Download Global Change and Coral Reefs

Global Change and Coral Reefs
(draft of text for presentation)
Joan A. Kleypas, National Center for Atmospheric Research, Boulder CO
Robert W. Buddemeier, Kansas Geological Survey, University of Kansas, Lawrence, KS
This is the rough text of this talk, and it will almost certainly differ from the actual
presentation in February. However, it should provide the main points that the authors
wish to convey during their talk.
INTRODUCTION
This talk follows that of Terry Done, which covered the local to regional impacts on coral
reefs. Now we will address the global impacts of climate change on coral reefs. I would
like to emphasize that the local to regional and global stresses on coral reefs do not
express themselves independently. We can sometimes walk up to a coral reef that
exhibits degradation and point the finger to some local stress (such as increases in
sediment load) or to some global stress (like warming), as the single cause, but usually
we’re looking at some combination of the two. However, it’s important to try to
determine the relative causes of coral reef decline, because that allows us to take action to
reverse that decline. And there is a trend in statements that are coming out of the coral
reef scientific community: that we should first address reducing the local to regional
impacts because 1) it’s feasible to do so (e.g., it’s easier to reduce nutrient runoff on reefs
than it is to slow down CO2 emissions); and 2) against a background of global stress, it
makes sense to minimize the local stresses acting on a reef. Here’s an analogy: when a
person’s health begins to decline, say due to some chronic illness, a doctor’s advice
always includes reducing any controllable stresses, such as smoking, eating habits, etc., at
least until he can find a way to tackle that chronic illness. If we consider global change
as a source of chronic stress, we would ideally like to cure that, but in the meantime, it
makes sense to reduce any of the other stresses that we can control.
As we pick up where Terry left off by reviewing how global change affects coral reef
ecosystems, I would like to put this in the context of the growing notion that climate
change is not likely to be smooth and predictable, and also that coral reef response is not
likely to be smooth and predictable.
SLIDE 1 (changes in coral reef topics over time)
To provide a quick historical perspective of how science has addressed climate change
and coral reefs, this slide displays an unofficial analysis of publications (using the ISI
index) on four major topics in coral reef science: Crown-of-thorns starfish (COTS)
infestations, coral disease, climate, and El Niño-Southern Oscillation (ENSO). This
graph illustrates that a mere twenty years ago, climate change was a rare topic in coral
reef research. Instead, there was a lot of concern about COTS. But over time, coral reef
publications have become increasingly focused on climate, particularly ENSO, and on
coral disease. Notice also that the COTS publications have dropped dramatically, despite
the fact that COTS remains a serious problem on many Indo-Pacific reefs. What this
graph shows is that climate change has become an increasing concern on coral reefs. It
also shows that coral disease has become a growing concern.
SLIDE 2/3 (how fossil fuel emissions affect coral reefs)
Global change here is defined as change due to increasing greenhouse gases, mainly CO2,
in the atmosphere. This chart illustrates the main ways that this affects coral reefs. The
best-known effect is global warming, which is the main problem behind the phenomenon
of coral bleaching; and also the driver behind sea level rise, which is likely to help some
reefs and to harm others. Another effect of increasing atmospheric CO2 is due to
changes in seawater chemistry. Adding CO2 to the atmosphere drives CO2 into the
ocean and makes it more acidic, and this is a very predictable change. This change in
ocean chemistry is important because there is good evidence that corals and other
organisms that build reefs secrete less calcium carbonate under such conditions. Finally,
there are a host of potential climate changes that are less certain to occur. These include
potential changes in storm activity, for example, which operate at a more regional scale;
and also potential changes in ENSO, which would certainly have impacts on coral reefs.
Here I will concentrate on two main consequences of increasing emissions on coral reefs:
1) increases in sea surface temperature (SST), and 2) changes in seawater chemistry.
SLIDE 4 (map of coral reefs on isotherms)
Coral reefs today occur mainly in the tropics, and obviously they are restricted to warm
waters. What’s interesting is that although they are limited by cold water, their
distribution doesn’t really tell us if they’re limited by warm waters. They occur even in
very warm waters.
SLIDE 5 (min versus max SST of coral reefs)
This demonstrates the same thing: the black dots on this plot show the minimum and
maximum temperatures for all of the shallow-water locations in the tropics. The red dots
show which of those dots are locations with reefs. It’s easy to draw a line where the
waters are too cold for reefs, but it’s not clear that we can draw a line where the oceans
are too warm. These warmest reefs are those from the Red Sea, and corals there can
tolerate unusually warm water. So one might come away from this graph thinking that
global warming is really not much of a problem for reefs, and that global warming will
actually be good for corals.
SLIDES 6/7 (the benefits)
In fact, many of the early predictions said just that. This plot shows the “limit” to coral
reef growth today, as defined by the 18°C isotherm. With global warming, that isotherm
will migrate poleward and expand the potential range for coral reef growth. There is
certainly evidence that this has happened in the past, and evidence that it’s happening
today (see Rich Aronson’s talk that follows this one). Unfortunately, that warming will
not likely lead to much expansion, and there is not a lot of area with suitable water depth
and other conditions that would allow reef growth.
SLIDE 8 (temperature rises are both chronic and episodic)
Instead of seeing potential benefits from global warming, what we have seen instead,
mainly, are problems. All of these problems have to do with coral health, and many are
related to both the chronic increase in background temperature (we normally think of
global warming as a gradual process), and the episodic increases in temperature.
SLIDE 9 (coral bleaching slide)
Coral bleaching has been the most dramatic and devastating result of sea surface
warming. Coral reefs scientists have known about coral bleaching for a long time, but
reports of bleaching had always been localized events. Corals happen to be sensitive to
changes in the environment and unusual changes in salinity, light or temperature, whether
it’s in your aquarium or in the field, will cause them to bleach. Despite the fact that we
have corals that are happy in 20°C water and corals that are happy in 30°C water, these
same corals can’t trade places and be happy. However, the large-scale coral bleaching
events, starting in 1982, was something of an “ecological surprise,” and the fact that coral
bleaching continued to increase almost year by year also was something of a surprise. I
believe the surprise was a combination of things: 1) that corals over very broad regions
and in all oceans could exhibit such a sensitivity to relatively small changes in
temperature, 2) that the temperature increases in the ocean were as high as they were, and
3) there were no indications in either recent observations or even folklore, that coral reefs
might experience such large-scale bleaching episodes.
Terry has already discussed coral bleaching to some extent, and I won’t repeat what he’s
covered. I’ll instead provide just a few bleaching statistics here, and to establish the link
with SST warming.
SLIDE 10 (bleaching statistics)
SLIDE 11 (bleaching thresholds)
This figure is from a recent paper in Science which was spearheaded by Terry Hughes of
James Cook University in Australia. It shows the most simplistic view of coral bleaching
response to SST, versus a more complex view. The top figure assumes that there is a
single threshold for coral bleaching which does not change through time – in other words,
there is no biological adaptation to warming temperatures. The middle figure illustrates
something much closer to what is actually observed: that is, each species on a coral reef
has a slightly different threshold to elevated SSTs. For example, the slower-growing
massive corals, like Porites, tend to have higher thresholds than faster-growing branching
corals, like Acropora. This graph says that, with continued rising of SSTs, we expect a
sequential loss of coral species on a reef, the less-tolerant ones disappearing first, and the
most tolerant ones hanging in there the longest. The bottom graph is slightly more
realistic still, because it allows some coral species to adapt to elevated SST with time. So
notice that while this species here starts off with a lower tolerance to SST than this
species, it has a higher capacity to adapt to increased SST and eventually becomes the
species more likely to survive. We can apply this concept – of different species having a
different capacity to adapt to increased temperature – to both real and modeled data.
SLIDE 12/13 (HADISST, CCSM)
Adaptation operates through a variety of mechanisms and on a range of time scales. If
corals are going to survive future changes in temperature, they need to adapt at a rate that
at least equals the rate of warming. Here is an illustration that tests two different rates of
adaptation: 25 years and 50 years. This approach was originally proposed by John Ware
(Ware, 1997), and it illustrates how the rate of adaptation versus the rate of SST warming
can determine whether a species is likely to suffer bleaching in the future. This graph
illustrates a species that has a time of adaptation of about 25 years – that is, its ability to
adapt is based on what it has experienced for the past 25 years. The black line is a
reconstruction of maximum annual SST for the past 150 years, produced by the Hadley
Center (HADISST, Rayner et al. 2003). The dashed line through that shows the 25-year
running average – this essentially mimics the average temperature the species has
“experienced” over the past 25 years. Mild bleaching (small stars) occurs if SST reaches
1°C above that 25 year average. Moderate bleaching (medium stars) occurs if SST
exceeds the norm by 1.5°C, and severe bleaching (large stars) occurs if SST exceeds the
norm by 2.0°C. This method accurately predicts the 1997/98 bleaching at all sites, and
the 1982/83 bleaching in the Galápagos. It also predicts bleaching at other times in the
Galápagos, and that seems to be reflected in the high variability in SST at this site.
Bleaching at these time periods are not documented, and in fact, the SST records prior to
the 1950s are based on fewer observations and so I wouldn’t put a lot of trust in these.
I’ve added to this graph the results of two model runs from the Community Climate
System Model (CCSM) at NCAR. The blue line is derived from a control run (one with
no anthropogenic greenhouse forcing) and the red line is from a run which simulates a
global warming scenario. These model runs sometimes get the temperature right, but as
you can see in the case of the Galápagos, sometimes not. But what’s more important here
is if the model captures the variability, and in all three cases, it does a pretty good job of
simulating that, although it doesn’t quite simulate events with the intensity of the 97/98
El Nino. But to illustrate the importance of adaptation here, we can use the model output
to predict future bleaching in this coral species, using the same 25-year adaptation criteria
as before; and there are periods of bleaching predicted for all three sites. Now, if we
change the period of adaptation to 50 years, the future looks worse; here the bleachings
occur fairly commonly.
SLIDE 14 (coral diseases)
Another recent problem with corals that is thought to be linked to temperature rise is that
of coral diseases. The Caribbean reefs have so far been much more affected by disease
than reefs in other others. But regardless of which ocean you’re working in, there has
been an increase in the number of diseases known to affect corals, as well as an increase
in the outbreaks of disease. The main reef builder in the Caribbean, a fast-growing
branching species, has been devastated by white band disease in many parts of the
Caribbean (Rich Aronson will address this in more detail). How does climate change
play a role in coral disease? First, corals that bleach are weakened and more susceptible
to pathogens. Second, there is strong evidence that when temperatures are warmer than
normal, regardless of whether they bleach or not, their fat reserves become depleted and
that leaves them less resilient to disease. Third, pathogens tend to thrive in warmer
water.
SLIDE 15 (is there an ocean thermostat?)
Is there any hope? There is one aspect of SST warming that is not always considered,
and that is the fact that in the open ocean, at least, there is a limit to how warm the oceans
can get. There is some evidence that the Pacific Warm Pool is already near that
maximum, which is believed to be about 30-31°C. So for those corals already in hot
water, there may be feedbacks that will keep those waters from getting a lot warmer. If
this is indeed the case, then we need to make sure that we include these areas in future
strategies to protect coral reefs.
SLIDE 16 (shift to seawater chemistry changes)
A second major aspect of climate change is that of seawater chemistry. We are
increasing the atmospheric pressure of carbon dioxide, and that in turn drives more CO2
into the ocean.
SLIDE 17/18 (carbonate chemistry tutorial)
This diagram illustrates how that process affects seawater chemistry. In the first stage,
CO2 combines with water to form carbonic acid. But carbonic acid is a weak acid and it
sheds a hydrogen ion rather quickly to form a bicarbonate ion, and this can dissociate
even further to form the carbonate ion. All three of these species: carbonic acid,
bicarbonate, and carbonate, occur simultaneously in seawater (together they constitute
“total CO2” or TCO2, but the relative proportions of each change in order to balance the
positive ions in the ocean, called the alkalinity. The higher the alkalinity, the more the
carbonate system shifts in favor of carbonate ion concentration. As a first approximation,
the carbonate ion concentration equals the difference between the alkalinity and the total
CO2 concentration. This shows just how those concentrations change between
preindustrial levels of atmospheric carbon dioxide, and a doubling of those levels. The
carbonate ion concentration decreases by about 30%. This is important, because it’s the
carbonate ion that is used in the formation of CaCO3 skeletons of corals and other reefbuilding organisms such as calcareous algae. Notice also how pH decreases. This looks
like a small increase, but recall that pH is a log10 representation of the actual value. This
table does not take into account the effect of temperature on CO2 chemistry in seawater.
SLIDE 19/20 (combined effects of pCO2 and temperature on [CO32–])
This slide shows the CO32– ion concentration under the combined effects of partial
pressure of CO2 and sea surface temperature. Over the last few million years, coral reefs
have had the luxury of high [CO32–] levels – this box delineates the conditions they’ve
experienced over the glacial/interglacial cycles. This box shows where the ocean
chemistry is headed – and it is obvious that the chemistry conditions will be far outside
the range under which corals have experienced for many years.
SLIDE 21 (photos of vulnerable marine calcifiers)
So why does this matter? I mentioned earlier that these changes in seawater chemistry
are likely to affect calcification in organisms that secrete CaCO3. Four major groups now
show these effects: the top two live predominantly in the open ocean as plankton:
coccolithophores and forams; the bottom two, calcareous algae and corals, are the main
builders of coral reefs. Almost all of the experiments to date are done within some kind
of laboratory setting, where the seawater chemistry is manipulated to simulate future
conditions, and nearly all have consistently shown that calcification rates are likely to
decrease in the future.
SLIDE 22 (calcification versus [CO32–] for coccolithophores and forams)
These graphs show the calcification rate versus carbonate ion concentration for
coccolithophores and forams.
SLIDE 23 (calcification versus [CO32–] for corals and coral communities)
These two graphs show the effects on individual coral species (most of this work was
done by Francesca Marubini, Marubini et al. 2003), and on coral communities (work in
the Biosphere 2, Langdon et al. 2000; 2003); and work in the Monaco mesocosm,
LeClercq et al. 2000, 2002). Individual coral species tend to show about a 15-30%
reduction in calcification under doubled CO2 conditions, while coral mesocosms tend to
show a stronger response, up to 50% reduction, and that probably reflects the fact that
these mesocosms include coralline algae, which seem to show a stronger calcification
response than corals do.
SLIDE 24 (“corals build reefs”... the calcium carbonate budget on a reef)
SLIDE 25 (reef growth versus sea level rise movie)
CaCO3 accumulation has been extremely important in the ability of coral reefs to keep up
with sea level rise. This little movie starts during the Last Glacial Maximum (18,000
years ago), and shows reef growth in response to sea level rise during that time. The four
posts provide the perspective of depth, and each represents 120 m. As the movie
progresses, reefs begin to grow upward – some reefs accumulate calcium carbonate fast
enough to keep up with sea level, and some don’t (those that don’t will turn white).
There are actually ten different coral types in this model, but only two really show up
well in this simulation: the massive corals, colored green, and the fast-growing branching
corals, shown in light blue. Watch what happens as sea level rises (run movie). What we
see are that the branching corals are the big contributors to the reef growth and that they
dominate the building of the reef. If this type of coral had been killed off, or if their
growth rates would have been 30% less, then many more of these reefs would have
“drowned” during sea level rise. Our barrier reef systems that we have today would look
very different. Fortunately, predictions of sea level rise within this century are on the
order of a meter, which is much less than the 120 m of the past 18,000 years.
SLIDE 26 (coral calcification and reef building)
So does calcification rate matter if reefs don’t have to worry about sea level rise? I
believe the answer is yes, at both the organism level and the reef level. We’ve yet to
conduct experiments on how a change in calcification will affect individual organisms.
Do they experience something akin to osteoporosis? Do they grow more slowly? And
reduced carbonate production (as well as increased dissolution in the more acidic
environment) reduces the CaCO3 budget on coral reefs. Does this mean that reef building
will slow down or reverse? The following movie illustrates two things: the importance
of corals to reef building, and particularly, the importance of certain fast-growing species.
SLIDE 27 (historical records of atmospheric CO2 concentration)
Finally, if we want to know where we’re going, we need to know where we’ve been.
These graphs represent the Earth’s atmospheric CO2 concentration, as determined by (top
graph) ice cores, and (bottom graph) various proxies. The top graph shows the excellent
record obtained by Petit et al. (1999) from the Vostok ice core. This record goes back
about ½ million years and shows about four glacial cycles. Throughout that time,
atmospheric CO2 remained within an envelope of about 200 to 300 ppmv. Today,
atmospheric CO2 concentration is about 370 ppmv, and it’s rising, with a projected
doubling of atmospheric levels by the middle of this century. CO2 data extending back
millions of years are scarce, but scientists have come up with some values based on the
“leaf stomatal index” of fossil leaves (Royer et al. 2001), and various marine isotopes
(Pearson and Palmer 2000). These tend to indicate that atmospheric CO2 concentrations
have remained below present values for about 24 million years. This illustrates the
statement that we are headed into a period for which there is no good analogue.
SLIDE 28 (the likelihood of climate surprises)
Despite the relatively small changes in atmospheric CO2 during the glacial/interglacial
fluctuation, climate sometimes changed rapidly. This figure is from a recent paper by
Robert Alley and others in Science. What this figure shows are two periods of rapid
climate change – the Younger-Dryas and the Little Ice Age – that show up in various
records across the globe. Such rapid climate changes are thought to be linked to changes
in ocean thermohaline circulation (THC). The very recent IGBP publication “Global
Change and the Earth System: A planet under pressure” (Steffen et al. 2004) supports the
growing view that climate change is likely to be bumpy, rather than smooth. That is,
there will most certainly be some climate “surprises” in our future.
SLIDE 30 (the likelihood of ecological surprises)
Coral bleaching is one example of an “ecological surprise.” It illustrates how thresholds
to climate change exist in organisms, and that we don’t always anticipate what these
thresholds are. We are prepared for rapid ecological responses to rapid climate change,
but ecological surprises are likely even when climate change is gradual. That is because
of the complex nature of ecosystems, and the inability to predict where the thresholds are
within ecosystems.
SLIDE 31 (conclusions)
1. Projected changes in climate will drive temperature and seawater chemistry to levels
outside the envelope of what coral reefs have experienced for many thousands of
years.
2. This hinders our ability to predict how coral reefs will respond to future climate
change – there is no analogue.
3. We should expect surprises in how coral reefs will respond:
a. surprises have already occurred
b. climate surprises are likely
c. ecological systems are complex and thresholds are difficult to predict
4. Our “best” predictions are that climate change will lead to major changes in coral reef
ecosystems, both biologically and geologically
a. species assemblages will change
b. geological functioning of coral reefs will change
5. The local stresses on reefs are likely to accelerate the effects of global change on
coral reefs.
REFERENCES
Hughes TP, AH Baird, DR Bellwood, M Card, SR Connolly, C Folke, R Grosberg, O
Hoegh-Guldberg, JBC Jackson, J Kleypas, JM Lough, P Marshall, M Nystrom,
SR Palumbi, JM Pandolfi, B Rosen and J Roughgarden. 2003. Climate change,
human impacts, and the resilience of coral reefs. Science 301: 929-933.
Langdon, C., Broecker, W. S., Hammond, D. E., Glenn, E., Fitzsimmons, K., Nelson, S.
G., Peng, T.-S., Hajdas, I., and Bonani, G.. 2003. Effect of elevated CO2 on the
community metabolism of an experimental coral reef. Global Biogeochem.
Cycles 17:1011, doi:10.1029/2002GB001941.
Langdon, C., Takahashi, T., Sweeney, C., Chipman, D., Goddard, J., Marubini, F.,
Aceves, H., Barnett, H., and Atkinson, M. J. 2000. Effect of calcium carbonate
saturation state on the calcification rate of an experimental coral reef. Global
Biogeochem. Cycles 14: 639–654.
Leclercq, N., Gattuso, J.-P., and Jaubert, J. 2000. CO2 partial pressure controls the
calcification rate of a coral community. Global Change Biol. 6:329–334.
Leclercq, N., Gattuso, J.-P., and Jaubert, J. 2002. Primary production, respiration, and
calcification of a coral reef mesocosm under increased CO2 partial pressure.
Limnol. Oceanogr. 47:558–564.
Marubini, F., Ferrier-Pages, C., and Cuif, J. P. 2003. Suppression of skeletal growth in
scleractinian corals by decreasing ambient carbonate-ion concentration: a crossfamily comparison. Proc. Roy. Soc. London Ser. B 270:179–184.
Pearson PN and MR Palmer. 2000. Atmospheric carbon dioxide concentrations over the
past 60 million years. Nature 406, 695-699.
Petit JR, J Jouzel, D Raynaud, NI Barkov, J-M Barnola, I Basile, M Bender, J
Chappellaz, M Davis, G Delaygue, M Delmotte, VM Kotlyakov, M Legrand, VY
Lipenkov, C Lorius, L Pepin, C Ritz, E Saltzman, and M Stievenard. 1999.
Climate and atmospheric history of the past 420,000 years from the Vostok ice
core, Antarctica. Nature 399: 429-436.
Rayner, N. A., Parker, D. E., Horton, E. B., Folland, C. K., Alexander, L. V., Rowell, D.
P., Kent, E.C., and Kaplan, A. 2003. Global analyses of sea surface temperature,
sea ice and night marine air temperature since the late nineteenth century. J.
Geophys. Res. 108(D14): Art. no. 4407.
Royer, D. L., Wing, S. L., Beerling, D. J., Jolley, D. W., Koch, P. L., Hickey, L. J., and
Berner, R. A. 2001. Paleobotanical evidence for near present-day levels of
atmospheric CO2 during part of the Tertiary. Science 292:2310–2313.
Steffen, W., Sanderson, A., Jäger, J., Tyson, P.D., Moore III, B., Matson, P.A.,
Richardson, K., Oldfield, F., Schellnhuber, H.-J., Turner II, B.L., Wasson, R.J.
2004. Global Change and the Earth System: A Planet Under Pressure. Springer
Verlag, Heidelberg, Germany. 336 pp.
Ware JR. 1997. The effect of global warming on coral reefs: Acclimate or die.
Proceedings of the 8th International Coral Reef Symposium, Panama, 1: 527-532.