首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 15 毫秒
1.
Studying the vegetation feedback during warm periods of the past can lead to better understanding of those in the future.In this study,we conducted several simulations to analyze vegetation feedback during the mid-Pliocene warm period.The results indicate that the main features of vegetation change in the mid-Pliocene were a northward shift of needleleaf tree,an expansion of broadleaf tree and shrub,and a northward expansion of grass,as compared to the pre-industrial period.The global annual mean warming ratio caused by vegetation feedback was 12.1％,and this warming ratio was much larger in northern middle and high latitudes.The warming caused by vegetation change was directly related to the surface albedo change and was further amplified by snow/sea ice-albedo feedback.  相似文献   

2.
Effects of Land Cover Conversion on Surface Climate   总被引：11，自引：0，他引：11  
This study investigates the effects of large-scale human modification of land cover on regional and global climate. A general circulation model (Colorado State University GCM) coupled to a biophysically-based land surface model (SiB2) was used to run two 15-yr climate simulations. The control run used current vegetation distribution as observed by satellite for the year 1987 to derive the vegetation's physiological and morphological properties. The twin simulation used a realistic approximation of vegetation type distribution that would exist in the absence of human disturbance.In temperate latitudes, where anthropogenic modification of the landscape has converted large areas of forest and grassland to cropland, conversion cools canopy temperatures up to 0.7 ° C in summer and 1.1 ° C in winter. This cooling results from both (1) morphological changes in vegetation which increase albedo and (2) physiological changes in vegetation which increase latent heat flux of crops compared with undisturbed vegetation during the growing season. In the tropics and subtropics, conversion warms canopy temperature by about 0.8 ° C year round. The warming results from a combination of morphological changes in vegetation offset by physiological changes that reduce latent heat flux of existing compared with undisturbed vegetation. If water efficient, tropical C4 grasses replace C3 vegetation, latent heat flux is further reduced.The overall effect of land cover conversion is cooling in temperate latitudes and warming in the tropics. Because the effects are opposite in sign in tropics and middle latitudes, they cancel each other when averaged globally. Over land, the surface temperature increased by 0.2 C in winter and remained essentially unchanged in summer. The effects on land surface hydrology were also small when averaged globally. The results suggest that the effects of land use change of the observed magnitude do not have a strong impact on the globally averaged climate but their signature at regional scales is significant and vary according to the type of land cover conversion.  相似文献   

3.
The aim of this study was to develop an advanced parameterization of the snow-free land surface albedo for climate modelling describing the temporal variation of surface albedo as a function of vegetation phenology on a monthly time scale. To estimate the effect of vegetation phenology on snow-free land surface albedo, remotely sensed data products from the Moderate-Resolution Imaging Spectroradiometer (MODIS) on board the NASA Terra platform measured during 2001 to 2004 are used. The snow-free surface albedo variability is determined by the optical contrast between the vegetation canopy and the underlying soil surface. The MODIS products of the white-sky albedo for total shortwave broad bands and the fraction of absorbed photosynthetically active radiation (FPAR) are analysed to separate the vegetation canopy albedo from the underlying soil albedo. Global maps of pure soil albedo and pure vegetation albedo are derived on a 0.5° regular latitude/longitude grid, re-sampling the high-resolution information from remote sensing-measured pixel level to the model grid scale and filling up gaps from the satellite data. These global maps show that in the northern and mid-latitudes soils are mostly darker than vegetation, whereas in the lower latitudes, especially in semi-deserts, soil albedo is mostly higher than vegetation albedo. The separated soil and vegetation albedo can be applied to compute the annual surface albedo cycle from monthly varying leaf area index. This parameterization is especially designed for the land surface scheme of the regional climate model REMO and the global climate model ECHAM5, but can easily be integrated into the land surface schemes of other regional and global climate models.  相似文献   

