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 present-day 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 present-day 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 all the aerosol forcings and atmospheric feedbacks, except those induced by the ocean–atmosphere interactions, enhanced fire activity induces a significant decrease of the MNDWS reaching a dozen of days in Quebec and in Eastern Siberia.

Highlights

  • The boreal regions have been characterised as a region very sensitive to climate change (Lemke et al, IPCC, chapter 4, 2007)

  • We considered the surface to be snow covered when the snow mass averaged over one day exceeds 0.01 kg m−2 (i.e. 0.01 mm. snow water equivalent)

  • Looking at the root mean square error (RMSE) between modelled and observed MNDWS (Fig. 2c), we see that our model describes quite well the snow cover duration over flat areas (RMS varying between 5 and 20)

Read more

Summary

Introduction

The boreal regions have been characterised as a region very sensitive to climate change (Lemke et al, IPCC, chapter 4, 2007). One reason for the amplification in arctic and subarctic surface warming in response to increased greenhouse gas concentrations is the snow and sea-ice albedo feedback, which decreases surface albedo as snow and sea ice further melt and disappear in response to warming by greenhouse gases (Serreze et al, 2006; Qu and Hall, 2007) Both sea-ice and snow-cover extents have been observed to shrink over the last decades in the Northern Hemisphere (Serreze et al, 2007; Shi et al, 2011). Over highly reflective surfaces like snow covered areas, this increase in the longwave flux can be higher than the decrease of the shortwave flux induced by atmospheric BC (Quinn et al, 2008). In addition to these direct radiative forcings, aerosols affect cloud microphysics, processes referred to as the aerosol indirect effects. Shindell (2007) and Shindell and Faluvegi (2009) estimate that both anthropogenic well-mixed greenhouses gases and short-lived species have contributed to the Arctic warming

Objectives
Results
Discussion
Conclusion
Full Text
Published version (Free)

Talk to us

Join us for a 30 min session where you can share your feedback and ask us any queries you have

Schedule a call