lmd_Boucher2014_abstracts.html

2014 .

N. Gedney, C. Huntingford, G. P. Weedon, N. Bellouin, O. Boucher, and P. M. Cox. Detection of solar dimming and brightening effects on Northern Hemisphere river flow. Nature Geoscience, 7:796-800, November 2014. [ bib | DOI | ADS link ]

Anthropogenic aerosols in the atmosphere have the potential to affect regional-scale land hydrology through solar dimming. Increased aerosol loading may have reduced historical surface evaporation over some locations, but the magnitude and extent of this effect is uncertain. Any reduction in evaporation due to historical solar dimming may have resulted in an increase in river flow. Here we formally detect and quantify the historical effect of changing aerosol concentrations, via solar radiation, on observed river flow over the heavily industrialized, northern extra-tropics. We use a state-of-the-art estimate of twentieth century surface meteorology as input data for a detailed land surface model, and show that the simulations capture the observed strong inter-annual variability in runoff in response to climatic fluctuations. Using statistical techniques, we identify a detectable aerosol signal in the observed river flow both over the combined region, and over individual river basins in Europe and North America. We estimate that solar dimming due to rising aerosol concentrations in the atmosphere around 1980 led to an increase in river runoff by up to 25% in the most heavily polluted regions in Europe. We propose that, conversely, these regions may experience reduced freshwater availability in the future, as air quality improvements are set to lower aerosol loading and solar dimming.

L. D. Rotstayn, E. L. Plymin, M. A. Collier, O. Boucher, J.-L. Dufresne, J.-J. Luo, K. von Salzen, S. J. Jeffrey, M.-A. Foujols, Y. Ming, and L. W. Horowitz. Declining Aerosols in CMIP5 Projections: Effects on Atmospheric Temperature Structure and Midlatitude Jets. Journal of Climate, 27:6960-6977, September 2014. [ bib | DOI | ADS link ]

P. J. Irvine, O. Boucher, B. Kravitz, K. Alterskjær, J. N. S. Cole, D. Ji, A. Jones, D. J. Lunt, J. C. Moore, H. Muri, U. Niemeier, A. Robock, B. Singh, S. Tilmes, S. Watanabe, S. Yang, and J.-H. Yoon. Key factors governing uncertainty in the response to sunshade geoengineering from a comparison of the GeoMIP ensemble and a perturbed parameter ensemble. Journal of Geophysical Research (Atmospheres), 119:7946-7962, July 2014. [ bib | DOI | ADS link ]

Climate model studies of the consequences of solar geoengineering are central to evaluating whether such approaches may help to reduce the harmful impacts of global warming. In this study we compare the sunshade solar geoengineering response of a perturbed parameter ensemble (PPE) of the Hadley Centre Coupled Model version 3 (HadCM3) with a multimodel ensemble (MME) by analyzing the G1 experiment from the Geoengineering Model Intercomparison Project (GeoMIP). The PPE only perturbed a small number of parameters and shares a common structure with the unperturbed HadCM3 model, and so the additional weight the PPE adds to the robustness of the common climate response features in the MME is minor. However, analysis of the PPE indicates some of the factors that drive the spread within the MME. We isolate the role of global mean temperature biases for both ensembles and find that these biases have little effect on the ensemble spread in the hydrological response but do reduce the spread in surface air temperature response, particularly at high latitudes. We investigate the role of the preindustrial climatology and find that biases here are likely a key source of ensemble spread at the zonal and grid cell level. The role of vegetation, and its response to elevated CO₂ concentrations through the CO₂ physiological effect and changes in plant productivity, is also investigated and proves to have a substantial effect on the terrestrial hydrological response to solar geoengineering and to be a major source of variation within the GeoMIP ensemble.

N. Huneeus, O. Boucher, K. Alterskjær, J. N. S. Cole, C. L. Curry, D. Ji, A. Jones, B. Kravitz, J. E. Kristjánsson, J. C. Moore, H. Muri, U. Niemeier, P. Rasch, A. Robock, B. Singh, H. Schmidt, M. Schulz, S. Tilmes, S. Watanabe, and J.-H. Yoon. Forcings and feedbacks in the GeoMIP ensemble for a reduction in solar irradiance and increase in CO₂. Journal of Geophysical Research (Atmospheres), 119:5226-5239, May 2014. [ bib | DOI | ADS link ]

The effective radiative forcings (including rapid adjustments) and feedbacks associated with an instantaneous quadrupling of the preindustrial CO₂ concentration and a counterbalancing reduction of the solar constant are investigated in the context of the Geoengineering Model Intercomparison Project (GeoMIP). The forcing and feedback parameters of the net energy flux, as well as its different components at the top-of-atmosphere (TOA) and surface, were examined in 10 Earth System Models to better understand the impact of solar radiation management on the energy budget. In spite of their very different nature, the feedback parameter and its components at the TOA and surface are almost identical for the two forcing mechanisms, not only in the global mean but also in their geographical distributions. This conclusion holds for each of the individual models despite intermodel differences in how feedbacks affect the energy budget. This indicates that the climate sensitivity parameter is independent of the forcing (when measured as an effective radiative forcing). We also show the existence of a large contribution of the cloudy-sky component to the shortwave effective radiative forcing at the TOA suggesting rapid cloud adjustments to a change in solar irradiance. In addition, the models present significant diversity in the spatial distribution of the shortwave feedback parameter in cloudy regions, indicating persistent uncertainties in cloud feedback mechanisms.

