Eleven coupled climate-carbon cycle models used a common protocol to study the coupling between climate change and the carbon cycle. The models were forced by historical emissions and the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emissions Scenarios (SRES) A2 anthropogenic emissions of CO 2 for the 1850-2100 time period. For each model, two simulations were performed in order to isolate the impact of climate change on the land and ocean carbon cycle, and therefore the climate feedback on the atmospheric CO 2 concentration growth rate. There was unanimous agreement among the models that future climate change will reduce the efficiency of the earth system to absorb the anthropogenic carbon perturbation. A larger fraction of anthropogenic CO 2 will stay airborne if climate change is accounted for. By the end of the twenty-first century, this additional CO 2 varied between 20 and 200 ppm for the two extreme models, the majority of the models lying between 50 and 100 ppm. The higher CO 2 levels led to an additional climate warming ranging between 0.1°and 1.5°C.All models simulated a negative sensitivity for both the land and the ocean carbon cycle to future climate. However, there was still a large uncertainty on the magnitude of these sensitivities. Eight models attributed most of the changes to the land, while three attributed it to the ocean. Also, a majority of the models located the reduction of land carbon uptake in the Tropics. However, the attribution of the land sensitivity to changes in net primary productivity versus changes in respiration is still subject to debate; no consensus emerged among the models.

Satellite data show increasing leaf area of vegetation due to direct (human land-use management) and indirect factors (climate change, CO 2 fertilization, nitrogen deposition, recovery from natural disturbances, etc.). Among these, climate change and CO 2 fertilization effect seem to be the dominant drivers. However, recent satellite data (2000–2017) reveal a greening pattern that is strikingly prominent in China and India, and overlapping with croplands world-wide. China alone accounts for 25% of the global net increase in leaf area with only 6.6% of global vegetated area. The greening in China is from forests (42%) and croplands (32%), but in India is mostly from croplands (82%) with minor contribution from forests (4.4%). China is engineering ambitious programs to conserve and expand forests with the goal of mitigating land degradation, air pollution and climate change. Food production in China and India has increased by over 35% since 2000 mostly due to increasing harvested area through multiple cropping facilitated by fertilizer use and surface/ground-water irrigation. Our results indicate that the direct factor is a key driver of the “Greening Earth”, accounting for over a third, and likely more, of the observed net increase in green leaf area. They highlight the need for realistic representation of human land-use practices in Earth system models.

The prevention of deforestation and promotion of afforestation have often been cited as strategies to slow global warming. Deforestation releases CO 2 to the atmosphere, which exerts a warming influence on Earth's climate. However, biophysical effects of deforestation, which include changes in land surface albedo, evapotranspiration, and cloud cover also affect climate. Here we present results from several large-scale deforestation experiments performed with a three-dimensional coupled global carbon-cycle and climate model. These simulations were performed by using a fully three-dimensional model representing physical and biogeochemical interactions among land, atmosphere, and ocean. We find that global-scale deforestation has a net cooling influence on Earth's climate, because the warming carbon-cycle effects of deforestation are overwhelmed by the net cooling associated with changes in albedo and evapotranspiration. Latitude-specific deforestation experiments indicate that afforestation projects in the tropics would be clearly beneficial in mitigating global-scale warming, but would be counterproductive if implemented at high latitudes and would offer only marginal benefits in temperate regions. Although these results question the efficacy of mid- and high-latitude afforestation projects for climate mitigation, forests remain environmentally valuable resources for many reasons unrelated to climate.

Observations have shown that the hydrological cycle of the western United States changed significantly over the last half of the 20th century. We present a regional, multivariable climate change detection and attribution study, using a high-resolution hydrologic model forced by global climate models, focusing on the changes that have already affected this primarily arid region with a large and growing population. The results show that up to 60% of the climate-related trends of river flow, winter air temperature, and snow pack between 1950 and 1999 are human-induced. These results are robust to perturbation of study variates and methods. They portend, in conjunction with previous work, a coming crisis in water supply for the western United States.

