Effects of a melted greenland ice sheet on climate, vegetation, and the cryosphere

This paper investigates the possible implications for the earth-system of a melting of the Greenland ice-sheet. Such a melting is a possible result of increased high latitude temperatures due to increasing anthropogenic greenhouse gas emissions. Using an atmosphere-ocean general circulation model (AOGCM), we investigate the effects of the removal of the ice sheet on atmospheric temperatures, circulation, and precipitation. We find that locally over Greenland, there is a warming associated directly with the altitude change in winter, and the altitude and albedo change in summer. Outside of Greenland, the largest signal is a cooling over the Barents sea in winter. We attribute this cooling to a decrease in poleward heat transport in the region due to changes to the time mean circulation and eddies, and interaction with sea-ice. The simulated climate is used to force a vegetation model and an ice-sheet model. We find that the Greenland climate in the absence of an ice sheet supports the growth of trees in southern Greenland, and grass in central Greenland. We find that the ice sheet is likely to regrow following a melting of the Greenland ice sheet, the subsequent rebound of its bedrock, and a return to present day atmospheric CO₂ concentrations. This regrowth is due to the high altitude bedrock in eastern Greenland which allows the growth of glaciers which develop into an ice sheet.     

Modelling the short-term response of the Greenland ice-sheet to global warming

 A two-dimensional vertically integrated ice flow model has been developed to test the importance of various processes and concepts used for the prediction of the contribution of the Greenland ice-sheet to sea-level rise over the next 350 y (short-term response). The mass balance is modelled by the degree-day method and the energy-balance method. The lithosphere is considered to respond isostatically to a point load and the time evolution of the bedrock follows from a viscous asthenosphere. According to the IPCC-IS92a scenario (with constant aerosols after 1990) the Greenland ice-sheet is likely to cause a global sea level rise of 10.4 cm by 2100 AD. It is shown, however, that the result is sensitive to precise model formulations and that simplifications as used in the sea-level projection in the IPCC-96 report yield less accurate results. Our model results indicate that, on a time scale of a hundred years, including the dynamic response of the ice-sheet yields more mass loss than the fixed response in which changes in geometry are not incorporated. It appears to be important to consider sliding, as well as the fact that climate sensitivity increases for larger perturbations. Variations in predicted sea-level change on a time scale of hundred years depend mostly on the initial state of the ice-sheet. On a time scale of a few hundred years, however, the variability in the predicted melt is dominated by the variability in the climate scenarios. Received: 21 August 1996/Accepted: 12 May 1997     

植被归一化植被指数对气候因子的空间敏感性

基于全球土地利用类型和覆盖度,利用生长季多年平均（1982～2015年）归一化植被指数（Normalized Difference Vegetation Index,NDVI）和气候平均态（气温、降水量）数据,讨论了全球植被格局与气候因子之间的关系,建立了两者之间的多元回归模型,并分析了植被对气温和降水气候态敏感性的特征。植被与气候因子在气候梯度上存在明显的对应关系,回归模型可较好拟合气候态NDVI的全球分布格局,拟合与观测NDVI的相关系数达0.90。其中,常绿阔叶林、混交林、常绿针叶林、落叶阔叶林、农田和木本稀树草原空间分布的拟合能力较好（r>0.8）。不同土地覆盖类型的NDVI对气温、降水气候态的空间敏感性特征不同。整体而言,植被对气温和降水的敏感性呈现反相关关系（r=-0.6）。不同土地覆盖类型对气温表现出正/负敏感性,寒带灌木对气温的敏感性最强,而农作物、草原、裸地对气温负敏感性较大;植被对降水的敏感性均表现出正敏感性,其中落叶针叶林、草原和稀树草原对降水的空间敏感性较强。     

Evaluating the Dependence of Vegetation on Climate in an Improved Dynamic Global Vegetation Model

The capability of an improved Dynamic Global Vegetation Model (DGVM) in reproducing the impact of climate on the terrestrial ecosystem is evaluated. The new model incorporates the Community Land ModelDGVM (CLM3.0-DGVM) with a submodel for temperate and boreal shrubs, as well as other revisions such as the two-leaf scheme for photosynthesis and the definition of fractional coverage of plant functional types (PFTs). Results show that the revised model may correctly reproduce the global distribution of tempera...     

