Afforestation impact on soil temperature in regional climate model simulations over Europe

In the context of the first phase of the Coordinated Regional Climate Downscaling Experiment in the European domain (EURO-CORDEX) flagship plot study on Land Use and Climate Across Scales (LUCAS), we investigate the biophysical impact of afforestation on the seasonal cycle of soil temperature over the European continent with an ensemble of 10 regional climate models. For this purpose, each ensemble member performed two idealized land cover experiments in which Europe is covered either by forests or grasslands. The multi-model mean exhibits a reduction of the annual amplitude of soil temperature (AAST) due to afforestation over all European regions, although this is not a robust feature among the models. In the Mediterranean, the spread of simulated AAST response to afforestation is between −4 and +2 ∘C at 1 m below the ground, while in Scandinavia the inter-model spread ranges from −7 to +1 ∘C. We show that the large range in the simulated AAST response is due to the representation of the summertime climate processes and is largely explained by inter-model differences in leaf area index (LAI), surface albedo, cloud fraction and soil moisture, when all combined into a multiple linear regression. The changes in these drivers essentially determine the ratio between the increased radiative energy at surface (due to lower albedo in forests) and the increased sum of turbulent heat fluxes (due to mixing-facilitating characteristics of forests), and consequently decide the changes in soil heating with afforestation in each model. Finally, we pair FLUXNET sites to compare the simulated results with observation-based evidence of the impact of forest on soil temperature. In line with models, observations indicate a summer ground cooling in forested areas compared to open lands. The vast majority of models agree with the sign of the observed reduction in AAST, although with a large variation in the magnitude of changes. Overall, we aspire to emphasize the biophysical effects of afforestation on soil temperature profile with this study, given that changes in the seasonal cycle of soil temperature potentially perturb crucial biochemical processes. Robust knowledge on biophysical impacts of afforestation on soil conditions and its feedbacks on local and regional climate is needed in support of effective land-based climate mitigation and adaption policies

Uncertainties in climate responses to past land cover change: First results from the LUCID intercomparison study

Seven climate models were used to explore the biogeophysical impacts of human-induced land cover change (LCC) at regional and global scales. The imposed LCC led to statistically significant decreases in the northern hemisphere summer latent heat flux in three models, and increases in three models. Five models simulated statistically significant cooling in summer in near-surface temperature over regions of LCC and one simulated warming. There were few significant changes in precipitation. Our results show no common remote impacts of LCC. The lack of consistency among the seven models was due to: 1) the implementation of LCC despite agreed maps of agricultural land, 2) the representation of crop phenology, 3) the parameterisation of albedo, and 4) the representation of evapotranspiration for different land cover types. This study highlights a dilemma: LCC is regionally significant, but it is not feasible to impose a common LCC across multiple models for the next IPCC assessment

Mid-Holocene greening of the Sahara: first results of the GAIM 6000 year BP experiment with two asynchronously coupled atmosphere/biome models

The mid-Holocene 'green' Sahara represents the largest anomaly of the atmosphere-biosphere system during the last 12 000 years. Although this anomaly is attributed to precessional forcing leading to a strong enhancement of the African monsoon, no climate model so far has been able to simulate the full extent of vegetation in the Sahara region 6000 years ago. Here two atmospheric general circulation models (LMD 5.3 and ECHAM 3) are asynchronously coupled to an equilibrium biogeography model to give steady-state simulations of climate and vegetation 6000 years ago, including biogeophysical feedback. The two model results are surprisingly different, and neither is fully realistic. ECHAM shows a large northward extension of vegetation in the western Dart of the Sahara only. LMD shows a much smaller and more zonal vegetation shift. These results are unaffected by the choice of 'green' or modern initial conditions. The inability of LMD to sustain a 'green' Sahara 6000 years ago is linked to the simulated strength of the tropical summer circulation. During the northern summer monsoon season, the meridional gradient of sea-level pressure and subsidence over the western Dart of northern Africa al-e both much weaker in ECHAM than in LMD in the present as well as the mid-Holocene. These features allow the surface moist air flux to penetrate further into northern Africa in ECHAM than in LMD. This comparison illustrates the importance of correct simulation of atmospheric circulation features for the sensitivity of climate models to changes in radiative forcing, particularly for regional climates where atmospheric changes are amplified by biosphere-atmosphere feedbacks. [References: 76

An attempt to quantify the impact of changes in wetland extent on methane emissions on the seasonal and interannual time scales

