Contribution of light-absorbing impurities in snow to Greenland’s darkening since 2009

Dumont, Marie; Brun, E.; Picard, Ghislain; Michou, Martine; Libois, Quentin; Petit, Jean‐Robert; Geyer, Michaël; Morin, Samuel; Josse, B.

doi:10.1038/ngeo2180

Cited by 202 publications

(229 citation statements)

References 27 publications

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“…Positive trends over seasonally snow-covered areas are most likely due to a decrease in snow coverage (e.g., Qu and Hall, 2007;Vaughan et al, 2013), and the trend in LCrRE that we find over northern Asia is consistent with studies showing recent declines in snow cover over this region (e.g., Dery and Brown, 2007;Brown and Robinson, 2011;Derksen et al, 2014). Possible reasons for the positive trends seen over Greenland include (1) increased snow metamorphism and black carbon deposition (e.g., Box et al, 2012;Keegan et al, 2014); (2) transport and deposition of dust and other light-absorbing impurities over the ice sheets due to increased dust source areas associated with increased snowfree area (Dumont et al, 2014); (3) higher melt extent across the Greenland ice sheet expose more fresh water at the surface (Tedesco et al, 2014); and (4) MODIS sensor degradation on the Terra satellite during recent years (Sun et al, 2014;Lyapustin et al, 2014), which would indicate a spurious decline in albedo. We also observe slightly negative LCrRE trends over Antarctica, which may be due to increases in snowfall that have decreased the duration that surface snow has to "age", thereby increasing surface albedo (Picard et al, 2012).…”

Section: Interannual Trendssupporting

confidence: 78%

The global land shortwave cryosphere radiative effect during the MODIS era

Singh¹,

Flanner²,

Perket³

2015

The Cryosphere

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Abstract. The shortwave cryosphere radiative effect (CrRE) is the instantaneous influence of snow and ice cover on Earth's top-of-atmosphere (TOA) solar energy budget. Here, we apply measurements from the MODerate resolution Imaging Spectroradiometer (MODIS), combined with microwave retrievals of snow presence and radiative kernels produced from four different models, to derive CrRE over global land during 2001-2013. We estimate global annualmean land CrRE during this period of −2.6 W m −2 , with variations from −2.2 to −3.0 W m −2 resulting from use of different kernels and variations of −2.4 to −2.6 W m −2 resulting from different algorithmic determinations of snow presence and surface albedo. Slightly more than half of the global land CrRE originates from perennial snow on Antarctica, whereas the majority of the northern hemispheric effect originates from seasonal snow. Consequently, the northern hemispheric land CrRE peaks at −6.0 W m −2 in April, whereas the southern hemispheric effect more closely follows the austral insolation cycle, peaking at −9.0 W m −2 in December. Mountain glaciers resolved in 0.05 • MODIS data contribute about −0.037 W m −2 (1.4 %) of the global effect, with the majority (94 %) of this contribution originating from the Himalayas. Interannual trends in the global annual-mean land CrRE are not statistically significant during the MODIS era, but trends are positive (less negative) over large areas of northern Asia, especially during spring, and slightly negative over Antarctica, possibly due to increased snowfall. During a common overlap period of 2001-2008, our MODIS estimates of the northern hemispheric land CrRE are about 18 % smaller (less negative) than previous estimates derived from coarse-resolution AVHRR data, though interannual variations are well correlated (r = 0.78), indicating that these data are useful in determining longer-term trends in land CrRE.

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Section: Interannual Trendssupporting

confidence: 78%

The global land shortwave cryosphere radiative effect during the MODIS era

Singh¹,

Flanner²,

Perket³

2015

The Cryosphere

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“…A potential climate impact of BC emissions is related to the deposition of BC on snow and ice which substantially reduces the surface albedo (e.g., Dumont et al, 2014). In this context, the question arises how strongly deposition rates in the Arctic and Antarctic depend on emission heights.…”

Section: Total Deposition Ratesmentioning

confidence: 99%

Fire emission heights in the climate system – Part 2: Impact on transport, black carbon concentrations and radiation

