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1.
Vegetation is a major component of the climate system because of its controls on the energy and water balance over land. This functioning changes because of the physiological response of leaves to increased CO2. A climate model is used to compare these changes with the climate changes from radiative forcing by greenhouse gases. For this purpose, we use the Community Earth System Model coupled to a slab ocean. Ensemble integrations are done for current and doubled CO2. The consequent reduction of transpiration and net increase of surface radiative heating from reduction in cloudiness increases the temperature over land by a significant fraction of that directly from the radiative warming by CO2. Large-scale atmospheric circulation adjustments result. In particular, over the tropics, a low-level westerly wind anomaly develops associated with reduced geopotential height over land, enhancing moisture transport and convergence, and precipitation increases over the western Amazon, the Congo basin, South Africa, and Indonesia, while over mid-latitudes, land precipitation decreases from reduced evapotranspiration. On average, land precipitation is enhanced by 0.03 mm day?1 (about 19 % of the CO2 radiative forcing induced increase). This increase of land precipitation with decreased ET is an apparent negative feedback, i.e., less ET makes more precipitation. Global precipitation is slightly reduced. Runoff increases associated with both the increased land precipitation and reduced evapotranspiration. Examining the consistency of the variations among ensemble members shows that vegetation feedbacks on precipitation are more robust over the tropics and in mid to high latitudes than over the subtropics where vegetation is sparse and the internal climate variability has a larger influence.  相似文献   

2.
The concentration of carbon dioxide in the atmosphere acts to control the stomatal conductance of plants. There is observational and modelling evidence that an increase in the atmospheric concentration of CO2 would suppress the evapotranspiration (ET) rate over land. This process is known as CO2 physiological forcing and has been shown to induce changes in surface temperature and continental runoff. We analyse two transient climate simulations for the twenty-first century to isolate the climate response to the CO2 physiological forcing. The land surface warming associated with the decreased ET rate is accompanied by an increase in the atmospheric lapse rate, an increase in specific humidity, but a decrease in relative humidity and stratiform cloud over land. We find that the water vapour feedback more than compensates for the decrease in latent heat flux over land as far as the budget of atmospheric water vapour is concerned. There is evidence that surface snow, water vapour and cloudiness respond to the CO2 physiological forcing and all contribute to further warm the climate system. The climate response to the CO2 physiological forcing has a quite different signature to that from the CO2 radiative forcing, especially in terms of the changes in the temperature vertical profile and surface energy budget over land.  相似文献   

3.
A recent modelling study has shown that precipitation and runoff over land would increase when the reflectivity of marine clouds is increased to counter global warming. This implies that large scale albedo enhancement over land could lead to a decrease in runoff over land. In this study, we perform simulations using NCAR CAM3.1 that have implications for Solar Radiation Management geoengineering schemes that increase the albedo over land. We find that an increase in reflectivity over land that mitigates the global mean warming from a doubling of CO2 leads to a large residual warming in the southern hemisphere and cooling in the northern hemisphere since most of the land is located in northern hemisphere. Precipitation and runoff over land decrease by 13.4 and 22.3%, respectively, because of a large residual sinking motion over land triggered by albedo enhancement over land. Soil water content also declines when albedo over land is enhanced. The simulated magnitude of hydrological changes over land are much larger when compared to changes over oceans in the recent marine cloud albedo enhancement study since the radiative forcing over land needed (?8.2?W?m?2) to counter global mean radiative forcing from a doubling of CO2 (3.3?W?m?2) is approximately twice the forcing needed over the oceans (?4.2?W?m?2). Our results imply that albedo enhancement over oceans produce climates closer to the unperturbed climate state than do albedo changes on land when the consequences on land hydrology are considered. Our study also has important implications for any intentional or unintentional large scale changes in land surface albedo such as deforestation/afforestation/reforestation, air pollution, and desert and urban albedo modification.  相似文献   

