2 years ago, Lina talked about the importance of sea ice and why studying it is essential. You can find her article here. I would like to expand on this discussion.
Note: This text was published as a thread on Twitter in August 2019. I rewrote it here as a blog post, you can find the original thread here.
Since a few weeks, I’m working on the introduction of my PhD thesis. I was making some visualization of the evolution of Arctic sea ice to illustrate the effect of Climate Change. This made me realize that we should maybe talk more about sea ice volume.
Sea ice volume is difficult to measure, as it is combining the measures of ice concentration and ice thickness. To get a reasonable estimate, we use a reanalysis. In short: The available data are assimilated into a sea ice model to bring it as close as possible to reality. The data that I use above come from the PIOMAS reanalysis.
We often talk about sea ice extent because sea ice cover influences the climate. The sea ice insulates the atmosphere from the ocean, and the white ice reflects sunrays back to space. The maps and evolution of sea ice extent and area can be found on the NSIDC.
Looking at these maps, like the one below, it sometimes feels like there is still a lot of ice. And that’s why I wanted to talk a bit about sea ice volume. Because sea ice volume is also important. Thinner ice will break more easily, move faster and melt faster. Old thick ice (5 years and older) has been a reservoir of negative energy, meaning that a large quantity of heat is necessary to melt it.
This old thick ice is the one melting away right now.
One would probably say that, if the sea ice area decreases, it is normal that the sea ice volume would also decrease. So let’s look at the data! I have been trying several visual ways to compare sea ice volume to the other observables, here sea ice extents and area.
And one I liked is the Pie chart. So let’s compare the September situation of years 1979 and 2018. I picked 1979 because it is the first year on record. The year of the first satellite observations of the Arctic sea ice. It is also the observed maximum of sea ice volume.
We can see that the sea ice area and extent decreased by roughly 1/3. (top left and right). At the same time, the sea ice volume decreased by almost 3/4! (bottom left) This means that we lost more than 1/2 of the mean ice thickness!! (bottom right)
So yes, sea ice is REALLY melting away. I can’t stress that enough, as many people have asked me.
And this has a critical influence: For the fragile Arctic ecosystems, for the communities living in the Arctic, and for the whole Earth climate. When I wrote this thread for twitter, we were in August, and the minimum of sea ice volume wouldn’t be known before the next month.
If one looks at the volume data from PIOMAS in the figure below, 2019 could have beaten 2012 as the worst year ever for the summer sea ice conditions. It didn’t, but we can see that it was very close to the 2012 volume and that the volume is still dramatically lower than the mean between 1979 and 2018.
During the coming 2020 summer, we will ask ourselves the question: How will the ice volume minimum look like in September? Lets talk again in July or August!