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 62 毫秒
1.
The purpose of this study is to perform a high-resolution general circulation model (GCM) experiment to quantify the sensitivity of regional climate to change in vegetation around the Mediterranean basin, corresponding to vegetation change during the Roman Classical Period (RCP), about 2000 years BP. First, an RCP vegetation distribution based on fossil pollen maps and historical records was defined. Second, the RCP vegetation inferred from palynology and other proxies was converted to the 12 vegetation types required by the biosphere model implemented in the GCM. The albedo change due to the change in vegetation significantly alters the atmospheric circulation over northern Africa and the Mediterranean. The consequences of this change involve a northward shift of the ITCZ in the African continent and a coupled circulation between northwestern Africa and the Mediterranean Sea. A large increase of precipitation occurs over the Sahel, the Nile valley and northwestern Africa. A smaller increase of precipitation occurs also over the Iberian Peninsula and the region corresponding to the south of the Caucasus range (Armenia). The increase of precipitation over northern Africa, the Iberian Peninsula and the Armenian region are consistent with the pollen, historical and geographical data. These results suggest that deforestation around the Mediterranean during the last 2000 years contributed to the dryness of the current climate.  相似文献   

2.
The effect of vegetation on the Younger Dryas (YD) climate is studied by comparing the results of four experiments performed with the ECHAM-4 atmospheric general circulation model (AGCM): (1) modern control climate, (2) simulation with YD boundary conditions, but with modern vegetation, (3 and 4) identical to (2), but with paleo-vegetation. Prescribing paleo-vegetation instead of modern vegetation resulted in temperature anomalies (both positive and negative) of up to 4°C in the Northern Hemisphere mid-latitudes, mainly as an effect of changes in forest cover (change in albedo). Moreover, changes in precipitation and evaporation were found, most notably during December–January–February (DJF) in the tropics and were caused by the replacement of forests by grasslands. These results are consistent with other model studies on the role of vegetation changes on climate and they suggest that it is important in paleoclimate simulation studies to prescribe realistic vegetation types, belonging to the period of interest. However, in our case the addition of YD vegetation did not improve the agreement with proxy data in Europe, as the temperatures were increasing during winter compared to the YD simulation with modern vegetation. It must be noted that this increase was not statistically significant. The model-data mismatch suggests that other factors probably played an important role, such as permafrost and atmospheric dust. We infer that during the last glacial-interglacial transition, the time lag between the first temperature increase and the northward migration of trees, estimated at 500–1000 years, could have delayed the warming of the Eurasian continent. The relatively open vegetation that existed during the early stages of the last glacial-interglacial transition had a relatively high albedo, thus tempering warming up of the Eurasian land surfaces.  相似文献   

3.
To drive an atmospheric general circulation model (AGCM), land surface boundary conditions like albedo and morphological roughness, which depend on the vegetation type present, have to be prescribed. For the late Quaternary there are some data available, but they are still sparse. Here an artificial neural network approach to assimilate these paleovegetation data is investigated. In contrast to a biome model the relation between climatological parameters and vegetation type is not based on biological knowledge but estimated from the available vegetation data and the AGCM climatology at the corresponding locations. For a test application, a data set for the modern vegetation reduced to the amount of data available for the Holocene climate optimum (about 6000 years B.P.) is used. From this, the neural network is able to reconstruct the complete global vegetation with a kappa value of 0.56. The most pronounced errors occur in Australia and South America in areas corresponding to large data gaps.  相似文献   

4.
The sensitivity of the ocean circulation to changes in North Atlantic surface fluxes has become a major factor in explaining climate variability. The role of the Antarctic Bottom Water in modulating this variability has received much less attention, limiting the development of a complete understanding of decadal to millennial time-scale climate change. New analyses indicate that the southern deepwater source may change dramatically (e.g., experience a decrease of as much as two thirds during last 800 years). Such change can substantially alter the ocean circulation patterns of the last millennium. Additional analyses indicate that the Southern Hemisphere led the Northern Hemisphere changes in some of the glacial cycles of Pleistocene, implying a seesaw-type oscillation of the global ocean conveyor. The potential for melting of sea ice and ice sheets in the Antarctica associated with global warming can cause a further slowdown of the southern deepwater source. These results demand an assessment of the role of the Southern Ocean in driving changes of the global ocean circulation and climate. Systematic model simulation targeting the ocean circulation response to changes in surface salinity in the high latitudes of both Northern and Southern Hemispheres demonstrate that meltwater impacts in one hemisphere may lead to a strengthening of the thermohaline conveyor driven by the source in the opposite hemisphere. This, in turn, leads to significant changes in poleward heat transport. Further, meltwater events can lead to deep-sea warming and thermal expansion of abyssal water, that in turn cause a substantial sea-level change even without a major ice sheet melting.  相似文献   

