Arctic sea ice loss is expected to continue over the next decades, with implications for the climate system and Arctic communities and ecosystems.
Arctic sea ice: A defining feature of the Arctic
Arctic sea ice forms from freezing ocean water during the dark, cold Arctic winters. In the 1980s, the sea ice stretched across the entire Arctic Ocean in winter, even reaching parts of the adjacent North Pacific and North Atlantic Oceans.
Each spring, as sunlight returns, the ice melts, but even at its lowest point in September, it still covers half of the Arctic Ocean. The Arctic Ocean’s year-round sea ice cover is a key feature of the Arctic as a region dominated by ice and snow.
The rapid decline of Arctic sea ice in recent decades
Over the past four decades, Arctic sea ice has declined significantly in both area and thickness. Since the start of continuous satellite observations in late 1978, we have seen a 50% reduction in the sea ice area in September (Fig. 1).
Therefore, during the seasonal minimum in September, the sea ice now covers only about 25% of the Arctic Ocean (Fig. 2). At the same time, the ice has also become thinner, as shown by measurements from submarines and, more recently, satellites.
Climate models show that the dramatic loss of Arctic sea ice over the past 40 years can only be explained by the influence of human-caused greenhouse gas emissions. When models are run with only natural factors, such as changes in solar radiation or volcanic activity, they fail to replicate the observed 50% reduction in Arctic sea ice area in September.
Impacts of Arctic sea ice loss
The sharp decline in Arctic sea ice cover in September is one of the most visible signs of human-driven greenhouse gas emissions (Fig. 2). However, the importance of sea ice loss goes far beyond symbolism. For example, the changing sea ice cover already affects Indigenous communities in the Arctic that rely on sea ice for hunting and transportation.
Furthermore, when sea ice melts, the bright, reflective surface of the Arctic is replaced by the darker ocean beneath. Unlike ice, which reflects much of the sun’s energy like a mirror, the darker ocean absorbs much more sunlight, warming the surface waters. This added heat accelerates sea ice melt, exposing even more dark oceans and further increasing heat absorption. This feedback loop is one of the key reasons why the Arctic has warmed two to four times faster than the global average and why sea ice loss has been so large in recent decades.
What happens in the Arctic doesn’t stay in the Arctic, as the region is closely linked to lower latitudes through ocean currents and atmospheric circulation. As a result, the increased energy absorbed by the Arctic Ocean from melting sea ice ultimately influences the climate in regions further south as well.
Projections of future Arctic sea ice loss and its impacts
Climate models agree that under continued global warming, the Arctic sea ice cover will continue to decrease in the coming decades.
This reduction is expected to persist through at least the mid- to late 2040s and across all months of the year, regardless of the greenhouse gas emission scenario considered. It is only in the late 2040s that differences in different possible future emissions pathways begin to significantly influence the extent of the Arctic sea ice cover.
As Arctic sea ice continues to decline, the Arctic Ocean absorbs increasing amounts of solar energy, triggering profound changes. One notable impact is a shift in ecosystems: as waters warm, fish species from the North Atlantic are likely to expand into newly hospitable areas of the Arctic Ocean. At the same time, the loss of sea ice threatens the survival of species such as polar bears and ringed seals, which depend on the ice for breeding and hunting. These ecological changes, coupled with the extended absence of sea ice near shorelines, will further disrupt the traditional hunting and transportation practices of Arctic Indigenous communities.
Less sea ice in the Arctic, especially in summer, also means it is more accessible to commercial shipping and tourism. In recent years, the number of ships crossing the Arctic has risen and is expected to grow as sea ice continues to decline. The reason for that is that the Arctic shipping route offers a faster and more cost-effective path between Europe or the US West Coast and Asia.
However, this increased traffic through the Arctic Ocean requires planning for search and rescue operations and disaster response in the remote Arctic Ocean. In addition, a more navigable Arctic Ocean has geopolitical implications.
Notably, all these impacts increase the more sea ice the Arctic loses.
An ice-free Arctic Ocean by 2050
Once the remaining sea ice area is equal to or less than 1 million km2, scientists consider the Arctic to be `practically ice-free.’ At that point, the remaining sea ice will be limited to the area north of Greenland and the Canadian Archipelago, leaving over 93% of the Arctic Ocean without sea ice.
The Arctic Ocean has not experienced ice-free conditions for over 80,000 years, meaning that modern humans have never encountered an ice-free Arctic.
Current climate models, which informed the 2021 Intergovernmental Panel on Climate Change Assessment Report, project a 66% probability that the Arctic will experience its first ice-free September by 2050. However, ice-free conditions could also occur decades earlier or later than 2050, as climate model projections are always probabilistic.
The probabilistic nature of climate projections arises from the chaotic behaviour of the atmosphere and ocean, which limits precise predictions, much like the inability to forecast weather beyond about ten days.
Climate models show that staying below global warming of 1.5°C is the only way to avoid ice-free conditions in the Arctic.
However, even at global warming of 1.5°C, occasional ice-free months may still occur. Thus, while completely preventing ice-free conditions may no longer be feasible, the models also show that reducing future greenhouse gas emissions can limit the extent of sea ice loss.
For instance, if warming exceeds 2.5°C, the ice-free season could extend for four months of the year, from July to October. Hence, any reduction in global greenhouse gas emissions decreases the area and duration of open water present in the Arctic Ocean, mitigating some of the impacts.
What will Arctic sea ice cover look like at the end of the 21st century?
The largest source of uncertainty in climate projections for the end of the 21st century is the amount of greenhouse gas emissions between now and then. Climate models simulate a range of possible outcomes for the Arctic sea ice cover based on different emission scenarios, which predict global warming levels between about 1.5°C and over 5°C by 2100.
Under lower warming scenarios (around 1.5°C), the Arctic sea ice cover in summer could still resemble its current state. However, with warming exceeding 3°C, the Arctic could become ice-free for several months each year (Fig. 3).
In summary, the future state of Arctic sea ice cover will depend entirely on global greenhouse gas emissions. Any reductions in emissions will mean that more sea ice will persist in the Arctic Ocean.
This article is based on research funded by the US National Science Foundation.