Thresholds for irreversible decline of the Greenland ice sheet

The Greenland ice sheet will decline in volume in a warmer climate. If a sufficiently warm climate is maintained for a few thousand years, the ice sheet will be completely melted. This raises the question of whether the decline would be reversible: would the ice sheet regrow if the climate cooled down? To address this question, we conduct a number of experiments using a climate model and a high-resolution ice-sheet model. The experiments are initialised with ice sheet states obtained from various points during its decline as simulated in a high-CO₂ scenario, and they are then forced with a climate simulated for pre-industrial greenhouse gas concentrations, to determine the possible trajectories of subsequent ice sheet evolution. These trajectories are not the reverse of the trajectory during decline. They converge on three different steady states. The original ice-sheet volume can be regained only if the volume has not fallen below a threshold of irreversibility, which lies between 80 and 90% of the original value. Depending on the degree of warming and the sensitivity of the climate and the ice-sheet, this point of no return could be reached within a few hundred years, sooner than CO₂ and global climate could revert to a pre-industrial state, and in that case global sea level rise of at least 1.3 m would be irreversible. An even larger irreversible change to sea level rise of 5 m may occur if ice sheet volume drops below half of its current size. The set of steady states depends on the CO₂ concentration. Since we expect the results to be quantitatively affected by resolution and other aspects of model formulation, we would encourage similar investigations with other models.     

The climatic impact of a Sonoran vegetation discontinuity

The international fence separating Mexico and the United States is marked by a sharp vegetation discontinuity in the Sonoran Desert. Due to overgrazing, the Mexican side of the border has shorter grasses, more bare soil, and a higher albedo compared to the adjacent lands in the United States. In this investigation, long-term climate records are analyzed to determine the magnitude of any climatic differences associated with the spatial variation in the vegetation regime. The results suggest that summertime maximum temperatures recorded at the Mexican stations are significantly higher (by nearly 2.5 °C) than the Arizona stations when latitude and elevation are held constant. When only elevation is held constant, the difference in the maximum temperature jumps to approximately 4 dgC. No discernible changes in monthly and/or summer season precipitation could be identified in the records. These findings add support to other site-specific field measurements suggesting warming in desert areas where vegetation cover is decreasing and albedo is increasing.     

Climate sensitivity to wetlands and wetland vegetation in mid-Holocene North Africa

 Wetland regions are important components of the local climate, with their own characteristic surface energy and moisture budgets. Realistic representation of wetlands, including the important vegetation component, may therefore be necessary for more accurate simulations of climate and climate change. However, many land-atmosphere coupled models either ignore wetlands or treat wetlands as bare, water-saturated soil, neglecting the vegetation present within wetland environments. This study investigates the possible response of the mid-Holocene climate of North Africa to changes in orbital forcing, both with and without the presence of wetlands. The location of these wetlands is guided by analysis of paleovegetation and wetland distribution. In this study, the wetland regime in the land surface component of a climate model was modified to incorporate vegetation. Field measurements have shown that vegetation affects water loss associated with evaporation (including transpiration) within a wetland area. Comparisons between non-vegetated wetland and vegetated wetland revealed an increase in local albedo that produced an associated decrease in net radiation, evaporation and precipitation in the vicinity of the wetlands regions. Based on an analysis of the model surface water balance, the calculated area of mid-Holocene wetland coverage for North Africa closely matches the observed. For the North African region as a whole, the effects of adding vegetation to the wetland produced relatively small changes in climate, but local recycling of water may have served to help maintain paleo wetland communities. Received: 16 March 1999 / Accepted: 17 May 2000     

Contribution of glacier melt to sea-level rise since AD 1865: a regionally differentiated calculation

 The contribution of glacier melt, including the Greenland ice-sheet, to sea-level change since AD 1865 is estimated on the basis of modelled sensitivity of glacier mass balance to climate change and historical temperature data. Calculations are done in a regionally differentiated manner to overcome the inhomogeneity of the global distribution of glaciers. A distinction is made between changes in summer temperature and in temperature over the rest of the year. Our best estimate of the ice melt in the period 1865–1990 in terms of sea-level change equivalent is 5.7 cm (2.7 cm for glaciers and 3.0 cm for the Greenland ice-sheet). Additional calculations show that simpler methods, like using annual or even global mean temperature anomaly give estimates that differ by up to 55%. Consequently, a regionally differentiating approach is advised for making projections of glacier melt with GCM output. Received: 6 December 1996/Accepted: 30 May 1997     