5.
Using a recently developed global vegetation distribution, topography, and shorelines for the Early Eocene in conjunction with the Genesis version 2.0 climate model, we investigate the influences that these new boundary conditions have on global climate. Global mean climate changes little in response to the subtle changes we made; differences in mean annual and seasonal surface temperatures over northern and southern hemispheric land, respectively, are on the order of 0.5°C. In contrast, and perhaps more importantly, continental scale climate exhibits significant responses. Increased peak elevations and topographic detail result in larger amplitude planetary 4 mm/day and decreases by 7–9 mm/day in the proto Himalayan region. Surface temperatures change by up to 18°C as a direct result of elevation modifications. Increased leaf area index (LAI), as a result of altered vegetation distributions, reduces temperatures by up to 6°C. Decreasing the size of the Mississippi embayment decreases inland precipitation by 1–2 mm/day. These climate responses to increased accuracy in boundary conditions indicate that “improved” boundary conditions may play an important role in producing modeled paleoclimates that approach the proxy data more closely.  相似文献   

6.
We use three measures of aridity, the Köppen climate classification, the UNEP aridity index and the Budyko dryness index, to estimate the possible effects of late 21st century climate change on the Mediterranean region under increased greenhouse gas concentrations (A2 and B2 IPCC emission scenarios) as simulated with a high resolution (20 km grid interval) regional climate model (the ICTP RegCM). A basic validation of the reference simulation along with a brief discussion of the surface climate changes for the A2 and B2 scenarios is also provided. Analysis of the changes in all three aridity measures indicates that by the end of the 21st century the Mediterranean region might experience a substantial increase in the northward extension of dry and arid lands, particularly in the central and southern portions of the Iberian, Italian, Hellenic and Turkish peninsulas and in areas of southeastern Europe (e.g. Romania and Bulgaria), the Middle East, northern Africa and major Islands (Corsica, Sardinia and Sicily). Most Ice-Cap areas of the Alps are also projected to disappear. These effects are due to a large warming and pronounced decrease in precipitation, especially during the spring and summer seasons. In addition, fine scale topography and coastline features affect the aridity change signal. We identify the southern Mediterranean as a region particularly vulnerable to water stress and desertification processes under climate change conditions.  相似文献   

7.
The climate sensitivity to specification of agricultural and urban land cover was investigated using the climate version of the Pennsylvania State University/National Center for Atmospheric Research Mesoscale Model (MM5) for 1990 over northeastern United States. The simulations were for 5 yr at a spatial resolution of 36 km. Urbanization resulted in near-surface temperature increases of more than 1 K over the urban sites during both winter and summer. The increase in summer temperature due to urbanization was more widespread than that due to the effect of agricultural land use. The conversion of forest to agricultural land resulted in a decrease in temperature of more than 0.5 K during winter and an increase of more than 1 K during summer over the sites of perturbation. The reduced temperature during winter is related to snow cover. Agricultural lands are covered by snow while the trees in non-agricultural areas protrude through the snow, reducing the albedo of the surface. The warming during summer reflects reduced evaporation. Urbanization also reduces the diurnal temperature range (DTR) by about 0.4 K.  相似文献   

8.
This study aimed to disclose impacts of environment changes on hydrologic regimes in the Hei River Watershed, Shaanxi Province in China. We investigated the effects of the man-made landscape (Jingpen Reservoir) on the rainstorm–flood processes using a proposed Kinematic Wave model, simulated impacts of land use and cover changes on surface runoff generation and river flow characteristics at monthly, seasonal, and annual scales through designed scenarios of different combinations of land use and cover and climate conditions on basis of the SWAT model, evaluated the climate change and human activities effects on water balance from 1954 to 2001. Through these investigations, the following results were achieved. Firstly, it showed that the man-made landscape (the Jingpen Reservoir) had altered the rainstorm–flood process, the flood wave damped right after it flowed out the Jingpen Reservoir. Secondly, changes of land use and cover led to river flow redistribution, soil moisture and recharge fluctuations. Evapotranspiration increased 12.9%, river flow discharge decreased 17.7%, runoff generation process accelerated 1.31 times in 2000 than in 1986, and water resources of the total watershed decreased 7.7% in 2000 compared to the land use and cover scenario in 1986. Finally, the interaction between climate change and human activities led to the total water resource decreased by 10.6% in 2000 compared to that in 1986 in the Hei River Watershed.  相似文献   