4.
Vegetation feedback under future global warming   总被引：2，自引：0，他引：2  
It has been well documented that vegetation plays an important role in the climate system. However, vegetation is typically kept constant when climate models are used to project anthropogenic climate change under a range of emission scenarios in the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emission Scenarios. Here, an atmospheric general circulation model, and an asynchronously coupled system of an atmospheric and an equilibrium terrestrial biosphere model are forced by monthly sea surface temperature and sea ice extent for the periods 2051?C2060 and 2090?C2098 as projected with 17 atmosphere?Cocean general circulation models participating in the IPCC Fourth Assessment Report, and by appropriate atmospheric carbon dioxide concentrations under the A2 emission scenario. The effects of vegetation feedback under future global warming are then investigated. It is found that the simulated composition and distribution of vegetation during 2051?C2060 (2090?C2098) differ greatly from the present, and global vegetation tends to become denser as expressed by a 21% (36%) increase in global mean leaf area index, which is most pronounced at the middle and high northern latitudes. Vegetation feedback has little effect on globally averaged surface temperature. On a regional scale, however, it induces statistically significant changes in surface temperature, in particular over most parts of continental Eurasia east of about 60°E where annual surface temperature is expected to increase by 0.1?C1.0?K, with an average of about 0.4?K for each future period. These changes can mostly be explained by changes in surface albedo resulting from vegetation changes in the context of future global warming.  相似文献   

5.
Transient experiments for the Eemian (128–113 ky BP) were performed with a complex, coupled earth system model, including atmosphere, ocean, terrestrial biosphere and marine biogeochemistry. In order to investigate the effect of land surface parameters (background albedo, vegetation and tree fraction and roughness length) on the simulated changes during the Eemian, simulations with interactive coupling between climate and vegetation were compared with additional experiments in which these feedbacks were suppressed. The experiments show that the influence of land surface on climate is mainly caused by changes in the albedo. For the northern hemisphere high latitudes, land surface albedo is changed partially due to the direct albedo effect of the conversion of grasses into forest, but the indirect effect of forests on snow albedo appears to be the major factor influencing the total absorption of solar radiation. The Western Sahara region experiences large changes in land surface albedo due to the appearance of vegetation between 128 and 120 ky BP. These local land surface albedo changes can be as much as 20%, thereby affecting the local as well as the global energy balance. On a global scale, latent heat loss over land increases more than 10% for 126 ky BP compared to present-day.  相似文献   

6.
In this study we investigate the impact of large-scale oceanic forcing and local vegetation feedback on the variability of the Sahel rainfall using a global biosphere-atmosphere model, the coupled GENESIS-IBIS model, running at two different resolutions. The observed global sea surface temperature in the twentieth century is used as the primary model forcing. Using this coupled global model, we experiment on treating vegetation as a static boundary condition and as a dynamic component of the Earth climate system. When vegetation is dynamic, the R30-resolution model realistically reproduces the multi-decadal scale fluctuation of rainfall in the Sahel region; keeping vegetation static in the same model results in a rainfall regime characterized by fluctuations at much shorter time scales, indicating that vegetation dynamics act as a mechanism for persistence of the regional climate. Even when vegetation dynamics is included, the R15 model fails to capture the main characteristics of the long-term rainfall variability due to the exaggerated atmospheric internal variability in the coarse resolution model. Regardless how vegetation is treated and what model resolution is used, conditions in the last three decades of the twentieth century are always drier than normal in the Sahel, suggesting that global oceanic forcing during that period favors the occurrence of a drought. Vegetation dynamics is found to enhance the severity of this drought. However, with both the observed global SST forcing and feedback from dynamic vegetation in the model, the simulated drought is still not as persistent as that observed. This indicates that anthropogenic land cover changes, a mechanism missing in the model, may have contributed to the occurrence of the twentieth century drought in the Sahel.  相似文献   

7.
This study examines the potential impact of vegetation feedback on the changes in the diurnal temperature range (DTR) due to the doubling of atmospheric CO2 concentrations during summer over the Northern Hemisphere using a global climate model equipped with a dynamic vegetation model. Results show that CO2 doubling induces significant increases in the daily mean temperature and decreases in DTR regardless of the presence of the vegetation feedback effect. In the presence of vegetation feedback, increase in vegetation productivity related to warm and humid climate lead to (1) an increase in vegetation greenness in the mid-latitude and (2) a greening and the expansion of grasslands and boreal forests into the tundra region in the high latitudes. The greening via vegetation feedback induces contrasting effects on the temperature fields between the mid- and high-latitude regions. In the mid-latitudes, the greening further limits the increase in T max more than T min, resulting in further decreases in DTR because the greening amplifies evapotranspiration and thus cools daytime temperature. The greening in high-latitudes, however, it reinforces the warming by increasing T max more than T min to result in a further increase in DTR from the values obtained without vegetation feedback. This effect on T max and DTR in the high latitude is mainly attributed to the reduction in surface albedo and the subsequent increase in the absorbed insolation. Present study indicates that vegetation feedback can alter the response of the temperature field to increases in CO2 mainly by affecting the T max and that its effect varies with the regional climate characteristics as a function of latitudes.  相似文献   

