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Australia: The Land Where Time Began |
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Greenland Ice Sheet Surface Melt Amplified by Snowline migration
and bare Ice Exposure
Mass loss from the Greenland ice sheet has increased recently as a
result of enhanced surface melt and runoff. As surface albedo critically
modulates melt, a prerequisite for forecasting accurately mass loss is
an understanding the processes and feedbacks that alter albedo. Ryan et
al. demonstrated the
importance of Greenland’s seasonally fluctuating snowline, which reduces
the albedo and enhances the melt by exposing dark bare ice, by using
satellite imagery. This process drove 53% of net shortwave radiation
variability in the ablation zone and amplified melting of the ice sheet
by 5 times more than hydrological and biological processes that darken
bare ice itself. Fluctuations of the snowline will exert an even greater
control of melt due to the flatter topography of the ice sheet at higher
elevations in a warmer climate. However, current climate models predict
inaccurately the elevations of the snowline during high melt years,
which portends an uncertainty that has not been foreseen in the
contribution to global sea level rise.
At the present, the Greenland Ice Sheet is the largest single
cryospheric contributor to global sea level rise, contributing 25% of
the total contribution of observed rise of the global ocean (Chen et
al., 2017). For the 21st century the enhanced loss of mass
from Greenland has been attributed to the increased runoff of surface
meltwater (van den Broeke et al., 2009; Enderlin et al., 2014; Van den
Broeke et al., 2016), of which ~93% is derived from the relatively small
ablation zone (~22% of the area of the ice sheet) along the margin of
the ice sheet (Box et al., 2012). During summer, as the winter snowpack
melts, bare glacial ice is exposed. Bare glacial ice absorbs more than
twice as much solar radiation and retains less melt water, because the
bare ice is darker and less porous than snow. Therefore, bare ice
produces a large proportion (~78%) of the total runoff to the ocean from
Greenland (Steger, Reijmer & van den Broeke, 2017), though exposed only
across a small area of the ice sheet during summer. It is, therefore,
critical to capture accurately the reduced albedo and the full extent of
bare ice in climate models in order to determine the current and future
runoff contributions from Greenland to sea level rise (van den Broeke et
al., 2017).
According to Ryan et al. it
is a challenge to represent bare ice albedo and extent in climate
models, as both impart positive feedbacks that are nonlinear between net
shortwave radiation and surface melt over seasonal time scales (Box et
al., 2012; van den Broeke et al., 2017; Tedesco et al., 2011; Tedesco et
al., , 2016). Bare ice albedo is reduced by a seasonal increase in
downwards shortwave radiation by melt processes that darken the surface
of the ice, notably the exposure of dust layers, pooling of surface
meltwater, increased content of interstitial water, and growth induced
by liquid meltwater of ice algal assemblages that are pigmented that
inhabit the surface of bare ice (Greuell, 2016; Wientjes et al., 2011;
Stibal et al., 2017; Tedstone et al., 2017; Williamson et al., 2018;
Ryan et al., 2018). It was argued that that these bare ice processes
have contributed substantially to a reduction in albedo and associated
increase in melt that has been observed across the ablation zone of
Greenland between 2000 and 2011 (van den Broeke et al., 2017; Stibal et
al., 2017; Tedstone et al., 2017), in spite of operating over a
relatively small area of the ice sheet. Ryan et
al. collectively term this
category of physical and biological melt-albedo processes that darken
bare ice the “the bare ice feedback”. The extent of bare ice through the
annual migration of the summer snowline is also increased by a seasonal
increase in downwards shortwave radiation. Glacial bare ice is exposed
which enhances further the adsorption of shortwave radiation by the ice
sheet, as sufficient energy is received at the surface to melt
completely the winter snowpack that has accumulated. Ryan et
al. term this melt albedo
feedback the “snowline-albedo feedback.” Ryan et
al. say snowlines have
received surprisingly little focus in Greenland beyond the pioneering
facies work in the 1960s and the 1990s (Benson, 1962; Fahnestock et al.,
1993; Partington, 1998), though the importance of this process has been
recognised for a long time in alpine glacier settings (Smith et al.,
1997; Klein & Isacks, 1999; Rabatel et al., 2012). The importance of the
snowline-albedo feedback in the amplification of melt, and its efficacy
relative to the bare ice-albedo feedback has, therefore, not yet been
evaluated.