9.
Climatic changes over the Mediterranean basin in 2031–2060, when a 2 °C global warming is most likely to occur, are investigated with the HadCM3 global circulation model and their impacts on human activities and natural ecosystem are assessed. Precipitation and surface temperature changes are examined through mean and extreme values analysis, under the A2 and B2 emission scenarios. Confidence in results is obtained via bootstrapping. Over the land areas, the warming is larger than the global average. The rate of warming is found to be around 2 °C in spring and winter, while it reaches 4 °C in summer. An additional month of summer days is expected, along with 2–4 weeks of tropical nights. Increase in heatwave days and decrease in frost nights are expected to be a month inland. In the northern part of the basin the widespread drop in summer rainfall is partially compensated by a winter precipitation increase. One to 3 weeks of additional dry days lead to a dry season lengthened by a week and shifted toward spring in the south of France and inland Algeria, and autumn elsewhere. In central Mediterranean droughts are extended by a month, starting a week earlier and ending 3 weeks later. The impacts of these climatic changes on human activities such as agriculture, energy, tourism and natural ecosystems (forest fires) are also assessed. Regarding agriculture, crops whose growing cycle occurs mostly in autumn and winter show no changes or even an increase in yield. In contrast, summer crops show a remarkable decrease of yield. This different pattern is attributed to a lengthier drought period during summer and to an increased rainfall in winter and autumn. Regarding forest fire risk, an additional month of risk is expected over a great part of the basin. Energy demand levels are expected to fall significantly during a warmer winter period inland, whereas they seem to substantially increase nearly everywhere during summer. Extremely high summer temperatures in the Mediterranean, coupled with improved climate conditions in northern Europe, may lead to a gradual decrease in summer tourism in the Mediterranean, but an increase in spring and autumn.  相似文献   

10.
Potential impacts of human-induced land cover change on East Asia monsoon   总被引：10，自引：0，他引：10  
As one major performance of anthropogenic activities, human-induced land use and land cover changes in East Asia have been one of the largest regions in the world. In the past 3000 years, more than 60% of the region has been affected by conversion of various categories of natural vegetation into farmland, conversion of grassland into semidesert and widespread land degradation. Such human-induced land cover changes result in significant changes of surface dynamic parameters, such as albedo, surface roughness, leaf area index and fractional vegetation coverage, etc.The results of a pair of numerical experiments in this paper have shown that by altering the complex exchanges of water and energy from surface to atmosphere, the changes in land cover have brought about significant changes to the East Asian monsoon. These include weakening of the summer monsoon and enhancement of winter monsoon over the region and a commensurate increase in anomalous northerly flow. These changes result in the reduction of all components of surface water balance such as precipitation, runoff, and soil water content. The consequent diminution of northward and inland moisture transfer may be a significant factor in explaining the decreasing of atmospheric and soil humidity and thus the trend in aridification observed in many parts of the region, particularly over Northern China during last 3000 years.The variation of East Asia monsoon presented here is the result of land cover changes only. It is very likely that the anthropogenic modification of monsoon system would have been occurred in the long history of civilization.  相似文献   

11.
The generation and development of dust storms are controlled by land surface conditions and atmospheric circulations. The latter, in turn, is influenced by the global ice–snow cover. In this study, we examine the relationship between the characteristics of dust storm activities in north China and the changes of global climate patterns. In particular, we are interested in whether Arctic ice–snow cover is related to the dust storm frequencies and intensities in north China. Our analysis, based on the monthly data for the period from 1954 to 1994, shows that this is indeed the case. This result suggests that the Arctic ice–snow cover can be used for the long-term prediction of dust storm activities in north China, and dust storm activities also serve as an indicator of global climate change.  相似文献   

12.
This paper analyses the effect of environmental changes observed in the 20th century on hydrology and water management in the southern Pyrenees, in terms of land use and climate. Moreover, a projected water-resource scenario for the 21st century is presented and discussed. Our results demonstrate that changes in precipitation, temperature, and snow accumulation, together with an increase in vegetation density in headwater regions, have led to a marked reduction in water availability in the region. Water resource managers have introduced major changes to dam operations to meet increasing water demand for irrigation purposes in lowland areas. Climatic and land-cover scenarios for the next century indicate that the sustainability of the equilibrium between available resources and water demand will be seriously threatened. These changes predicted for the Pyrenees may be representative of the changes that will occur within many other Mediterranean mountain sectors with similar climatic and socio-economic conditions.  相似文献   

