November 2018 issue

Last update: 10/12/2018

There were many papers and reports related to sea level in November. A lot more than I could read so I focus here on just a few highlights: Climatic impact of Antarctic freshwater, ocean tides importance for basal melt under Antarctic ice shelves, the small glaciers that disappeared, and a relation between atmospheric oxygen concentration and ocean heat content.

A particular paper that caught my attention was from Ben Bronselaer and colleagues. They looked into the climatic impact of a very large freshwater flux from Antarctica (up to around 20.000 Gt/year, 50 times more than current rates) following the DeConto and Pollard 2016 projections. Impact on precipitations and surface air temperature are discussed in the paper but also the warming up of subsurface temperature around the ice shelves of Antarctica. In the GFDL model that they analyse the warming of ocean water at 400 meters deep is four times larger than a normal RCP8.5 scenario without additional freshwater from Antarctic melting (1.6ºC instead of 0.4ºC). Meltwater leads to subsurface warming which in turn leads to increase meltwater production. This is a positive feedback that could be important for the future of Antarctica (n.b. for non-scientists: positive feedback does not mean positive consequences, in this case quite the opposite). This finding confirms a previous hypothesis that James Hansen and colleagues published in a controversial paper: “Ice melt, sea level rise and superstorms”. I quickly checked last week what happened in our climate model EC-EARTH and it seems that it also exhibits this positive feedback so I am working on quantifying the possible effect for future sea level. If you know other groups that have run their climate model with additional freshwater fluxes around Antarctica let me know.

Another missing link in climate models to model the interaction between the ocean and Antarctica is the tides. Tides are typically parameterised in climate models mostly by increasing vertical mixing at locations of steep bottom topography. It turns out that tides also influence basal melt under ice shelves (basal melt means at the base, to make the difference with surface melt). This effect is challenging to model even in regional ocean model as discussed by Naughten and colleagues. But Jourdain and colleagues provide a method to prescribe tidal effect on ice shelf melt in models that cannot solve tides. This is very promising for future climate models that will resolve the ocean circulation under ice shelves and will be coupled to ice sheet/ice shelves models.

When trying close the sea level budget of the 20th century we try to estimate how much each source contributed to sea level. For glaciers it is difficult because there are many of them. Another difficulty is that some glaciers melted during the 20th century and were already gone when modern data bases were constructed. A new paper by Parks and Marzeion provides a first estimate for this “forgotten’” contribution that is between 2 and 5 cm. This is a large effect compared to the estimates of total sea level rise that is 10 to 20 cm.

At the beginning of November a paper by Resplandy and colleagues made the headlines. The authors argued that the world oceans had accumulated more heat than had been measured by traditional instruments (ARGO, CTD, XBT…) over the last 25 years. The new method developed in that paper is based on measuring atmospheric oxygen concentration. A warmer ocean can contain less oxygen so this effect could be measured to get an integrated measure of total ocean heat content. Unfortunately shortly after the publication, a few problems have been found that make the uncertainty so large that it encompasses all previous estimates. See the series of four blog posts by Nic Lewis on the issue. What I take from this story is that the reproducibility crisis is not only in psychology and biology, it is also an issue in climate science. With such a simple analysis as done in that paper it was possible for someone to check the computation. For most papers it is not the case. This issue is particularly important for science with a direct societal impact like sea level. Last year our method for sea level projections was audited by Rijkswaterstaat, the institute responsible for the design, construction, management and maintenance of the main infrastructure facilities in the Netherlands. They checked the reliability, reproducibility and transparency of our method. The conclusion was that we can do better, which I agree with so I am working on it.

A few reports containing some chapter on sea level were also published but I didn’t have time to read them so I just list them here:

- The new UK climate projections:

https://www.metoffice.gov.uk/research/collaboration/ukcp

- US Fourth National Climate Assessement:

https://nca2018.globalchange.gov/#sf-1

- Sea level report from J. Curry:

https://judithcurry.com/2018/11/27/special-report-on-sea-level-rise/#more-24504