[Coral-List] Coral bleaching and light levels

Thomas Goreau goreau at bestweb.net
Fri Feb 13 13:51:44 EST 2009

It has been well known for about 50 years from direct experimentation  
for at least that sudden changes in light level, both high and low,  
can induce bleaching. I do not at all deny that high light exacerbates  
the rate of bleaching at high temperatures, which I measured directly  
20 years ago. But I do not think anyone can seriously claim that  
increased light has caused the mass bleaching that we have seen in the  
last three decades, although it was obvious that it was a contributing  
factor from field evidence in the very first mass bleaching events,  
but it plays a secondary role, not a primary one. You should not  
confuse "blanching" (what happens to corals that are exposed to full  
sunlight at extreme low tides) with "bleaching" sensu strictu as Brown  

The question is whether light levels have changed at the surface in a  
way that could explain mass bleaching apart from changes in  
temperature. Variations in cloudiness are not new, although they are  
becoming more extreme. But there is a strong and clear tendency to  
increased cloudiness over the oceans, so this is resulting in reduced  
light stress, not more!

There is in fact a large literature on changes in global surface  
irradiation, and the results have shown that solar output is  
essentially unchanged but that changes in atmospheric transparency  
have had a global impact. This was stimulated by the widespread  
prediction that decreases in the ozone layer caused by increases in  
atmospheric nitrous oxide and halocarbon gases should result in  
increases in surface UV. But the long term data conclusively showed  
the opposite, that UV had been decreasing for decades. The same turned  
out also to be true for surface visible light, and also for total  
irradiation as measured by global meteorological pan evaporation  
rates. The explanation was global dimming caused by sulfur and  
nitrogen aerosols generated from combustion. As industrial pollution  
has begun to be controlled, global dimming has decreased, allowing a  
slight increase in surface irradiation in recent years. This is a  
double whammy because global dimming had masked an appreciable portion  
of the global warming caused by CO2 in the short term, so that now  
that these pollutants are reduced, global warming rates will greatly  
accelerate. Increased pollution has been proposed by some as a devil's  
bargain to geo-engineer against global warming, but this is a fool's  
errand because the CO2 remains in the atmosphere absorbing heat for  
centuries, but the aerosols are rained out in days to weeks (or a year  
or so in the stratosphere) so you would need to pollute with aerosols  
at exponentially increasing rates just to stay even. There are many  
excellent papers on this, but Meinrat Andreae provides the clearest  
recent explanation.

At any rate, light cannot explain the large scale spatio-temporal  
bleaching patterns we see, although it clearly contributes to the  
small-scale patterns. Juvenile corals, which prefer to settle in dark  
undersides, are protected for that reason. Peter Glynn did excellent  
work with various filters in which he showed that it was  
photosynthetically active radiation was causing the increased rate of  
bleaching in high light exposed corals above the bleaching threshold  
temperature, and that UV increases made no difference. This is as  
would be expected by the very high levels of UV screening pigments.  
Charlie Mazel and I dived on severely bleached reefs at night in 1990  
measuring UV absorbance and fluorescence of bleached corals, and we  
found that even totally bleached corals were completely opaque to UV  
light. So increased UV cannot be the cause of the large scale field  
bleaching, even though it can be an experimental cause of bleaching.

I'm fully aware of the microphysical patterns you mention discussing  
light attenuation and scattering. But the reason I do not think they  
are relevant is because what I have seen is that waters in most reefs  
are far more turbid than they used to be and light penetration is  
greatly reduced. In many places I see large areas of deep reef 100-200  
feet down that used to be healthy and are now dead because they are no  
longer getting the light they used to, for example in Jamaica and  
Panama. In places where as a boy I would watch my father diving 300  
feet below from the surface, the waters that used to be clear and blue  
are now dark and green. The whole reef light looks completely  
different. So I am sure that increased light exposure is not the cause  
of mass bleaching.

You may think that the changes global atmosphere patterns are  
"simplistic" and not "scientific" with regard to microphysical water  
measurements, but I can assure you that they are well documented in  
the atmospheric literature (having degrees in Planetary Physics (MIT),  
Planetary Astronomy (Caltech), and Atmospheric Biogeochemistry  
(Harvard) and having worked on these issues from the top down as well  
as the coral's bottom up view).

