Impacts of Irrigation and Vegetation Growth on Summer Rainfall in the Taklimakan Desert

In recent decades, a greening tendency due to increased vegetation has been noted around the Taklimakan Desert(TD), but the impact of such a change on the local hydrological cycle remains uncertain. Here, we investigate the response of the local hydrological cycle and atmospheric circulation to a green TD in summer using a pair of global climate model(Community Earth System Model version 1.2.1) simulations. With enough irrigation to support vegetation growth in the TD, the modeling suggests first, that significant increases in local precipitation are attributed to enhanced local recycling of water, and second, that there is a corresponding decrease of local surface temperatures. On the other hand, irrigation and vegetation growth in this low-lying desert have negligible impacts on the large-scale circulation and thus the moisture convergence for enhanced precipitation. It is also found that the green TD can only be sustained by a large amount of irrigation water supply since only about one-third of the deployed water can be "recycled " locally. Considering this,devising a way to encapsulate the irrigated water within the desert to ensure more efficient water recycling is key for maintaining a sustainable, greening TD.     

Atmosphere-land surface interactions and their influence on extreme rainfall and potential abrupt climate change over southern Africa

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.     

Enhanced albedo feedback in North Africa from possible combined vegetation and soil-formation processes

It has long been recognized that albedo related vegetation feedbacks amplify climate variability in North Africa. Recent studies have revealed that areas of very high albedo associated with certain desert soil types contribute to the current dry climate of the region. We construct three scenarios of North African albedo, one based on satellite measurements, one where the highest albedo resembles that of soils in the desert transition zones, and one based on a vegetation map for the “green Sahara” state of the middle Holocene, ca. 6,000 years ago. Using a series of climate model simulations, we find that the additional amplitude of albedo change from the middle Holocene to the present caused by the very bright desert soils enhances the magnitude of the June-to-August precipitation change in the region of the present Sahara from 0.6 to 1.0 mm/day on average. We also find that albedo change has a larger effect on regional precipitation than changes in either the Earth’s orbit or sea surface temperatures between 6,000 years ago and today. Simulated precipitation agrees rather well with present observations and mid Holocene reconstructions. Our results suggest that there may exist an important climate feedback from soil formation processes that has so far not been recognized.     

Fully coupled climate/dynamical vegetation model simulations over Northern Africa during the mid-Holocene

 The climate and vegetation patterns of the middle Holocene (6000 years ago; 6 ka) over Northern Africa are simulated using a fully-synchronous climate and dynamical vegetation model. The coupled model predicts a northward shift in tropical rainforest and tropical deciduous forest vegetation by about 5 degrees of latitude, and an increase in grassland at the present-day simulated Saharan boundaries. The northward expansion of vegetation over North Africa at 6 ka is initiated by an orbitally-induced amplification of the summer monsoon, and enhanced by feedback effects induced by the vegetation. These combined processes lead to a major reduction in Saharan desert area at 6 ka relative to present-day of about 50%. However, as shown in previous asynchronous modelling studies, the coupled climate/vegetation model does not fully reproduce the vegetation patterns inferred from palaeoenvironmental records, which suggest that steppe vegetation may have existed across most of Northern Africa. Orbital changes produce an intensification of monsoonal precipitation during the peak rainy season (July to September), whilst vegetation feedbacks, in addition to producing further increases in the peak intensity, play an important role in extending the rainy season from May/June through to November. The orbitally induced increases in precipitation are relatively uniform from west to east, in contrast to vegetation feedback-induced increases in precipitation which are concentrated in western North Africa. Annual-average precipitation increases caused by vegetation feedbacks are simulated to be of similar importance to orbital effects in the west, whilst they are relatively unimportant farther to the east. The orbital, vegetation and combined orbital and vegetation-induced changes in climate, from the simulations presented in this study, have been compared with results from previous modelling studies over the appropriate North African domain. Consequently, the important role of vegetation parametrizations in determining the magnitude of vegetation feedbacks has been illustrated. Further modelling studies which include the effects of changes in ocean temperature and changes in soil properties may be needed, along with additional observations, to resolve the discrepancy between model predictions of vegetation and palaeorecords for North Africa. Received: 15 June 1999 / Accepted: 14 December 1999     

West African monsoon response to greenhouse gas and sulphate aerosol forcing under two emission scenarios