4.
C. Hatté  J. Guiot 《Climate Dynamics》2005,25(2-3):315-327
A modified version of the Biome4 vegetation model for simulation of the mean δ13C of plant communities is presented, and used to reconstruct palaeoprecipitation. We treat all fractionations by C3 and C4 plants in all coexistent Plant Functional Types, weighted by their respective net primary production. We constrain the range of variation in the intracellular versus atmospheric CO2 concentration by fixing a lower limit. Finally, we replace some constant parameters by functions of external forcing to account for their responses to environmental variation. The new version of Biome4 was applied as an inverse model and tested on three modern data sets. The fit between observations and simulations is very close to the 1:1 relationship, with respective slopes of 0.90±0.02 (r 2=0.98, n=29) for δ13C and 0.97±0.06 (r 2=0.90, n=29) for precipitation. Inverse modelling was applied using the Metropolis-Hastings algorithm to the Nußloch loess sequence. Over the last glaciation, simulated palaeoprecipitation varies between 240 mm year?1 and 400 mm year?1. This study clearly demonstrates atmospheric teleconnections with the Greenland ice-sheet extension, by matching Dansgaard-Oeschger events with precipitation increase of ca. 100–200 mm year?1.  相似文献   

5.
The radiative forcings and feedbacks that determine Earth’s climate sensitivity are typically defined at the top-of-atmosphere (TOA) or tropopause, yet climate sensitivity itself refers to a change in temperature at the surface. In this paper, we describe how TOA radiative perturbations translate into surface temperature changes. It is shown using first principles that radiation changes at the TOA can be equated with the change in energy stored by the oceans and land surface. This ocean and land heat uptake in turn involves an adjustment of the surface radiative and non-radiative energy fluxes, with the latter being comprised of the turbulent exchange of latent and sensible heat between the surface and atmosphere. We employ the radiative kernel technique to decompose TOA radiative feedbacks in the IPCC Fourth Assessment Report climate models into components associated with changes in radiative heating of the atmosphere and of the surface. (We consider the equilibrium response of atmosphere-mixed layer ocean models subjected to an instantaneous doubling of atmospheric CO2). It is shown that most feedbacks, i.e., the temperature, water vapor and cloud feedbacks, (as well as CO2 forcing) affect primarily the turbulent energy exchange at the surface rather than the radiative energy exchange. Specifically, the temperature feedback increases the surface turbulent (radiative) energy loss by 2.87 W m?2 K?1 (0.60 W m?2 K?1) in the multimodel mean; the water vapor feedback decreases the surface turbulent energy loss by 1.07 W m?2 K?1 and increases the surface radiative heating by 0.89 W m?2 K?1; and the cloud feedback decreases both the turbulent energy loss and the radiative heating at the surface by 0.43 and 0.24 W m?2 K?1, respectively. Since changes to the surface turbulent energy exchange are dominated in the global mean sense by changes in surface evaporation, these results serve to highlight the fundamental importance of the global water cycle to Earth’s climate sensitivity.  相似文献   

