Forced Changes in the Arctic Freshwater Budget Emerge in the Early 21st Century

Abstract

Arctic liquid freshwater (FW) storage has shown a large increase over the past decades, posing the question: Is the Arctic FW budget already showing clear signs of anthropogenic climate change, or are the observed changes the result of multidecadal variability? We show that the observed change in liquid and solid Arctic FW storage is likely already driven by the changing climate, based on ensemble simulations from a state‐of‐the‐art climate model. Generally, the emergence of forced changes in Arctic FW fluxes occurs earlier for oceanic fluxes than for atmospheric or land fluxes. Nares Strait liquid FW flux is the first flux to show emergence outside the range of background variability, with this change potentially already occurring. Other FW fluxes have likely started to shift but have not yet emerged into a completely different regime. Future emissions reductions have the potential to avoid the emergence of some FW fluxes beyond the background variability.

Dominant Terms in the Freshwater and Heat Budgets of the Subpolar North Atlantic Ocean and Nordic Seas From 1992 to 2015

“…Although a correlation with wind exists, freshening and cooling are likely not driven directly by changes in wind. While Arctic freshening has been shown to be principally anthropogenic (Jahn & Laiho, 2020), this is not currently the case for the SPNA. Arctic‐North Atlantic exchanges will likely play an important role in the future of global climate as it can be expected that accumulated Arctic freshwater will eventually outflow through major gateways into the SPNA.…”

Arctic Ocean Freshening Linked to Anthropogenic Climate Change: All Hands on Deck

“…The CESM experiments reported by Jahn and Laiho (2020) make clear projections about future change (Figure 1). They delineate the sequence of emergence of forced signals and they characterize the variability of different elements of the freshwater system.…”

“…The question is what causes these freshwater changes? Now Alexandra Jahn and Rory Laiho of the University of Colorado at Boulder have provisionally answered this question (Jahn & Laiho, 2020). They show that the Arctic Ocean liquid freshwater storage increase is anthropogenic, like the sea ice decline.…”

“…The middle subplots show the freshwater storage in the Arctic Ocean as sea ice (solid, top) and liquid (bottom) freshwater (km 3 relative to 34.80). Results from the Community Earth System Model (CESM) control (gray), large ensemble (LE, purple), and low warming (LW, green) experiments are shown in each case, adapted from Jahn and Laiho's (2020) Figure 2. The subplots show the times when the models show emergence of a forced, anthropogenic signal (meaning the time of first permanent departure from the ±3.5 σ envelope of control variability, where σ is the standard deviation; horizontal and vertical lines).…”

“…They examine the simulated Arctic Ocean freshwater cycle from the 21st century projections of the Coupled Earth System Model (CESM), a climate model from the National Center for Atmospheric Research (thin colored lines in Figure 1). Jahn and Laiho (2020) use output from an ensemble of CESM runs, which is a set of many model projections that differ only in their natural, unforced climate variations. They consider two CESM ensemble experiments for the 21st century: the large ensemble (CESM‐LE, Kay et al, 2015, purple lines) and the low warming ensemble (CESM‐LW, Sanderson et al, 2017, green lines).…”