Why I think that the 'cold blob' in the North Atlantic cannot be explained by wind changes (a question raised in the @nytimes) - a thread. Keep in mind that we're talking about the long-term evolution, not short-term variability. @PdeMenocal nytimes.com/interactive/20…
1. Halldór Björnsson of the Icelandic weather service has shown that short-term temperature fluctuations correlate with the heat exchange through the sea surface, but that this does not explain the longer-term development of the ‘cold blob’ over decades. realclimate.org/index.php/arch…
2. What we observe is a specific cold/warm "fingerprint", which is highly specific for an AMOC slowdown. I haven't seen any study that can explain this by wind changes. The physics behind this is well-understood ocean dynamics (vorticity conservation). realclimate.org/index.php/arch…
3. Climate model historic runs show a widely differing AMOC slowdown, and the 'cold blob' temperatures (x-axis) correlate strongly with AMOC slowdown (y-axis). (Caesar et al., Nature 2008.) This correlation is also found in the CMIP6 models. (Menary et al., GRL 2020.)
4. In our new paper, of the 11 time series we show, only 5 have to do with temperature or heat content - 6 are completely independent confirmations of the AMOC weakening. Here's my popular thread on this paper again.
5. At the end of the time series, since 2004, the trend we deduce from the 'cold blob' agrees well with the weakening trend measured by the RAPID project. Also the heat transport change related to the estimated weakening agrees with the observed cooling. (Cold blob image: NASA.)
To sum up, we have a bunch of independent and consistent evidence pointing to an AMOC weakening. I have not seen any coherent alternative explanation of the 'cold blob' (long term!) in the literature. Let me know if I missed something.
If you want to know more about the ocean's role in climate change, you're warmly invited to watch my recent video lecture!

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More from @rahmstorf

26 Feb
Frisch erschienen: Unser neues Paper in Nature Geoscience erhärtet die Belege für eine einzigartige Abschwächung des Golfstromsystem (Atlantische Umwälzströmung, 𝗔𝗠𝗢𝗖) ! @NatureGeosci
Thread. 1/12
Erstautorin Levke Caesar (Maynooth University) hat eine Reihe von veröffentlichten AMOC Rekonstruktionen aus Proxydaten (v.a. aus Tiefseesedimenten) zusammengestellt, die bis 1600 Jahre zurück reichen. Sie zeigen Strömungsgeschwindigkeit, Wassermassen, Wärmetransport an. 2/12
Diese Daten zeigen übereinstimmend eine klare Abschwächung im 20. Jh; in den letzten Jahrzehnten ist die Strömung am schwächsten. In 9 der 11 Datenreihen ist dies statistisch signifikant. 3/12
Read 12 tweets
25 Feb
Just out: our new paper affirming the unprecedented slowdown of the Gulf Stream System (aka Atlantic meridional overturning circulation, 𝗔𝗠𝗢𝗖) in Nature Geoscience! @NatureGeosci A thread. 1/11
Lead author Levke Caesar compiled a range of different, published proxy data reconstructions of the AMOC - thanks to all their authors for sharing them. The longest ones go back 1600 years! They represent different AMOC facets: flow speed, water masses, heat transport. 2/11
These data consistently show an AMOC decline in the 20th Century, with the weakest AMOC state of the whole series in the last decades. This finding is statistically significant in 9 of the 11 time series. 3/11
Read 13 tweets
17 Feb
Two competing effects influence our northern winters:

#1 Global warming.

#2 Increasing polar air outbreaks due to stratospheric polar vortex disturbances.

In the long run #1 wins - our winters are getting warmer. 1/🧵 Image
That's expected as #2 just happens occasionally, and polar air is also getting warmer.

Some people point to long-term warming trends to cast doubt on whether #2 is even real, but that is a non-sequitur. It just means #1 wins in the long run. 2/🧵 Image
That disturbed stratospheric polar vortex states are linked to polar air outbreaks and thus cold extremes either in Eurasia or the US is shown by data analysis: nature.com/articles/s4161… 3/🧵
Read 9 tweets
27 Oct 20
The US elections are now climate elections, as Trump focuses on discrediting Biden's statement about transitioning from oil to renewable energy. It's the fossil fuel lobby versus sensible climate policy. The 20th Century pitched against the 21st. (1/🧵)
Stopping #GlobalWarming is only possible by getting beyond fossil fuels. All nations on this planet agreed in Paris to stop warming well below 2°C. Most of the fossil fuel resources must remain in the ground for that. 2/🧵
China plans to be climate neutral by 2060. Japan, Canada and the EU by 2050. The world is getting out of coal, oil and gas. Trying to scandalise this while the country burns and is ravaged by storms and rising seas, is about as smart as recommending to drink disinfectant. 3/🧵
Read 4 tweets
15 Sep 20
Und nun die Wettervorhersage für Dienstag, den 15. September...
Es freut mich, dass viele hier auf den Unterschied von Wetter und Klima hinweisen! Aber wussten Sie auch, dass der Klimawandel inzwischen schon im täglichen Wetter nachweisbar ist? ethz.ch/de/news-und-ve…
Wer sich für die langfristige Temperaturentwicklung in 🇩🇪 interessiert:
Read 6 tweets
17 Aug 20
News about the death of the Greenland Ice Sheet is greatly exaggerated. What the new paper shows is an observed increase in solid ice discharge into the ocean, which has been at a higher rate than before during the past 15 years. The authors basically say: if this continues, ...
...then the Greenland Ice Sheet will continue to lose mass.
But due to that, the ice front retreats from the ocean, as nicely shown e.g. in Andy Aschwanden's detailed simulations:
And when the ice loses contact with the ocean, the ice discharge stops.
The tipping point for losing the Greenland Ice Sheet altogether therefore depends on surface melt permanently exceeding snowfall and does not involve solid ice discharge. Whether this real tipping point of Greenland has been crossed, we do not know.
Read 7 tweets

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