6.
With the implementation of the Chinese Natural Forest Conservation Program (NFCP) in 1998, over millions of hectares of forest in northeastern China have been protected through natural restoration (closure of hills). The impact of this program on the carbon budget of soil has not been evaluated until now. This paper presents results from a 6-year study of total CO2 efflux from both soil and litter (R total), CO2 flux from soil (R soil), soil organic matter (SOM), soil microbe density, and litter input and root biomass at an uncut larch (Larix gmelinii) forest and at a natural restoration site. The natural restoration area is a clear-cut site that was formerly part of a continuous portion of the uncut larch forest. Our objectives were to: (1) quantify the magnitude of CO2 efflux from typical sites in northeastern China; (2) explore the changes in thermal conditions, SOM, and annual CO2 flux during the 6-year natural restoration, and (3) evaluate the impact of NFCP on soil carbon processes. The annual R soil at the clear-cut site (58.6–68.2 mol m???2 year???1) was 113.6–228.4% (mean 141.5%) higher than that at the uncut larch site (29.6–58.4 mol m???2 year???1). At the same time, annual CO2 from litter at the clear-cut site (2.0–14.2 mol m???2 year???1) was only 23.5–84.5% (mean 52.5%) of that at the uncut larch site (5.4–16.8 mol m???2 year???1). SOM at the surface layer of the clear-cut site was 75% of that at the uncut larch site, but the soil microbial biomass (carbon) at the clear-cut site was much higher than that at the larch site (p?<?0.05). The percentage of bacteria, fungi and actinomycetes also were largely different between both sites. Natural restoration at the clear-cut site strongly affected thermal conditions. Although the soil temperature (T soil) and effective accumulated $T_{\rm soil} > 0^{\circ}$ C at the clear-cut site was much higher, the temperature sensitivity (Q 10) was much lower than that at the uncut larch site, and their differences decreased linearly from 2001 to 2006 (p?<?0.05). Moreover, Q 10 at the clear-cut site significantly increased with the progress of natural restoration, which diminished the Q 10 difference between the two sites (slope?=???0.2792, r 2?=?0.4744, p?<?0.05). These data imply that the NFCP natural restoration process has positively recovered the thermal condition of the clear-cut site to the level of uncut larch forest during the 6-year period. However, linear regression analysis showed that the 6-year natural restoration only slightly affected the annual soil CO2 efflux and SOM at both sites, and also did not diminish the differences between the two sites (p?>?0.10), indicating that a much longer time is necessary to restore the soil carbon in the clear-cut site.  相似文献   

7.
A regional atmospheric climate model with multi-layer snow module (RACMO2) is forced at the lateral boundaries by global climate model (GCM) data to assess the future climate and surface mass balance (SMB) of the Antarctic ice sheet (AIS). Two different GCMs (ECHAM5 until 2100 and HadCM3 until 2200) and two different emission scenarios (A1B and E1) are used as forcing to capture a realistic range in future climate states. Simulated ice sheet averaged 2 m air temperature (T2m) increases (1.8–3.0 K in 2100 and 2.4–5.3 K in 2200), simultaneously and with the same magnitude as GCM simulated T2m. The SMB and its components increase in magnitude, as they are directly influenced by the temperature increase. Changes in atmospheric circulation around Antarctica play a minor role in future SMB changes. During the next two centuries, the projected increase in liquid water flux from rainfall and snowmelt, together 60–200 Gt year?1, will mostly refreeze in the snow pack, so runoff remains small (10–40 Gt year?1). Sublimation increases by 25–50 %, but remains an order of magnitude smaller than snowfall. The increase in snowfall mainly determines future changes in SMB on the AIS: 6–16 % in 2100 and 8–25 % in 2200. Without any ice dynamical response, this would result in an eustatic sea level drop of 20–43 mm in 2100 and 73–163 mm in 2200, compared to the twentieth century. Averaged over the AIS, a strong relation between $\Updelta$ SMB and $\Updelta\hbox{T}_{2{\rm m}}$ of 98 ± 5 Gt w.e. year?1 K?1 is found.  相似文献   