8.
近年来遥感技术的发展为多圈层中陆面过程和边界层研究提供了有力的工具。文章分析了目前用于陆面过程参数化研究的重要遥感信息源。并评述遥感信息在陆面过程参数化研究中的基本应用和存在问题，最后，提出发展方向和展望。  相似文献   

9.
To explore processes involved in glacial inception at 116 kaBP, the response of an atmospheric general circulation model (AGCM) to changes in lower boundary conditions is investigated. Two 116 kaBP experiments are conducted to examine the importance of sea surface conditions (sea surface temperature and sea ice distribution): one with the present-day sea surface conditions, and the other with 116 kaBP sea surface conditions. These two different sea surface conditions are obtained from simulations using an earth system climate model of intermediate complexity. Perennial snow cover occurred over the Canadian Archipelago under 116 kaBP orbital and CO2 forcing with present-day "warm" sea surface conditions, and further expanded over northeastern Canada when 116 kaBP "cool" sea surface conditions were applied. The net positive accumulation in northeastern Canada, with little in Alaska, is in good agreement with geological records. Two additional 116 kaBP experiments are conducted to examine the combined importance of sea surface conditions and land surface conditions (vegetation): one with the present-day sea surface and modified land surface conditions, and the other with 116 kaBP sea surface and modified land surface conditions. Modifying vegetation, based on cooling during summer induced by 116 kaBP sea surface conditions, leads to much larger areas of perennial snow cover. Only when 116 kaBP sea surface conditions are applied, is a realistic global net snow accumulation rate obtained. Contrary to the earlier ice age hypothesis, our results suggest that the capturing of glacial inception at 116 kaBP requires the use of "cooler" sea surface conditions than those of the present climate. Also, the large impact of vegetation change on climate suggests that the inclusion of the vegetation feedback is important for model validation, at least, in this particular period of Earth history.  相似文献   

10.
The effects of vegetation and its seasonal variation on energy and the hydrological cycle were examined using a state-of-the-art Community Atmosphere Model （CAM3）. Three 15-year numerical experiments were completed： the first with realistic vegetation characteristics varying monthly （VEG run）, the second without vegetation over land （NOVEG run）, and the third with the vegetation characteristics held at their annual mean values （VEGMEAN run）. In these models, the hydrological cycle and land surface energy budget were widely affected by vegetation. Globaland annual-mean evapotranspiration significantly increased compared with the NOVEG by 11.8% in the VEG run run, while runoff decreased by 13.2% when the realistic vegetation is incorporated. Vegetation plays different roles in different regions. In tropical Asia, vegetation-induced cooling of the land surface plays a crucial role in decreasing tropical precipitation. In middle latitudes and the Amazon region, however, the vegetation-induced increase of evapotranspiration plays a more important role in increasing precipitation. The seasonal variation of vegetation also shows clear influences on the hydrological cycle and energy budget. In the boreal mid-high latitudes where vegetation shows a strong seasonal cycle, evapotranspiration and precipitation are higher in the summer in the VEG run than in the VEGMEAN run.  相似文献   

11.
The Common Land Model（CoLM） was coupled with the IAP Dynamic Global Vegetation Model（IAPDGVM）, and the performance of this combined CoLMIAP model was evaluated. Offline simulations using both the original Common Land Model（CoLM-LPJ） and CoLM-IAP were conducted. The CoLM-IAP coupled model showed a significant improvement over CoLMLPJ, as the deciduous tree distribution decreased over temperate and boreal regions, while the distribution of evergreen trees increased over the tropics. Some biases in CoLM-LPJ were preserved, including the overestimation of evergreen trees in tropical savanna, the underestimation of boreal evergreen trees, and the absence of boreal shrubs. However, most of these biases did not exist in a further coupled simulation of IAP-DGVM with the Community Land Model（CLM）, for which the parameters of IAP-DGVM were optimized. This implies that further improvement is needed to deal with the differences between CoLM and CLM in parameterizations of landbased physical and biochemical processes.  相似文献   

