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Stefan Rahmstorf

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A physicist and oceanographer by training, Stefan Rahmstorf has moved from early work in general relativity theory to working on climate issues.

He has done research at the New Zealand Oceanographic Institute, at the Institute of Marine Science in Kiel and since 1996 at the Potsdam Institute for Climate Impact Research in Germany (in Potsdam near Berlin).

His work focuses on the role of ocean currents in climate change, past and present.

In 1999 Rahmstorf was awarded the $ 1 million Centennial Fellowship Award of the US-based James S. McDonnell foundation.

Since 2000 he teaches physics of the oceans as a professor at Potsdam University.

Rahmstorf is a member of the Academia Europaea and served from 2004-2013 in the German Advisory Council on Global Change (WBGU). He was also one of the lead authors of the 4th Assessment Report of the IPCC. In 2007 he became an Honorary Fellow of the University of Wales and in 2010 a Fellow of the American Geophysical Union.

More information about his research and publication record can be found here.

All posts by stefan.

Global Temperature News

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There are two interesting pieces of news on the global temperature evolution.

First, today a paper by Grant Foster and Stefan Rahmstorf was published by Environmental Research Letters, providing a new analysis of the five available global (land+ocean) temperature time series. Foster and Rahmstorf tease out and remove the short-term variability due to ENSO, solar cycles and volcanic eruptions and find that after this adjustment all five time series match much more closely than before (see graph). That’s because the variability differs between the series, for example El Niño events show up about twice as strongly in the satellite data as compared to the surface temperatures. In all five adjusted series, 2009 and 2010 are the two warmest years on record. For details have a look over at Tamino’s Open Mind.
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References

  1. G. Foster, and S. Rahmstorf, "Global temperature evolution 1979–2010", Environmental Research Letters, vol. 6, pp. 044022, 2011. http://dx.doi.org/10.1088/1748-9326/6/4/044022

Contributors

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The current contributors to content on this site are:

William Connolley was a contributor, but has now left academia; Ray Bradley, David Archer, and Ray Pierrehumbert are no longer active; Jim Bouldin was a contributor from 2009 and Caspar Ammann and Thibault de Garidel were early supporters of the site. Group posts can be assumed to be the from the whole set of current contributers, or will be individually signed.

Don’t estimate acceleration by fitting a quadratic…

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… if your data do not look like a quadratic!

This is a post about global sea-level rise, but I put that message up front so that you’ve got it even if you don’t read any further.

The reputable climate-statistics blogger Tamino, who is a professional statistician in real life and has published a couple of posts on this topic, puts it bluntly:

Fitting a quadratic to test for change in the rate of sea-level rise is a fool’s errand.

I’d like to explain why, with the help of a simple example. Imagine your rate of sea-level rise changes over 100 years in the following way:
[Read more…] about Don’t estimate acceleration by fitting a quadratic…

Is Sea-Level Rise Accelerating?

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A few months ago a paper by Jim Houston and Bob Dean in the Journal of Coastal Research (JCR) cast doubt on whether global sea level rise has accelerated over the past century or so. As things go these days, ‘climate sceptics’ websites immediately heralded this as a “bombshell”. A rebuttal by myself and Martin Vermeer has now been published in JCR.
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The water south of Greenland has been cooling, so what causes that?

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Sea surface temperature trend 1993 – 2018, from European Atlas of the Seas

Let’s compare two possibilities by a back-of-envelope calculation.

(1) Is it due to a reduced heat transport of the Atlantic Meridional Overturning Circulation (AMOC)?

(2) Or is it simply due to the influx of cold meltwater as the Greenland Ice Sheet is losing ice?

The latter is often suggested. The meltwater also contributes indirectly to slowing the AMOC, but not because it is cold but because it is freshwater (not saline), which contributes to the first option (i.e. AMOC decline).

AMOC heat transport

For that we take the AMOC flow rate times the temperature difference of 15 °C between the northward upper branch and southward deep return flow to obtain the heat transport.

17,000,000 m3/s x 15 K x 1025 kg/m3 x 4 kJ/kgK = 1 PW (1)

(Here, 1 PW = 1015 Watt and 4 kJ/kgK is the heat capacity of water.)

An AMOC weakening by 15 % thus cools the region at a rate of 0.15 PW = 1.5 x 1014 W and according to model simulations can fully explain the observed cooling trend (2). Of course, this slowdown is not only due to Greenland meltwater – other factors like increasing precipitation probably play a larger role, but the impact of Greenland melting is not negligible, as we argue in (3).

Greenland ice melt

Here we start by taking the Greenland mass loss rate into the ocean, times the temperature difference between the meltwater and the water it replaces. Note we are interested in the longer-term temperature trend over decades over the region with the meltwater properly mixed in, not at some temporary patches of meltwater floating locally at the surface.

Total Greenland mass loss has been on average 270 Gt/year for the last two decades (4).

Most of that evaporates though, and what ends up in the ocean of this, according to a recent study by Jason Box (5), is around 100 Gt/year, about 30% of which in form of ice and 70% in form of meltwater.

100 Gt/year = 3000 tons/second – that sounds a lot but the AMOC flow is more than 5000 times larger.

Assuming the ice and meltwater runoff occurs at 0 °C and replaces water that is 10 °C (a very high assumption corresponding to summer conditions and not the long-term average), the cooling rate is:

3,000,000 kg/s x 10 K x 4 kJ/kgK = 1.2 x 1011 W

So in comparison, the cooling effect of a 15 % AMOC slowdown is over 1,000 times larger than the direct cooling effect of the Greenland meltwater.

For the part entering the ocean as ice, we must also consider that to melt ice requires energy. The heat of fusion of water is 334 kJ/kg so that adds 900 tons/s x 334 kJ/kg = 3 x 1011 W.

So it turns out that those suggesting that ‘cold’ meltwater might cause the cold blob in the northern Atlantic are doubly wrong: if we talk about the direct impact of stuff coming off Greenland, than ice is the dominant factor and the energy that’s required to melt the ice. But both the direct effect of meltwater and of icebergs entering the ocean are completely dwarfed by the weakening of the AMOC (regardless of whether we take the numbers of Box et al. or other estimates). And Greenland’s contribution to that is not because the meltwater is ‘cold’, but because it is fresh – it contains no salt and dilutes the saltiness of the ocean water, thereby reducing its density.

As an additional observation: the cooling patch shown above often vanishes in summer, covered up by a warm surface layer – just when the Greenland melt season is on – only to resurface when deeper mixing starts in autumn. Which again supports the idea that it is not due to a direct effect of cold meltwater influx. Also compare the temperature change directly at the Greenland coast, where the meltwater enters, in the image above.

Finally, some have suggested that the cold blob south of Greenland has been caused by increased heat loss to the atmosphere. That of course is relevant for short-term weather variability – if a cold wind blows over the ocean it will of course cool the surface – but I do not think it can explain the long-term trend, as we discussed earlier here at Realclimate.

References

1.            Trenberth, K. E. & Fasullo, J. T. (2017) Atlantic meridional heat transports computed from balancing Earth’s energy locally, Geophys. Res. Let. 44: 1919-1927.

2.            Caesar, L., Rahmstorf, S., Robinson, A., Feulner, G., & Saba, V. (2018) Observed fingerprint of a weakening Atlantic Ocean overturning circulation, Nature 556: 191-196.

3.            Rahmstorf, S., J.E. Box, G. Feulner, M.E. Mann, A. Robinson, S. Rutherford, and E.J. Schaffernicht, 2015: Exceptional twentieth-century slowdown in Atlantic Ocean overturning circulation. Nature Climate Change, 5, 475–480, doi:10.1038/nclimate2554.

4.            NASA Vital Signs, https://climate.nasa.gov/vital-signs/ice-sheets/

5.            Box, J. E., et al. (2022), Greenland ice sheet climate disequilibrium and committed sea-level rise, Nature Clim. Change, 12(9), 808-813, doi: 10.1038/s41558-022-01

Sealevelgate

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Translations: (Italian)

Imagine this. In its latest report, the IPCC has predicted up to 3 meters of sea level rise by the end of this century. But “climate sceptics” websites were quick to reveal a few problems (or “tricks”, as they called it).

First, although the temperature scenarios of IPCC project a maximum warming of 6.4 ºC (Table SPM3), the upper limit of sea level rise has been computed assuming a warming of 7.6 ºC. Second, the IPCC chose to compute sea level rise up to the year 2105 rather than 2100 – just to add that extra bit of alarmism. Worse, the IPCC report shows that over the past 40 years, sea level has in fact risen 50% less than predicted by its models – yet these same models are used uncorrected to predict the future! And finally, the future projections assume a massive ice sheet decay which is rather at odds with past ice sheet behaviour.

Some scientists within IPCC warned early that all this could lead to a credibility problem, but the IPCC decided to go ahead anyway.

Now, the blogosphere and their great media amplifiers are up in arms. Heads must roll!

[Read more…] about Sealevelgate

Why global emissions must peak by 2020

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(by Stefan Rahmstorf and Anders Levermann)

In the landmark Paris Climate Agreement, the world’s nations have committed to “holding the increase in the global average temperature to well below 2 °C above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5 °C above pre-industrial levels”. This goal is deemed necessary to avoid incalculable risks to humanity, and it is feasible – but realistically only if global emissions peak by the year 2020 at the latest.

Let us first address the importance of remaining well below 2°C of global warming, and as close to 1.5°C as possible. The World Meteorological Organization climate report[i] for the past year has highlighted that global temperature and sea levels keep rising, reaching record highs once again in 2016. Global sea ice cover reached a record low, and mountain glaciers and the huge ice sheets in Greenland and Antarctica are on a trajectory of accelerating mass loss. More and more people are suffering from increasing and often unprecedented extreme weather events[ii], both in terms of casualties and financial losses. This is the situation after about 1°C global warming since the late 19th Century. [Read more…] about Why global emissions must peak by 2020

Science Story: the Making of a Sea Level Study

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Guest commentary by Martin Vermeer

On December 7, 2009 the embargo expired, and my and Stefan’s joint paper ‘Global sea level linked to global temperature’ appeared in the Proceedings of the U.S. National Academy of Sciences. It had been a long time coming! But this post is not so much about the science as about the process, and about how a geodesist from Helsinki and an oceanographer from Potsdam, who to this day have never even met, came to write, to the surprise of both of us, a joint paper on sea level rise.
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What makes sea-level rise?

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Last week the science community was shocked by the claim that 42% of the sea-level rise of the past decades is due to groundwater pumping for irrigation purposes. What could this mean for the future – and is it true?

The causes of global sea level rise can be roughly split into three categories: (1) thermal expansion of sea water as it warms up, (2) melting of land ice and (3) changes in the amount of water stored on land. There are independent estimates for these contributions, and obviously an important question is whether their sum is consistent with the total sea level rise actually observed.


foto (c) Stefan Rahmstorf 2012
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