By: Jake Aylmer
The futures of Arctic and Antarctic sea ice are tough to pin down partly because of local weather mannequin uncertainty. Latest work reveals totally different ocean behaviours which have a crucial impression on sea ice, highlighting a possible means to constrain projections.
For the reason that late Seventies, satellites have monitored the frozen floor of the Arctic Ocean. The decline in Arctic sea ice cowl—about 12% space misplaced per decade—is a putting and well-known sign of local weather change. In addition to long-term retreat of the ocean ice edge, the ice is changing into thinner and extra fragmented, making it extra susceptible to excessive climate and an more and more precarious setting for human actions and polar wildlife. On the reverse pole, sea ice surrounding Antarctica has not, on the entire, modified considerably regardless of international warming—a conundrum but to be totally resolved.
There’s excessive confidence that Arctic sea ice will proceed to retreat all through the twenty-first century, however uncertainties stay within the specifics. For example, when will the primary ice-free summer time happen? Such questions are inherently unsure because of the chaotic nature of the local weather system (inner variability). Nevertheless, totally different local weather fashions give vastly totally different solutions starting from the 2030s to 2100 or past, indicating a contribution of mannequin biases within the projected charges of sea ice loss.
My co-authors and I are notably within the position the ocean may play in setting such mannequin biases. Research present that the ocean circulation has a powerful affect on sea ice extent in fashions and observations, related to its transport of warmth into the polar areas (e.g., Docquier and Koenigk, 2021). If there may be variation on this ocean warmth transport throughout local weather fashions, this might have a knock-on impact on the ocean ice and thus assist clarify uncertainties in future projections. To discover this, we should first perceive how the connection between the ocean warmth transport and sea ice happens.
We checked out simulations of the pre-industrial period, which exclude international warming and thus act as management experiments isolating pure, inner variability. In all fashions examined, when there’s a spontaneous enhance in internet ocean warmth transport in the direction of the pole, there’s a corresponding lower in sea ice space. That is intuitive—extra warmth, much less ice. It happens independently at each poles, however how the ocean warmth reaches sea ice is totally different between the 2.
Within the Arctic, the warmth is launched across the sea ice edge. It doesn’t lengthen far underneath the majority of the ice pack as a result of there are restricted deep-ocean routes into the Arctic Ocean, which is itself shielded from rising warmth by recent floor water. However, the ocean warmth transport contributes to sea ice soften nearer the north pole, assisted by atmospheric transport appearing as a ‘bridge’ to larger latitudes. For Antarctic sea ice, the method is extra easy with the warmth being merely launched underneath the entire sea ice pack—the Southern Ocean doesn’t have the identical oceanographic obstacles because the Arctic, and there’s no atmospheric position (Fig. 1). These totally different pathways lead to totally different sensitivities of the ocean ice to modifications in ocean warmth transport, and are remarkably constant throughout totally different fashions (Aylmer, 2021; Aylmer et al. 2022).
Determine 1: Completely different pathways by which additional ocean warmth transport (OHT) reaches sea ice within the Arctic (crimson) the place it’s ‘bridged’ by the ambiance to succeed in nearer to north pole, in comparison with the Antarctic (darkish blue), the place it’s merely launched underneath the ice. Schematic tailored from Aylmer et al. (2022).
We will additionally clarify how a lot sea ice retreat happens per change in ocean warmth transport utilizing a simplified ‘toy mannequin’ of the polar local weather system, constructing on our earlier work creating principle underlying why sea ice is extra delicate to oceanic than atmospheric warmth transport (Aylmer et al., 2020; Aylmer, 2021). This work, which is ongoing, accounts for the totally different pathways proven in Fig. 1, and we now have proven it to quantitatively seize the local weather mannequin behaviour (Aylmer, 2021).
There’s mounting proof that the ocean performs a key position sooner or later evolution of Arctic and Antarctic sea ice, however questions stay open. For example, what position does the ocean play within the sea ice sensitivity to international warming—one thing that’s persistently underestimated by fashions (Rosenblum and Eisenman, 2017)? Our toy-model principle is at the moment unable to discover this as a result of it’s designed to know the variations amongst fashions, not their offset from observations. As a part of a brand new undertaking because of begin in 2023, we’ll adapt it for this objective and embrace extra detailed sea ice processes that we hypothesise might clarify this bias. As extra ocean observations change into out there, it’s doable that our work might assist to constrain future projections of the Arctic and Antarctic sea ice.