In this paper Ryan et al.
assess the importance of the snowline-albedo feedback and the influence
it has on the meltwater production of the Greenland Ice Sheet using new
remotely sensed products of a bare ice presence. They derived this
product from daily Moderate resolution Imaging Spectroradiometer (MODIS)
satellite imagery acquired by NASA’s Terra satellite and validated by
use of Landsat 5,7 and 8 satellite imagery and in situ field
observations. They used their product to map the variability of the
snowline across Greenland from 2001-2017, as well as to evaluate the
impact on the total net shortwave radiation relative to processes that
darken bare ice and firn/snow. Then they combined their snowline dataset
with the surface topography to investigate how the strength of the
snowline-albedo feedback changes as snowlines rise to higher elevations
under a warming climate. As a result of their heavy use for the
prediction into the future of the Greenland Ice Sheet melting and
contribution of runoff to global sea level rise, they assessed whether
regional climate models (RCMs) accurately determine snowline elevations.
Discussion
Ryan et al. suggest that in
principle, the current generation of physical RCMs should be capable of
capturing the current and future strength of the snowline-albedo
feedback, as they couple sophisticated multilayer snow models with
realistic ice sheet topography (van den Broeke et al., 2016; Steger et
al., 2017; Fettweis et al., 2017; Noël et al., 2018; Noël et al., 2015).
Ryan et al. found, however,
that 2 RCMs that are commonly used to forecast meltwater runoff from
Greenland [MAR3.9 (Fettweis et al., 2017) and Regional Atmospheric
Climate Model (RACMO) 2.3p2 (Noël et al., 2018) do not capture maximum
snowline elevations and the extent of bare ice accurately. Compared to
the remotely sensed bare ice presence metrics of Ryan et
al. found that, on average,
RACMO2.3p2 overestimates by 13% during the 2001-2017 study period. They
also found that these discrepancies are correlated significantly with
total summer runoff. It is suggested by this that MAR3.9 and RACMO2.3p2
do not sufficiently capture the role of the snowline-albedo feedback
during extreme melt years (e.g., 2010, 2012, and 2016). The failure of
RCM to predict accurately snowline elevation and the extent of bare ice
during high melt years, given that bare ice exposure is a primary
control on the production of meltwater and that extreme melt events are
projected to increase in the future (Vizcaino et al., 2014), raises
uncertainty in 21st century forecasts of future runoff from
Greenland contributions to global sea level rise. Also, uncertainty in
bare ice extent adds an additional challenge for modelling the
spatiotemporal variability of bare ice albedo and its impact on runoff
in the future (Tedstone et al., 2017; Ryan et al., 2018).
It is indicated by the findings of Ryan et
al. that future projections
of ice sheet runoff by the current generation of semi-empirical models
(e.g., positive degree day or temperature index) must also be treated
with caution. It is assumed by these models, which are typically
calibrated to RCM-modelled runoff, that the strength of melt-albedo
feedbacks that were observed in the past remains constant into the
future (Franco et al., 2013; Mengel et al., 2016; Mengel et al., 2018).
Since the snowline-albedo feedback is not captured accurately by RCMs
and there is an increase nonlinearly in exposure of bare ice as
snowlines rise, it appears this assumption is not justified. Ryan et
al. suggest that they should
account for the increasing strength of melt-albedo feedbacks, such as is
induced by snowline migration, if semi-empirical models are to be used
for forecasts of the future contributions to sea level rise from
Greenland.
The importance of the snowline in Greenland in the amplification of ice
sheet melt was quantified for the first time by this study by Ryan et
al. They found that
substantial seasonal and interannual variation was exhibited by
snowlines and they are the dominant control on the absorption of
shortwave energy in the production of meltwater in ablation zones.
Hydrological and biological processes are secondary to the extent of
bare ice exposure that is associated with fluctuations of the snowline,
though they also darken bare ice and influence the absorption of
shortwave energy and the generation of runoff. Snowlines will rise to
higher elevations in a warmer climate and amplify melt seasonally even
more than they do at present due to the hypsometry of the ice sheet, if
they are not offset by enhanced accumulation of snow in winter. Rising
snowlines will reduce the albedo of the ice sheet as well as the
capacity of the ice sheet to retain meltwater, as bare ice has a much
lower porosity than snow and firn. Also, the exposure of bare ice will
increase the rate of turbulent heat transfer from the atmosphere to the
surface of the ice sheet, as bare ice has substantially higher surface
roughness than snow. It is therefore critical for the accuracy of
projecting future runoff contribution from the Greenland Ice Sheet to
global sea level rise, the representation of snowlines and bare ice
exposure in climate models.
Ryan, J. C., et al. (2019). "Greenland Ice Sheet surface melt amplified
by snowline migration and bare ice exposure." Science Advances
5(3): eaav3738.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |