December 2018-January 2019 issue

Last update: 07/03/2019

Since I am a little late for this review I will cover December and January together. Main subjects are: past observations of ocean heat uptake, the melting of Wilkes Land (East Antarctica) and decadal variability of sea level in the North Atlantic.

Many papers on ocean heat uptake came out in that period. This is a subject I do not often cover in these monthly reviews so it is a good opportunity to catch up. First, let me point you to a very short and interesting historical review on the RealClimate blog. I was surprised to learn that it is only in 2000 that the first global reconstruction of ocean heat content change from 1950 onwards was put together. There was then a lot of improvements in bias correction of the observations from ship, “eXpendable Bathy-Thermographs” and ARGO. The current view is that since the beginning of observations in the 1950th the ocean has been accumulating heat and the warming rate seems to be accelerating since the 1990th. A short perspective paper by Cheng et al. (2019) in Science make the point that post IPCC AR5 reconstructions of ocean heat uptake between 1971 and 2010 are larger than the values assessed in AR5. This makes the reconstructions closer to the CMIP5 climate models and the argument there is too much ocean heat uptake in climate models does not hold anymore. This point was disputed by Nic Lewis in a blog post but it seems that there is at least an agreement that the difference between models and observationally based products decreased.

It is more difficult to reconstruct the ocean heat uptake before 1950 because there are only few direct observations at depth. Two papers tackle this issue by using surface temperature measurements and present day ocean transport reanalysis products to advect past SST anomalies to the deep ocean and thereby reconstruct ocean heat uptake. The two approaches and results are compared in this blog post. Over the common period that both studies investigate (1871-2015) the two reconstructions find similar total heat uptake. However, this is the result of a different vertical distribution. Zanna et al. (2019) find that the deep ocean, below 2000 m, had a constant heat content until the 1930th followed by a slow warming. In contrast, Gebbie and Hyubers (2019) argue that because of the lingering effect of the Little Ice age the deep ocean cooled down until around 1970 and then slowly warmed up. And the deep Pacific ocean is still cooling down now. This has a large implications for the interpretation of CMIP5 climate models and the reconstruction of sea level from them. The traditional approximation in the CMIP exercise is that the ocean was at equilibrium in 1850 when the historical simulations start. If it was cooling, as Gebbie and Huybers (2019) argue, then deep ocean heat content from climate models is overestimated. Since current reconstructions of sea level since 1900 using CMIP5 models cannot explain all the sea level from tide gauges products, Slangen et al. 2016 and 2017 suggested that the deep ocean was warming or that ice sheets were losing mass. If the deep ocean was actually cooling it could mean that other contributors to sea level are underestimated. The usual suspects being glaciers and ice sheets.

An important paper by Rignot et al. (2019) reconstruct Antarctic mass loss since 1979. The main point is that East Antarctica, in particular Wilkes Land, has been loosing mass at least since 1979. This is contradictory with the mainstream view that mass balance in East Antarctica is dominated by surface mass balance which is expected to increase (add mass) as the climate warms. As the authors put it:

“Our observations challenge the traditional view that the East Antarctic Ice Sheet is stable and immune to change. An immediate consequence is that closer attention should be paid to East Antarctica.”

Last but not least, an important paper for the understanding of sea level in the North Atlantic and at the Dutch coast in particular. Chafik et al. (2019) investigate the relation between the North Atlantic subpolar gyre and sea level. They show that when the subpolar gyre transitions between a strong to a weak state sea level rises faster in the southern North Sea compared to global sea level. This happened during the period 1993-2004. The opposite effect happened in the period 2005-2016 resulting in slower sea level rise in the Southern North Sea. This mode of climate variability might be important to explain why the acceleration of sea level rise that is measured globally is not yet measured at the Dutch coast.

References:

Chafik, L., Nilsen, J. E. Ø., Dangendorf, S., Reverdin, G., & Frederikse, T. (2019). North Atlantic Ocean Circulation and Decadal Sea Level Change During the Altimetry Era. Scientific Reports, 9(1), 1041. http://doi.org/10.1038/s41598-018-37603-6

Cheng, L., Abraham, J., Hausfather, Z., & Trenberth, K. E. (2019). How fast are the oceans warming? Science, 363(6423), 128–129. http://doi.org/10.1126/science.aav7619

Gebbie, G., & Huybers, P. (2019). The Little Ice Age and 20th-century deep Pacific cooling. Science, 363(6422), 70–74. http://doi.org/10.1126/science.aar8413

Rignot, E., Mouginot, J., Scheuchl, B., van den Broeke, M., van Wessem, M. J., & Morlighem, M. (2019). Four decades of Antarctic Ice Sheet mass balance from 1979–2017. Proceedings of the National Academy of Sciences, 201812883. http://doi.org/10.1073/pnas.1812883116

Slangen, A. B. A., Church, J. A., Agosta, C., Fettweis, X., Marzeion, B., & Richter, K. (2016). Anthropogenic forcing dominates global mean sea-level rise since 1970. Nature Climate Change, (April), 11–16. http://doi.org/10.1038/nclimate2991

Slangen, A. B. A., Meyssignac, B., Agosta, C., Champollion, N., Church, J. A., Fettweis, X., … Spada, G. (2017). Evaluating Model Simulations of Twentieth-Century Sea Level Rise. Part I: Global Mean Sea Level Change. Journal of Climate, 30(21), 8539–8563. http://doi.org/10.1175/JCLI-D-17-0110.1

Zanna, L., Khatiwala, S., Gregory, J. M., Ison, J., & Heimbach, P. (2019). Global reconstruction of historical ocean heat storage and transport. Proceedings of the National Academy of Sciences, 116(4), 1126–1131. http://doi.org/10.1073/pnas.1808838115