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Prof. Stefan Rahmstorf 🌏 🦣 Profile picture
@rahmstorf
Feb 17, 2021 • 9 tweets • 4 min read • Read on X
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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/🧵
Objective cluster analysis of polar vortex states shows that cluster 4 is linked to cold in the US, cluster 5 to cold in Eurasia. 4/🧵 Image
Do these clusters increase in prevalence? The reanalysis data shows they do over the period 1979-2018. And this can explain why Eurasian winters haven't warmed more: agupubs.onlinelibrary.wiley.com/doi/full/10.10… But of course that could be just due to random variability. 5/🧵 Image
On the other hand there are plausible mechanisms that link this increasing polar vortex instability to Arctic warming and sea ice loss. Causal effect network analysis (a data correlation method able to tease out causal links) suggests that: journals.ametsoc.org/view/journals/… 6/🧵 Image
Furthermore, analysis of 35 CMIP5 models shows that the sea ice loss effect, though weaker in the models than in the observational data, "can explain all of the projected ensemble-mean stratospheric polar vortex weakening": wcd.copernicus.org/articles/1/715… 7/🧵 Image
To finish off, a fun fact: in 2014 right-wing US commentator Rush Limbaugh claimed the polar vortex was invented by the left... Against that speaks that it's already in the 1959 AMS Glossary of Meteorology. 8/🧵 Image
And a bunch of relevant references. Image

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More from @rahmstorf

Prof. Stefan Rahmstorf 🌏 🦣 Profile picture
Mar 26
A new study by van Westen et al. shows that the #tippingpoint of the Atlantic overturning circulation #AMOC is also found in a high-resolution ocean model which resolves ocean eddies. A first, but no surprise to AMOC experts. 🧵
To find the tipping point you need to do a very slow, long hysteresis model run. That is so computationally expensive that it hasn't been tried before. All models which have tried show this tipping point - we published a first intercomparison in 2005. 🧵 agupubs.onlinelibrary.wiley.com/doi/10.1029/20…
So all kind of models since Stommel's 1961 box model show it, due to the destabilising salt transport feedback. The new model also shows the well-known sea surface temperature fingerprint pattern of an #AMOC shutdown: the 'cold blob', and warming along the American coast. 🧵 Image
Read 5 tweets
Prof. Stefan Rahmstorf 🌏 🦣 Profile picture
Mar 15
Zur Erinnerung: Kernfusion ist /keine/ Klimalösung sondern klimaschädlich. (Nur falls jemand auf die Idee kommt, diese Träume von Merz mit Klimaschutzgeldern zu subventionieren.)
Kurzer Thread in 5 Bildern. Image
Image
Image
Read 7 tweets
Prof. Stefan Rahmstorf 🌏 🦣 Profile picture
Jan 12
2024 war global das heißeste Jahr seit Beginn der Aufzeichnungen, mit 1,6 Grad über dem Temperaturniveau des späten 19. Jahrhunderts. Kleiner Thread mit Datengrafiken dazu. 🧵 Image
In Deutschland war es ebenfalls das wärmste Jahr seit Beginn der Aufzeichnungen - allerdings schon 3,1 Grad wärmer. Weil Deutschland ein Landgebiet ist, erwärmt es sich doppelt so schnell wie der globale Mittelwert, der 71% Meeresfläche enthält. 🧵 Image
Diese moderne Erderwärmung ist praktisch komplett vom Menschen verursacht. Natürliche Faktoren haben weniger als + oder - 0,1 °C beigetragen. Das ist eine Kernaussage des Weltklimarats IPCC. Eher - als +, weil die Sonnenaktivität etwas abgenommen hat. 🧵 Image
Read 7 tweets
Prof. Stefan Rahmstorf 🌏 🦣 Profile picture
Nov 19, 2024
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, 2024
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, 2024
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

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