The past and projected freshwater flux from Arctic land ice and its impact on ocean circulation and climate

We have developed a freshwater flux (FWF) time series aimed at providing a benchmark data set for testing the sensitivity of ocean and coupled GCMs to realistic, plausible future FWF forcing alongside a 70 year reconstruction of past fluxes. Here we build on previous work that reconstructed the FWF from Arctic glaciers and the Greenland Ice Sheet (GrIS) from reanalysis (Bamber et al., 2018). First, we use ERA5 reanalyses, a regional climate model and satellite observations to reconstruct the FWF for all Arctic land ice from 1950-2021, partitioned into solid and liquid phases around the coastline of glaciated sectors of the Arctic. We then project the FWF forward until 2300 using estimates of GrIS melt derived from a structured expert judgement assessment for two temperature scenarios that approximate business as usual and a Paris Agreement limit to warming (Bamber et al., 2022). Fluxes from glaciers and ice caps are derived from projections for equivalent temperature scenarios. We develop projections for both the median and 95^th percentile melt estimates to provide FWF forcings that encompass the plausible future range from Arctic land ice. We assumed a linear increase in mass loss from 2021 such that the integral up to 2100 matches the projection for the GrIS by that date. The geographic distribution of melt anomalies are scaled according to the present-day spatial “fingerprint” of mass loss. For the high end case (business as usual, 95^th percentile) this equates to a FWF anomaly from the GrIS of about 0.16 Sv by mid century and 0.3 Sv by 2100, representing an unlikely but plausible FWF entering, primarily, the sub-polar North Atlantic.

We use the historical time series and ensemble of projections to examine their influence on the hydrography of the North Atlantic in a suite of sensitivity studies using the moderate resolution coupled model HadCM3, tuned to present-day transport at 34 degs south. We present preliminary findings of these forcing experiments compared to a control run with a climatological mean FWF.  Even for the most extreme FWF scenario (~ 0.3 Sv) we do not see an AMOC collapse but a monotonic decline that is approximately a linear response to forcing. Interestingly, we do observe a modest cooling compared to the control, of about 0.3 degs for the historical period (1950-2021) in the sub-polar North Atlantic, which appears to be driven by recent ice sheet melt.

Bamber, J. L., M. Oppenheimer, R. E. Kopp, W. P. Aspinall, and R. M. Cooke (2022), Ice Sheet and Climate Processes Driving the Uncertainty in Projections of Future Sea Level Rise: Findings From a Structured Expert Judgement Approach, Earth's Future, 10(10), e2022EF002772, doi:https://doi.org/10.1029/2022EF002772.

Bamber, J. L., A. J. Tedstone, M. D. King, I. M. Howat, E. M. Enderlin, M. R. van den Broeke, and B. Noel (2018), Land Ice Freshwater Budget of the Arctic and North Atlantic Oceans: 1. Data, Methods, and Results, Journal of Geophysical Research: Oceans, 123(3), 1827-1837, doi:10.1002/2017jc013605.