Reducing spread in climate model projections of a September ice-free Arctic

Abstract

This paper addresses the specter of a September ice-free Arctic in the 21st century using newly available simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5). We find that large spread in the projected timing of the September ice-free Arctic in 30 CMIP5 models is associated at least as much with different atmospheric model components as with initial conditions. Here we reduce the spread in the timing of an ice-free state using two different approaches for the 30 CMIP5 models: (i) model selection based on the ability to reproduce the observed sea ice climatology and variability since 1979 and (ii) constrained estimation based on the strong and persistent relationship between present and future sea ice conditions. Results from the two approaches show good agreement. Under a high-emission scenario both approaches project that September ice extent will drop to ∼1.7 million km 2 in the mid 2040s and reach the ice-free state (defined as 1 million km 2 ) in 2054-2058. Under a medium-mitigation scenario, both approaches project a decrease to ∼1.7 million km 2 in the early 2060s, followed by a leveling off in the ice extent.A rctic sea ice has undergone dramatic decline in recent years (1). The minimum sea ice extent set on September 16, 2012 (3.41 million km 2 , ref.2) was 48.5% below the long-term mean and broke the previous record minimum set on September 18, 2007. The last six years (2007)(2008)(2009)(2010)(2011)(2012) have featured the lowest September ice extents during the satellite era. This decline raises the specter of a September ice-free Arctic in the coming decades, which would have significant impacts on Arctic maritime activities and ecosystems, biogeochemical feedbacks, and extreme weather and climate in mid and high latitudes (3, 4).The Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4) concluded that "Arctic sea ice responds sensitively to warming, . . . late-summer sea ice is projected to disappear almost completely towards the end of the 21st century under the A2 scenario in some models" (5). Subsequent research suggested the ice-free Arctic might occur from as early as the late 2030s to the end of the 21st century under the Special Report on Emissions Scenarios (SRES) A1B and A2 scenarios based on the IPCC AR4 model simulations (also referred to as the CMIP3). An abrupt reduction in sea ice cover (i.e., 2-6 million km 2 within a decade) during the 21st century seems to be a common feature in a number of climate projections by the IPCC AR4 models. This could result in ice-free September conditions (less than ∼1.5 million km 2 ) by 2040 according to one simulation from the Community Climate System Model (CCSM) (6). Using the observed September sea ice extent in 2007 as a starting point, together with the projections of a subset of the IPCC AR4 models that better reproduce the observed climatological September ice extent, one study suggested the Arctic could be ice-free in September (less than 1 million km 2 ) in the late 2030s (with a large uncertainty...

Patterns of Sea Ice Retreat in the Transition to a Seasonally Ice-Free Arctic

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Atmospheric response to the autumn sea-ice free Arctic and its detectability

“…Daily sea-ice area (SIA) is below 0.1 million km 2 during September and October in the last 20 years of the ProjBCM. That is far lower than 1 million km 2 which is used as the threshold to define the nearly ice-free Arctic in previous studies (Liu et al 2013;Overland and Wang 2013).…”

“…Future climate projections by Coupled Model Intercomparison Project Phase 5 (CMIP5) models indicate that the Arctic Ocean would likely be sea-ice free in autumn at mid-twenty first century (Liu et al 2013) or even earlier (Overland and Wang 2013). Sea-ice plays an important role in the climate system because of its high albedo,…”

Description and basic evaluation of Beijing Normal University Earth System Model (BNU-ESM) version 1

“…In Antarctica, BNU-ESM estimates reasonable sea ice extents for February, but overestimates them in September (26 million km 2 ), which is somewhat above the range of 42 CMIP5 models. BNU-ESM and CCSM/CESM adopt similar sea ice schemes, and both models can simulate both the September Arctic sea ice extent and the rate of Arctic sea ice decline over recent decades better than many other CMIP5 models (Liu et al, 2013). While for Antarctica BNU-ESM and CCSM both have a tendency to overestimate sea ice extent.…”