12.
Uncertainties in the climate response to a doubling of atmospheric CO2 concentrations are quantified in a perturbed land surface parameter experiment. The ensemble of 108 members is constructed by systematically perturbing five poorly constrained land surface parameters of global climate model individually and in all possible combinations. The land surface parameters induce small uncertainties at global scale, substantial uncertainties at regional and seasonal scale and very large uncertainties in the tails of the distribution, the climate extremes. Climate sensitivity varies across the ensemble mainly due to the perturbation of the snow albedo parameterization, which controls the snow albedo feedback strength. The uncertainty range in the global response is small relative to perturbed physics experiments focusing on atmospheric parameters. However, land surface parameters are revealed to control the response not only of the mean but also of the variability of temperature. Major uncertainties are identified in the response of climate extremes to a doubling of CO2. During winter the response both of temperature mean and daily variability relates to fractional snow cover. Cold extremes over high latitudes warm disproportionately in ensemble members with strong snow albedo feedback and large snow cover reduction. Reduced snow cover leads to more winter warming and stronger variability decrease. As a result uncertainties in mean and variability response line up, with some members showing weak and others very strong warming of the cold tail of the distribution, depending on the snow albedo parametrization. The uncertainty across the ensemble regionally exceeds the CMIP3 multi-model range. Regarding summer hot extremes, the uncertainties are larger than for mean summer warming but smaller than in multi-model experiments. The summer precipitation response to a doubling of CO2 is not robust over many regions. Land surface parameter perturbations and natural variability alter the sign of the response even over subtropical regions.  相似文献   

13.
We present results from a coupled atmosphere-biosphere model CCM3/IBIS (the Community Climate Model coupled to the Integrated BIosphere Simulator), which is designed to study the dynamic interactions between climate and vegetation and the global carbon cycle. We analyze the climate simulated by CCM3/IBIS with fixed vegetation conditions and we compare it to the climate simulated by the standard CCM3, which includes the LSM (land surface model) land-surface package. Important differences between the two models include simple parametrizations of lakes, wetlands and crops in CCM3/LSM not taken into account in CCM3/IBIS. CCM3/IBIS and CCM3/LSM share common biases (compared to observations) in the temperature field in boreal winter and in the precipitation field annually, making the atmospheric model the most probable cause of those biases. The models differ in the temperature field and surface energy balance in the Sahara annually and in the mid-to high latitudes from spring through fall. CCM3/IBIS simulates global annual air temperatures that are on average 1.7 °C higher than CCM3/LSM and 0.5 °C higher than the observed climatology. Differences in albedo and/or snow parametrization explain most of the Sahara and high-latitude temperature disagreement. Our sensitivity study with CCM3/LSM shows that the presence of lakes and wetlands in CCM3/LSM can account for about half of the difference in temperature in summer over the lake and wetland regions of the mid-latitudes. A second sensitivity study shows that higher surface roughness length in CCM3/IBIS can also explain part of the difference in summer surface temperature in the mid-latitudes. Surface roughness length affects the surface temperature through a feedback mechanism linking surface wind speed, planetary boundary layer height, low level cloudiness and radiation  相似文献   

14.
A full global atmosphere-ocean-land vegetation model is used to examine the coupled climate/vegetation changes in the extratropics between modern and mid-Holocene (6,000 year BP) times and to assess the feedback of vegetation cover changes on the climate response. The model produces a relatively realistic natural vegetation cover and a climate sensitivity comparable to that realized in previous studies. The simulated mid-Holocene climate led to an expansion of boreal forest cover into polar tundra areas (mainly due to increased summer/fall warmth) and an expansion of middle latitude grass cover (due to a combination of enhanced temperature seasonality with cold winters and interior drying of the continents). The simulated poleward expansion of boreal forest and middle latitude expansion of grass cover are consistent with previous modeling studies. The feedback effect of expanding boreal forest in polar latitudes induced a significant spring warming and reduced snow cover that partially countered the response produced by the orbitally induced changes in radiative forcing. The expansion of grass cover in middle latitudes worked to reinforce the orbital forcing by contributing a spring cooling, enhanced snow cover, and a delayed soil water input by snow melt. Locally, summer rains tended to increase (decrease) in areas with greatest tree cover increases (decreases); however, for the broad-scale polar and middle latitude domains the climate responses produced by the changes in vegetation are relatively much smaller in summer/fall than found in previous studies. This study highlights the need to develop a more comprehensive strategy for investigating vegetation feedbacks.  相似文献   