大规模植被恢复对东亚气候的可能影响

利用动态植被模型CLM4-CNDV、区域气候模式RegCM4.6-CLM3.5和全球气候模式CAM4探究了当前气候状态下东亚区域可能的自然植被分布以及自然植被恢复对东亚区域气候产生的可能影响。结果表明,当前气候条件下,农作物区可能分布的自然植被为:蒙古高原以北、东北、华北平原和四川盆地的部分地区为裸土;东亚东南部及蒙古高原以北地区主要为林地;四川盆地及山东半岛主要为灌木;东北地区、东南沿海和长江中下游地区主要为草地。将农作物区恢复为自然植被后将对区域气候产生显著影响。其中,东亚东部大部分地区由于植被叶面积指数增加引起的蒸散发增强,使得夏季降水增加且温度降低显著;华北、四川盆地和广东中部平原地区植被叶面积指数减小,伴随区域内夏季降水显著减少且温度升高。而蒙古高原地区的气候变化不仅受区域内植被覆盖变化影响,还可能与印度地区和我国东南部植被变化引起的大气环流调整有关,使得蒙古高原西部冬季温度降低,而其东部夏季温度升高,同时夏季降水减少显著。研究所采用的试验方案是在相对理想的情况下进行的,但其结果为进一步区分不同地区植被覆盖变化的影响提供一定的参考。     

Soil carbon and nitrogen dynamics in southern African savannas: the effect of vegetation-induced patch-scale heterogeneities and large scale rainfall gradients

Savanna ecosystems are mixed plant communities in which trees and grasses co-exist thereby providing a heterogeneous landscape with a mosaic of tree-dominated and grass-dominated soil patches. Despite the important role that nutrient availability plays in these systems, detailed knowledge of differences in carbon and nitrogen cycling in soil patches predominantly covered by tree canopies or by grasses is still lacking. In this study, a process-based model was used to investigate the carbon and nitrogen dynamics in soil plots located in grass-dominated and tree/shrub-dominated soil patches along the Kalahari Transect (KT). The KT in southern Africa traverses a dramatic aridity gradient, across relatively homogenous soils, providing an ideal setting for global change studies. Here we show that there are distinctly different dynamics for soil moisture, decomposition and nitrogen mineralization between soil plots located under tree canopies and in open canopy areas, especially at the dryer end of the KT. Such differences diminished when approaching the wetter end of this transect. This study shows that in savanna ecosystems, water availability determines the patterns and rates of nutrient cycling at large scales, while at the local scales, vegetation patchiness plays an important role in nutrient cycling. Savannas are relatively stable ecosystems, resilient to small rainfall modifications, although irreversible changes may occur with stronger shifts in climate conditions.     

Long-term ice sheet–climate interactions under anthropogenic greenhouse forcing simulated with a complex Earth System Model

Several multi-century and multi-millennia simulations have been performed with a complex Earth System Model (ESM) for different anthropogenic climate change scenarios in order to study the long-term evolution of sea level and the impact of ice sheet changes on the climate system. The core of the ESM is a coupled coarse-resolution Atmosphere–Ocean General Circulation Model (AOGCM). Ocean biogeochemistry, land vegetation and ice sheets are included as components of the ESM. The Greenland Ice Sheet (GrIS) decays in all simulations, while the Antarctic ice sheet contributes negatively to sea level rise, due to enhanced storage of water caused by larger snowfall rates. Freshwater flux increases from Greenland are one order of magnitude smaller than total freshwater flux increases into the North Atlantic basin (the sum of the contribution from changes in precipitation, evaporation, run-off and Greenland meltwater) and do not play an important role in changes in the strength of the North Atlantic Meridional Overturning Circulation (NAMOC). The regional climate change associated with weakening/collapse of the NAMOC drastically reduces the decay rate of the GrIS. The dynamical changes due to GrIS topography modification driven by mass balance changes act first as a negative feedback for the decay of the ice sheet, but accelerate the decay at a later stage. The increase of surface temperature due to reduced topographic heights causes a strong acceleration of the decay of the ice sheet in the long term. Other feedbacks between ice sheet and atmosphere are not important for the mass balance of the GrIS until it is reduced to 3/4 of the original size. From then, the reduction in the albedo of Greenland strongly accelerates the decay of the ice sheet.     

