Contribution of the Greenland Ice Sheet to sea level over the next millennium

Abstract

The Greenland Ice Sheet holds 7.2 m of sea level equivalent and in recent decades, rising temperatures have led to accelerated mass loss. Current ice margin recession is led by the retreat of outlet glaciers, large rivers of ice ending in narrow fjords that drain the interior. We pair an outlet glacier-resolving ice sheet model with a comprehensive uncertainty quantification to estimate Greenland's contribution to sea level over the next millennium. We find that Greenland could contribute 5 to 33 cm to sea level by 2100, with discharge from outlet glaciers contributing 8 to 45% of total mass loss. Our analysis shows that uncertainties in projecting mass loss are dominated by uncertainties in climate scenarios and surface processes, whereas uncertainties in calving and frontal melt play a minor role. We project that Greenland will very likely become ice free within a millennium without substantial reductions in greenhouse gas emissions.

Fig. 1. Time series of air temperature anomalies, cumulative contribution to GMSL since 2008, and rate of GMSL rise due to mass changes of the Greenland Ice Sheet.

(A) Ensemble minimum and maximum (thin lines) and mean (thick lines) of RCP 2.6, 4.5, and 8.5 temperature anomalies with respect to 2006–2015 derived from four GCM simulations that extend until 2300. Beyond 2300, the linear 2200–2300 trend was extrapolated to 2500, after which the 2500 value was kept constant (see Materials and Methods). The area between ensemble minimum and maximum is shaded. (B) Cumulative contribution to global mean sea level (GMSL) since 2008 (ΔGMSL). (C) Rates of GMSL in millimeter sea level equivalent (SLE) per year, $\dot{M}$. (D) Contribution of ice discharge to $\dot{M}$ given as ${\dot{D}}_{\dot{M}}=\dot{D}/(\dot{D}+\stackrel{\dot{};}{R})\dot{M}$, where $\dot{D}$ and $\dot{R}$ are ice discharge rate and surface runoff rate, respectively. (E) Ratio of ice discharge rate and the total of ice discharge rate and surface runoff rate, ${\dot{D}}_{\%}=\dot{D}/(\dot{D}+\dot{R})$. (B to E) Uncertainties are shaded between 16th and 84th percentile of the 500 ensemble members, the solid line is the median, and the thin dashed line is the control simulation. Some simulations under RCP 8.5 lose all ice, thus the 84th percentile of the cumulative contribution tapers out (B) and the rates decline (C). Rates in (C) to (E) are 11-year running means. The ensemble mean is used for the control simulation.

Fig. 2. Observed 2008 state and simulations of the Greenland Ice Sheet at year 3000.

(A) Observed 2008 ice extent (53). (B to D) Likelihood (percentiles) of ice cover as percentage of the ensemble simulations with nonzero ice thickness. Likelihoods less than the 16th percentile are masked. (E) Multiyear composite of observed surface speeds (61). (F to H) Surface speeds from the control simulation. Basin names shown in (A) in clockwise order are southwest (SW), central-west (CW), northwest (NW), north (NO), northeast (NE), and southeast (SE). RCP 2.6 (B and F), RCP 4.5 (C and G), and RCP 8.5 (D and H). Topography in meters above sea level (m a.s.l.) [(A) to (H)].

Fig. 3. Evolution of ice sheet area and mass balance components for the control simulation for each RCP scenario.

(A to C) Ice area evolution. (D to F) Partitioning of ice sheet wide mass balance rates into snow accumulation, runoff, and ice discharge into the ocean shown in Gt year⁻¹ (D to F) and kg m⁻² year⁻¹ (left axis) and m year⁻¹ ice equivalent (right axis) (G to I). We distinguish between runoff due to climate warming and runoff due to surface elevation lowering. (A) to (I) are plotted as 11-year running means.

Fig. 4. Retreat of two outlet glaciers in a similar climatic setting between 2015 and 2315 for the RCP 4.5 scenario.

(A) Upernavik Isstrøm South (UIS) shows a gradual retreat of about 50 km over the next 200 years. (B) Store Gletscher (SG) is currently in a very stable position on a bedrock high. It takes almost a hundred years before substantial retreat happens. However, once the glacier loses contact with the bedrock high, retreat of 25 km occurs in less than a decade. The glacier retreats quickly until it is out of the water. Every line represents a year (A and B). (C) Location of the two outlet glaciers on the west coast, present-day observed surface speeds (61), and flow lines of Upernavik Isstrøm South and Store Gletscher (white dashed lines). Small inset shows area where the two glaciers are located.