Limiting global warming to 2 deg C is unlikely to save most coral reefs: a discussion

The new paper in Nature Climate Change by Katja Frieler, a number of other colleagues and I warns that limiting global warming to 2 deg C Celsius, the threshold long-discussed at the policy level and now enshrined in the Cancun Accord, is unlikely to be enough to spare most coral reefs from climate change.

The paper might look like the same old, same old from scientists about climate change and coral reefs, a bunch of sky-is-falling pessimists out to spoil the release of the 3D version of Finding Nemo. Certainly, go ahead and see the movie, it provides a pretty scientifically accurate picture of life on a “healthy” coral reef in terms of the oceanography and the behavior of reef organisms (except for the small matter of the talking fish, and, really, if a Great Barrier Reef clownfish did evolve the power of speech, wouldn't it have an Aussie accent?).

It's worth also learning about what is new in this paper, and what we understand about the threat coral reefs face.

The general findings of Frieler et al. might not come as a surprise to people following the scientific research on climate change and coral reefs. The paper does, however, employ some important new methods and offer valuable new analyses. I'll point to three keys:

1. The focus on "global" temperature thresholds

Frieler et al. relates the projected frequency of heat stress events – what I often call ocean “heat waves” – in coral reefs worldwide – that can cause coral bleaching to global mean temperature change, the metric discussed so often in policy circles, the public and the press.  In past studies, including several of my own (Donner et al., 2005, 2007), we estimated the frequency of bleaching events under different future greenhouse gas scenarios. In those studies, we are able to show the difference in the outcome for coral reefs between futures with different levels of atmospheric carbon dioxide (and other greenhouse gases). For example, my “Coping with Commitment" (open access) paper concludes  with a discussion of the level of atmospheric CO2 necessary to avoid one definition of "dangerous" impacts to coral reefs.


In those past analyses, though, if you wanted to know what the coral reefs picture looked like under some specific global mean temperature increase – like, say, the proposed 2 deg C threshold – you’d need to take apart the results and find the point in the emission scenarios were global average warming reaches (or, depending on your question, stabilizes) at that level. In response to a request in advance of the UN Copenhagen climate summit in 2009, a group of colleagues and I did that analysis (pdf) for the Caribbean and found a clear difference between the coral reef outcomes in +1.5 C and a +2 C world. Frieler et al is the first paper to do a comprehensive analysis of coral reef outcomes under different temperature thresholds for the whole planet.

2. More robust analysis of uncertainty

Frieler et al. is based on output from a huge range of climate models and scenarios, unlike previous work which employed only one or a few models. The study employed output from 19 different climate models running up to seven different scenarios, working to 32,000 model “years”. Are the numbers just inside baseball? Actually, the approach is important for two reasons. First, the vast quantity of information about the possible futures allowed the team to essentially invert the results; instead of looking at the coral reef results in the individual scenarios, the climate model output was all combined together to create a relationship between global average temperature change and the temperature change at all the reef sites around the world. This enabled the previously mentioned analysis: rather than look at the bleaching frequency for different reefs under different emissions scenarios, this study was able to look at the bleaching frequency for different reefs based on the global mean temperature change.

Second, with so many models included in the analysis, the study was also able to calculate a more complete range of uncertainty in the results. My past studies, for example, employed output from individual models which were determined, from analysis, to best represent the month-to-month and year-to-year variability in tropical ocean temperatures (important since coral bleaching is a response to unusual warmth, or “heat waves”). The approach is quite sensible, but naturally led to the question: what do the other models say? Frieler et al. answers that question, and the answer is that, in general, the results are robust across all climate models.

3. Influence of ocean acidification

Frieler et al. estimates the possible influence of ocean acidification on coral bleaching. Past studies, including my own, have roughly looked at the potential “positive” effect of thermal adaptation by individual coral species (physiological acclimation or actual genetic adaptation to warmer temperatures) or communities of coral species (the tough species survive, etc.) on future projections of coral bleaching and reef health. Until now, however, they have not looked at the possible inverse effect of rising levels of carbon in the ocean. There is some experimental and observational evidence that with more carbon dioxide dissolving in the ocean, corals may bleach at lower temperatures (e.g. Anthony et al., 2008). This not a certain outcome in every, or any, case; research on synergies between bleaching and acidification is still ongoing.

So in Frieler et al., we do a sensitivity analysis: how would this opposite effect of ocean acidification on bleaching thresholds affect the future bleaching projections? The general answer is, not very much, because by time the acidification effect is large enough to substantially alter the bleaching thresholds, most coral reefs are already experiencing dangerously frequent bleaching events. I recommend digging into the online supplement that accompanies the article for more on this subject.

So what?


In the end, perhaps what’s most striking about the study is that despite many methodological differences from previous work, the results are not surprising. In that sense, the paper shows how robust the overall climate forecast is for coral reefs.

Frieler et al. also points to the importance of identifying the corals, habitats or entire reefs that are more resistant (can withstand a heat wave) or more resilient (can recover from a heat wave) than the norm, figuring out what exactly makes them tougher, and then targeting conservation at reefs with the right characteristics. This was the subject of a recent analysis by McClanahan et al. in PLoS-One (open access), and is the inspiration for my field program in Kiribati.

Read More...

Summary only...

Are Caribbean coral reefs on the verge of collapse?

