What's The Latent-Heat Tipping Point?

I received an email suggesting that I'd overlooked the positive feedback from latent heat in my list of positive feedbacks contributing to the Arctic Death Spiral.

I thought long and hard about this but I could see only negative feedback from the latent heat of water/ice. The physics works as follows:

  1. Suppose the Arctic was in an equilibrium.
  2. Suppose we applied enough heat to the planet that it would warm by 1°C, as long as no ice melted.
  3. Of course (we observe) we end up with less ice on the new, warmer planet.
  4. Effectively we've melted some ice, so this will have absorbed latent heat in the transition from ice to water and removed thermal energy from the Earth system.
  5. So the Arctic/world ends up a bit less than 1°C warmer.
  6. That's negative feedback!

Aside: ice is doing a tremendous job of slowing down global warming, as some of the extra energy we're forcing into the atmosphere and oceans melts ice instead of raising the temperature. This effect is small on a planetary scale, compared to the heat absorbed by the oceans, but has a huge effect in the Arctic, for which we should be thankful, while it lasts.

Then my correspondent pointed out the Latent-Heat Tipping Point eg https://arctic-news.blogspot.com/p/latent-heat.html, which occurs when and where the Arctic Ocean is ice-free.

The idea that latent heat causes a tipping point didn't make sense to me, because I thought a tipping point should require positive feedback and I couldn't find any caused by latent heat. I looked up the definition of a tipping point:

Tipping point - the point at which a series of small changes or incidents becomes significant enough to cause a larger, more important change.

Then I finally got my head round the idea (at least I think I have)...

The Latent-Heat Tipping Point comes not from any positive feedback but from a loss of negative feedback!

When the Arctic Ocean is ice-free, the negative feedback from latent heat no longer occurs, because no ice is there to be melted. Any new energy that arrives from the tropics via the jet stream and ocean currents, and from the sun, heats the Arctic environment instead of melting ice. This is a significant enough change to justify calling it a tipping point.

Note: even where the Arctic Ocean is ice-free, some negative feedback continues, as latent heat is absorbed by the evaporation of water into the atmosphere.

There's more information under "Latent Feedback" on https://climatetippingpoints.info/2019/04/02/fact-check-will-an-ice-free-arctic-trigger-a-climate-catastrophe/.

However, the periodic loss of negative feedback makes existing positive feedbacks worse - see below.

Does This Matter?

During the Arctic summer a large part of the Arctic Ocean is already ice-free, compared to a few decades ago (see Arctic Sea-Ice Area Loss), and the Latent-Heat Tipping Point is already happening in those locations. For example https://arctic.noaa.gov/Report-Card/Report-Card-2019/ArtMID/7916/ArticleID/840/Sea-Surface-Temperature reports:

August 2019 mean SSTs were around 1-7°C warmer than the 1982-2010 August mean in the Beaufort, Chukchi, and Laptev Seas

The maps on figures 1 & 2 on the same page show the warmer water creeping into the Arctic Ocean through the Bering Strait, where there would previously have been sea ice cooling it.

As the ice-free area in the Arctic increases in extent and duration, the following will occur during the ice-free periods, with increasing severity:

  • These existing positive feedbacks will be exacerbated: reflective snow and ice are lost faster and further across on the neighbouring land; a driver for the AOC (Atlantic overturning current), which removes heat and CO₂ into the deep ocean, weakens; methane is released faster from the Arctic seabed and surrounding permafrost.
  • The temperature gradient between the tropics and the Arctic helps power the jet stream; as the gradient decreases, no one knows what will happen to the jet stream and the Northern hemisphere weather patterns!
  • Extinction of species that can't survive or are outcompeted in the warmer waters.

Note: as the Arctic moves into winter and the sun is lost and temperatures fall, the loss of negative feedback applies again in the opposite direction: the ice-free areas cool relatively rapidly until they hit 0°C. Once the system hits 0°C, we get negative feedback again and the temperature drops slowly as water freezes releasing latent heat.

However, the consequences of the temporarily increased positive feedbacks will remain: the AOC has lost a little extra momentum; the permafrost has melted more than it otherwise would have; there's more methane in the atmosphere.

Not-a-Fact Check

I had intended to use the following quote from Peter Wadhams's book A Farewell to Ice p122 as evidence of the Latent-Heat Tipping Point:

During an ... August 2014 voyage ... the US Coastguard icebreaker 'Healy' recorded extraordinary sea surface temperatures in the Chukchi Sea; on our way north towards the Bering Strait, off Nome, we experienced ... sea surface temperatures of 17°C.

The Chukchi Sea is part of the Arctic Ocean. Nome, however, is on the SW side of Alaska, on the Pacific Ocean, several hundred km from the Chukchi Sea and the Arctic Ocean.

Credits

Big thanks to Phil Lister (citizen scientist) for bring this to my attention and to my friends Jon Woolf and Andrew Fitzgibbon for their patience in helping me work this out.



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