Via P Gosselin above 10. December 2021
A German analysis found that the imminent death threat scenarios of all coral reefs in a recent study were not justifiable.
Corals off East Africa about to go extinct?
By Frank Bosse
(Translated, edited by P. Gosselin)
An agency report startled readers: “All coral reefs in the western Indian Ocean are at risk of dying“. The report mentions this research, highlighting a range of risks, such as overfishing in the coming years, that threaten coral reefs.
However, how fishing will develop over the next 50 years remains unclear. Of particular interest is the climate context, which also comes to the fore in the reports, as climate models can be used here.
If the water is too warm, then coral bleaching results.”
In fact, such coral bleaching has been observed many times and occurs more often at water temperatures above 30°C. This happened now and then locally in the Pacific Ocean, especially especially during El Niño events. So now total collapse is predicted in 2070.
We took a closer look at the study, focusing on projected climate stress. The study area is depicted in Figure 1 of the study, along with threats from various factors:
Figure 1, Copy of Figure 1 of the study. Criterion C is what is directly related to climate. However, this identifies particularly vulnerable areas (red) in the final result (left), which is the deciding criterion for particularly important spaces.
In the study, an emission scenario is assumed, that is, the scenario with the second strongest emissions. It provides an additional anthropogenic radiation force of 6W/m² in 2100, it is called “RCP 6.0” according to IPCC AR5. RCP 8.5 provides the highest warmth, it was purposely not used in the study, which is related to works classifying it as impractical.
Now RCP 6.0 is in use. We wondered if this assumed over the past 40 years would correspond to the observations. The first thing to note is that there are reefs at some water depth (up to 60 meters). So the first question is whether surface temperatures are representative of these depths of water. For this purpose, we use observations of the “Argo” system in the study area (Figure 1), which also evaluates measurements at water depths.
Figure 2: Water temperature down to a depth of 100m from 2004 to 2020 in the observation area. The source: Argo.
We see: Up to about 60 meters, the highest water temperature of about 28°C occurs from February to April (southern hemisphere summer) every year. In this respect and during these months, sea surface temperatures (SST) are therefore representative.
Models are overrated
How does the average of the models used (CMIP5) represent the observations of SST?
For comparison, we restricted ourselves to the months with the highest SCT, as they were the decisive months for the bleaching of the mentioned corals. We compared the MMM (Multi Model Mean), driven by RCP 6.0 used in the study, with observations according to ERSSTv5.
Figure 3: Observed SST (blue) and modeled SST (red) with their respective linear trends from 1980 to 2020. Note the peaks in the 1997 El Nino events , 2010 and 2016.
The linear gain of the model mean is 30% higher than that observed.
The RCP 6.0 scenario used overestimates the observed warming over the past 40 years by 30%.
So the question is: Are claims of impending coral bleaching from climate model observations strong?
In Figure 1, the particularly endangered areas are also depicted in great spatial detail. What do models do in this field? We looked at this and compared the spatially resolved correlation of the model mean with observations:
Figure 4: Spatial correlation between actual observations and the model mean during the evaluation period. In large parts, it only reaches values below 0.5, which is not a satisfactory result. Charts created with KNMI Climate Explorer.
Therefore, the question of whether the models used are really suitable to describe conditions off the east coast of Africa around Madagascar, must therefore be returned to the study’s authors. .
We conclude: rather not.
Therefore, the study’s conclusions should be classified as “overconfident” after a substantial examination.
The models do not accurately represent the observed warming rate (over 30%), nor its spatial representation. Models with high uncertainty simply cannot do that. Therefore, the paper should be classified as describing one of several possibilities.
The agency report’s title again conveys threatening scenarios of the impending demise of “all coral reefs,” which cannot be justified or supported.