Assessing Unstoppable Change: Ocean Heat Storage and Antarctic Glacial Ice Melt

Prediction of sea level rise from understanding and modeling of glacial and land-based ice sheet melt is difficult at best, yet of critical importance for future climate prediction. Antarctic glacial melt is particularly difficult, leading to the Antarctic’s contribution to sea level rise being downplayed during IPCC assessment IV. Numerous observation and modeling studies cite the ocean as providing the source of heat for the recently observed acceleration of the Antarctic melt rate. That melt is concentrated in the West Antarctic, at the coastal margin of the Amundsen/Bellingshausen Seas (ABS). We approach this project with 17 years of gridded ocean data adjacent to the West Antarctic Peninsula (WAP) upstream of the West Antarctic Ice Sheet (WAIS) primary drainage basin. These data show that the ocean heat content on the WAP shelf (QWAP) has been rising steadily since the early 1990s, and dramatically since the 1950s, qualitatively consistent with the dramatic increase in the observed glacial melt, and with the required ocean heat. This warm water, Upper Circumpolar Deep Water (UCDW) is available for melting ice in the WAP and WAIS. We desire to determine the ultimate source of this increased ocean heat content, to estimate future warming associated with the source. 

The world oceans have been absorbing heat at their surface from the warming atmosphere, and some of that heat has penetrated to depth, leading to excess ocean heat content (Qexcess); multiple studies argue that the observed Qexcess is due to absorption of anthropogenic heat. Some of this heat will reach, or is already within, southward flowing deep currents transporting it to the Antarctic Circumpolar Current (ACC), where it warms the warm deep subsurface water it already transports. The ACC transports this warmed water to the ABS shelves (the only shelves in the Antarctic where the ACC flows along the shelf-break) fueling accelerated glacier melt. The goal of this project is to assess what fraction of QWAP warming is from this Qexcess and via extrapolation, how much of this Qexcess would still be available for accelerated glacial melt in the ABS, even if there is a reduction or elimination of global warming. 

The analyses will involve assessment of global historical data, beginning as a natural extension to the series of studies that have analyzed historical data to show that the ocean heat content has risen since the 1950s. We will use previously developed objectively analyzed 5° gridded composites to deal with the sparse data deeper than 300 m in earlier decades, updated to correct for XBT and Argo float biases. In our case the focus is more on change as a function of time and space, in an effort to track potential paths of heat transfer to the south (and time scales of the transfer). 

We expect to reveal that amount of heat (with uncertainties) that will still be available for glacial melt regardless of changes in the rate of global warming (or even better, as a function of total global warming). In other words, climate change is already committed to accelerated glacial melt from this stored heat — knowing the magnitude and timescale of its delivery is an essential component required in our ability to model the contribution of glaciers and land-based ice sheets to future global sea level rise. Alternative methods for explaining increasing QWAP (e.g., changes in the strength of the polar westerlies) are being investigated elsewhere, but preliminary analysis of the post-1990s data suggest that both mechanisms contribute.