Guest essay by Eric Worrall

Even 1.5C would kill 99% of all coral, according to the study. But I think there is room for a little doubt.

Corals doomed even if global climate goals met: study

by Marlowe Hood
FEBRUARY 6, 2022

Coral reefs that anchor a quarter of marine wildlife and the livelihoods of more than half-a-billion people will most likely be wiped out even if global warming is capped within Paris climate goals, researchers said Tuesday.

An average increase of 1.5 degrees Celsius above pre-industrial levels would see more than 99 percent of the world’s coral reefs unable to recover from ever more frequent marine heat waves, they reported in the journal PLOS Climate.

At two degrees of warming, mortality will be 100 percent according to the study, which used a new generation of climate models with an unprecedented resolution of one square kilometre.

“The stark reality is that there is no safe limit of global warming for coral reefs,” lead author Adele Dixon, a researcher at the University of Leeds’ School of Biology, told AFP.

“1.5C is still too much warming for the ecosystems on the frontline of climate change.”

…

Read more: https://phys.org/news/2022-02-corals-doomed-global-climate-goals.html

The abstract of the study;

Future loss of local-scale thermal refugia in coral reef ecosystems

Adele M. Dixon, Piers M. Forster, Scott F. Heron, Anne M. K. Stoner, Maria Beger

Published: February 1, 2022

Thermal refugia underpin climate-smart management of coral reefs, but whether current thermal refugia will remain so under future warming is uncertain. We use statistical downscaling to provide the highest resolution thermal stress projections (0.01°/1 km, >230,000 reef pixels) currently available for coral reefs and identify future refugia on locally manageable scales. Here, we show that climate change will overwhelm current local-scale refugia, with declines in global thermal refugia from 84% of global coral reef pixels in the present-day climate to 0.2% at 1.5°C, and 0% at 2.0°C of global warming. Local-scale oceanographic features such as upwelling and strong ocean currents only rarely provide future thermal refugia. We confirm that warming of 1.5°C relative to pre-industrial levels will be catastrophic for coral reefs. Focusing management efforts on thermal refugia may only be effective in the short-term. Promoting adaptation to higher temperatures and facilitating migration will instead be needed to secure coral reef survival.

Read more: https://journals.plos.org/climate/article?id=10.1371/journal.pclm.0000004

Given that there is at least 6C climatic range from the subtropical Southern to the tropical Northern tip of the Great Barrier Reef, it is impossible that 0.5-1C of warming would wipe out the entire reef.

So where did the study authors go wrong?

Delving into the study, it appears they pulled the marine version of the agriculture study trick.

Agricultural studies which predict imminent climate doom usually make completely unrealistic assumptions, like assuming farmers won’t try anything new, that they’ll keep dumbly planting exactly the same crop mix, year after year, regardless of what happens to yield.

In a similar way, the authors of this coral study appear to have assumed low coral mobility.

A lack of coral mobility is not a reasonable representation of nature. Mature corals might be stuck in place, anchored to rocks or other corals, but every year corals spawn countless billions of highly mobile larvae, which seek out favourable sites to colonise.

So any overheated reefs would be rapidly colonised by coral larvae whose parents are adapted to warmer water.

The researchers assigned a temperature value to each point on a very fine grid, compared it to the modelled pre-industrial temperature, and set a threshold which according to climate models will shortly be breached. This led to the unrealistic study conclusion that stressed grid points will rapidly increase from 6.8% of the study region (according to their assumptions) to 99-100% of the study region.

There is no serious upper temperature limit to coral heat endurance. Hot water extremophile corals like those from the Persian Gulf endure far hotter water temperatures than any open ocean, yet they are just as mobile as any other coral.

Even if the world were to experience insanely implausible levels of global warming, the extremophile corals would erupt out of their limited hot water ecological niches and colonise the entire rest of the ocean. The mix of species might change, but the reef as a whole would stay healthy.

These studies have a place, as a mathematically modelling exercise, or a baseline exercise. But the suggestion of real world applicability, the suggestion that corals cannot survive a small long term temperature shift is utterly contradicted by the wide climatic range of real world reefs like the Great Barrier Reef, which contain more or less continuous stretches of different climatic conditions, from the subtropics to the far Northern high tropics.