The Guardian reported yesterday that Caribbean coral reefs are "on the verge of collapse": 

The decline of the reefs has been rapid: in the 1970s, more than 50% showed live coral cover, compared with 8% in the newly completed survey. The scientists who carried it out warned there was no sign of the rate of coral death slowing.

Coral reefs in the Caribbean certainly are, in a mean sense, in a serious state of decline. The presentation of the data in the Guardian, however, could lead to confusion.

The data is based on the preliminary report (pdf) of a Global Coral Reef Monitoring Network (GCRMN) workshop held earlier this year. The graph at right, taken from the report shows the decline in "percent coral cover" averaged across all reefs with observations in the GCRMN data (black) and an earlier study by Gardner et al. The percent cover dips to ~8% at the end of the longer GCRMN dataset.

That means that the average percent coral cover at the reef sites in the dataset is 8%. It does not mean that less than 8% of reefs had any live corals.

This two statements are the difference between the optimist saying glass is half-full and a mistaken pessimist saying only half of the glasses have any water at all. If less than 8% of the reefs in the Caribbean featured any live coral, then Caribbean coral reefs would not be on the verge of collapse. They would have already collapsed.

This is merely a potential (mis?)interpretation of the wording of the article. The greater concern about the Guardian coverage is that it appears to have missed the three key findings listed right in the Executive Summary. Here's a snapshot:

The first two points stress that the status of coral reefs varies greatly across the Caribbean, from the relatively high cover reefs of the Cayman Islands to the lower cover in places like the US Virgin Islands, which were affected by recent bleaching events. The headline should be that "some" Caribbean coral reefs appear to be on the verge of collapse.

This matters. The sites that have bucked the downward trend just might be able to teach us something about how coral reef resilience to climate change and local disturbance, and maybe even give us some insight into management and marine park design. That's point #3, and the one of the major inspirations for doing this type of analysis in the first place.

Read More...

Summary only...

Banning the trade of shark fins

A note unrelated to climate change: The NY Times reports that the state of California is considering a ban on the sale and possession of shark fins. Frankly, it is hard to believe that dealing in shark fins is not already illegal in North America. There is no better example of a cultural obsession with a particular "delicacy" - in this case, shark fin's soup - contributing to the demise of a wild population.

I took the picture at left just a couple weeks back in a market on the Pacific Coast of Central America. Frankly, it the photo could be from the Indian Ocean, the Caribbean or the western Pacific, I've seen the same thing on the coasts of all the tropical oceans.
You'll notice the fin-less sharks in the bin are all small: the larger sharks are dumped at sea after being "finned". The fin is worth so much more than the meat that there's no value in bringing a big heavy dead shark back ashore, even though it is obviously a potential food source. In Kiribati, people do catch sharks and sell the fins, but at least they also take the shark home to eat.

You don't have to be the least bit interested in marine conservation or the dwindling global shark population to see the practice as finning and leaving the shark behind as ridiculously wasteful. Perhaps a ban of shark fishing may be politically or logistically impossible in today's world, I don't know. I can say that if we are going to continue to catch sharks for human consumption, let's at least catch the whole shark.

Read More...

Summary only...

Summary of Caribbean Bleaching in PLoS-One by Eakin et al.

The new paper by Eakin et al. in PLoS-One (open access) summarizes the extent of the 2005 coral bleaching event in the Caribbean, an event discussed at length on Maribo. The paper includes data from dozens of coral reef sites across the Caribbean that were affected by the prolonged period of water water in the late summer and fall of 2005, the same unusual warmth which helped promote the strong Atlantic hurricane season. 

The figures at right give an idea of the scale of the event, and the scale of the data collected by Eakin et al. The top panel shows the heat stress ("degree heating weeks") experienced by reefs across the Caribbean; the lower panel summarize the percent of corals in each region that bleached (data is collected in some cases by counting number of bleaching colonies, in others by estimating the percent of total coral cover that bleached). The figure shows that bleaching tended to be the most extensive in the areas that experienced the greatest heat stress, particularly the core of the "hot spot" in the Lesser Antilles.

We've seen a near repeat of this event in the past few months; the effect of the follow-up event on living coral cover won't be known for some time. From a climate standpoint. The fact that it has happened again five years later is, in itself, remarkable. Eakin et al. report that the sea surface temperatures in the fall of 2005 were the warmest since records began in the mid-1800s. The new record appears to have only lasted five years.

Read More...

Summary only...

Climate change and the Caribbean coral bleaching, again

There's an upswing of skepticism about climate change of late, thanks in part to the crazy season in the US (and I'm not talking about the baseball playoffs, although they have been pretty surprising too). While the pundits argue, the climate keeps on moving.

When I give talks about climate change and coral reefs, I almost always use the two slides on the right. The first slide is a map of "degree heating weeks" (DHW), a measure of accumulated heat stress experienced by corals, in the Caribbean in mid-October in 2005. Severe bleaching and coral morality is typically observed when the values of DHW exceed 8 deg C-week. Basically, the same long period of warm water temperatures that helps spawn the destructive 2005 Atlantic hurricane season caused unprecedented coral bleaching event in the eastern Caribbean.

