Will climate change turn the Arctic green?

The Arctic is in the hotseat of climate change, warming four times faster than anywhere else on Earth.

Already, the Arctic Ocean has experienced immense change. Sea ice which once covered a vast expanse of ocean year-round is now seasonal, melting and disappearing from large areas during the summer months.

Of the ice that remains, much is thinning, and snow depth is observed to have been declining.

ESA's CryoSat mission has been a key asset for tracking the Arctic's melting ice. After 14 years in space, it has one of the longest unbroken records of polar ice thickness in existence.

Now, thanks to research published in Geophysical Research Letters, it can add another string to its bow—measuring light-penetration through Arctic sea ice.

Lighting up the Arctic Ocean

Rapid warming of the Arctic affects the marine ecosystem—and not just polar bears and walruses.

"As ice and snow gets thinner, more light penetrates to the bottom of sea ice," said Julienne Stroeve of the University of Manitoba and the University of Colorado. "This changing light regime has the potential to impact the entire marine ecosystem, which all begins with algae."

On the undersurface of the ice, a lawn of algae expands every year. Like open-water phytoplankton, which grow to cover huge expanses of ocean that can be spotted from space, the ice-bound algae bloom over wide areas and support an intricate food web.

As algae bloom, zooplankton graze on them from underneath, which go on to feed a variety of animals including fish, which go on to feed seals, and in turn the polar bears.

We still have much to learn about how this ecosystem will be affected due to climate change, therefore algae—the primary producers in the ecosystem—are a great place to start.

Mapping that from space has been a challenge.

"Unlike in the open ocean, we can't see the algae within sea ice from space," said Karley Campbell of the University of Tromsø. "What we can do is start by estimating light availability. Light, harvested by ice algae to make organic compounds in photosynthesis, is a major factor driving marine production.

"If we can map the light reaching the algae, we can get an idea of when and how much they might bloom."

To understand that you need to know how thick the ice is, and how much snow is on top. Thicker ice and snow mean less light can reach algae under the ice.

That's where CryoSat comes in. Along with Copernicus Sentinel-3 and NASA ICESat-2 data, scientists estimated Arctic sea-ice thickness over CryoSat's 14-year lifetime in orbit.

Applying algorithms to understand how much light penetrates through ice, snow and using models to predict historic snow and ice cover, it was then possible to model where and when algae might start to bloom.

Data from 2011 to 2022 showed more southerly Arctic regions would experience earlier algal blooms, which varied year-on-year. Snow appeared to be a large factor.

The model suggested that a particularly snowy 2017 resulted in a deeper snow cover preventing large areas from blooming due to insufficient light.