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Prof. Stefan Rahmstorf 🌏 🦣 Profile picture
@rahmstorf
Feb 25, 2021 • 13 tweets • 6 min read • Read on X
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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 Image
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
This decline is as predicted by climate models in response to #globalwarming & responsible for a particular "finger print" pattern of sea surface temperature change including the northern Atlantic '𝗰𝗼𝗹𝗱 𝗯𝗹𝗼𝗯' or '𝘄𝗮𝗿𝗺𝗶𝗻𝗴 𝗵𝗼𝗹𝗲'. See realclimate.org/index.php/arch… 4/11
What the last IPCC reports said on the AMOC decline. We know IPCC is very conservative - let's see what they make of the new and stronger evidence in the next report. 5/11
Here I discuss a couple of further recent studies supporting an AMOC decline - one based on sea level data, the other based on salinity data from the South Atlantic. realclimate.org/index.php/arch…

Graph from @chrispiecuch nature.com/articles/s4146… 6/11
This 'cold blob' is another climate model prediction come true, and in the models it's due to human-caused AMOC decline. A discussion of various pro and con arguments (as of three years ago): realclimate.org/index.php/arch…
(The image of observed SST trends is from IPCC AR5.) 7/11
How much AMOC weakening do the latest (CMIP6) climate models predict by the year 2100? They find "the AMOC might decline between 6 and 8 Sv (34–45%) by 2100." 8/11 agupubs.onlinelibrary.wiley.com/doi/full/10.10…
And what about the risk of crossing the tipping point for an AMOC breakdown? I discuss that here: realclimate.org/index.php/arch… 9/11
Here's a nice Washington Post story on our paper, by @chriscmooney. washingtonpost.com/climate-enviro… 10/11
Since it mentions The Day After Tomorrow I've gotta add this clip... I reviewed the movie at the time it came out: pik-potsdam.de/~stefan/tdat_r… 11/11
Should read SAT trends of course. Surface air temperature.
Open access link to our paper:
rdcu.be/cfPxA

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More from @rahmstorf

Prof. Stefan Rahmstorf 🌏 🦣 Profile picture
Nov 19
Important new study shows that current climate models underestimate the human-caused slowing of the #AMOC (Atlantic Meridional Overturning Circulation), because they neglect freshwater influx from Greenland melt and other sources. /1
nature.com/articles/s4156…
The study shows "that accounting for upper-end meltwater input in historical simulations significantly improves the data–model agreement on past changes in the Atlantic Meridional Overturning Circulation, yielding a slowdown of 0.46 sverdrups per decade since 1950." /2
In our 2018 Nature article (Caesar et al.) we estimated ~3 Sv slowing since 1950, i.e. -0.4 Sv/decade, based on the observed 'cold blob' in the Atlantic west of Britain. /3 Image
Read 5 tweets
Prof. Stefan Rahmstorf 🌏 🦣 Profile picture
Nov 3
Latest NASA global temperature data.
Earth has never been hotter since Homo sapiens discovered agriculture in the early Holocene. Likely even since 120,000 years ago.
Fossil coal, oil and gas emissions caused it.
We need to stop making it worse.
Yes, we can if we want to. 🧵 Image
Here is the last 2023 years of data for CO2 (from Antarctic ice core data) and global temperature (from numerous sources of proxy data from around the world, such as sediment and ice cores). Check it out: pastglobalchanges.org/science/wg/2k-…Image
And here's global temperature for the past 24,000 years - since the last Ice Age! Earth is now warming 20 times faster than at the end of the last Ice Age.
(Ice ages are caused by the Earth orbit's Milankovich cycles - modern warming is not.)
Source: nature.com/articles/s4158…Image
Read 7 tweets
Prof. Stefan Rahmstorf 🌏 🦣 Profile picture
Oct 21
One feature of global warming is the *energy imbalance* of the Earth: we are absorbing more energy from the sun than we send back to space in form of thermal radiation.
If Earth’s climate were in equilibrium, these two numbers would exactly balance.
The main reason they don’t is the thermal inertia of the ocean. Because the ocean takes a long time to warm up, the warming of the surface ocean lags behind the warming of the land areas.
So the ocean remains cooler and therefore emits less thermal radiation.
93% of the energy imbalance is due to that relatively cool ocean.
If the ocean surface warming didn’t lag behind the land areas, the imbalance would thus largely disappear.
I’ve seen some crazy claims, like: if the ocean did not absorb most of the energy imbalance, then that amount of heat would end up in the atmosphere, heating the Earth by 36 degrees Celsius.
That’s not how this works.
The ocean with its large heat capacity and therefore large heat uptake causes most of the energy imbalance of our planet at a time of rapid global warming. If the ocean didn’t do that, the Earth would only take up a fraction of the heat it does now. It would be a little bit warmer (a few tenths of a degree C) but nothing like 36 C!
That misunderstanding of ocean thermal inertia, is linked to another one: That the Earth will keep warming for decades after we reach zero CO2 emissions, as the oceans catch up with warming. That’s also incorrect.
That idea is not fundamentally wrong, but there is a balancing effect: the CO2 uptake inertia. While the ocean continues warming for some decades, it will also continue taking up CO2 for some decades after we stopped emitting, because of a CO2 concentration imbalance between atmosphere and upper ocean. So the CO2 concentration will decline, and from the point where we reach zero emissions, the warming will likely stop right away.