In 2007, my colleagues and I published a study examining of the likelihood of the 2005 "hot spot" occurring with and without human influence on the climate system. The study contrasted model simulations of the Caribbean with historical data and then computed the statistics of extreme ocean temperature events. The second slide summarizes some of the key results of from study. In a nutshell, our best analysis concluded the 2005 Caribbean "heat wave" would likely be on the order of a once in a thousand year event, had there been no human-generated greenhouse gas or aerosol emissions since the Industrial Revolution ("natural forcing"). By the 1990s, the human forcings increased the odds to once in 10-50 years. And continued warming under "business as usual" would make such heat waves happen in three out of every four years.

Five years later, a Caribbean "heat wave" has happened again. I've been writing for months that there was a strong likelihood of extensive coral bleaching in the Caribbean this fall according to NOAA's advance forecast of sea surface temperatures (in fact, we had a good inkling of this last summer). Now we're getting reports of bleaching from observers in the Caribbean. Add this to the observations (following predictions, once again!) from Southeast Asia and the Equatorial Pacific, and we have what may be the most, or second most, extensive "global" coral bleaching event in recorded history.

For all those writing about this event, keep in mind the predictions. This is what the scientific community predicted was likely to happen. An event which we calculated would be a once in a millennium occurrence without human impact on the climate, happened again five years later.

There are caveats, for sure. There is uncertainty in the model simulations of interannual variability which can affect the specific calculations of odds (see the 2007 paper for some details). And once-in-a-thousand year events can in reality happen five years apart; we'd need to collect data for thousands and thousands of years to properly calculate the statistics. Obviously, that's not feasible, which is the very reason we have computers help us do the math on these problems.

The real climate doesn't care about the political climate.

Read More...

Summary only...

The second "global" coral bleaching event

The NY Times has an article on the observations of coral bleaching across the planet over the past 12 months, an subject I've raised here in the past few months.

The NY Times article calls the concurrence of events "only the second known global bleaching of coral reefs". Though the characterization is awkward and not terribly accurate, as the bleaching is not really "global", it does point to the right comparison. The spatial extent of coral bleaching in 2010 is likely to be second to that observed in 1998, which people have taken to calling the first "global bleaching event". It is no coincidence that these "global" events occurred during two of the warmest, if not the two warmest, years in observed history.

It is also no coincidence that both 1998 and 2010 began with strong El Nino conditions, which later flipped to La Nina conditions. In a simplistic sense, that "maximizes" the area of ocean which experiences anomalous warmth that tends trigger mass coral bleaching events. This year, the El Nino event causes anomalous warmth in the eastern and central Equatorial Pacific. The migration of the West Pacific Warm Pool back westwards, as El Nino subsided, helped cause the high temperatures and bleaching in Southeast Asia and the Western Pacific. Atmospheric teleconnections may be responsible for some of the anomalous warmth in the Caribbean, as has been suggested in past studies. And as the article indicates, the recent switch to La Nina conditions is increasing concern about bleaching in the southwest Pacific, including the Great Barrier Reef, over the southern hemisphere summer.

One area of uncertainty in the science community is to what extent can coral reefs which have experienced bleaching in the recent past, whether 1998 or more recently, can become more resilient to future temperature stress. The year 2010 may serve as a great biophysical experiment.

[UPDATE: The Atlantic and Gulf Rapid Reef Assessment is inviting divers in the Caribbean to join them in "monitoring simple, ecosystem-
level pigmentation changes in live corals and any associated changes
in live coral cover using the newly updated BLAGGRA Line Transects
protocol (www.agrra.org/BLAGRRA). Sites can be very quickly and
repeatedly surveyed by small teams of 1-2 experienced divers. A
representative assessment can be made of reefs in the area affected by
bleaching, and/or sampling can be focused on special-interest sites
(such as within and outside of MPAs)"]

Read More...

Summary only...

A year of extreme weather: Is more bleaching to come in the Caribbean?

This year, which is on pace to to be one the warmest years, if not the warmest year, in the instrumental record, has seen a number of severe heat waves like the intense event in Russia. 

The heat waves have not only been on land. I mentioned a few weeks ago that high ocean temperatures has led to widespread coral bleaching over the past year, including occurrences in the Central Equatorial Pacific and Indonesia.

More may be to come. This is the NOAA Coral Reef Watch bleaching outlook for the fall, based on current temperatures and an experimental long-term forecast. It calls for the possibility of "Alert Level 2" thermal stress, which in general tends to cause mass coral bleaching and some coral mortality (in reality: the response is highly variable in time and space). If the forecast is correct, observations of bleaching would probably start to happen sometime in September.

Read More...

Summary only...

2C or not 2C: Copenhagen and global temperature threshold (Part II)

In the previous post, we discussed the origin of the 2 deg C threshold for dangerous impacts of climate change.

An example is happening at Copenhagen. The Association of Small Island States (AOSIS) and the Caribbean nations are lobbying for establishing a warming limit of 1.5 C rather than 2C. The real-world value of setting a temperature threshold notwithstanding, the AOSIS position demonstrates that the acceptable limit of warming is a value judgment based on scientific results rather than an immutable law of nature.

Why 1.5 degrees C, rather than 2C? The argument is based on the effect that sea level rise (and storm surge height) and coral reef degradation from bleaching and acidification would have on the economy and society.

Read More...

Summary only...