Projecting the future vegetation–climate system over East Asia and its RCP-dependence

The future vegetation–climate system over East Asia, as well as its dependence on Representative Concentration Pathways (RCPs), is investigated using a regional climate–vegetation model driven with boundary conditions from Flexible Global Ocean–Atmosphere–Land System Model: Grid-point Version 2. Over most of the region, due to the rising CO₂ concentration and climate changes, the model projects greater vegetation density (leaf area index) and gradual shifts of vegetation type from bare ground to grass or from grass to trees; the projected spatial extent of the vegetation shift increases from RCP2.6 to RCP8.5. Abrupt shifts are projected under RCP8.5 over northeast China (with grass replacing boreal needleleaf evergreen trees due to heat stress) and India (with tropical deciduous trees replacing grass due to increased water availability). The impact of vegetation feedback on future precipitation is relatively weak, while its impact on temperature is more evident, especially during DJF over northeast China and India with differing mechanisms. In northeast China, the projected forest loss induces a cooling through increased albedo, and daytime high temperature (T_max) is influenced more than nighttime low temperature (T_min); in India, increased vegetation cover induces an evaporative cooling that outweighs the warming effect of an albedo decrease in DJF, leading to a weaker impact on T_max than on T_min. Based on a single model, the qualitative aspects of these results may hold while quantitative assessment will benefit from a follow-up regional model ensemble study driven by multiple general circulation models.

     

Impact of climate sensitivity and polar amplification on projections of Greenland Ice Sheet loss

The future rate of Greenland Ice Sheet (GrIS) deglaciation and the future contribution of GrIS deglaciation to sea level rise will depend critically on the magnitude of northern hemispheric polar amplification and global equilibrium climate sensitivity. Here, these relationships are analyzed using an ensemble of multi-century coupled ice-sheet/climate model simulations seeded with observationally-constrained initial conditions and then integrated forward under tripled preindustrial CO₂. Polar amplifications and climate sensitivities were varied between ensemble members in order to bracket current uncertainty in polar amplification and climate sensitivity. A large inter-ensemble spread in mean GrIS air temperature, albedo and surface mass balance trends stemming from this uncertainty resulted in GrIS ice volume loss ranging from 5 to 40 % of the original ice volume after 500 years. The large dependence of GrIS deglaciation on polar amplification and climate sensitivity that we find indicates that the representation of these processes in climate models will exert a strong control on any simulated predictions of multi-century GrIS evolution. Efforts to reduce polar amplification and equilibrium climate sensitivity uncertainty will therefore play a critical role in constraining projections of GrIS deglaciation and sea level rise in a future high-CO₂ world.     

Transient responses of North-American grasslands to changes in climate

Our objective was to evaluate the transient responses of grasslands in the central grassland region of North America to changes in climate. We used an individual plant-based gap dynamics simulation model (STEPPE-GP) linked with a soil water model (SOILWAT) to evaluate the effects of changes in climate on the composition and structure of grassland vegetation. Five functional types of plants were simulated based upon lifeform, physiology, and rooting distribution with depth. C₃ and C₄ perennial grasses with either a shallow or deep rooting distribution, and deeply rooted C₃ shrubs were simulated under current climatic conditions and under a GFDL climate change scenario for nine sites representative of the temperature and precipitation regimes in the grassland region.Although vegetation at the sites responded differently to climate change, shifts in functional types occurred within 40 years of the start of the climate change. C₄ grasses increased in dominance or importance at all sites with a change in climate, primarily as a result of increases in temperature in all months at all sites. The coolest sites that arc currently dominated by C₃ grasses were predicted to shift to a dominance by C₄ grasses, whereas sites that are currently dominated by C₄ grasses had an increase in importance of this functional type with a change in climate. Current annual temperature was the best predictor of changes in C₃ biomass, and C₃ and C₄ biomass combined; current annual precipitation was the best predictor of changes in C₄ biomass. These predicted shifts in dominance and importance of C₃ versus C₄ grasses would have important implications for the management of natural grasslands as well as the cultivation of crops in the central grassland region.     

Modeling diurnal to seasonal water and heat exchanges at European Fluxnet sites

Summary The importance of linking measurements, modeling and remote sensing of land surface processes has been increasingly recognized in the past years since on the diurnal to seasonal time scale land surface–atmosphere feedbacks can play a substantial role in determining the state of the near-surface climate. The worldwide Fluxnet project provides long term measurements of land surface variables useful for process-based modeling studies over a wide range of climatic environments.In this study data from six European Fluxnet sites distributed over three latitudinal zones are used to force three generations of LSMs (land surface models): the BUCKET, BATS 1E and SiB 2.5. Processes simulating the exchange of heat and water used in these models range from simple bare soil parameterizations to complex formulations of plant biochemistry and soil physics.Results show that – dependent on the climatic environment – soil storage and plant biophysical processes can determine the yearly course of the land surface heat and water budgets, which need to be included in the modeling system. The Mediterranean sites require a long term soil water storage capability and a biophysical control of evapotranspiration. In northern Europe the seasonal soil temperature evolution can influence the winter energy partitioning and requires a long term soil heat storage scheme. Plant biochemistry and vegetation phenology can drive evapotranspiration where no atmospheric-related limiting environmental conditions are active.     