The impact of increased greenhouse gases (GHG) and aerosols concentrations upon the West African monsoon (WAM) is investigated for the late twenty-first century period using the Météo-France ARPEGE-IFS high-resolution atmospheric model. Perturbed (2070–2100) and current (1961–2000) climates are compared using the model in time-slice mode. The model is forced by global sea surface temperatures provided by two transient scenarios performed with low-resolution coupled models and by two GHG evolution scenarios, SRES-A2 and SRES-B2. Comparing to reanalysis and observed data sets, the model is able to reproduce a realistic seasonal cycle of WAM despite a clear underestimation of the African Easterly Jet (AEJ) during the boreal summer. Mean temperature change indicates a global warming over the continent (stronger over North and South Africa). Simulated precipitation change at the end of the twenty-first century shows an increase in precipitation over Sudan-Sahel linked to a strong positive feedback with surface evaporation. Along Guinea Gulf coast, rainfall regimes are driven by large-scale moisture advection. Moreover, results show a mean precipitation decrease (increase) in the most (less) enhanced GHG atmosphere over this region. Modification of the seasonal hydrological cycle consists in a rain increase during the monsoon onset. There is a significant increase in rainfall variance over the Sahel, which extends over the Guinea coast region in the moderate emission scenario. Enhanced precipitation over Sahel is linked to large-scale circulation changes, namely a weakening of the AEJ and an intensification of the Tropical Easterly Jet.     

Applying the energy- and water balance model for incorporation of the cryospheric component into a climate model. Part II. Modeled mass balance on the green land ice sheet surface

The Greenland ice sheet is a very important potential source of fresh water inflow to the World Ocean under warming climate conditions. Apparently, it was the same during the Last Interglacial 130-115 thousand years ago. In order to quantify input of the Greenland ice sheet to the rise of the global mean sea level in the past or in the future, we include a surface mass balance model block into the Earth System Model. The computational algorithm is based on the calculation of energy balance on the ice sheet surface. The key tuning parameter of the model is the daily amplitude of air surface temperature. It defines the area and the rate of snow or ice melting. The range of possible values of this parameter is determined during a series of numerical experiments. High sensitivity of meltwater runoff volume to surface air temperature amplitude is revealed.     

陆面过程对全球变暖的响应及可能机制—基于CMIP3的多模式集合分析

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。     

Impact of vegetation changes on the dynamics of the atmosphere at the Last Glacial Maximum

Much work is under way to identify and quantify the feedbacks between vegetation and climate. Palaeoclimate modelling may provide a mean to address this problem by comparing simulations with proxy data. We have performed a series of four simulations of the Last Glacial Maximum (LGM, 21,000 years ago) using the climate model HadSM3, to test the sensitivity of climate to various changes in vegetation: a global change (according to a previously discussed simulation of the LGM with HadSM3 coupled to the dynamical vegetation model TRIFFID); a change only north of 35°N; a change only south of 35°N; and a variation in stomatal opening induced by the reduction in atmospheric CO₂ concentration. We focus mainly on the response of temperature, precipitation, and atmosphere dynamics. The response of continental temperature and precipitation mainly results from regional interactions with vegetation. In Eurasia, particularly Siberia and Tibet, the response of the biosphere substantially enhances the glacial cooling through a positive feedback loop between vegetation, temperature, and snow-cover. In central Africa, the decrease in tree fraction reduces the amount of precipitation. Stomatal opening is not seen to play a quantifiable role. The atmosphere dynamics, and more specifically the Asian summer monsoon system, are significantly altered by remote changes in vegetation: the cooling in Siberia and Tibet act in concert to shift the summer subtropical front southwards, weaken the easterly tropical jet and the momentum transport associated with it. By virtue of momentum conservation, these changes in the mid-troposphere circulation are associated with a slowing of the Asian summer monsoon surface flow. The pattern of moisture convergence is slightly altered, with moist convection weakening in the western tropical Pacific and strengthening north of Australia.     

NUMERICAL SIMULATION OF THE INFLUENCE OF VEGETATION COVER FACTOR ON BOUNDARY LAYER CLIMATE IN SEMI-ARID REGION

A numerical model has been developed for simulating land-surface processes and atmosphericboundary layer climate of vegetation and desert in semi-arid region.Dynamically,thermal andhydrological processes take place in the atmospheric boundary layer.Vegetation and surface layerof soil are included in the soil-vegetation-atmosphere coupled system,in which,vegetation isconsidered as a horizontally uniform layer,soil is divided into 13 layers and the horizontaldifferences of variables in the system are neglected.The influence of local boundary layer climateby vegetation cover factor is simulated with the coupled model in the semi-arid region of NorthwestChina (around 38°N,105°E).Results indicate that due to significant differences of water andenergy budgets in vegetation and desert region,the air is colder and wetter over the vegetation andcorrespondingly an obvious local circulation in the lower atmosphere is formed.Simulating results also show that maximum updraft and downdraft occur around thevegetation-desert marginal area,where the dynamical and thermodynamical properties of PBL(Planetary Boundary Layer) are uncontinuous.It is stronger at daytime,weaker and reverse atnighttime.In the simulation,the moisture inversion phenomena are analyzed.Finally.theinfluences of vegetation cover factor exchange on local boundary layer climate are simulated.Thesimulating results bring to light that water may be conserved and improved by developing treeplanting and afforestation,and improving cover factor of vegetation in local ecoenvironment,andthis is an important way of transforming local climate in arid and semi-arid area.Results indicatethat the coupled model can be used to study the soil-vegetation-atmosphere interaction and localboundary layer climate.     

