Abstract
Abstract. Forecasting the future sea level relies on accurate modeling of the response of the Greenland and Antarctic ice sheets to changing temperatures. The surface mass balance (SMB) of the Greenland Ice Sheet (GrIS) has a nonlinear response to warming. Cold and warm anomalies of equal size do not cancel out and it is therefore important to consider the effect of interannual fluctuations in temperature. We find that the steady-state volume of an ice sheet is biased toward larger size if interannual temperature fluctuations are not taken into account in numerical modeling of the ice sheet. We illustrate this in a simple ice sheet model and find that the equilibrium ice volume is approximately 1 m SLE (meters sea level equivalent) smaller when the simple model is forced with fluctuating temperatures as opposed to a stable climate. It is therefore important to consider the effect of interannual temperature fluctuations when designing long experiments such as paleo-spin-ups. We show how the magnitude of the potential bias can be quantified statistically. For recent simulations of the Greenland Ice Sheet, we estimate the bias to be 30 Gt yr−1 (24–59 Gt yr−1, 95 % credibility) for a warming of 3 °C above preindustrial values, or 13 % (10–25, 95 % credibility) of the present-day rate of ice loss. Models of the Greenland Ice Sheet show a collapse threshold beyond which the ice sheet becomes unsustainable. The proximity of the threshold will be underestimated if temperature fluctuations are not taken into account. We estimate the bias to be 0.12 °C (0.10–0.18 °C, 95 % credibility) for a recent estimate of the threshold. In light of our findings it is important to gauge the extent to which this increased variability will influence the mass balance of the ice sheets.
Highlights
Ice sheet mass balance has a nonlinear dependence on temperature. This behavior is observed in simple ice sheet models (Weertman, 1961, 1964, 1976; Roe and Lindzen, 2001) and in regional climate modeling of Greenland surface mass balance (SMB) (Fettweis et al, 2013), and the nonlinear effect of temperature on melt has been observed in Greenland river discharge
Temperature fluctuations may be explicitly accounted for by forcing the ice sheet model with climate model output that reproduces the magnitude of the observed interannual temperature variability
We find that the steady-state ice sheet volume in Oer03 is 0.5–1 m SLE smaller when the minimal model is forced with fluctuating temperatures compared to constant temperature (Fig. 2)
Summary
Ice sheet mass balance has a nonlinear dependence on temperature. This behavior is observed in simple ice sheet models (Weertman, 1961, 1964, 1976; Roe and Lindzen, 2001) and in regional climate modeling of Greenland surface mass balance (SMB) (Fettweis et al, 2013), and the nonlinear effect of temperature on melt has been observed in Greenland river discharge (van As et al, 2017). On the scale of sub-annual climatology, there are large uncertainties as the estimates of the SMB differ significantly depending on the simplifying assumptions used in the PDD formulation, highlighting the need to accurately model both spatial and temporal variability These findings are built upon by Wake and Marshall (2015), who found that the standard deviation of monthly average temperature may be represented as a quadratic function of monthly average temperature. 4 we estimate the consequences of temperature fluctuations on a recent long-term ice sheet study (Robinson et al, 2012), assuming the effect of temperature fluctuation presented here is not already accounted for The limitations of this approach, as well as further possible applications, are discussed in Sect.
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