More reliable thermal readings of Arctic sea ice

The retreat of Arctic sea ice is a key concern of climate researchers and a focal point of media coverage on climate change. Distressing images of polar bears on melting ice floes are everywhere, and indeed, satellites have confirmed that ice is covering less and less of the Arctic Ocean. The summer of 2012 set a record low for sea ice cover.

Sea ice thickness is significant

Scientists still have a great deal to learn about the ice cover around the North Pole, not least about the full meaning of the thickness of sea ice. Norwegian and Chinese researchers have now collected a wealth of data about the optical properties and thickness of sea ice, providing a sound basis for further research. These efforts were part of the project Advancing Modelling and Observing solar Radiation of Arctic sea-ice -- understanding changes and processes (AMORA). The Research Council of Norway provided much of the project's funding.

One thing is certain: global warming is reduced by snow and sea ice reflecting solar energy back into space. (The reflecting power of a surface is called its albedo.) Conversely, when there is less Arctic sea ice, the ocean absorbs more heat from the sun, adding to global warming.

Unmanned research platform developed

This marks the first time that Norwegian and Chinese researchers have collaborated on a project to study Arctic sea ice and snow cover.

A core component of the project was the development of a Spectral Radiation Buoy (SRB). Outfitted with specialised instruments, the buoy functions as an unmanned research platform for continuous measurements of solar radiation absorption and other critical properties of Arctic snow and ice.

Big difference from 2012 to 2013

The researchers set the buoy adrift throughout the summer seasons of 2012 and 2013. It drifted from its North Pole starting point and into the Fram Straight between Greenland and Svalbard. Each course took roughly half a year. In its 2013 deployment the buoy went missing, but the researchers had already received large amounts of unique data from the buoy via satellite transmission.

In the summer of 2012 a record low for Arctic sea ice was set, while the ice cover in summer 2013 was somewhat more normal, with the 6th lowest amount of sea ice since comprehensive records began in 1979. The AMORA project researchers have collected a large quantity of data that document how differently these two Arctic summers turned out.

Sunlight reflection an important climatic factor

Calculations of the Earth's albedo, i.e. its reflectivity of sunlight, are important for constructing reliable climate models.

Albedo is a ratio of how much sunlight (and its thermal radiation) is reflected back into space by a given surface. The albedo of newly fallen snow may approach 90 per cent, while the far-more-absorbent surface of seawater has a low albedo. Thus the amount of snow and ice cover in the Arctic during the high-sunlight season is assumed to have a major impact on global climate.

"Because access to the Arctic is so difficult, there is not much reliable data on albedo there," explains Sebastian Gerland, who headed the AMORA project and is a research scientist at the Norwegian Polar Institute.

All the previous albedo data had been collected by manned monitoring stations, which as a rule are only active for short periods. Measurement expeditions typically have to rely on a sea-going vessel and are costly and challenging to carry out.

"The unmanned SRB buoy we built made it possible for the first time to generate continuous data on albedo and other properties of sea ice over a long period," says Dr Gerland.

Data capture southwards from the Pole

The SRB buoy was set up on an ice floe near the North Pole, and was left to eventually drift southwards with the ice.

"The buoy worked well," continues Dr Gerland, "measuring light reflection off the ice as well as how much light penetrated into the ocean beneath the ice."

"Via satellite we received a continuous data feed on changes in albedo on the ice floe where we placed the SRB. Along the floe's drift course southwards from the Pole, we could see in detail at what point the ice began melting. In addition we received detailed data on changes in the thermal radiation reaching the seawater through the ice."

"This knowledge about how Arctic sea ice melts over the course of the summer season will be valuable in further research," he says. "It will help us to find clearer answers as to whether the Arctic sea ice melts primarily due to higher temperatures or whether the sea ice is shrinking due to changes in wind and ocean currents."

Role of snow cover revealed

The SRB data has also helped climate researchers to understand how the snow cover on Arctic sea ice affects melting of the ice.

"We confirmed that the snow also plays an important role in melting," says Dr Gerland. "The puddles that form atop the sea ice are the melted snow. These puddles are dark, so they reduce the albedo ratio and accelerate ice melt. This is one of several ways snow affects the sea ice and climate in the Arctic."

Sunlight that penetrates the ice is also critical for algae and plankton of the Arctic Ocean. The Arctic ecosystem is finely adapted to the amount of light that penetrates the ice. Scientists have also learned more about this solar radiation from the project.

Successful Norwegian-Chinese cooperation

An important component of the AMORA project was to establish ties and share knowledge between Norwegian and Chinese scientists. Researchers from Finland, the USA and Germany also participated in the project.

China is investing relatively heavily in research on the Arctic and Antarctic, with publicly funded institutions as well as universities active in the field. The two Chinese institutions participating in the AMORA project were the Polar Research Institute of China Shanghai and the Dalian University of Technology.

Several of the Chinese researchers, from fellowship holders to senior scientists, have had a research stay in Norway in connection with AMORA project activities. Workshops and project meetings were held in China (Shanghai and Dalian), Norway (Tromsø) and Finland (Helsinki).

"Without a doubt," concludes Dr Gerland, "this cooperation has taught us a great deal about how the Chinese polar researchers work, and they have gained a deeper understanding of our work in this field."