The role of sea ice in the temperature-precipitation feedback of glacial cycles

The response of the hydrological cycle to climate variability and change is a critical open question, where model reliability is still unsatisfactory, yet upon which past climate history can shed some light. Sea ice is a key player in the climate system and in the hydrological cycle, due to its strong albedo effect and its insulating effect on local evaporation and air-sea heat flux. Using an atmospheric general circulation model with specified sea surface temperature and sea-ice distribution, the role of sea ice in the hydrological cycle is investigated under last glacial maximum (LGM) and present day conditions, and by studying its contribution to the “temperature-precipitation feedback”. By conducting a set of sensitivity experiments in which the albedo and thickness of the sea ice are varied, the various effects of sea ice in the hydrological cycle are isolated. It is demonstrated that for a cold LGM like state, a warmer climate (as a result of reduced sea-ice cover) leads to an increase in snow precipitation over the ice sheets. The insulating effect of the sea ice on the hydrological cycle is found to be larger than the albedo effect. These two effects interact in a nonlinear way and their total effect is not equal to summing their separate contribution.     

Decadal variability of rainfall in the Sahel: results from the coupled GENESIS-IBIS atmosphere-biosphere model

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.     

Vegetation Feedback at the Mid-Pliocene

A global atmospheric general circulation model and an asynchronously coupled global atmosphere-biome model are used to simulate vegetation feedback at the mid-Pliocene approximately 3.3 to 3.0 million years ago.For that period,the simulated vegetation differed from present conditions at 62%of the global ice-free land surface.Vegetation feedback had little overall impact on the global climate of the mid-Pliocene.At the regional scale,however,the interactive vegetation led to statistically significant increases in annual temperature over Greenland,the high latitudes of North America,the mid-high latitudes of eastern Eurasia,and western Tibet,and reductions in most of the land areas at low latitudes,owing to vegetation-induced changes in surface albedo.     

Holocene hydrological cycle changes in the Southern Hemisphere documented in East Antarctic deuterium excess records

Four Holocene-long East Antarctic deuterium excess records are used to study past changes of the hydrological cycle in the Southern Hemisphere. We combine simple and complex isotopic models to quantify the relationships between Antarctic deuterium excess fluctuations and the sea surface temperature (SST) integrated over the moisture source areas for Antarctic snow. The common deuterium excess increasing trend during the first half of the Holocene is therefore interpreted in terms of a warming of the average ocean moisture source regions over this time. Available Southern Hemisphere SST records exhibit opposite trends at low latitudes (warming) and at high latitudes (cooling) during the Holocene. The agreement between the Antarctic deuterium excess and low-latitude SST trends supports the idea that the tropics dominate in providing moisture for Antarctic precipitation. The opposite trends in SSTs at low and high latitudes can potentially be explained by the decreasing obliquity during the Holocene inducing opposite trends in the local mean annual insolation between low and high latitudes. It also implies an increased latitudinal insolation gradient that in turn can maintain a stronger atmospheric circulation transporting more tropical moisture to Antarctica. This mechanism is supported by results from a mid-Holocene climate simulation performed using a coupled ocean-atmosphere model. Received: 7 July 1999 / Accepted: 21 July 2000     

Coupling between subtropical cloud feedback and the local hydrological cycle in a climate model

