# Two equally probable, but vastly different transitions, to a seasonally ice-free Arctic

## Implications for global warming communication

Sandrine Trotechaud1, Bruno Tremblay1,2
1
McGill University, 2Columbia University Lamont-Doherty Earth Observatory

### Introduction

The decline in the minimum sea ice extent (Min SIE) has been the center of attention since a few years. Global climate models project a seasonally ice-free Arctic within the next two/three decades. Of the 40 ensemble members (EM) from the CESM-LE model, two are of interest:

• EM 27 projects a relatively continuous decline in the Min SIE until reaching a seasonally ice-free state
• EM 13 projects a considerable recovery in the Min SIE that reaches a level similar of the 2000s followed by a rapid decline.

Both EMs however lead to a seasonally ice-free Arctic.

A recovery in the Min SIE can have an impact on the communication of global warming, because it could cast doubts in people's mind about the existence of climate change. Therefore, the objective of the project is to understand the mechanisms responsible for such trends in the Min SIE.

### Method

CESM-LE
• Fully coupled global climate model with nominal 1∘ resolution in all components
• 40 Ensemble Members initialized with perturbations in the temperature field of 10-14 K
• Annual data from 1920 to 2100
RD

A rapid decline (RD) is defined as being steeper than -0.3 million km 2 per year in 5-year running mean. Gradual declines (GD) identified are less steep, but still important in the study.

### Method (continued)

Spatial Domain

The spatial domain of this study consists of the Arctic Ocean. It includes the Barents Sea Opening (BSO), the Fram Strait and the Bering Strait. The continental shelves, characterized by depths shallower than 425 meters, are shown on the figure below as grey shaded area.

OHT

Ocean heat transport defined through a gate:

OHT=cp$$\rho$$UTAcross

OHT=cp$$\rho$$$$\sum$$i,kFi,kAi$$\Delta$$zk

### Results : EM 27

The EM 27 has two key periods identified: a gradual decline from 1999 (black) to 2016 (cyan) and a rapid decline from 2016 to 2022 (green).

### Results : EM 27 (continued)

According to a 20-year sliding window correlation of the anomalies:

• The first half of the GD isn’t significantly correlated with any OHTs. When comparing the trend in the Min SIE during the first half of the GD with the model mean thermodynamic tendency (not shown here), it is shown that it is less than climatology.
• The second half of the GD and the RD are significantly anti-correlated with the BSO (r=-0.65) and the Fram Strait OHTs (r=0.60).

### Results: EM 13 (continued)

The EM 13 has three key periods identified: a gradual decline from 2000 (black) to 2014 (green), a recovery period from 2014 to 2020 (pink) followed by a rapid decline from 2020 to 2029 (cyan).

Shown by the correlation of the anomalies:

• The GD is only anti-correlated significantly with the Bering Strait at the beginning and the end of the period (r=-0.55). However, the significance level remains above 80% the whole period with r=-0.40.
• The recovery period and the RD are significantly anti-correlated with the Bering Strait at r=-0.45.

The SW and LW heat fluxes are significantly anti-correlated with the Min SIE (resp. r=-0.95 and r=-0.75). The LW heat flux undergo a considerable increase during the RD.

### Discussion

From the results and with further investigation, we can note that:

• Recoveries are still possible (to the observed 1994 level) even from a nearly ice-free Arctic initial condition.
• During the rapid decline following the recovery period, the LW heat flux has a significative role in inhibiting

the formation of the ice during the winter. The sea ice stays relatively young and thin which predisposes a great ice loss during the next few years, consisting of a RD.
• The SW heat flux role varies between a moderate and an effective amplifier of the melting through the ice-albedo feedback, but is rarely the primary cause.
• The OHTs are the primary cause in the key periods identified in this study with a focused importance of the Bering Strait OHT in EM 13 and Fram Strait/BSO OHTs in EM 27.
• Although the Bering Strait OHT is of smaller amplitude, it interacts with the sea ice over expanded continental shelves which promotes considerable melting.

Multiple scenarios are plausible for the future trend in the Min SIE due to great climate variability. A considerable recovery in the Min SIE is possible even at a nearly ice-free Arctic condition through internal variability. However, as we saw in the case of EM 13, it is not sustainable and the EM still projects a seasonal ice-free cover within a few years of EM 27.

Hence, recoveries in the Arctic sea ice cover are possible due to climate variability, even with the presence of climate change.