13.
Environmental change in the Sahel–Sudan zone of West Africa has been a major issue in development debates over the last decades. Using remote sensing based land cover change analysis, archival data, national and international statistical data, and household interviews, we analyze the drivers of environmental change in Eastern Saloum in Central East Senegal as well as the local perceptions of these changes and adaptation. Being part of the ground nut basin, Eastern Saloum has witnessed rapid environmental degradation caused by the conversion of forest and savanna areas to agricultural land during the last 20–30 years and by a combination of decline in precipitation, soil degradation, a diversity of policies with little concern for the environment, fluctuating markets and population pressure. Farmers perceive the environmental change mainly as land degradation and poor soil fertility, though recent extensification of agriculture counters this effect and has led to increased vegetation cover in marginal areas. They identified erratic climate, agricultural policies, insufficient food production and desire to increase income as the main drivers of change in the area. We conclude that while climate variability has influenced environmental change in the area, various types of State interventions in agriculture and global market fluctuations appear to have been the main underlying causes of environmental degradation.  相似文献   

14.
The climate cooling and vegetation changes in the Miocene/Pliocene are generally well documented by various proxy data. Some important ecosystem changes occurred at that time. Palaeobotanical evidence suggests that the Sahara desert first appeared in the Pliocene, whereas in the Miocene North Africa was green. In the present study, we investigate the Late Miocene climate response to the appearance of the Sahara desert from a climate modelling sensitivity experiment. We compare a model experiment, which includes a full set of Late Miocene boundary conditions, with another one using the same boundary conditions except that the North African vegetation refers to the present-day situation. Our sensitivity study demonstrates that the introduction of the Sahara desert leads to a cooling and an aridification in Africa. In addition, we observe teleconnection patterns related to the North African desertification at around the Miocene/Pliocene boundary. From our sensitivity experiment, we observe that the Sahara contributes to a cooling in Central Asia and in North America. As compared to hypsodonty data for Central Asia, an increased aridity is underestimated in the Sahara experiment. Finally, we observe that the introduction of the Sahara leads to a cooling in the northern high latitudes. Hence, our sensitivity experiment indicates that the appearance of the Sahara desert is one piece to better understand Late Cenozoic climate cooling being most pronounced in the high latitudes.  相似文献   

15.
This paper examines the cloud radiative forcing and its impacts on the surface climate for global climate model simulations that use reduced ozone concentrations and land fractions as boundary conditions. In one simulation using present-day land continents, ozone concentrations are reduced to zero and compared to the present-day climate simulation. In the second set of simulations under global ocean conditions, the implied poleward transport of heat by the ocean is varied. The removal of ozone causes an increase in longwave cloud radiative forcing at the top of the atmosphere and the surface. The increase in longwave forcing melts sea-ice and snow at high latitudes leading 10–14°C warmer temperatures and globally a 2°C increase. The global ocean simulations lead to higher cloud fractions than present-day simulation. Without poleward transport of heat by the ocean, surface temperatures cool as a result of higher cloud fractions. Increasing the ocean heat transport by a factor of 3.33 brings about ice-free conditions. An 11°C difference in globally averaged surface air temperatures is found between the enhanced and zero poleward oceanic heat transport simulations. The longwave cloud radiative forcing from high cloud fractions enhance the surface warming in the polar regions during the winter season. Conversely, during the summer season, a high cloud fraction increases the shortwave cloud radiative forcing producing only moderately warm temperatures in the polar regions. High cloud fractions in polar regions during warm periods throughout geologic times may help to explain the reduced equator to pole temperature gradient.  相似文献   

16.
Land fraction and the solar energy at the top of the atmosphere (solar constant) may have been significantly lower early in Earth's history. It is likely that both of these factors played some important role in the climate of the early earth. The climate changes associated with a global ocean(i.e. no continents) and reduced solar constant are examined with a general circulation model and compared with the present-day climate simulation. The general circulation model used in the study is the NCAR CCM with a swamp ocean surface. First, all land points are removed in the model and then the solar constant is reduced by 10% for this global ocean case.Results indicate that a 4 K increase in air temperature occurs with global ocean simulation compared to the control. When solar constant is reduced by 10% under global ocean conditions a 23 K decrease in air temperature is noted. The global ocean warms much of the troposphere and stratosphere, while a reduction in the solar constant cools the troposphere and stratosphere. The largest cooling occurs near the surface with the lower solar constant.Global mean values of evaporation, water vapor amounts, absorbed solar radiation and the downward longwave radiation are increased under global ocean conditions, while all are reduced when the solar constant is lowered. The global ocean simulation produces sea ice only in the highest latitudes. A frozen planet does not occur when the solar constant is reduced—rather, the ice line settles near 30° of latitude. It is near this latitude that transient eddies transport large amounts of sensible heat across the ice line acting as a negative feedback under lower solar constant conditions keeping sea ice from migrating to even lower latitudes.Clouds, under lower solar forcing, also act as a negative feedback because they are reduced in higher latitudes with colder atmospheric temperatures allowing additional solar radiation to reach the surface. The overall effect of clouds in the global ocean is to act as a positive feedback because they are slightly reduced thereby allowing additional solar radiation to reach the surface and increase the warming caused by the removal of land. The relevance of the results to the “Faint-Young Sun Paradox” indicates that reduced land fraction and solar forcing affect dynamics, heat transport, and clouds. Therefore the associated feedbacks should be taken into account in order to understand their roles in resolving the “Faint-Young Sun Paradox”.  相似文献   

