Latent heat

[ feedback #14: Latent Heat ]

Above image shows a number of feedbacks that are accelerating warming in the Arctic. Feedback #14 refers to the heat that is (less and less) going into melting the sea ice. The reason this heat is called latent (hidden) heat, is that it doesn't raise the temperature of the water, but instead gets consumed in the process of melting the ice.

Thinner Sea Ice

Heat is melting Arctic sea ice from below, as it enters the Arctic Ocean from the Atlantic Ocean and the Pacific Ocean. This is a self-reinforcing feedback loop, i.e. as the sea ice gets thinner every year, an increasing proportion of this heat is no longer consumed by melting the ice the next year, but will instead get absorbed by the water and thus contribute to accelerated warming of the water of the Arctic Ocean.

As illustrated by above combination image, the thickness of the sea ice is now substantially less than it used to be. The image compares June 1, 2021 (left), with June 1, 2015 (right).

The animation below was run on September 15, 2020, and shows Arctic sea ice thickness over 30 days (last 8 frames are forecasts for September 16 - September 23, 2020).

Latent Heat Tipping Point

There is a point where the ice gets so thin that, instead of consuming most heat, it will consume virtually no heat anymore, so from that point, most further heat that is flowing from the Atlantic and Pacific Oceans into the Artctic Ocean will go into raising the temperature of the water. 

As long as there is sea ice in the water, this sea ice will keep absorbing heat as it melts, so the temperature will not rise at the sea surface and remain at zero°C. The amount of energy that is consumed in the process of melting the ice is as much as it takes to heat an equivalent mass of water from zero°C to 80°C.

The video below, created by Stuart Trupp, shows how added heat at first (A) goes mainly into warming up water that contains ice cubes. From about 38 seconds into the movie, all heat starts going into the transformation of the ice cubes into water, while the temperature of the water doesn't rise (B). More than a minute later, as the ice cubes have melted (C), the temperature of the water starts rising rapidly again.

Ocean Heat

2019 analysis concludes that the latent heat tipping point gets crossed when the sea surface temperature anomaly on the Northern Hemisphere gets higher than 1°C above 20th century's temperature and when there is little or no thick sea ice left. As the image below indicates, the temperature anomaly of 1°C above the 20th century average looks set to be crossed in the course of the year 2021.

Let's take a look at where global heating is going. Oceans are absorbing over 90% of global heating, as illustrated by above image, and over the decade from 2093 to 2003, Arctic sea ice consumed 0.8% of global heating. As the amount of heat that is entering the Arctic Ocean is increasing, and as the sea ice is getting thinner each year, the tipping point is getting ever closer each year. 

As the tipping point gets crossed, there will still be a thin layer of ice at the surface, at least as long as air tempartures are low enough to keep it frozen. This thin layer will still consume some heat, but at the same time it acts as a seal, preventing heat from the Arctic Ocean to enter the atmosphere. Once the tipping point is crossed, the bulk of the heat from the influx of warm water will go into heating up the water of the Arctic Ocean, including the deeper parts of the water, and each year this will occur earlier in the year.  

As illustrated by the image below by Nico Sun, the accumulation of energy going into melting the sea ice was at record high for the time of year on June 8, 2021. 

Methane Hydrates Tipping Point

As discussed in a 2020 post, the loss of subsurface sea ice is only one of ten tipping points hitting the Arctic. Once this tipping point is crossed and as the temperature of the oceans keeps rising, more heat will reach sediments at the seafloor of the Arctic Ocean that contain vast amounts of methane, as discussed in this page and this post. The danger is that this heat will destabilize hydrates in the sediments, resulting in huge releases of methane from the hydrates and from methane that is present in the form of free gas underneath the hydrates.

The methane hydrates tipping point is estimated to get crossed as ocean temperature anomalies on the Northern Hemisphere become higher than 1.35°C above the 20th century average, as the image further above indicates.

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