In HadGEM2-A, AMIP experiments forced with observed sea surface temperatures respond to uniform and patterned +4 K SST perturbations with strong positive cloud feedbacks in the subtropical stratocumulus/trade cumulus transition regions. Over the subtropical Northeast Pacific at 137°W/26°N, the boundary layer cloud fraction reduces considerably in the AMIP +4 K patterned SST experiment. The near-surface wind speed and the air-sea temperature difference reduces, while the near-surface relative humidity increases. These changes limit the local increase in surface evaporation to just 3 W/m² or 0.6 %/K. Previous studies have suggested that increases in surface evaporation may be required to maintain maritime boundary layer cloud in a warmer climate. This suggests that the supply of water vapour from surface evaporation may not be increasing enough to maintain the low level cloud fraction in the warmer climate in HadGEM2-A. Sensitivity tests which force the surface evaporation to increase substantially in the +4 K patterned SST experiment result in smaller changes in boundary layer cloud and a weaker cloud feedback in HadGEM2-A, supporting this idea. Although global mean surface evaporation in climate models increases robustly with global temperature (and the resulting increase in atmospheric radiative cooling), local values may increase much less, having a significant impact on cloud feedback. These results suggest a coupling between cloud feedback and the hydrological cycle via changes in the patterns of surface evaporation. A better understanding of both the factors controlling local changes in surface evaporation and the sensitivity of clouds to such changes may be required to understand the reasons for inter-model differences in subtropical cloud feedback.     

Regional modelling of future African climate north of 15°S including greenhouse warming and land degradation

Previous studies have highlighted the crucial role of land degradation in tropical African climate. This effect urgently has to be taken into account when predicting future African climate under enhanced greenhouse conditions. Here, we present time slice experiments of African climate until 2025, using a high-resolution regional climate model. A supposable scenario of future land use changes, involving vegetation loss and soil degradation, is prescribed simultaneously with increasing greenhouse-gas concentrations in order to detect, where the different forcings counterbalance or reinforce each other. This proceeding allows us to define the regions of highest vulnerability with respect to future freshwater availability and food security in tropical and subtropical Africa and may provide a decision basis for political measures. The model simulates a considerable reduction in precipitation amount until 2025 over most of tropical Africa, amounting to partly more than 500 mm (20–40% of the annual sum), particularly in the Congo Basin and the Sahel Zone. The change is strongest in boreal summer and basically reflects the pattern of maximum vegetation cover during the seasonal cycle. The related change in the surface energy fluxes induces a substantial near-surface warming by up to 7°C. According to the modified temperature gradients over tropical Africa, the summer monsoon circulation intensifies and transports more humid air masses into the southern part of West Africa. This humidifying effect is overcompensated by a remarkable decrease in surface evaporation, leading to the overall drying tendency over most of Africa. Extreme daily rainfall events become stronger in autumn but less intense in spring. Summer and autumn appear to be characterized by more severe heat waves over Subsaharan West Africa. In addition, the Tropical Easterly Jet is weakening, leading to enhanced drought conditions in the Sahel Zone. All these results suggest that the local impact of land degradation and reduction of vegetation cover may be more important in tropical Africa than the global radiative heating, at least until 2025. This implies that vegetation protection measures at a national scale may directly lead to a mitigation of the expected negative implications of future climate change in tropical Africa.     

Vegetation feedback under future global warming

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.     

Role of dynamic vegetation in regional climate predictions over western Africa

This study examines the role of vegetation dynamics in regional predictions of future climate change in western Africa using a dynamic vegetation model asynchronously coupled to a regional climate model. Two experiments, one for present day and one for future, are conducted with the linked regional climate-vegetation model, and the third with the regional climate model standing alone that predicts future climate based on present-day vegetation. These simulations are so designed in order to tease out the impact of structural vegetation feedback on simulated climate and hydrological processes. According to future predictions by the regional climate-vegetation model, increase in LAI is widespread, with significant shift in vegetation type. Over the Guinean Coast in 2084–2093, evergreen tree coverage decreases by 49% compared to 1984–1993, while drought deciduous tree coverage increases by 56%. Over the Sahel region in the same period, grass cover increases by 31%. Such vegetation changes are accompanied by a decrease of JJA rainfall by 2% over the Guinean Coast and an increase by 23% over the Sahel. This rather small decrease or large increase of precipitation is largely attributable to the role of vegetation feedback. Without the feedback effect from vegetation, the regional climate model would have predicted a 5% decrease of JJA rainfall in both the Guinean Coast and the Sahel as a result of the radiative and physiological effects of higher atmospheric CO₂ concentration. These results demonstrate that climate- and CO₂-induced changes in vegetation structure modify hydrological processes and climate at magnitudes comparable to or even higher than the radiative and physiological effects, thus evincing the importance of including vegetation feedback in future climate predictions.     

