We interviewed researcher Carolina Gabarró to find out how the study of the oceans from space has progressed in recent years and her contribution to the MOSAiC expedition, the most important polar campaign to date.
Is it true that the ocean can be studied from space?
Yes, it is. Since the early 1970s, we can study the Earth from space with data obtained from satellites. The first satellite specially designed to study the oceans arrived in 1978 and was called SeaSat. It was equipped with several active instruments, such as a radar altimeter, SAR (Synthetic Aperture Radar) and a scatterometer that measured wind, sea surface temperature and wave height, among other parameters.
Is this the solution to studying areas as remote as the Arctic?
There is no doubt that satellites are a great tool for monitoring and studying remote and difficult-to-access areas such as poles. In situ measurements are very expensive and the instruments need to be able to withstand extreme conditions, which means that in many polar regions there are still no measurements of variables such as salinity. Satellites have provided a great deal of information that would be impossible to obtain in any other way, although in situ data is still essential to validate data acquired from space.
How does this relate to the MOSAiC expedition?
In MOSAiC we have conducted an experiment related to the European Space Agency (ESA) SMOS mission, in which we have been working for many years. The main instrument of this mission is a radiometer that measures the humidity of the ground, the salinity of the oceans and the thickness of sea ice. Normally, sea ice thickness is derived from radiometric measurements -brightness temperature- using ice emissivity models, which have their limitations and do not take into account some unknown effects such as the snow that seems to modify the brightness temperature of the ice.
For this reason, we decided to participate in the MOSAiC campaign and take measurements with a radiometer like the one in the SMOS but much simpler, the ARIEL, which we placed on a sled. ARIEL allowed us to observe ice and snow in different conditions and periods of the annual cycle. Simultaneously, other instruments measured ice and snow thickness, temperature and ice salinity with other sensors. This experiment was carried out in collaboration with scientists from the University of Bremen, the University of Hamburg, the ESA and the Swiss Federal Research Institut, among others.
What is all this for?
We are currently processing the data and discussing the first results. We believe the final results will take a year or two to be validated. The experiment aims to understand the sensitivity of emissivity -measured by radiometers- to parameters such as snow thickness or ice salinity to improve emissivity models that will allow us to infer sea ice thickness.
Why is it so important to know how sea ice is evolving?
The temperature increase in the Arctic is double the increase observed in the tropics, which is causing a rapid decline in the Arctic sea ice extent and volume. In fact, there has been a 40% reduction in summer ice extent compared to the 1980s. The term 'Arctic amplification' explains why the observed changes in the Arctic are growing in a non-linear and retro-feeding manner. For example, the increase in temperature leads to a reduction in Arctic ice, and since the water absorbs more solar radiation than the ice because it has less albedo, the ocean warms even more. In turn, the higher air temperature makes it easier for the atmosphere to accumulate more humidity, which leads to more precipitation in the Arctic, and contributes to the melting of sea ice.
What does the reduction in ice cover mean?
With less sea ice, the ocean absorbs more energy and, as a result, there is more wind and waves. This makes the ice thinner and provokes that it breaks up faster. Also, it favours the mixing of stratified Arctic waters and the emergence of warmer and saltier deep waters, which again contributes to the shrinking of sea ice.
Finally, the surrounding continental regions are also undergoing dramatic changes because rising temperatures are causing the permafrost -continental ice that never melts- to melt. This is making the Earth absorb more heat and leading to more evaporation, which contributes to more humidity in the atmosphere. On the other hand, some of the water from melting land ice goes into rivers, and this causes the input of these continental waters into the Arctic to increase, as has been observed in recent years.
What about Greenland?
It is also melting and losing ice mass, especially during the years 2012 and 2019. This continental meltwater is flowing into the Arctic Ocean, causing an increase in freshwater and a rise in sea level.
How does this affect the Earth's climate?
These changes cause variations in natural atmospheric and oceanic patterns, leading to extreme weather events in mid-latitudes of the northern hemisphere. As temperatures in the Arctic rise more sharply than in the rest of the world, the jet stream circulation could be destabilised, and the same could happen with the Gulf Stream, which is fundamental to the global thermohaline circulation. This shows that the climate system in the Arctic is a perfect fit, but if one thing is destabilised it can throw the whole system out of whack.
Could your research be used to study climate change in other regions of the planet where there is also ice?
Yes, our experiment at MOSAiC aims to improve both the sea ice thickness maps of SMOS and NASA's SMAP satellites, as well as those of ESA's future CIMR mission. This will allow to accurately measure the thickness of both Arctic and Antarctic sea ice, which is essential to quantify the volume of sea ice and helps to better understand the impacts caused by climate change.
What does it take to plan a campaign on the scale of MOSAiC?
MOSAiC was the largest polar campaign to date. The marine research centre AWI (Alfred Wagner Institut) in Germany started planning it 10 years ago and its work has been excellent. More than 400 scientists from more than 20 countries have been involved, working on 7 research icebreakers. To organise all this, the participants have attended several meetings where we reviewed the work plans, logistics, teams, leaders, etc.... We had clear objectives, deadlines and the coordinators sent precise and brief messages. Even now we are still holding regular meetings to coordinate results, publications, the presentation of results at conferences, etc. I have learned a lot thanks to this huge project.