Understanding the Relationship Between Arctic Sea Ice Loss and Planetary Boundaries
Seung-Ki Min
Pohang University of Science and Technology, South Korea
“Disappearance of Arctic sea ice even under a low emission scenario calls for stronger policies and earlier achievement of net zero target”
The Arctic sea ice regulates global warming by keeping the Arctic cool. It reflects much of incoming solar radiation and insulates the atmosphere from the warmer water below. Without this protective sea ice buffer, the Arctic warming will intensify quickly, accelerating global warming. Arctic sea ice has been rapidly melting during the past decades due to human-induced greenhouse warming, and the ice-free Arctic condition was expected to occur in September, when the sea ice area reaches its annual minimum, near the mid-21st century unless we reduce greenhouse gas emissions aggressively (Allan et al., IPCC 2021). However, there remains a large uncertainty in this prediction and our recent study has found that ice-free Arctic conditions can arrive as early as the 2030s (Figure 1). This is faster by around a decade than recently reported by the Intergovernmental Panel on Climate Change (IPCC). Moreover, our results show that an ice-free Arctic condition will occur in September irrespective of our effort to reduce emissions, which was not initially expected. This means that we need to prepare adaptation measures for the faster Arctic warming and associated impacts such as possible changes in atmospheric circulations and more frequent heatwaves and wildfires in the mid-latitudes. The low emission scenario we considered is the Shared Socioeconomic Pathways (SSP) 1-2.6 and its path assumes net zero emissions achieved around 2070 (Lee et al., IPCC 2023). To keep the Arctic sea ice present during summertime and thus avoid associated impacts, we need to achieve net zero emissions earlier, around 2050, which is equivalent to achieving the 1.5 degrees warming target of the Paris Agreement.
Figure 1. Future changes in September Arctic see ices area. Adopted from Kim et al., 2023.
Our results confirm that a stronger emission reduction would delay the arrival of an ice-free Arctic in September and, more importantly, avoid the expansion of the ice-free Arctic into other summer months. Compared to the low emissions scenario, higher emissions scenarios project ice-free August and October before the 2080s, indicating that higher emissions will surely bring stronger impacts. The current policies based on nationally determined contributions (NDCs), i.e. all countries’ action plans to cut emissions following the requirement of the Paris Agreement, will be close to the SSP2-4.5 scenario (intermediate emissions path) which projects about 2.7 degrees of global warming relative to the preindustrial condition (Lee et al., IPCC 2023). In this scenario, our results indicate that the first ice-free September will be experienced as early as in the 2040s, also expanding into August and October before the 2070s. However, if we can achieve the 1.5-degrees warming target, we can see Arctic sea ice remain in most of the months with some overall reduced area compared to the current condition. This implies that Arctic sea ice can be recovered under stronger mitigation policies since the Arctic sea ice area is closely related to global warming levels.
To reduce errors and improve the reliability of ice-free Arctic projections, we have adjusted future projections of the Arctic sea ice area using a rigorous method of comparison between observations and climate model simulations. Using this method, we have found that greenhouse gas emissions dominated the reduction in observed sea ice area during the past four decades and that models on average underestimate this observed trend. We have applied this insight on model underestimation and constrained the models’ future projections accordingly. Global climate models we used are recent versions of computer programs that can simulate Earth’s climate system based on physical laws considering atmosphere-ocean-sea ice interactions at a 1–2-degrees resolution. To find the causes of the observed Arctic sea-ice changes, we used climate model simulations performed under different observed forcings including greenhouse gas increases only, anthropogenic aerosol changes only, and solar and volcanic activities. After estimating the response patterns of the Arctic sea ice area to each forcing, we related these ‘fingerprint’ patterns to the observations using a technique that enables the breakdown of observed trends into each forcing contribution, and we found that increase in greenhouse gas emissions explains most of the observed sea ice reduction throughout the year, whereas other forcing factors exert much weaker influences. This procedure is important for reliable future projections which are basically determined by greenhouse gas emissions scenarios.
After scaling our model simulations based on observational data, we have confirmed an even faster timing of Arctic sea ice depletion than previous IPCC predictions. In particular, we have provided the timing of a 10-year faster arrival on average of ice-free Arctic, under different emissions scenarios, which will provide important implications for policymakers in Arctic regions as well as remote regions affected through global climate feedbacks. Our projection results warn that we need to be vigilant about the potential disappearance of Arctic sea ice, regardless of carbon neutrality policies, and that the goal of net zero emission needs to be achieved much earlier than the 2050s. We therefore hope that the various climate change impacts resulting from the disappearance of Arctic sea ice and the developing adaptation measures alongside carbon emission reduction policies will be reassessed very soon based on our study and subsequent analyses. The worst scenario of losing the Arctic sea ice within a decade should be considered seriously as an important factor of accelerating the breakdown of multiple planetary boundaries including biosphere integrity, atmospheric aerosol loading, and freshwater change.
Our findings highlight for the first time that the extinction of Arctic sea ice is possible irrespective of achieving carbon neutrality, indicating the possibility of faster collapse of the climate change boundary. The accelerated decline of Arctic sea ice, faster than previously anticipated, is expected to have significant impacts not only on the Arctic region but also on human societies and ecosystems worldwide, affecting multiple planetary boundaries including atmospheric aerosol loading and freshwater use. The reduction of sea ice can result in changes in atmospheric circulations by intensifying Arctic warming. The stronger Arctic warming, so-called Arctic amplification, is expected to reduce temperature difference between the Arctic and mid-latitude Asia, Europe, and North America, resulting in increased chances of persistent high-pressure weather patterns, so-called ‘blocking’ events. The increased chances of atmospheric steady conditions will cause more frequent occurrences of extreme weather events such as heat waves, wildfires, and heavy rainfalls over the Northern Hemisphere mid-latitudes including North America, Europe, and Asia. These stagnant weather patterns can also degrade air quality, given the same atmospheric aerosol loading. In addition, Arctic warming can fasten the thawing of the Siberian permafrost, which will intensify global warming by releasing methane and carbon dioxide. The accelerated Arctic warming will exert adverse impacts on global precipitation patterns, changing the rainfall distributions, and thereby affect freshwater use as well. Therefore, it is urgently required to assess the possible influences of the faster ice-free Arctic on these global planetary boundaries. Once assessed, we will be able to prepare better mitigation and adaptation strategies to overlooked worst-case scenarios, and hopefully minimize undesired impacts that could result from abrupt Arctic changes.
References
Allan, Richard P., Paola A. Arias, Sophie Berger, Josep G. Canadell, Christophe Cassou, Deliang Chen, Annalisa Cherchi et al. Intergovernmental Panel on Climate Change (IPCC). Summary for Policymakers. In Climate change 2021: The physical science basis. Contribution of working group I to the sixth assessment report of the intergovernmental panel on climate change. Cambridge University Press, 2023. pp. 3-32.
Lee, Hoesung, Katherine Calvin, Dipak Dasgupta, Gerhard Krinner, Aditi Mukherji, Peter Thorne, Christopher Trisos et al. IPCC, 2023: Climate Change 2023: Synthesis Report, Summary for Policymakers. Contribution of Working Groups I, II and III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, H. Lee and J. Romero (eds.)]. IPCC, Geneva, Switzerland, 2023. pp. 1-34.
Kim, Yeon-Hee, Seung-Ki Min, Nathan P. Gillett, Dirk Notz, and Elizaveta Malinina. Observationally-constrained projections of an ice-free Arctic even under a low emission scenario. Nature Communications 14, no. 1, 2023. Article number 3139.
