[Coral-List] Poor terminology in coral reef research 4: El Nino caused bleaching

Thomas Goreau goreau at bestweb.net
Sun Nov 5 11:24:58 EST 2006

For some years this phrase has become very popular. Every bleaching  
event and weather anomaly is blamed on El Nino, whether or not there  
is legitimate reason to do so, often by people with little  
understanding of what El Nino is.

The basic El Nino Southern Oscillation pattern has been well  
understood since it was first discovered about a century ago by a  
meteorological  genius who found that droughts in India were strongly  
correlated with the barometric pressure difference between Darwin,  
Australia, and Papeete, Tahiti (The ENSO Index). El Nino is simply an  
atmospheric pressure wave in which the high and low pressure poles  
oscillate irregularly between the East and West Pacific, thereby  
influencing winds, surface currents, and upwelling. The  
teleconnections (correlations between pressure at any point on earth  
with ENSO are well mapped too, and some places correlate strongly  
positively with ENSO, particularly the East Pacific, and others  
correlate strongly negatively, particularly the West Pacific, with  
other places having much smaller positive or negative correlations,  
in most places not very significant.

During an El Nino the East Pacific gets very hot and rainy, but the  
West Pacific gets cold, reducing evaporation and rainfall, and  
leading to droughts and forest fires as now taking place in Borneo as  
they did in 1998. Peter Glynn's brilliant work in the East Pacific  
during the 1982-1983 El Nino was the first to show large scale  
bleaching correlated to the high temperatures El Nino caused in that  
restricted region. But it is high temperature per se, not El Nino,  
that causes bleaching, and El Nino related bleaching only takes place  
in regions with strong positive ENSO correlations. In 1998 severe  
bleaching took place in the East Pacific, West Pacific, Indian Ocean,  
and Caribbean, regardless of the local ENSO correlation. That  
happened because by coincidence several other regional pressure  
oscillations in the Indian Ocean, North Pacific, and Atlantic  
happened to peak around the same time as ENSO, as they are bound  
every now and then. Severe bleaching takes place in non El Nino years  
wherever and whenever it gets too hot, which is why the HotSpot  
method works so perfectly. The spatial and temporal pattern of  
bleaching is surely a global warming signal and not an ENSO one! Sure  
there are many local and regional fluctuations, but it is the rising  
global temperature baseline that is the ultimate cause of bleaching  
even in those few places where El Nino might be a proximate cause. We  
saw thousand year old corals die from heat in 1998 that had been  
through hundreds of El Ninos in the past with no damage, as shown by  
their morphology.

Why is it that so many people never say "bleaching" without prefacing  
it with "El Nino caused"? That is because this phrase has been a  
convenient way by certain countries (that I dare not name here  
without being banned from the list server for life) as a way to  
suggest that coral bleaching is a purely natural phenomenon, a rare  
and unusual weather anomaly that will never happen again, and  
certainly has no (putative) relationship to any (hypothesized)  
greenhouse effect or (alleged) rises in global temperature and CO2,  
so that there is just no need to worry about them at all and action  
would be premature and a foolish waste of money that could be better  
used to buy bombs and other neat toys for the boys.

Let us be clear, it is high temperatures  that have caused  
essentially all of the large scale bleaching events since 1982, that  
we have lost most of our corals since then, that we are already at or  
past coral's upper tolerance limits almost everywhere, and that they  
CAN TAKE NO FURTHER WARMING without reefs being condemned to  
extinction from heatstroke caused directly by human-caused global  

And let us admit that complete and effective global action on this  
NOW is the sine qua non for the future of coral reefs and more than  
100 countries for whom they are the major source of marine  
biodiversity, fisheries, tourism, and shore protection, even if we  
reduce all local and regional stresses.

Thomas J. Goreau, PhD
Global Coral Reef Alliance
37 Pleasant Street, Cambridge MA 02139
goreau at bestweb.net

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