M. Ménégoz, G. Krinner, Y. Balkanski, O. Boucher, A. Cozic, S. Lim, P. Ginot, P. Laj, H. Gallée, P. Wagnon, A. Marinoni, and H. W. Jacobi. Snow cover sensitivity to black carbon deposition in the Himalayas: from atmospheric and ice core measurements to regional climate simulations. Atmospheric Chemistry & Physics, 14:4237-4249, April 2014. [ bib | DOI | ADS link ]

We applied a climate-chemistry global model to evaluate the impact of black carbon (BC) deposition on the Himalayan snow cover from 1998 to 2008. Using a stretched grid with a resolution of 50 km over this complex topography, the model reproduces reasonably well the remotely sensed observations of the snow cover duration. Similar to observations, modelled atmospheric BC concentrations in the central Himalayas reach a minimum during the monsoon and a maximum during the post- and pre-monsoon periods. Comparing the simulated BC concentrations in the snow with observations is more challenging because of their high spatial variability and complex vertical distribution. We simulated spring BC concentrations in surface snow varying from tens to hundreds of μg kg^-1, higher by one to two orders of magnitude than those observed in ice cores extracted from central Himalayan glaciers at high elevations (6000 m a.s.l.), but typical for seasonal snow cover sampled in middle elevation regions (6000 m a.s.l.). In these areas, we estimate that both wet and dry BC depositions affect the Himalayan snow cover reducing its annual duration by 1 to 8 days. In our simulations, the effect of anthropogenic BC deposition on snow is quite low over the Tibetan Plateau because this area is only sparsely snow covered. However, the impact becomes larger along the entire Hindu-Kush, Karakorum and Himalayan mountain ranges. In these regions, BC in snow induces an increase of the net short-wave radiation at the surface with an annual mean of 1 to 3 W m^-2 leading to a localised warming between 0.05 and 0.3 degC.

O. Boucher, P. M. Forster, N. Gruber, H.-D. Minh, M. G. Lawrence, T. M. Lenton, A. Maas, and N. E. Vaughan. Rethinking climate engineering categorization in the context of climate change mitigation and adaptation. Wiley Interdisciplinary Reviews: Climate Change, Volume 5, Issue 1, pages 23â#128#14735, 5:23, January 2014. [ bib | DOI | ADS link ]

J. C. Moore, A. Rinke, X. Yu, D. Ji, X. Cui, Y. Li, K. Alterskjær, J. E. Kristjánsson, H. Muri, O. Boucher, N. Huneeus, B. Kravitz, A. Robock, U. Niemeier, M. Schulz, S. Tilmes, S. Watanabe, and S. Yang. Arctic sea ice and atmospheric circulation under the GeoMIP G1 scenario. Journal of Geophysical Research (Atmospheres), 119:567-583, January 2014. [ bib | DOI | ADS link ]

We analyze simulated sea ice changes in eight different Earth System Models that have conducted experiment G1 of the Geoengineering Model Intercomparison Project (GeoMIP). The simulated response of balancing abrupt quadrupling of CO₂ (abrupt4xCO2) with reduced shortwave radiation successfully moderates annually averaged Arctic temperature rise to about 1degC, with modest changes in seasonal sea ice cycle compared with the preindustrial control simulations (piControl). Changes in summer and autumn sea ice extent are spatially correlated with temperature patterns but much less in winter and spring seasons. However, there are changes of 20% in sea ice concentration in all seasons, and these will induce changes in atmospheric circulation patterns. In summer and autumn, the models consistently simulate less sea ice relative to preindustrial simulations in the Beaufort, Chukchi, East Siberian, and Laptev Seas, and some models show increased sea ice in the Barents/Kara Seas region. Sea ice extent increases in the Greenland Sea, particularly in winter and spring and is to some extent associated with changed sea ice drift. Decreased sea ice cover in winter and spring in the Barents Sea is associated with increased cyclonic activity entering this area under G1. In comparison, the abrupt4xCO2 experiment shows almost total sea ice loss in September and strong correlation with regional temperatures in all seasons consistent with open ocean conditions. The tropospheric circulation displays a Pacific North America pattern-like anomaly with negative phase in G1-piControl and positive phase under abrupt4xCO2-piControl.