15.
使用2014年5—9月塔克拉玛干沙漠腹地塔中大气环境观测实验站地表辐射观测资料,分析了塔中人工绿地与自然沙面在各种典型天气条件下的辐射分量特征差异。结果表明:人工绿地与自然沙面辐射平衡各分量最小值均出现在夜间,最大值出现在中午前后,总辐射和反射辐射日最大值出现在12:00;大气长波辐射日变化振幅较小,地面长波辐射日变化呈现不对称分布。晴天,人工绿地与自然沙面总辐射和净辐射变化幅度较小,自然沙面总辐射高于人工绿地;阴天,地面长波辐射略有减小,绿地大气长波辐射略有增加,总辐射和反射辐射减少,净辐射的变化受总辐射的影响,但减弱幅度小于总辐射;沙尘暴天气下,沙尘对辐射各分量影响明显,辐射各分量日变化不规则,人工绿地与自然沙面总辐射被明显削弱,日变化波动大。  相似文献   

16.
陆地生态系统模型及其与气候模式耦合的回顾   总被引：5，自引：2，他引：3  
陆地生态系统和气候系统通过能量通量、水汽通量、物质交换相互影响、作用。作者对陆地生态系统模型及其与气候模式耦合的研究进行综述和讨论,总结了当代5类主要全球陆地生态系统模型,即生物地理模型、生物地球化学模型、森林林窗模型、陆面生物圈模型和动态全球植被模型,以及它们与气候模式耦合的研究进展。阐述了动态全球植被模型及其与气候模式耦合研究在全球变化研究的重要作用。最后,对未来模拟研究的方向进行了分析。  相似文献   

17.
华文剑  陈海山 《大气科学》2011,35(1):121-133
利用“国际耦合模式比较计划” (Phase 3 of the Coupled Model Intercomparison Project, CMIP3) 12个模式对20世纪 (The Twentieth-Century Climate in Coupled Models, 20C3M) 和21世纪SRES (Special Report on Emissions Scenarios) A1B 情景下的模拟结果, 通过21世纪 (2001～2099年) 与20世纪 (1901～1999年) 陆面能量和水文变量的对比分析, 揭示了陆面过程对全球变暖响应的基本特征, 并探讨了其可能的响应机制。结果表明, 与20世纪相比, 21世纪全球陆面平均的表面温度、 地表净辐射、 潜热通量明显增加; 而感热通量有所减小。降水、 径流、 蒸发等地表水循环分量也表现出不同程度的增加, 而土壤含水量有减小趋势。通过分析近地层主要大气强迫变量与陆面变量之间的联系, 发现陆面能量平衡过程对全球变暖的响应主要受向下长波辐射和气温变化的影响, 而温度的变化对陆面水文过程的影响起决定性的作用。进一步分析表明, 陆面过程对全球变暖的响应存在明显的区域性差异, 陆面温度和感热对全球变暖响应最显著的区域位于北半球中高纬, 而净辐射和潜热对全球变暖的响应在亚洲中部和非洲大陆最显著。相对于20世纪, 21世纪主要是长波辐射和温度对陆面能量平衡过程的贡献重要。对于陆面水文过程, 径流和土壤含水量对全球变暖的响应在亚洲中部以及北美最显著。在全球变暖背景下, 21世纪相对于20世纪, 温度对陆面水循环的影响更加显著, 主要体现在北半球中纬度地区。  相似文献   