carbonbrief.org/explainer-will…
A third misunderstanding (that one promoted by climate skeptics) is that we do not need to reduce our CO2 emissions to zero in order to stabilize the concentration, because the ocean takes up 25% of our emissions. However, that is primarily just due to a temporary imbalance and will stop after a few decades, just like the heat uptake will. Much of the increased CO2 will actually remain for many tens of thousands of years in the atmosphere (unless our descendants actively pull it out of the atmosphere).
Read 4 tweets
Prof. Stefan Rahmstorf 🌏 🦣 Profile picture
Sep 14
Wie hängen solche Extremniederschläge wie derzeit mit dem Klimawandel zusammen? Kurzer🧵. 1/x Image
Erstens enthält mit Feuchtigkeit gesättigte Luft pro Grad Erwärmung 7% mehr Wasserdampf. Das ist ein Gesetz von 1834, ich habe es hier erklärt:
Zweitens kommt mehr Wassernachschub durch Verdunstung, wenn das Mittelmeer wie derzeit wärmer ist. 2/x spiegel.de/wissenschaft/c…
Image
Das nennen wir in der Forschung thermodynamische Effekte. Hinzu kommen Veränderungen in der Wetterdynamik, etwa dem Verlauf des Jetstreams oder der zunehmenden Dauer von Wetterlagen (relevant bei mehrtägigem Dauerregen). Das erklärt unser Kommentar. 3/x iopscience.iop.org/article/10.108…
Read 9 tweets
Prof. Stefan Rahmstorf 🌏 🦣 Profile picture
Aug 17
Wo macht ihr in der #Klimakrise lieber Urlaub? Izmir oder Menorca?

Sorry wenn ich mir nach 30 Jahren des Warnens manchmal eine Prise Sarkasmus nicht verkneifen kann.
Read 5 tweets
Prof. Stefan Rahmstorf 🌏 🦣 Profile picture
Aug 12
Vor 22 Jahren - am 12./13. August 2002 - fiel eine nie zuvor in Deutschland innnert 24 Stunden gemessene Regenmenge: 312 mm. Bald stand Dresden unter Wasser, es gab über 11 Milliarden € Schaden. Mein Welt-Interview von damals. 🧵
pik-potsdam.de/~stefan/welt-i…
Inzwischen ist die von mir erwähnte Zunahme der Extremniederschläge noch viel deutlicher in den Messdaten der Wetterstationen hervorgetreten.
Die Häufung liegt inzwischen weit außerhalb der historischen Schwankungen. nature.com/articles/s4161…
2002 dachten wir noch, Begrenzung der Erwärmung auf 2 Grad würde reichen. Doch im Lichte neuer Erkenntnisse, u.a. über Klimakipppunkte, hat sich die Weltgemeinschaft 2015 im Pariser Abkommen auf die Begrenzung auf 1,5 Grad verständigt. Inzwischen leider kaum noch zu schaffen.
Read 4 tweets

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