Sea ice holds the key to understanding the entire Arctic marine ecosystem. But it’s melting faster than we can unravel its complexity.
In our planet’s far north, an extraordinary ocean exists. The Arctic Ocean is special in many ways but its sea ice, floating on the frigid water, holds the key to a food web and ecosystem that supports creatures from inconspicuous crustaceans to mighty polar bears. But all of this is under threat from climate change.
Every year, Arctic sea ice reaches its largest annual extent in March and shrinks back to its smallest by September. This March, it reached 14 million square kilometres — nearly half the size of Africa — before returning to less than 5 million square kilometres last month.
These might seem like very large numbers, and they are. But they’re much smaller than they used to be.
In the 40-plus years that satellites have watched the Arctic, its sea ice has fallen faster than scientists and their modelling had projected. The Arctic Ocean is expected to be ice-free during summer before mid-century if we don’t keep global warming below a 2 degree Celsius increase relative to pre-industrial levels.
“Positive albedo feedback” is one of the main reasons why the Arctic Ocean is warming so much faster than the rest of our planet. Sea ice, and the snow on top of it, reflect more sunlight than seawater. When global warming melts the ice, the darker water left behind reflects less sunlight and absorbs more heat, amplifying the melt of the sea ice that remains.
Sea ice plays a vital role in keeping global temperatures in check by reflecting sunlight — but it provides other services for the Arctic too Sea ice is more than just frozen water. It contains a complex mix of pure solid ice, gas bubbles and pockets of salty water. An entire and unique ecosystem develops within these pockets, ranging from viruses, bacteria, to fungi, algae and little grazing creatures such as microscopic crustaceans. The only thing these life forms have in common is that they can all fit into the pockets of salty water and they can survive in the dark, cold, salty environment of the sea ice.
The most abundant organisms here are tiny algae. Along with phytoplankton in the water, these algae form the foundation of the entire Arctic marine food web. When there is no other food available, algae feed many organisms living within and under the sea ice.
Arctic cod are a key species in transferring this energy along the entire food web — up to top predators like the polar bear. Microscopic creatures feed on the algae and the Arctic cod graze mainly on them. The cod are in turn eaten by other creatures, including ringed seals and belugas.
A whole chain of Arctic species like the ringed seal depend on the sea ice for their food.
But human-induced global warming can cause major stresses at any level of the marine food webs, including where ice algae and phytoplankton sit. By the mid-century, we will have lost the ability to understand the summertime Arctic ecosystem as it had existed for millennia. With less ice, the increased sunlight available earlier in the season can turbo-charge ice algae and phytoplankton. But this early extra food might not coincide with the peak of the grazers’ life cycle, with implications for the Arctic cod. The organisms that feed on the cod are then also affected. With a gap in the ecosystem, other species, like capelin and killer whales, can move in from the south. There are no rules for who eats whom when species invade like this, and it can have far-reaching effects on other species, changing the amount and quality of food available to everyone.
A change that begins with the melting sea ice disrupts the entire food web.
Since the Arctic Ocean is remote and its climate is harsh, it remains barely accessible and, because of this, poorly understood. We know so little about the consequences of the rapid loss of sea ice that scientists call it a “crisis discipline” — urgent decisions need to be made in the face of incomplete knowledge. Our ability to manage the enormous effects is hampered by huge gaps in our understanding.
But protecting the sea ice ecosystem is fundamental to guarantee all the ecosystem services it provides. It is a habitat, nursery and feeding ground for many, a source of food, a climate regulator, and it supports local and indigenous knowledge, tourism and scientific research. If we do not slow down climate change, the disruption of the Arctic will continue to outpace science.