Boreal and temperate snow cover variations induced by black carbon emissions in the middle of the 21st century

Abstract

Abstract. We used a coupled climate-chemistry model to quantify the impacts of aerosols on snow cover north of 30 • N both for the present-day and for the middle of the 21st century. Black carbon (BC) deposition over continents induces a reduction in the mean number of days with snow at the surface (MNDWS) that ranges from 0 to 10 days over large areas of Eurasia and Northern America for the presentday relative to the pre-industrial period. This is mainly due to BC deposition during the spring, a period of the year when the remaining of snow accumulated during the winter is exposed to both strong solar radiation and a large amount of aerosol deposition induced themselves by a high level of transport of particles from polluted areas. North of 30 • N, this deposition flux represents 222 Gg BC month −1 on average from April to June in our simulation. A large reduction in BC emissions is expected in the future in all of the Representative Concentration Pathway (RCP) scenarios. In particular, considering the RCP8.5 in our simulation leads to a decrease in the spring BC deposition down to 110 Gg month −1 in the 2050s. However, despite the reduction of the aerosol impact on snow, the MNDWS is strongly reduced by 2050, with a decrease ranging from 10 to 100 days from presentday values over large parts of the Northern Hemisphere. This reduction is essentially due to temperature increase, which is quite strong in the RCP8.5 scenario in the absence of climate mitigation policies. Moreover, the projected sea-ice retreat in the next decades will open new routes for shipping in the Arctic. However, a large increase in shipping emissions in the Arctic by the mid-21st century does not lead to significant changes of BC deposition over snow-covered areas in our simulation. Therefore, the MNDWS is clearly not affected through snow darkening effects associated with these Arctic ship emissions. In an experiment without nudging toward atmospheric reanalyses, we simulated however some changes of the MNDWS considering such aerosol ship emissions. These changes are generally not statistically significant in boreal continents, except in Quebec and in the West Siberian plains, where they range between −5 and −10 days. They are induced both by radiative forcings of the aerosols when they are in the snow and in the atmosphere, and by all the atmospheric feedbacks. These experiments do not take into account the feedbacks induced by the interactions between ocean and atmosphere as they were conducted with prescribed sea surface temperatures. Climate change by the mid-21st century could also cause biomass burning activity (forest fires) to become more intense and occur earlier in the season. In an idealised scenario in which forest fires are 50 % stronger and occur 2 weeks earlier and later than at present, we simulated an increase in spring BC deposition of 21 Gg BC month −1 over continents located north of 30 • N. This BC deposition does not impact directly the snow cover through snow darkening effects. However, in an experiment considering ...

Snow cover sensitivity to black carbon deposition in the Himalayas: from atmospheric and ice core measurements to regional climate simulations

“…All the experiments were conducted with the present-day global aerosol emission inventory described in Lamarque et al (2009), a decadal resolved inventory made for the Coupled Model Inter-comparison Project Phase 5 (CMIP5, CLI-VAR special issue, 2011). We further used the detailed representation of snow cover implemented in ORCHIDEE by Krinner et al (2006) and used in Ménégoz et al (2013a). This includes a two-layer scheme describing the snow albedo as a function of snow grain size and aerosol content in the snow based on Wiscombe and Warren (1980).…”

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Ice Cores from the St. Elias Mountains, Yukon, Canada: Their Significance for Climate, Atmospheric Composition and Volcanism in the North Pacific Region

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Concentrations and source regions of light-absorbing particles in snow/ice in northern Pakistan and their impact on snow albedo

“…BC deposited on snow in the Himalayan region induces an increase in net shortwave radiation at the snow surface with an annual mean of about 1 to 3 W m −2 , producing an estimated 0.05-0.3 • C warming (Ménégoz et al, 2014). The deposition of anthropogenic BC has been observed to contribute significantly to the decrease in snow cover extent over recent decades (Déry et al, 2007) and a shortening of the duration of the snow cover season by several days (Ménégoz et al, 2013). The climate warming efficiency of BC in snow is greater than the warming efficiency of other anthropogenic pollutants, including carbon dioxide (Hansen et al, 2005).…”