Hurricane Bill, the Atlantic hurricane season and Pacific warming

The first Atlantic hurricane of the season, Hurricane Bill, is on its way north towards the east coast of Canada (and creating huge waves in Bermuda and the northeastern US). Forecasters expect Bill to pass the coast of Nova Scotia on Sunday and run towards Newfoundland and Labrador.

The offshore path of the first and only hurricane of the Atlantic season brings to mind an interesting paper published earlier this summer, that warrants more attention than (I think) it received. In this post a few weeks back, before I disappeared for the Ontario leg of the Canadian Summer of ’09 heat wave tour (ah, Rex Murphy, what happened to global cooling?), I briefly mentioned the paper by Kim et al. that looked at the response of Atlantic hurricane activity to different types of El Nino events.

The conventional thinking is that Atlantic hurricane activity is low during El Nino events. Basically, the increase in eastern and central equatorial Pacific Ocean temperatures that happens during El Ninos shifts upper-level atmospheric circulation, which in turn, creates wind shear in Atlantic that disrupts hurricane activity.

Kim et al. went an important a step further. They found that the relationship depends on the nature of the Pacific warming.

The above figure tells the story. When the warming occurs throughout the eastern equatorial Pacific (EPW), the more classic El Nino, there are significantly fewer hurricanes in the in the Caribbean and up the eastern seaboard of North America. When warming is centered in central equatorial Pacific (CPW), as happened in 2002 and 2004, there actually is a significant increase in hurricanes reaching North America. Central Pacific warming events basically cause less vertical wind shear.

The ongoing development of El Nino conditions in the Pacific is the major reason for predictions of a less active than normal hurricane season in the Atlantic (and an active eastern Pacific season). That Pacific surface temperature anomaly is currently centered in the eastern Pacific, which suggests that all other factors being equal, which of course they never are, there is a higher likelihood of hurricane track density depicted in Part A of the figure above.

Now one hurricane does not a season make: you could argue that Bill's track is loosely bucking that prediction. Nonetheless, it will be interesting to follow hurricane development this year, and during the next episode of central Pacific warming, to see whether Kim et al. are correct in asserting that location of all those flapping butterflies in the Pacific predictably determines the development of storms in the Atlantic.

Read More...

Summary only...

El Nino and the likelihood of mass coral bleaching

The seasonal forecasting system recently developed by NOAA Coral Reef Watch suggests that coral reefs in the Caribbean and in part of the central equatorial Pacific (the Line Islands, including Kiritimati or "Christmas Island") are at risk of coral bleaching in the coming months due to warm ocean temperatures. The forecast is due to the seeming return of El Nino in the Pacific.

The word from the NOAA Climate Prediction Center is that El Nino conditions are expected to prevail though the winter. Climate buffs can "see" the El Nino development in maps of sea surface temperature anomalies (warm water in the central and eastern Pacific) the thermocline (deepening in the eastern Pacific, meaning less upwelling off of South America) and sea surface height (the wind reversal on the equator means higher water in the central and eastern Pacific).

Not all El Nino events are created equal, and the models currently disagree on the current trajectory. This is especially important to remember when predicting the effect that El Nino conditions may have on other parts of the planet, what we luddites call "teleconnections". For example, a terrific paper by Kim et al. in Science demonstrated clear differences between the effect of "central pacific warming" and "eastern pacific warming" - which can both be classified as an El Nino event, depending what metric is applied - on ocean temperatures and hurricane tracks in the Caribbean.

One silver lining: For people living in the central Pacific, especially the parched islands of the southern Gilberts (Kiribati) Islands like Arorae, the development of El Nino conditions hopefully also means an end to the two-year drought that has claimed many of the coconut trees.

UPDATE: More on the Caribbean bleaching threat here

Read More...

Summary only...

Photo of the week

In the past year, I've heard from a couple colleagues who work in the Caribbean about the large spread of lionfish, a colorful reef fish native to the Indo-Pacific. It is thought that a few lionfish were accidentally introduced through a broken tank at a large private aquarium in Miami and/or by some hobby aquarium owners.

You can find them now in many parts of the Caribbean, apparently including Cuba, where I spotted this fellow under a rock a couple weeks back. I'm definitely not an expert on lionfish, but you don't need to be an icthyologist to spot this aptly named species!

A welcome new addition to Caribbean reefs? Not quite. As if Caribbean coral reef ecosystems were not already under enough pressure, lionfish are invasive predators that are capable of removing much of the native reef fish stock and altering ecosystem dynamics [any experts wish to elaborate in the comments?].

If you are interested, the US Geological Survey has a terrific Google Map showing all the reported sightings across the Caribbean, including the recent sightings in the Florida Keys.

Read More...

Summary only...

Accidents in the Arctic

This summer has seen a record number of boats - cruise ships, commercial vessels, military ships, and boats full of scientists - in the Arctic. Aware that the downward trend in ice cover is prompting an increase in boat traffic, the Canadian military decided to rehearse for any possible accidents:

Beginning tomorrow , the army, navy and air force will begin Operation Nanook 08, the latest in a series of manoeuvres designed to boost Canada's Arctic sovereignty and increase the military's ability to respond to emergencies.

Operation Nanook will simulate an outbreak of disease on a cruise ship, a hostage-taking on a cruise ship, a fuel spill and a fire on a Russian cargo ship.

