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Penny Holliday

Things Penny Says on Podcasts

Where to Find Them

Penny Holliday has been a guest on The Climate Question .

Recently: “AMOC: How the changing Atlantic could reshape our world” on The Climate Question (November 2026).

What They Said

“In order to detect a weakening of the AMOC, you have to measure the ocean for long enough to be able to characterise all that natural variability. We've only been measuring the AMOC for 20 years. We expect it to be at least another 10 years or 20 years before we can really detect whether there's a climate change signal over the natural variability.” — Penny Holliday, The Climate Question

The host pointed to the thousands of instruments used to monitor the Atlantic overturning circulation and asked whether they'd give warning of a weakening. The answer: yes, but the record is still too short to separate a climate signal from natural swings.

The Climate Question · 2026-11-01 Permalink → Listen →
The Climate Question Around 16:15 into the episode
Speaker 4

…or not being completely knowledgeable about the physics of the AMOC, we can't get that to be reproduced in the climate models. So, there's work to be done to improve how the AMOC is represented in climate models so that we can reduce that envelope of uncertainty that we have at the moment.

Speaker 2

…nearly 500 kilometres of wiring to just measure what you're talking about and 3,000 pieces of equipment out there monitoring. So, it's not through lack of trying. I guess all of that equipment and all of that wiring, will it give us a warning if weakening is happening? Will we get time to prepare?

Speaker 4

It will do. I'm absolutely sure of that. One thing to understand, I think, is that the ocean does vary, it does change, it has fluctuations on lots of time scales. So, you're all familiar with the idea that there's tides, okay? So, we have a 12-hour fluctuation in sea level based on tides. We also have storm surges and we have different processes that happen on quite short time scales. And there are similar cycles of changes in the ocean over long time scales, too. It varies according to the season, it varies on time scales of one year to another, 10 years to another, 10 years. And in order to detect a weakening of the AMOC, you have to measure the ocean for long enough to be able to characterise all that natural variability and look to see whether you can detect a long-term climate change signal in it. We've only been measuring the AMOC for 20 years. We expect it to be at least another 10 years or 20 years before we can really detect whether there's a climate change signal over the natural variability that the AMOC also contains. Based on the science we have available now, before the next decade of data that we might get, what's…

Speaker 2

I've got a bit of a weird question to end on with, Penny, if that's okay. What's your favourite bit of the ocean?

Speaker 4

My favourite bit of the ocean is probably the subpolar North Atlantic. So that's the area between Canada, Greenland, Iceland, and the UK. It's cold and it's stormy, but it's incredibly beautiful. I've spent a lot of time out on ships there. The wildlife is spectacular.

Speaker 2

Polar bears?

Speaker names come from our own diarization. The position is estimated from where the line sits in the episode.
“The thing that's not realistic about the film is that it happens sort of overnight or over a few days. And that wouldn't happen. It's plausible that it could weaken quite fast in ocean time scales. And that means over probably a few decades rather than just a day or two.” — Penny Holliday, The Climate Question

Asked whether the Atlantic current collapse in The Day After Tomorrow is accurate, the oceanographer says less heat reaching Europe could cool it, but the film gets the speed wrong. She prefers "weakening" to "collapse."

The Climate Question · 2026-11-01 Permalink → Listen →
The Climate Question Around 09:26 into the episode
Speaker 1

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Graihagh Jackson

…that film the day after tomorrow, and in that film, the AMOC collapses, and New York basically freezes over. And indeed, there's a lot of concern that people have about the AMOC collapsing, at least online. And I just wonder, is that an accurate depiction of what could happen if it does collapse?

Speaker 4

There are some aspects of it, which are similar to how things might evolve in the future, in that if the AMOC weakens, less heat is carried northwards by that ocean current. And that means there's less heat available to warm places like Europe and the UK. So with under a weaker AMOC, there is the potential for there to be a net cooling effect in Europe and the UK. But the thing that's not realistic about the film is that it happens sort of overnight or over a few days. And that wouldn't happen. The AMOC could potentially weaken. It's plausible that it could weaken quite fast in ocean time scales. And that means over probably a few decades rather than just a day or two. And it's really important to consider the terminology we're using here. So there is talk of AMOC collapse, but we prefer to talk about AMOC weakening because most likely what will happen under climate change is that the AMOC will slowly start to weaken. There may be a point, some models do show a point where it collapses, it gets to a weaken point where it can't recover. But many models…

Speaker 2

Has the AMOC ever collapsed before

Speaker 4

or weakened? Before? Yeah, we think the AMOC, there's plenty of evidence to show that the AMOC can exist in a weaker state than it is at the moment. There's some evidence too that the AMOC during the period in which human civilization has expanded across the world, the AMOC's been in a really…

Speaker 2

Can I ask how extreme some of the changes

Speaker names come from our own diarization. The position is estimated from where the line sits in the episode.

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Climate & Energy