Thomas J. Goreau, PhD
President, Global Coral Reef Alliance
Coordinator, United Nations Commission on Sustainable Development  
Partnership in New Technologies for Small Island Developing States
37 Pleasant Street, Cambridge MA 02139
goreau at bestweb.net

On Feb 13, 2009, at 12:25 PM, Richard Dunne wrote:

> Tom
> It is wrong to state so unequivocally that light is "a secondary  
> factor". It can be a primary factor as publications have shown (e.g  
> Brown et al - plenty of pubs). It is much more scientific and  
> accurate to say that it is an interplay between light and  
> temperature - see the diagram and discussion in the Fitt et al.  
> Review in Coral Reefs. Sometimes light is more important and  
> sometimes temperature.
> It is also incorrect to say that "light levels are basically  
> unchanged". Changes in regional meterology occur as a result of  
> climate change which can lead to coral reefs being exposed to much  
> higher seasonal irradiance which can tip the balance between corals  
> bleaching or not. In addition, changes in sea level can be a very  
> important factor, particularly for shallow reefs. The underwater  
> light field is a lot more complicated, and affected by many physical  
> parameters (e.g., cloud cover, water surface roughness, sediment  
> levels, water depth, bottom type, zenith angle of the light field,  
> differential spectral attentuation, etc). Your statement about  
> surface light going down (evidence?) is far too simplistic to be  
> accurate.
> I know this is just Coral List but please can we be a bit more  
> scientific.
> Richard P Dunne
> Thomas Goreau wrote:
>>> From: Thomas Goreau <goreau at bestweb.net>
>>> Date: February 13, 2009 10:11:18 AM EST
>>> To: billraymond10 at yahoo.com
>>> Subject: Re: [Coral-List] Coral bleaching and ocean acidification
>>> Dear Bill,
>>> Absolutely! But it is a secondary factor.
>>> 20 years ago I did a bunch of experiments in Jamaica with corals in
>>> tanks at different temperatures and light level combinations.
>>> Bleaching took place only above a certain temperature, but the more
>>> light the faster it happened. Peter Glynn did similar experiments in
>>> Okinawa and reached the same conclusions. The reason I did this was
>>> our field observations in the first Caribbean bleaching event in
>>> 1987, where shaded corals were much less bleached, in fact where
>>> they were shaded by overhangs you could see the shadow, and we've
>>> seen that everywhere in the Indian Ocean and Pacific since too.
>>> But the discussion was about bleaching versus acidification, so I
>>> did not get into secondary factors affecting bleaching. And
>>> temperature has been rising worldwide whereas light levels are
>>> basically unchanged because solar radiation variations are so small.
>>> In fact surface light has been going down for decades because of
>>> atmospheric pollution, but now starting to rise slowly as
>>> atmospheric sulfur and nitrogen pollution are controlled.
>>> Hans Creek is an interesting area, and you may know that I proposed
>>> it as a coral reef restoration site. I still hope this is possible,
>>> but it really depends on blocking the marina and golf course. Which
>>> I guess is still a pending issue. There is a small chance that I may
>>> get back to BVI later this year.
>>> Best wishes,
>>> Tom
>>> Thomas J. Goreau, PhD
>>> President, Global Coral Reef Alliance
>>> Coordinator, United Nations Commission on Sustainable Development
>>> Partnership in New Technologies for Small Island Developing States
>>> 37 Pleasant Street, Cambridge MA 02139
>>> 617-864-4226
>>> goreau at bestweb.net
>>> http://www.globalcoral.org
>>> On Feb 13, 2009, at 9:57 AM, Bill Raymond wrote:
>>>> Tom Goreau:
>>>> Don't you consider solar intensity a factor in coral bleaching? In
>>>> Tortola I found bleached corals in late October, offshore, but
>>>> mangrove-shaded Siderastreas in warm Hans Creek
>>>> were not bleached.
>> _______________________________________________
>> Coral-List mailing list
>> Coral-List at coral.aoml.noaa.gov
>> http://coral.aoml.noaa.gov/mailman/listinfo/coral-list

More information about the Coral-List mailing list