The rapidly rising CO2 level in the atmosphere has led to proposals of climate stabilization by ''geoengineering'' schemes that would mitigate climate change by intentionally reducing solar radiation incident on Earth's surface. In this article we address the impact of these climate stabilization schemes on the global hydrological cycle. By using equilibrium climate simulations, we show that insolation reductions sufficient to offset global-scale temperature increases lead to a decrease in global mean precipitation. This occurs because solar forcing is more effective in driving changes in global mean evaporation than is CO2 forcing of a similar magnitude. In the model used here, the hydrological sensitivity, defined as the percentage change in global mean precipitation per degree warming, is 2.4% K ؊1 for solar forcing, but only 1.5% K ؊1 for CO2 forcing. Although other models and the climate system itself may differ quantitatively from this result, the conclusion can be understood based on simple considerations of the surface energy budget and thus is likely to be robust. For the same surface temperature change, insolation changes result in relatively larger changes in net radiative fluxes at the surface; these are compensated by larger changes in the sum of latent and sensible heat fluxes. Hence, the hydrological cycle is more sensitive to temperature adjustment by changes in insolation than by changes in greenhouse gases. This implies that an alteration in solar forcing might offset temperature changes or hydrological changes from greenhouse warming, but could not cancel both at once.climate change ͉ global hydrology ͉ radiative forcing ͉ mitigation T he rapid rise in the rate of fossil fuel emission in recent years has revived the discussion of mitigating climate change by ''geoengineering'' schemes (1-4). The proposed schemes fall into two categories. The first involves reducing the solar radiation absorbed by the climate system by an amount that balances the reduction in outgoing terrestrial radiation because of the increase in the atmospheric CO 2 and other greenhouse gases (1,(5)(6)(7)(8)(9)(10)(11)(12). The other class of schemes typically removes the atmospheric CO 2 and sequesters it in terrestrial vegetation, in deep geologic formations, or in the oceans.Climate modeling studies have investigated the viability of the first category of schemes. The first equilibrium simulation studies on this subject (13,14) show that if the model configuration is designed such that the incoming solar radiation (''insolation'') is reduced by an appropriate percentage, it could largely mitigate even regional and seasonal climate change from a doubling or quadrupling of CO 2 , even though the spatial and temporal pattern of radiative forcing from greenhouse gases differs markedly from that of sunlight. These modeling studies find that residual temperature changes in a climate with increased greenhouse gases and appropriately reduced insolation are much smaller than the changes caused by CO 2 increases alone.Further mode...

An increase in atmospheric carbon dioxide (CO 2 ) concentration influences climate both directly through its radiative effect (i.e., trapping longwave radiation) and indirectly through its physiological effect (i.e., reducing transpiration of land plants). Here we compare the climate response to radiative and physiological effects of increased CO 2 using the National Center for Atmospheric Research (NCAR) coupled Community Land and Community Atmosphere Model. In response to a doubling of CO 2 , the radiative effect of CO 2 causes mean surface air temperature over land to increase by 2.86 AE 0.02 K (AE1 standard error), whereas the physiological effects of CO 2 on land plants alone causes air temperature over land to increase by 0.42 AE 0.02 K. Combined, these two effects cause a land surface warming of 3.33 AE 0.03 K. The radiative effect of doubling CO 2 increases global runoff by 5.2 AE 0.6%, primarily by increasing precipitation over the continents. The physiological effect increases runoff by 8.4 AE 0.6%, primarily by diminishing evapotranspiration from the continents. Combined, these two effects cause a 14.9 AE 0.7% increase in runoff. Relative humidity remains roughly constant in response to CO 2 -radiative forcing, whereas relative humidity over land decreases in response to CO 2 -physiological forcing as a result of reduced plant transpiration. Our study points to an emerging consensus that the physiological effects of increasing atmospheric CO 2 on land plants will increase global warming beyond that caused by the radiative effects of CO 2 .global warming | runoff | evapotranspiration | hydrological cycle | plant stomata I ncreased atmospheric CO 2 content affects global climate not only through its greenhouse radiative effect, but also through its effect on plant physiology. Plant stomata open less widely under elevated CO 2 concentrations, leading to reduced plant transpiration (1-3). A decrease in canopy transpiration tends to reduce evapotranspiration (the sum of canopy evaporation, canopy transpiration, and soil evaporation), triggering changes in atmospheric water vapor and clouds, and affecting surface radiative fluxes, thus producing changes to temperature and the water cycle. This driver of climate change, referred to as "CO 2 -physiological forcing," has been detected in both field experiments (4, 5) and climate modeling studies (3,(6)(7)(8)(9)(10)(11).In this study, we examine the climate effect of CO 2 -physiological forcing using a coupled global atmosphere-land surface model (12,13). While previous studies have looked at the response of temperature and runoff to CO 2 -physiological forcing, the focus of this study is to examine the nature of climate response to CO 2 -physiological forcing in terms of both magnitude and pattern, and contrast it with the effect of CO 2 -radiative forcing. Most previous modeling studies on the climate effect of CO 2 -physiological forcing (6-11) were performed within the modeling framework of the Met office Hadley Center models using the "MOSES" scheme (7) as...

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Abstract.To counteract anthropogenic climate change, several schemes have been proposed to diminish solar radiation incident on Earth's surface. These geoengineering schemes could reverse global annual mean warming; however, it is unclear to what extent they would mitigate regional and seasonal climate change, because radiative forcing from greenhouse gases such as CO2 differs from that of sunlight. No previous study has directly addressed this issue. In the NCAR CCM3 atmospheric general circulation model, we reduced the solar luminosity to balance the increased radiative forcing from doubling atmospheric COs. Our results indicate that geoengineering schemes could markedly diminish regional and seasonal climate change from increased atmospheric COs, despite differences in radiative forcing patterns. Nevertheless, geoengineering schemes could prove environmentally risky.