Are the exercises necessary? More than you might think. Apparently the opening of the Northwest Passage is already drawing yachtees tired of the Caribbean:

A total of 26 commercial cruises are planned in the Canadian Arctic this season, a historic high and an increase of four trips over last summer. As well, at least eight private vessels are thought to be sailing in and around the Northwest Passage.

Franklin would be jealous.

Read More...

Summary only...

Corals return to Bikini

This has not been a good news year for the world’s oceans. Scientists found an increase in the number of dead zones, documented acidification along the Pacific coast of North America, produced a startling map of the human impact on the global oceans, warned about the decline of Caribbean coral reefs, and detected an increase in the unproductive mid-ocean "deserts". The UNEP Rapid Response Report on the status of the world's fisheries, released in February, had the ominous title "In Dead Water".

For a change, here is a story about the recovery of a marine ecosystem from the most extreme of disturbances. A team of scientists conducted a survey of the coral communities of infamous Bikini Atoll in the Marshall Islands. The results were published earlier this year in a fascinating paper by Richards et al. in the journal Marine Pollution Bulletin that I finally had the chance to read.

Between 1946 and 1958, the U.S. government conducted 23 nuclear tests at Bikini Atoll. You can easily see some of the impact craters using Google Earth the photo at right is a 23 kton test in 1946). The Bikinians were forcibly removed, a story that cannot be told enough.

The Bikinians today live on Majuro Atoll, the capital of now independent Marshall islands. I took this photo of the displaced Bikini Town Hall during a short stay in Majuro a few years back. (It's a a long story; if your only way home involves the only plane of Air Nauru, a bankrupt airline from a rapidly depopulating island nation, you do what the ticket says).

The experience of the Bikinians – or the people of Rongelap, or the Banabans forced to buy an island in Fiji, or the i-Kiribati moved to the Phoenix Islands and back – is a lesson to all those who think it would be “easy” to move communities from islands threatened by climate change. Forced resettlements, whether due to colonial management, World War II, or environmental degradation, have been in many cases disastrous for those leaving their home, those already living in the new home, and the more powerful actors “orchestrating” the migration. But that's an issue for another day. Back to the corals.

A taxonomic study before the nuclear tests found 172 species of hard corals. Richards et al. equated this to 126 species according modern coral taxonomy. The nuclear tests, no surprise, devastated the coral communities. From Richards et al.:

Post-test descriptions of environmental impacts include: surface seawater temperatures raised by 55,000 C after air-borne tests; blast waves with speeds of up to 8 m/s; and shock and surface waves up to 30 m high with blast columns reaching the floor of the lagoon... Coral fragments were reported to have landed on the decks of the target fleet deployed within the lagoon... The most publicized of the Bikini tests, ‘Bravo’... destroyed three islands causing millions of tonnes of sand, coral, plant and sea life from Bikini’s reef to become airborne. The sediment regime in Bikini was fundamentally altered by the nuclear events because millions of tonnes of sediment were pulverized, suspended, transported and then deposited throughout the lagoon by wind-driven lagoonal current patterns.

Five decades later, Richards et al. found 183 species of hard corals living around Bikini (note: the increase from the pre-tests results is likely due to more thorough surveys and better taxonomic knowledge). It is a remarkable recovery considering the complete devastation caused by the nuclear tests. The study concluded that 28 species were lost. Most of those were calm (lagoon) water specialists, whose habitat was most severely and permanently disturbed by the nuclear tests.

The authors find a tragic irony in this tale of recovery:

If the disturbance event were to be repeated in the modern day, recovery would not be expected to be as high, due to the combination of additional stressors associated with climate change (Anthony et al., 2007; Lesser, 2007) and a possibly much altered atoll environment due to an additional 50 years of human occupation. Thus, in a twist of fate, the radioactive contamination of northern Marshall Island Atolls has enabled the recovery of the reefs of Bikini Atoll to take place in the absence of further anthropogenic pressure. Today Bikini Atoll provides a diverse coral reef community and a convincing example of partial resilience of coral biodiversity to non-chronic disturbance events.

Read More...

Summary only...

Coral bleaching and limits to ocean warming

A new paper by Kleypas et al in Geophysical Research Letters suggests that a possible upper limit to ocean temperatures – proposed by the “ocean thermostat” hypothesis – may spare some coral reefs from future coral bleaching events. The science has been more or less massacred by some of the press, so it’s worth explaining what is actually going on.

The “ocean thermostat” theory predicts that there is an upper limit to sea surface temperatures – somewhere around 30-34 C depending on the region. First things first: that does not mean if you put some ocean water on the stove, it is impossible to get the temperature over 34 C. The theory is based on a number of negative feedback mechanisms which only operate at relatively large scales, larger, say than the pot sitting on your stove. So the limit is expected to be 30-31 C in the open ocean, like the Pacific, and a bit higher, 33-34 C in enclosed seas like the Red Sea.

We’re used to hearing about positive feedbacks – temperature rises, sea ice melts, a darker surface (water) that reflects less radiation is exposed, causing temperatures to rise even more. In this case, rising water temperature would drive processes that would slow or stop the temperature rise. There are several candidate mechanisms. If the ocean thermostat really exists, it is likely caused by a combination of the mechanisms. One example is that as the ocean warms, more water evaporates, creating more clouds and reducing incoming solar radiation. Another is that warming of one area of ocean will create pressure gradients (with other cooler areas) creating currents that redistribute the water.