17.
  相似文献   

18.
Today, most land surface process models have prescribed seasonal change of vegetation with regard to the exchange processes between land and the atmosphere. However, in order to consider the real interaction between vegetation and atmosphere and represent it best in a climate model, the vegetation growth process should be included. In other words, “life” should be brought into climate models. In this study, we have coupled the physical and biological components of AVIM (Atmosphere–Vegetation Interaction Model), a land surface model including plant ecophysiological processes, into the IAP/LASG L9 R15 GOALS GCM. To exhibit terrestrial vegetation information, the vegetation is given a high resolution of 1.5° by 1.5° to nest and couple the fine grid cells of land with the coarse grid cells of atmosphere, which is 7.5° longitude and 4.5° latitude. The simulated monthly mean surface air temperature and precipitation is close to the observations. The monthly mean Leaf Area Index (LAI) is consistent with the observed data. The global annual mean net primary production (NPP) simulation is also reasonable. The coupled model is stable, providing a good platform for research on two-way interaction between land and atmosphere, and the global terrestrial ecosystem carbon cycle.  相似文献   

19.
One response of vegetation to future increases in atmospheric CO2 may be a widespread increase in stomatal resistance. Such a response would increase plant water usage efficiency while still allowing CO2 assimilation at current rates. The associated reduction in transpiration rates has the potential of causing significant modifications in climate on regional and global scales.This paper describes the effects of a uniform doubling of the stomatal resistance parameterization in a global climate model (GENESIS). The model includes a land-surface transfer scheme (LSX) that accounts for the physical effects of vegetation, including stomatal resistance and transpiration, which is described in detail in an appendix. The atmospheric general circulation model is a heavily modified version of the NCAR Community Climate Model version 1 with new treatments of clouds, penetrative convection, planetary boundary layer mixing, solar radiation, the diurnal cycle, and semi-Lagrangian transport of water vapor. The other surface models include multi-layer models of soil, snow and sea ice, and a 50-m slab ocean mixed layer.The effects of doubling the stomatal resistance parameterization are largest in heavily forested regions: tropical South America, and parts of the Northern Hemispheric boreal forests in Canada, Russia and Siberia in summer. The primary surface changes are a decrease in evapotranspiration, an increase in upward sensible heat flux, and a surface-air warming. Secondary effects include shifts in the ITCZ which cause large increases in precipitation, soil moisture and runoff in western tropical South America, and decreases in these quantities in northern subtropical Africa. Noticeable changes in relative humidity, cloudiness and meridional circulation occur throughout the troposphere. The global effects on atmospheric temperature and specific humidity are small fractions of those found in other doubled CO2 experiments. However, unlike doubled CO2 the signs of those changes combine to give relatively large reductions in relative humidity and cloudiness. It is suggested that the stomatal-resistance effect and other plant responses to large-scale environmental perturbations should be included in models of future climate.  相似文献   

20.
The microcharcoal content (particles < 180 µm) of overlapping sedimentary sequences from two crater lake basins in central Turkey are used to reconstruct the regional fire history of the East Mediterranean oak–grass parkland zone from the Last Glacial Maximum to the present-day. These results are correlated with stable isotope and pollen data from the same cores in order to assess the changing role of climate, vegetation and human activity in landscape burning. This indicates that climatically-induced variation in biomass availability was the main factor controlling the timing of regional fire activity during the Last Glacial–Interglacial climatic transition, and again during Mid-Holocene times, with fire frequency and magnitude increasing during wetter climatic phases. Spectral analysis of the Holocene part of the record from Eski Acıgöl indicates significant cyclicity with a periodicity of ~ 1500 years that may be linked with large-scale climate forcing. Although proto-agricultural societies were established in this region as early as 10,000 years ago, it is only during the last two to three millennia that the pacing of wildfire cycles appears to have become decoupled from climate and linked instead to human-induced changes in land cover and fuel load availability.  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