18.
徐康  何金海  祝从文 《气象学报》2011,69(4):570-580
最近50年全球变暖,陆地增温幅度大于海洋,主要的增温中心位于亚洲北部、欧洲和北美等地区。因此,全球变暖有可能通过改变大尺度季风环流而影响中国气候变化。利用美国国家航空航天局空间研究中心(GISS)的逐月地表气温资料、NCEP/NCAR再分析资料及中国604个站逐月气温和降水观测资料,重点讨论了1951—2007年中国东部夏季降水与同期的北半球大陆地表气温年代际尺度变化关系。结果表明,近50年中国东部夏季降水异常主要表现为南旱北涝与南涝北旱两者年代际异常之间的转换,但在1996年之后,伴随北方干旱区向南发展,呈现出华北和长江中下游地区降水同时减少的特征。研究发现中国华北地区夏季降水与同期的环贝加尔湖地表气温在年代际尺度上存在显著的负相关关系;贝加尔湖地区地表气温增暖可能导致蒙古高原对流层出现异常的暖性反气旋,使得位于蒙古高原的气旋频数减少和强度减弱。由于华北降水与蒙古气旋的活动直接相关,从而导致华北地区夏季降水的持续性减少。自1996年开始贝加尔湖地区的地表气温进一步升高,导致中国北方干旱化加剧。由于环贝加尔湖地区是过去50年全球变暖的最显著地区之一,因此,全球变暖可能是通过关键区域的温度变化对中国的气候变化产...  相似文献   

19.
During the early and mid-Pliocene, the period from 5 to 3 million years ago, approximately, the Earth is believed to have been significantly warmer than it is today, but the reasons for the higher temperatures are unclear. This paper explores the impact of recent findings that suggest that, at that time, cold surface waters were absent from the tropical and subtropical oceanic upwelling zones. El Niño was in effect a perennial rather than intermittent phenomenon, and sea surface temperatures in low latitudes were essentially independent of longitude. When these conditions are specified as the lower boundary condition for an atmospheric GCM, we find that the trade winds along the equator, and hence the Walker Circulation, collapse. The low-level stratus clouds in low latitudes diminish greatly, thus reducing the albedo of the Earth. The atmospheric concentration of water vapor increases, and enhanced latent heat release due to stronger evaporation warms up the tropical atmosphere, particularly between 40°S and 20°N. Moreover, teleconnection patterns from the Pacific induce a warming over North America that is enhanced by surface albedo feedback, a process that may have helped to maintain this region ice-free before 3 Ma. The results presented here indicate that the suggested absence of cold surface waters from the tropical and subtropical oceanic upwelling zones could have contributed significantly to the Pliocene warmth.  相似文献   

20.
In a changing climate, changes in rainfall variability and, in particular, extreme rainfall events are likely to be highly significant for environmentally vulnerable regions such as southern Africa. It is generally accepted that sea-surface temperatures play an important role in modulating rainfall variability, thus the majority work to date has focused on these mechanisms. However past research suggests that land surface processes are also critical for rainfall variability. In particular, work has suggested that the atmosphere-land surface feedback has been important for past abrupt climate changes, such as those which occurred over the Sahara during the mid-Holocene or, more recently, the prolonged Sahelian drought. Therefore the primary aim of this work is to undertake idealised experiments using both a regional and global climate model, to test the sensitivity of rainfall variability to land surface changes over a location where such abrupt climate changes are projected to occur in the future, namely southern Africa. In one experiment, the desert conditions currently observed over southwestern Africa were extended to cover the entire subcontinent. This is based on past research which suggests a remobilisation of sand dune activity and spatial extent under various scenarios of future anthropogenic global warming. In the second experiment, savanna conditions were imposed over all of southern Africa, representing an increase in vegetation for most areas except the equatorial regions. The results suggest that a decrease in rainfall occurs in the desert run, up to 27% of total rainfall in the regional model (relative to the control), due to a reduction in available moisture, less evaporation, less vertical uplift and therefore higher near surface pressure. This result is consistent across both the regional and global model experiments. Conversely an increase in rainfall occurs in the savanna run, because of an increase in available moisture giving an increase in latent heat and therefore surface temperature, increasing vertical uplift and lowering near surface pressure. These experiments, however, are only preliminary, and form the first stage of a wider study into how the atmosphere-land surface feedback influences rainfall extremes over southern Africa in the past (when surface i.e. vegetation conditions were very different) and in the future under various scenarios of future climate change. Future work will examine how other climate models simulate the atmosphere-land surface feedback, using more realistic vegetation types based on past and future surface conditions.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号