The sea surface temperatures across most of the tropics are well below the theoretical temperature limit suggested by the ocean thermostat theory. The maximum annual temperature could rise 3-4 C in most coral reef regions, well beyond the threshold for the corals, without hitting the thermostat limit. That's the case in the Caribbean, the eastern Pacific, much of the Indian Ocean, much of the Great Barrier Reef, etc.

The exception is in the Western Pacific. The warmest open ocean waters is the world are found in a zone that can stretch from the Indian Ocean well into the Central Pacific. It is affectionately known as the West or Western Pacific Warm Pool or WPWP. In a pathetic attempt to give you the illusion scientists are hip and clever, I will use the short form WP2.

You probably know of WP2, if not by name. For one, the Pacific section can be (loosely) thought of as the body of warm water in the equatorial Pacific that migrates eastward during El Nino events. Also, the Pacific section is home to the majority or at least a sizeable fraction, of what’s left of the world’s un-canned tuna and, in turn, to the majority of the world’s illegal fishing boats.

Many years, the WP2 is usually centered in SE Asian archipelago or the expansive territorial waters of the Pacific Island nation of the Federated States of Micronesia. Of course, the exact location changes each year, bringing the core of warm open ocean waters maybe closer to the Solomon Islands, or Palau, or Kiribati, or Tuvalu, or other neighbouring countries. The fishing vessels flying flags of convenience from Mongolia or Western Sahara or Suriname are usually close behind.

When the edge of the warm pool moves, say, western Kiribati, the rise in water temperatures (1-2 C) is quite likely to cause a coral bleaching event. If you take a look at the January degree heating week maps – that’s the accumulated heat stress metric we use to predict bleaching events – for the past several years, you’ll see that almost every year, there’s a hot spot somewhere outside of WP2’s home base. This has been the proximate cause of bleaching events in the Kiribati (Gilbert) Islands, the Phoenix Islands, Tuvalu and the Solomons in recent years - this may be, in part, a manifestation of long-term temperature trends.

That’s where Kleypas et al. comes in. The paper finds that open ocean waters with maximum annual SSTs above 29 C – in other words, our friend WP2 – have warmed less than any other parts of the ocean since 1950. The closer the SSTs are to 30-31 C, the lower the rate of warming. The data appears to support, at least, circumstantially the existence of the ocean thermostat. If correct, this would imply corals in the warm region could be spared from a dangerous temperature rise.

That’s what spawned the “corals saved from ocean thermostat” headlines. Here’s the catch – ok, three catches. These are not criticisms of the paper rather crucial issues touched on in the paper which were not addressed in the news pieces (and may lead to abuse of the results):

First, the notion of a temperature ceiling sparing coral reefs is limited to this very warm open ocean region. In fact, one of the most interesting results of the paper is that while WP2 has not warmed as rapidly as other parts of the tropical ocean, it has expanded. In other words, the edges have warmed. That, as we’ve seen, has caused bleaching events in a number of countries like Kiribati

Second, the extent and severity of coral bleaching is related to experience. Kleypas et al. compiled bleaching reports from islands within WP2. In these locations, coral bleaching tended to occur with temperatures only 0.2-0.3 C greater than the usual maximum experience by the corals. The threshold is generally thought to be 1-2 C. Corals, like any organisms, adapt or acclimate over time to their environment. Since temperatures historically have shown less variation in the WP2 than other parts of the tropics, the corals living there may be naturally more sensitive to temperature stress. If so, the ocean thermostat might not provide the protection that was suggested in the media reports.

Third, the ocean thermostat theory is predicated in part on warming being limited to the surface ocean. Historical temperature profile data suggest ocean warming is also occurring at greater depths. The implications of the warming at depth needs to be fully investigated before we propose an upper limit to surface ocean temperatures in the future.

In other words, there's much more to come on this story.

Read More...

Summary only...

Status of the Coral Reefs of the Caribbean

If you've come across a headline "Hurricanes and Global warming devastating Caribbean coral reefs", here's why. The IUCN and the Global Coral Reef Monitoring network just released The Status of the Coral Reefs of the Caribbean after Bleaching and Hurricanes in 2005, a long-awaited report compiled by dozens of scientists including myself. The report examines the impacts of the events of 2005 and long-term forecast for Caribbean reefs. The latter based in part on our ongoing work on the effect of climate change on coral reefs.

Here is take-home message, from the executive summary:

"This is a pivotal moment for the coral reefs. The world is already committed to some further warming due to past greenhouse gas emissions and the expected emissions from existing world energy infrastructure (Chapter 2). Thanks to more than a century of ‘committed warming’; events like 2005 are expected to occur more frequently by the 2030s. The only possible way to sustain some live coral on the reefs around the world will be to carefully manage the direct pressures like pollution, fishing and damaging coastal developments, and hope that some coral species are able to adapt to the warmer environment. However, a dramatic reduction in
greenhouse gas emissions in the next 20 years will be critical to control further warming and dangerously high CO2 levels that will probably reduce the robustness and competitive fitness of corals and limit the habitats for many other organisms living on Caribbean coral reefs."

Read More...

Summary only...

