Coral bleaching and limits to ocean warming

A new paper by Kleypas et al in Geophysical Research Letters suggests that a possible upper limit to ocean temperatures – proposed by the “ocean thermostat” hypothesis – may spare some coral reefs from future coral bleaching events. The science has been more or less massacred by some of the press, so it’s worth explaining what is actually going on.

The “ocean thermostat” theory predicts that there is an upper limit to sea surface temperatures – somewhere around 30-34 C depending on the region. First things first: that does not mean if you put some ocean water on the stove, it is impossible to get the temperature over 34 C. The theory is based on a number of negative feedback mechanisms which only operate at relatively large scales, larger, say than the pot sitting on your stove. So the limit is expected to be 30-31 C in the open ocean, like the Pacific, and a bit higher, 33-34 C in enclosed seas like the Red Sea.

We’re used to hearing about positive feedbacks – temperature rises, sea ice melts, a darker surface (water) that reflects less radiation is exposed, causing temperatures to rise even more. In this case, rising water temperature would drive processes that would slow or stop the temperature rise. There are several candidate mechanisms. If the ocean thermostat really exists, it is likely caused by a combination of the mechanisms. One example is that as the ocean warms, more water evaporates, creating more clouds and reducing incoming solar radiation. Another is that warming of one area of ocean will create pressure gradients (with other cooler areas) creating currents that redistribute the water.

The sea surface temperatures across most of the tropics are well below the theoretical temperature limit suggested by the ocean thermostat theory. The maximum annual temperature could rise 3-4 C in most coral reef regions, well beyond the threshold for the corals, without hitting the thermostat limit. That's the case in the Caribbean, the eastern Pacific, much of the Indian Ocean, much of the Great Barrier Reef, etc.

The exception is in the Western Pacific. The warmest open ocean waters is the world are found in a zone that can stretch from the Indian Ocean well into the Central Pacific. It is affectionately known as the West or Western Pacific Warm Pool or WPWP. In a pathetic attempt to give you the illusion scientists are hip and clever, I will use the short form WP2.

You probably know of WP2, if not by name. For one, the Pacific section can be (loosely) thought of as the body of warm water in the equatorial Pacific that migrates eastward during El Nino events. Also, the Pacific section is home to the majority or at least a sizeable fraction, of what’s left of the world’s un-canned tuna and, in turn, to the majority of the world’s illegal fishing boats.

Many years, the WP2 is usually centered in SE Asian archipelago or the expansive territorial waters of the Pacific Island nation of the Federated States of Micronesia. Of course, the exact location changes each year, bringing the core of warm open ocean waters maybe closer to the Solomon Islands, or Palau, or Kiribati, or Tuvalu, or other neighbouring countries. The fishing vessels flying flags of convenience from Mongolia or Western Sahara or Suriname are usually close behind.

When the edge of the warm pool moves, say, western Kiribati, the rise in water temperatures (1-2 C) is quite likely to cause a coral bleaching event. If you take a look at the January degree heating week maps – that’s the accumulated heat stress metric we use to predict bleaching events – for the past several years, you’ll see that almost every year, there’s a hot spot somewhere outside of WP2’s home base. This has been the proximate cause of bleaching events in the Kiribati (Gilbert) Islands, the Phoenix Islands, Tuvalu and the Solomons in recent years - this may be, in part, a manifestation of long-term temperature trends.

That’s where Kleypas et al. comes in. The paper finds that open ocean waters with maximum annual SSTs above 29 C – in other words, our friend WP2 – have warmed less than any other parts of the ocean since 1950. The closer the SSTs are to 30-31 C, the lower the rate of warming. The data appears to support, at least, circumstantially the existence of the ocean thermostat. If correct, this would imply corals in the warm region could be spared from a dangerous temperature rise.

That’s what spawned the “corals saved from ocean thermostat” headlines. Here’s the catch – ok, three catches. These are not criticisms of the paper rather crucial issues touched on in the paper which were not addressed in the news pieces (and may lead to abuse of the results):

First, the notion of a temperature ceiling sparing coral reefs is limited to this very warm open ocean region. In fact, one of the most interesting results of the paper is that while WP2 has not warmed as rapidly as other parts of the tropical ocean, it has expanded. In other words, the edges have warmed. That, as we’ve seen, has caused bleaching events in a number of countries like Kiribati

Second, the extent and severity of coral bleaching is related to experience. Kleypas et al. compiled bleaching reports from islands within WP2. In these locations, coral bleaching tended to occur with temperatures only 0.2-0.3 C greater than the usual maximum experience by the corals. The threshold is generally thought to be 1-2 C. Corals, like any organisms, adapt or acclimate over time to their environment. Since temperatures historically have shown less variation in the WP2 than other parts of the tropics, the corals living there may be naturally more sensitive to temperature stress. If so, the ocean thermostat might not provide the protection that was suggested in the media reports.

Third, the ocean thermostat theory is predicated in part on warming being limited to the surface ocean. Historical temperature profile data suggest ocean warming is also occurring at greater depths. The implications of the warming at depth needs to be fully investigated before we propose an upper limit to surface ocean temperatures in the future.

In other words, there's much more to come on this story.