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Papa, Fabrice/D-3695-2009 Papa, Fabrice/0000-0001-6305-6253 Agence Nationale pour la Recherche (ANR); Commissariat a l'Energie Atomique (CEA) This research was supported by the project Impact-Boreal, funded by the Agence Nationale pour la Recherche (ANR). We also thank M. Jackowicz-Korczynski and T. Christensen for the Abisko data. Computing support was provided by Commissariat a l'Energie Atomique (CEA). 42 AMER GEOPHYSICAL UNION WASHINGTON GLOBAL BIOGEOCHEM CYClimate variability impacts CH4 wetland sources as changes in flux density per unit area and via expansion or contraction of wetland areas in response to surface hydrological processes. This paper is a first attempt to isolate the role of varying wetland area on the seasonal and interannual variability of CH4 wetland emissions over the past decade. Wetland area extent at monthly intervals was provided over the period 1993-2000 by a suite of satellite observations from multiple sensors. The regionally variable fraction of wetland area was optimized using satellite observations of flooded area as a first estimate and further adjusted to match the seasonal cycle of CH4 fluxes retrieved from a global atmospheric inversion. Wetland flux densities of CH4 were calculated by coupling the ORCHIDEE global vegetation model with a process-based wetland CH4 emission model, calibrated by optimizing its parameters at the site level against representative CH4 flux time series. For boreal bogs north of 50 degrees N, we found that variations in area contributed about 30% to the annual flux. For temperate and tropical wetlands, the variations in area has almost no influence on the annual CH4 emissions but contributes significantly to the seasonal behavior, accounting for 40% and 66% of the seasonal amplitude of fluxes, respectively. In contrast, the interannual variability of wetland area appears to be the dominant cause of interannual variations in regional CH4 emissions from wetlands at all latitudes (largest in the tropics), with up to 90% of annual flux anomalies explained by wetland area anomalies in some years. For example, in 1998, boreal wetlands north of 50 degrees N contributed to approximately 80% of the positive anomaly according to our calculations. We also found that climate anomalies can lead to both increased emitting areas and decreased flux densities at the same time, with opposite effects on the total CH4 flux entering the atmosphere. With a view to forecasting the future trajectory of atmospheric methane content, our results point to the absolute necessity to be able to predict the variations in wetland extent, a hydrological problem, in order to affirm the reliability of simulations of changing methane emissions perturbed by climate

Changes in climate and land use have a larger direct impact than rising CO2 on global river runoff trends

The significant worldwide increase in observed river runoff has been tentatively attributed to the stomatal “antitranspirant” response of plants to rising atmospheric CO2 [Gedney N, Cox PM, Betts RA, Boucher O, Huntingford C, Stott PA (2006) Nature 439: 835–838]. However, CO2 also is a plant fertilizer. When allowing for the increase in foliage area that results from increasing atmospheric CO2 levels in a global vegetation model, we find a decrease in global runoff from 1901 to 1999. This finding highlights the importance of vegetation structure feedback on the water balance of the land surface. Therefore, the elevated atmospheric CO2 concentration does not explain the estimated increase in global runoff over the last century. In contrast, we find that changes in mean climate, as well as its variability, do contribute to the global runoff increase. Using historic land-use data, we show that land-use change plays an additional important role in controlling regional runoff values, particularly in the tropics. Land-use change has been strongest in tropical regions, and its contribution is substantially larger than that of climate change. On average, land-use change has increased global runoff by 0.08 mm/year2 and accounts for ≈50% of the reconstructed global runoff trend over the last century. Therefore, we emphasize the importance of land-cover change in forecasting future freshwater availability and climate

Land-Use/Land Cover Change as Driver of Earth System Dynamics: past progress, future priorities, and new data and models for advancing the science

Human land-use activities have resulted in large changes to the biogeochemical and biophysical properties of the Earth surface, with impacts on climate. The activities which alter vegetation cover, biomass, and phenology, directly influence regional to global climate through modification of surface biophysical properties and the surface energy balance. Land-use activities are also impact climate through changes in carbon and nitrogen balances and greenhouse gas emissions. Moreover, land-use activities are likely to expand and/or intensify further to meet future demands for food, feed, fiber, and energy. The fifth phase of the Coupled Model Intercomparison Project (CMIP5) achieved a qualitative scientific advance in studying the effects of land-use on climate, for the first time explicitly accounting for the effects of global gridded land-use changes (past-future) in coupled carbon-climate model projections. Enabling this advance, the first consistent gridded land-use dataset (past-future) was developed, linking historical land-use data to future projections from Integrated Assessment Models, in a standard format required by climate models (“Land-use Harmonization”). Results from climate models indicate that the effects of land-use on climate, while uncertain, are sufficiently large and complex to warrant an expanded treatment of land-use. Here, we review past progress, future priorities, and present on new data and models designed to improve the understanding of the effects of land-use on climate (past-future). The work is organized through the Land Use Model Intercomparison Project (LUMIP) in preparation for CMIP6

The significance of land-atmosphere interactions in the Earth system-iLEAPS achievements and perspectives

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