The odd couple: hurricanes and coral bleaching

The Atlantic hurricane season is like the baseball season. Officially, it officially begins in the spring. Every year, the fans get all riled up for opening day. They then spend the subsequent months disappointed by the lack of drama and begin calling for the team to fire its manager (or hurricane forecaster). When the summer starts to wind down, the fans are reminded, once again, that all the big (ok, regular season) games tend to happen in late August and September.

With a trail of wreckage from Dean and Felix bearing down on Honduras and Belize (right), once again we're being reminded that hurricane activity often peaks right around now. Not in June, the time of the first peak in hurricane media coverage. Not in July, when everyone starts whining about the hurricane forecast. It happens in late August and September, when ocean temperatures in the tropical north Atlantic and the Caribbean tend to peak.

Coral bleaching events also happen during the annual peak in sea surface temperatures (SSTs). There are key differences, of course. Bleaching is driven largely by anomalously warm SSTs; UV light levels play a crucial role as well, although since the two variables tend to be correlated, temperature is a decent proxy. Hurricanes, on the other hand, are driven by a variety of factors, of which, the SSTs are only one.

You could say the ongoing furor over human-induced climate change in hurricane activity - the subject of Chris Mooney's recent book - is centered on this problem. The climate change --> warmer SSTs link is more or less straight-forward to test. The warmer SSTs --> more intense or more frequent storms is not. So, for example, the role of climate change in the 2005 Caribbean bleaching event, as we did in the paper published earlier this year, can be evaluated with more certainty than evaluating the role of climate change in the 2005 hurricane season.

While the SST link to hurricanes is more complicated than that to coral bleaching, it is not a complete surprise that strong hurricanes and coral bleaching events often occur in the same places each year. This year, there's been bleaching off Oman, near the region hit by Cyclone Gonu; in northern Madagascar, an area hit by a sequence of cyclones; in Okinawa, near the areas where typhoons have passed. So, the NOAA Coral Reef Watch data products like the Hotspot or DHW animations give a picture of the development of bleaching events and also a very rough idea of places that have experienced major hurricanes (try comparing the animations to this Google map of strong 2007 hurricanes). You can see that although thermal stress on corals is too low in much of the central and southern Caribbean where Dean and Felix have passed to cause coral bleaching, but temperatures are still above normal.

Like other odd couples, the combination of a coral bleaching event and a hurricane can be an utter disaster for the inhabitants of the area (e.g., corals weakened or killed by bleaching are then broken apart by the wave activity). A paper published earlier this year found an example of an opposite effect; cold water upwelling caused by the passage of hurricanes in 2005 reduced the thermal pressure on corals in the US Virgin Islands. Of course, the 2005 hurricanes and the bleaching were both driven by the warm water, so perhaps we would not have had one event without the other.

Read More...

Summary only...

Coral decline in the Indo-Pacific

The decline of coral reefs has entered the literary world: a recent New Yorker featured Kimiko Hahn’s poem The Fever about coral bleaching. Take a look before it disappears from the online site.

The poem includes the passage "I wonder if it’s, yet again, the ozone layer". Hopefully the author is playing off the history human impacts on the environment not suggesting ozone depletion is the cause of coral bleaching. UV light does play a role; thankfully no corals are growing on the shores of Antarctica.

The poem's appearance is well-timed. First, in the past month, there have been warm-water anomalies in Florida, in Okinawa (Ishigaki) and the Northern Mariana Islands. In each case, people on the ground reported coral bleaching was underway. The predictive ability never ceases to amaze me. You can see the temperatures maps at NOAA's Coral Reef Watch (the data is best visualized using Google Earth).

Second, the IPCC's full Working Group II report - that's the climate change impacts section - is now available. The impact of climate change on corals reefs is one of several "cross-cutting" themes including the impact of climate change on coral reefs. Rather than look flip through the 20 chapters of the full report for information about corals, you can just read the case studies at the end (scroll down, it is before the appendix). The summary is a pretty solid survey of the science; unfortunately, since the IPCC report was finished a while back and could not include some more recent results.

Third, a terrific meta-analysis by John Bruno and Elizabeth Selig of UNC published a few weeks ago in the free (yes, free to anyone) online journal Public Library of Science documents the decline of coral cover across the Indo-Pacific. Bruno and Selig went through the arduous task of analysing every survey of coral cover conducted in the past 40 years. They found coral cover across Indo-Pacific reefs may have declined from over 40% in the 1980s to closer to 20% today. The method of averaging the data over such a large area may be questioned; there may be bias in where we choose to study corals. But the study makes it very clear that coral cover is decreasing. The paper has caused quite a stir (see Coral Bones and Climate Shifts for some accessible discussion).

The decline is believed to be caused by the usual suspects: overfishing, destructive fishing practices, disease, pollution / sedimentation, and coral bleaching due to rising ocean temperatures. The gradual decline in coral cover may seem to point to the direct human threats rather than coral bleaching, the rationale being that coral bleaching events like the 1997/1998 event are widespread and should result in step changes in coral cover (punctuated equilibrium over gradualism?). A tempting argument, but the data does not support it.

In truth, individual events are never that widespread. Coral bleaching has yet to occur in concert across globe or even across the massive and diverse region in the Bruno and Selig study. For example, the famed 1997/1998 was not really global: events occurred in the Indian Ocean, the Caribbean, and parts of the Pacific, due in part to El Nino. The "Indo-Pacific" covers SE Asia and Australia east to Tahiti, an area largely not affected by the 1997/1998 El Nino (the W Pacific is, if anything, cooler during El Nino events; bleaching on the Great Barrier Reef happened when the Pacific flipped to La Nina conditions in 1998).