Quantifying the role of biosphere-atmosphere feedbacks in climate change: coupled model simulations for 6000 years BP and comparison with palaeodata for northern Eurasia and northern Africa

 The LMD AGCM was iteratively coupled to the global BIOME1 model in order to explore the role of vegetation-climate interactions in response to mid-Holocene (6000 y BP) orbital forcing. The sea-surface temperature and sea-ice distribution used were present-day and CO₂ concentration was pre-industrial. The land surface was initially prescribed with present-day vegetation. Initial climate “anomalies” (differences between AGCM results for 6000 y BP and control) were used to drive BIOME1; the simulated vegetation was provided to a further AGCM run, and so on. Results after five iterations were compared to the initial results in order to identify vegetation feedbacks. These were centred on regions showing strong initial responses. The orbitally induced high-latitude summer warming, and the intensification and extension of Northern Hemisphere tropical monsoons, were both amplified by vegetation feedbacks. Vegetation feedbacks were smaller than the initial orbital effects for most regions and seasons, but in West Africa the summer precipitation increase more than doubled in response to changes in vegetation. In the last iteration, global tundra area was reduced by 25% and the southern limit of the Sahara desert was shifted 2.5 °N north (to 18 °N) relative to today. These results were compared with 6000 y BP observational data recording forest-tundra boundary changes in northern Eurasia and savana-desert boundary changes in northern Africa. Although the inclusion of vegetation feedbacks improved the qualitative agreement between the model results and the data, the simulated changes were still insufficient, perhaps due to the lack of ocean-surface feedbacks. Received: 5 December 1996 / Accepted: 16 June 1997     

Climate sensitivity and response

Results from climate change simulations indicate a reasonably robust proportionality between global mean radiative forcing and global mean surface air temperature response. The "constant" of proportionality is a measure of the overall strength of climate feedback processes and hence of global climate sensitivity. Geographically, however, temperature response patterns are generally not proportional to, nor do they resemble, their parent forcing patterns. Temperature response patterns, nevertheless, exhibit a remarkable additivity whereby the sum of response patterns for different forcings closely resembles the response pattern for the sum of the forcings. The geographical distribution of contributions to the climate sensitivity/feedback are obtained diagnostically from simulations with the Canadian Centre for Climate Modelling and Analysis (CCCma) coupled global climate model (GCM). There is positive feedback over high-latitude oceans, over northern land areas, and over the equatorial Pacific. The remaining regions over oceans and tropical land areas exhibit negative feedback. The feedback results are decomposed into components associated with short-and longwave radiative processes and in terms of cloud-free atmosphere/surface and cloud feedbacks. While the geographic pattern of the feedbacks may generally be linked to local processes, all feedback processes display regions of both positive and negative values (except for the solar atmosphere/surface feedback associated with the retreat of ice and snow which is positive) and all vary from place to place so that there is no simple physical picture that operates everywhere. The stable geographical pattern of the feedback is a consequence of the balance between local physical processes rather than the dominance of a particular process. The feedback results indicate that, to first order, temperature response patterns are determined by the geographical pattern of local feedback processes. The feedback processes act to localize forcing changes and to generate temperature response patterns which depend firstly on the pattern of feedbacks and only secondarily on the pattern of the forcing. The geographical distribution of feedback processes can be regarded as a feature of the climate model (and by inference of the climate system) and not (or only comparatively weak) functions of forcing and climate state. An illustrative model is able to reproduce qualitatively the kinds of forcing/temperature response behavior seen in the CCCma GCM including the quasi-independence of forcing and response patterns, the additivity of temperature response patterns, and the resulting "non-constancy" of the global climate sensitivity.     

Natural and anthropogenic climate change: incorporating historical land cover change,vegetation dynamics and the global carbon cycle

This study explores natural and anthropogenic influences on the climate system, with an emphasis on the biogeophysical and biogeochemical effects of historical land cover change. The biogeophysical effect of land cover change is first subjected to a detailed sensitivity analysis in the context of the UVic Earth System Climate Model, a global climate model of intermediate complexity. Results show a global cooling in the range of –0.06 to –0.22 °C, though this effect is not found to be detectable in observed temperature trends. We then include the effects of natural forcings (volcanic aerosols, solar insolation variability and orbital changes) and other anthropogenic forcings (greenhouse gases and sulfate aerosols). Transient model runs from the year 1700 to 2000 are presented for each forcing individually as well as for combinations of forcings. We find that the UVic Model reproduces well the global temperature data when all forcings are included. These transient experiments are repeated using a dynamic vegetation model coupled interactively to the UVic Model. We find that dynamic vegetation acts as a positive feedback in the climate system for both the all-forcings and land cover change only model runs. Finally, the biogeochemical effect of land cover change is explored using a dynamically coupled inorganic ocean and terrestrial carbon cycle model. The carbon emissions from land cover change are found to enhance global temperatures by an amount that exceeds the biogeophysical cooling. The net effect of historical land cover change over this period is to increase global temperature by 0.15 °C.