the hillslope scale is given.The model presented performed reasonable calculations of Hortonian surface runoff and subsequent erosion processes, given detailed information of initial soil water content and soil hydraulic conditions. The vertical and lateral soil moisture dynamics were also represented quite well. However, the given examples of model applications show that quite detailed climatic and soil data are required to obtain satisfactory results. The limitations of detailed hillslope hydrological modelling arise from different points: difficulties in the representations of certain processes (e.g. surface crusting, unsaturated–saturated soil moisture flow, macropore flow), problems of small-scale variability, a general scarcity of detailed soil data, incomplete process parametrization and problems with the interdependent linkage of several hillslopes and channel–hillslope interactions. Copyright © 1999 John Wiley & Sons, Ltd.

The Effect of Global Environmental Regimes: A Measurement Concept

Abstract: The article outlines a method to measure the effect which international regimes have on solving global environmental problems such as global climate change. By using political-economic cost/benefit analysis, a no-regime counterfactual and a collective optimum (lower and upper bounds) are derived. By comparing the actual performance of a regime to these bounds, a simple coefficient of regime effectiveness can be computed. After theoretically deriving the various bounds, the authors discuss the determining factors and provide guidance on how such a research agenda could be pursued empirically. The authors conclude with suggestions to further refine the measurement concept and its merit for public policy.

Sensitivity of Ventilation Rates and Radiocarbon Uptake to Subgrid-Scale Mixing in Ocean Models

Abstract: The sensitivity of ventilation timescales and radiocarbon ( 14C) uptake to subgrid-scale mixing parameterization is studied in a global ocean model. Seven experiments are examined that are identical in every manner except their representation of subgrid-scale mixing of tracers. The cases include (i) two runs with traditional Cartesian mixing (HOR), (ii) a run with enhanced isopycnal mixing (ISO), and (iii) four runs in which the effects of eddies on the mean ocean flow are parameterized following Gent and McWilliams (GM). Horizontal, isopycnal, and isopycnal-thickness diffusion coefficients are varied sequentially in the model runs. Of particular interest is the role of the tracer mixing schemes in influencing longer timescale ventilation processes—centennial and beyond—such as deep water mass renewal and circulation. Simulated ventilation timescales and 14C vary greatly between the three mixing schemes. The isopycnal mixing run exhibits the most rapid water mass renewal due to strong diffusion effects and excessive surface convective overturn, particularly in the Southern Ocean. In contrast, the GM cases show much more gradual renewal of deep and bottom waters, with limited vertical convection of surface waters and slower abyssal currents. Under GM, a background horizontal diffusion or altered isopycnal mixing do not significantly change interior ocean ventilation rates. This means modelers can adjust these background diffusion coefficients under GM (for numerical purposes) without significantly changing model ventilation rates. Reducing the GM isopycnal thickness diffusivity, on the other hand, noticeably increases simulated deep water ventilation rates. In comparison with the HOR runs, deep and bottom water ventilation timescales are reduced by about 30% in ISO, and increased by 30%‐40% under GM. Comparison is made between model simulated and observed 14C. The GM runs appear to be the least successful in the North Atlantic Ocean, exhibiting very gradual and only shallow water-mass renewal compared to observations. In the Pacific and Indian Oceans, the HOR and ISO runs are ventilated too rapidly due to strong convection and water-mass contribution from the Southern Ocean. In contrast, the GM runs simulate spuriously old and 14C-depleted bottom and middepth water. The GM cases do, however, capture realistic 14C in the upper 1500 m of the Indian and Pacific Oceans. Overall, none of the model cases reproduce global ocean ventilation rates over centennial timescales (under the chosen set of parameter values). Higher horizontal resolution and a spatially varying GM thickness diffusivity may be required before global models capture long timescale ocean renewal processes with some degree of fidelity.

Adaptation to climate variability and change: methodological issues

Abstract: The Intergovernmental Panel on Climate Change (IPCC) convened a Workshop on Adaptation to Climate Variability and Change in Costa Rica in 1998 that involved more than 200 expects and incorporated views from many research communities This paper summarizes the recommendations from the Workshop and profiles the contributions to the advancement of methodologies for adaptation science

On the stability of the high-latitude climate-vegetation system in a coupled atmosphere-biosphere model

Abstract: Summary In order to investigate the hypothesis that the Earth's climate and vegetation patterns may have more than one basic state, we use the fully coupled GENESIS-IBIS model. GENESIS is an atmospheric general circulation model. IBIS is a dynamic global vegetation model that integrates biophysical, physiological, and ecological processes. GENESIS and IBIS are coupled by way of a common land surface interface to allow for the full and transient interaction between changes in the vegetation structure and changes in the general circulation of the atmosphere. We examine two modern climate simulations of the coupled model initialized with two different initial conditions. In one case, we initialize the model vegetation cover with the modern observed distribution of vegetation. In the other case, we initialize the vegetation cover with evergreen boreal forests extending to the Arctic coast, replacing high-latitude tundra. We interpret the coupled model's behaviour using a conceptual model for multistability and demonstrate that in both simulations the climate-vegetation system converges to the same equilibrium state. In the present climate, feedbacks between land, ocean, sea ice, and the atmosphere do not result in the warming required to support an expanded boreal forest.