气候与植被覆盖变化对中国西南亚高山区流域碳水循环的影响模拟

正确认识气候变化对流域森林植被和水文的影响对于林业经营管理与流域生态修复具有重要意义。为了揭示气候与植被覆盖变化对西南亚高山区流域碳水循环过程的影响,用生物物理/动态植被模型SSiB4/TRIFFID（Simplified Simple Biosphere model version 4, coupled with the Top-down Representation of Interactive Foliage and Flora Including Dynamics model）与流域地形指数水文模型TOPMODEL（Topographic Index Model）的耦合模型（以下记为SSiB4T/TRIFFID）模拟了不同气候情景下西南亚高山区的梭磨河流域植被演替和碳水循环过程。结果表明,所有试验流域植被经历了从C3到苔原灌木最后到森林的变化;控制试验流域蒸散在流域植被主要为苔原灌木时达到最大而径流深最小;增温5 ℃并且增雨40%试验[记为T+5, (1+40%) P试验]流域蒸散在流域为森林覆盖时达到最大而径流深最小。随着温度增加,森林蒸腾、冠层截留蒸发和蒸散的增加幅度明显大于草和苔原灌木,导致森林从控制试验的增加径流量变为减小径流量。从控制试验到T+5, (1+40%) P试验,温度增加使森林净初级生产力有所增加,但对草和苔原灌木的净初级生产力影响很小;植被水分利用效率随温度增加明显减小。西南山区随着海拔高度降低（温度升高）,森林从增加径流量转变为减少径流量,植被水分利用效率也相应明显减小。西南山区气候的垂直地带性对森林—径流关系和水分利用效率的空间变化有着重要的影响。     

Snowpack model estimates of the mass balance of the Greenland ice sheet and its changes over the twentyfirst century

The performance of a snow cover model in capturing the ablation on the Greenland ice sheet is evaluated. This model allows an explicit calculation of the formation of melt water, of the fraction of melt water which re-freezes, and of runoff in the ablation region. The input climate variables to the snowpack model come from two climate models. While the higher resolution general circulation model (ECHAM 4), is closest to observations in its estimate of accumulation, it fails to give accurate results in its predictions of runoff, primarily in the southern half of the ice sheet. The two-dimensional low-resolution climate model (MIT 2D LO) produces estimates of runoff from the Greenland ice sheet within the range of uncertainty of the Inter governmental Panel on Climate Change (IPCC1) 1995 estimates. Both models reproduce some of the characteristics of the extent of the wet snow zone observed with satellite remote sensing; the MIT model is closer to observations in terms of areal extent and intensity of the melting in the southern half of the ice-sheet in July and August while the ECHAM model reproduces melting in the northern half of the ice sheet well. Changes in runoff from Greenland and Antarctica are often cited as one of the major concerns linked to anthropogenic changes in climate. Because it is based on physical principles and relies on the surface energy balance as input, the snow cover model can respond to the current climatic forcing as well as to future changes in climate on the century time scale without the limitations inherent in empirical parametrizations. For a reference climate scenario similar to the IPCC's IS92a, the model projects that the Greenland ice sheet does not contribute significantly to changes in the level of the ocean over the twenty-first century. Increases in accumulation over the central portion of the ice sheet offset most of the increase in melting and runoff, which takes place along the margins of the ice sheet. The range of uncertainty in the predictions of sea-level rise is estimated by repeating the calculation with the MIT model for seven climate change scenarios. The range is –0.5 to 1.7 cm.     

Potential impacts of land-use on climate variability and extremes

This study aims at exploring potential impacts of land-use vegetation change (LUC) on regional climate variability and extremes. Results from a pair of Australian Bureau of Meteorology Research Centre (BMRC) climate model 54-yr (1949-2002) integrations have been analysed. In the model experiments, two vegetation datasets are used, with one representing current vegetation coverage in China and the other approximating its potential coverage without human intervention. The model results show potential impacts ...