et al. 2015

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Abstract. Wildfires represent a major source for aerosols impacting atmospheric radiation, atmospheric chemistry and cloud micro-physical properties. Previous case studies indicated that the height of the aerosol-radiation interaction may crucially affect atmospheric radiation, but the sensitivity to emission heights has been examined with only a few models and is still uncertain. In this study we use the general circulation model ECHAM6 extended by the aerosol module HAM2 to investigate the impact of wildfire emission heights on atmospheric long-range transport, black carbon (BC) concentrations and atmospheric radiation. We simulate the wildfire aerosol release using either various versions of a semiempirical plume height parametrization or prescribed standard emission heights in ECHAM6-HAM2. Extreme scenarios of near-surface or free-tropospheric-only injections provide lower and upper constraints on the emission height climate impact. We find relative changes in mean global atmospheric BC burden of up to 7.9 ± 4.4 % caused by average changes in emission heights of 1.5-3.5 km. Regionally, changes in BC burden exceed 30-40 % in the major biomass burning regions. The model evaluation of aerosol optical thickness (AOT) against Moderate Resolution Imaging Spectroradiometer (MODIS), AErosol RObotic NETwork (AERONET) and Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) observations indicates that the implementation of a plume height parametrization slightly reduces the ECHAM6-HAM2 biases regionally, but on the global scale these improvements in model performance are small. For prescribed emission release at the surface, wildfire emissions entail a total sky top-of-atmosphere (TOA) radiative forcing (RF) of −0.16 ± 0.06 W m −2 . The application of a plume height parametrization which agrees reasonably well with observations introduces a slightly stronger negative TOA RF of −0.20 ± 0.07 W m −2 . The standard ECHAM6-HAM2 model in which 25 % of the wildfire emissions are injected into the free troposphere (FT) and 75 % into the planetary boundary layer (PBL), leads to a TOA RF of −0.24 ± 0.06 W m −2 . Overall, we conclude that simple plume height parametrizations provide sufficient representations of emission heights for global climate modeling. Significant improvements in aerosol wildfire modeling likely depend on better emission inventories and aerosol process modeling rather than on improved emission height parametrizations.

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“…A recent study (Dumont et al, 2014) concluded that dust deposition has been increasing over much of the GrIS and that this is driving lowered albedo across the ice sheet. That conclusion was based on trends of an "impurity index", which is the ratio of the logarithm of albedo in the 545-565 nm MODIS band (where LAI affect albedo) to the logarithm of albedo in the 841-876 nm band (where they do not).…”

Section: Trends In Aod Over Greenlandmentioning

confidence: 99%

“…In the Dumont et al (2014) study, this correction was made using simulations of atmospheric aerosols by the Monitoring Atmospheric Composition and Climate (MACC) model. Their resulting impurity index shows positive trends, and these are attributed in part (up to 30 %) to increases in atmospheric aerosol not accounted for by the model, and the remainder to increases in snow LAI.…”

Section: Trends In Aod Over Greenlandmentioning

confidence: 99%

The darkening of the Greenland ice sheet: trends, drivers, and projections (1981–2100)

et al. 2016

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Abstract. The surface energy balance and meltwater production of the Greenland ice sheet (GrIS) are modulated by snow and ice albedo through the amount of absorbed solar radiation. Here we show, using space-borne multispectral data collected during the 3 decades from 1981 to 2012, that summertime surface albedo over the GrIS decreased at a statistically significant (99 %) rate of 0.02 decade −1 between 1996 and 2012. Over the same period, albedo modelled by the Modèle Atmosphérique Régionale (MAR) also shows a decrease, though at a lower rate (∼ −0.01 decade −1 ) than that obtained from space-borne data. We suggest that the discrepancy between modelled and measured albedo trends can be explained by the absence in the model of processes associated with the presence of light-absorbing impurities. The negative trend in observed albedo is confined to the regions of the GrIS that undergo melting in summer, with the drysnow zone showing no trend. The period 1981-1996 also showed no statistically significant trend over the whole GrIS. Analysis of MAR outputs indicates that the observed albedo decrease is attributable to the combined effects of increased near-surface air temperatures, which enhanced melt and promoted growth in snow grain size and the expansion of bare ice areas, and to trends in light-absorbing impurities (LAI) on the snow and ice surfaces. Neither aerosol models nor in situ and remote sensing observations indicate increasing trends in LAI in the atmosphere over Greenland. Similarly, an analysis of the number of fires and BC emissions from fires points to the absence of trends for such quantities. This suggests that the apparent increase of LAI in snow and ice might be related to the exposure of a "dark band" of dirty ice and to increased consolidation of LAI at the surface with melt, not to increased aerosol deposition. Albedo projections through to the end of the century under different warming scenarios consistently point to continued darkening, with albedo anomalies averaged over the whole ice sheet lower by 0.08 in 2100 than in 2000, driven solely by a warming climate. Future darkening is likely underestimated because of known underestimates in modelled melting (as seen in hindcasts) and because the model albedo scheme does not currently include the effects of LAI, which have a positive feedback on albedo decline through increased melting, grain growth, and darkening.

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Contribution of light-absorbing impurities in snow to Greenland’s darkening since 2009

Cited by 202 publications

References 27 publications

The global land shortwave cryosphere radiative effect during the MODIS era

The global land shortwave cryosphere radiative effect during the MODIS era

Fire emission heights in the climate system – Part 2: Impact on transport, black carbon concentrations and radiation

The darkening of the Greenland ice sheet: trends, drivers, and projections (1981–2100)

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