8.
The soil C balance is determined by the difference between inputs (e.g., plant litter, organic amendments, depositional C) and outputs (e.g., soil respiration, dissolved organic C leaching, and eroded C). There is a need to improve our understanding of whether soil erosion is a sink or a source of atmospheric CO2. The objective of this paper is to discover the long-term influence of soil erosion on the C cycle of managed watersheds near Coshocton, OH. We hypothesize that the amount of eroded C that is deposited in or out of a watershed compares in magnitude to the soil C changes induced via microbial respiration. We applied the erosion productivity impact calculator (EPIC) model to evaluate the role of erosion–deposition processes on the C balance of three small watersheds (∼1 ha). Experimental records from the USDA North Appalachian Experimental Watershed facility north of Coshocton, OH were used in the study. Soils are predominantly silt loam and have developed from loess-like deposits over residual bedrock. Management practices in the three watersheds have changed over time. Currently, watershed 118 (W118) is under a corn (Zea mays L.)–soybean (Glycine max [L.] Merr.) no till rotation, W128 is under conventional till continuous corn, and W188 is under no till continuous corn. Simulations of a comprehensive set of ecosystem processes including plant growth, runoff, and water erosion were used to quantify sediment C yields. A simulated sediment C yield of 43 ± 22 kg C ha−1 year−1 compared favorably against the observed 31 ± 12 kg C ha−1 year−1 in W118. EPIC overestimated the soil C stock in the top 30-cm soil depth in W118 by 21% of the measured value (36.8 Mg C ha−1). Simulations of soil C stocks in the other two watersheds (42.3 Mg C ha−1 in W128 and 50.4 Mg C ha−1 in W188) were off by <1 Mg C ha−1. Simulated eroded C re-deposited inside (30–212 kg C ha−1 year−1) or outside (73–179 kg C ha−1 year−1) watershed boundaries compared in magnitude to a simulated soil C sequestration rate of 225 kg C ha−1 year−1 and to literature values. An analysis of net ecosystem carbon balance revealed that the watershed currently under a plow till system (W128) was a source of C to the atmosphere while the watersheds currently under a no till system (W118 and W188) behaved as C sinks of atmospheric CO2. Our results demonstrate a clear need for documenting and modeling the proportion of eroded soil C that is transported outside watershed boundaries and the proportion that evolves as CO2 to the atmosphere.  相似文献   

9.
The surface energy budget is closely related to freeze-thaw processes and is also a key issue for land surface process research in permafrost regions.In this study,in situ data collected from 2005 to 2015 at the Tanggula site were used to analyze surface energy regimes,the interaction between surface energy budget and freeze-thaw processes.The results confirmed that surface energy flux in the permafrost region of the Qinghai-Tibetan Plateau exhibited obvious seasonal variations.Annual average net radiation(Rn)for 2010 was 86.5 W m-2,with the largest being in July and smallest in November.Surface soil heat flux(G0)was positive during warm seasons but negative in cold seasons with annual average value of 2.7 W m-2.Variations in Rn and G0 were closely related to freeze-thaw processes.Sensible heat flux(H)was the main energy budget component during cold seasons,whereas latent heat flux(LE)dominated surface energy distribution in warm seasons.Freeze-thaw processes,snow cover,precipitation,and surface conditions were important influence factors for surface energy flux.Albedo was strongly dependent on soil moisture content and ground surface state,increasing significantly when land surface was covered with deep snow,and exhibited negative correlation with surface soil moisture content.Energy variation was significantly related to active layer thaw depth.Soil heat balance coefficient K was>1 during the investigation time period,indicating the permafrost in the Tanggula area tended to degrade.  相似文献   

10.
A coupled climate–carbon cycle model composed of a process-based terrestrial carbon cycle model, Sim-CYCLE, and the CCSR/NIES/FRCGC atmospheric general circulation model was developed. We examined the multiple temporal scale functions of terrestrial ecosystem carbon dynamics induced by human activities and natural processes and evaluated their contribution to fluctuations in the global carbon budget during the twentieth century. Global annual net primary production (NPP) and heterotrophic respiration (HR) increased gradually by 6.7 and 4.7%, respectively, from the 1900s to the 1990s. The difference between NPP and HR was the net carbon uptake by natural ecosystems, which was 0.6 Pg C year?1 in the 1980s, whereas the carbon emission induced by human land-use changes was 0.5 Pg C year?1, largely offsetting the natural terrestrial carbon sequestration. Our results indicate that monthly to interannual variation in atmospheric CO2 growth rate anomalies show 2- and 6-month time lags behind anomalies in temperature and the NiNO3 index, respectively. The simulated anomaly amplitude in monthly net carbon flux from terrestrial ecosystems to the atmosphere was much larger than in the prescribed air-to-sea carbon flux. Fluctuations in the global atmospheric CO2 time series were dominated by the activity of terrestrial vegetation. These results suggest that terrestrial ecosystems have acted as a net neutral reservoir for atmospheric CO2 concentrations during the twentieth century on an interdecadal timescale, but as the dominant driver for atmospheric CO2 fluctuations on a monthly to interannual timescale.  相似文献   

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