So, if bleaching was a major cause of a decrease in coral cover, a gradual decline, not a step change, in total average coral cover for a large region, or for the entire globe, is exactly what we should expect to see. That's not to say bleaching is "the" cause, rather one of the causes.

This year, the N Marianas and Okinawa. Next year?

Read More...

Summary only...

More on Gonu

The remnants of Cyclone / Hurricane Gonu have caused severe flooding in southern Iran. The overall death toll on Oman and Iran has passed 70, including 12 killed in Iran from flash floods. These are places not accustomed to severe rainfalls. Based on the little decent news coverage (not to defend the source) I was able to find, it appears the Iranian villages were unprepared for a deluge of this size.

This image (scroll down) shows the cyclone tracks in the northwest Indian Ocean since 1985. Notice no storms passed into the Gulf of Oman (upper left). The global image on Wikipedia shows just how rare it is for cyclone to even approach the Arabian peninsula or Iran.

The anamously warm sea surface temperatures off Oman played a role in steering the cyclone on that unusual track. NOAA's Coral Reef Watch has a "bleaching watch" for the coast of Oman. That means sea surface temperatures are nearing a level at which some coral bleaching may occur. Since both can be driven by warm ocean temepratures, the concurrence of intense hurricanes and coral bleaching is not so unusual; the largest coral bleaching event in the Caribbean took place in 2005, also the strongest hurricane season.

Given that this blog is devoted in large part to climate change science and policy, I'm normally wary of writing about a specific storm or weather event as it can give the false impression that one could reasonably argue that that storm or event is a clear result of climate change.

In this case, I'm writing about the storm because it is a rare event, because of the human impact, and because, frankly, the North American news coverage has been just abominable. CNN.com interrupts its coverage of the human toll to discuss oil prices. This is a direct quote:

At least 35 people were dead, most of them in Oman, and 30 were missing.
The storm spared the region's oil installations, and oil futures fell Friday on a wave of profit-taking that followed a surge in prices a day earlier. News that Cyclone Gonu had spared major oil installations in the Gulf of Oman also alleviated supply concerns. Light, sweet crude for July delivery fell 62 cents to $66.31 U.S. a barrel in morning trading on the New York Mercantile Exchange after dropping as low as $65.55 early in the session.
At least 32 Gonu-related deaths were reported in Oman, including members of police rescue squads, and 30 others were reported missing, police said. Rescue teams were searching for victims using helicopters and boats, he said.

Read More...

Summary only...

Climate change and Caribbean coral reefs

Our latest research on climate change and coral reefs in the Caribbean was published (online) today in the Proceedings of the National Academy of Sciences.

In the study, my colleagues and I used historical temperature data and climate models developed to examine the probability of the widespread coral bleaching happening in the Caribbean, like in 2005, with and without human influence on the climate. For a short summary of our findings, take a look at the department's press release.

The abstract:

Episodes of mass coral bleaching around the world in recent decades have been attributed to periods of anomalously warm ocean temperatures. In 2005, the sea surface temperature (SST) anomaly in the tropical North Atlantic that may have contributed to the strong hurricane season caused widespread coral bleaching in the Eastern Caribbean. Here, we use two global climate models to evaluate the contribution of natural climate variability and anthropogenic forcing to the thermal stress that caused the 2005 coral bleaching event.

Historical temperature data and simulations for the 1870-2000 period show that the observed warming in the region is unlikely to be due to unforced climate variability alone. Simulation of background climate variability suggests that anthropogenic warming may have increased the probability of occurrence of significant thermal stress events for corals in this region by an order of magnitude. Under scenarios of future greenhouse gas emissions, mass coral bleaching in the Eastern Caribbean may become a biannual event in 20-30 years. However, if corals and their symbionts can adapt by 1-1.5°C, such mass bleaching events may not begin to recur at potentially harmful intervals until the latter half of the century. The delay could enable more time to alter the path of greenhouse gas emissions, although long-term "committed warming" even after stabilization of atmospheric CO2 levels may still represent an additional long-term threat to corals.

Read More...

Summary only...

The hurricane season

I’m in New Orleans for a meeting. Naturally, it has me thinking about hurricanes, climate change, and just how toxic the subject has become.

The 2006 Atlantic cyclone season came and went with far fewer storms than originally forecast. Why? El Nino conditions caused greater upper-level wind shear, slowing hurricane development and helped divert the storms that did develop safely into the middle of the Atlantic. This was relief for many coastal dwellers in the Caribbean, the Gulf Coast and the Atlantic seaboard.

It should also be a cautionary tale for everyone out there talking about the threat posed by climate change.

Hurricane Katrina and the devastating 2005 hurricane season provided a legitimate opportunity to raise concerns about the possible future effects of climate change. Not because those events were caused by climate change, or that the overpopulation of vulnerable coastal areas is a huge, separate problem, but because some research suggested it was the type of event that could occur more frequently in a warmer climate.

Hurricanes have become a rallying cry for the greenhouse gas reduction movement – just look at the posters for An Inconvenient Truth. The 2006 hurricane season was promoted like the upcoming fall television season. As I remarked back in June, the press covered “opening day” of the hurricane season, a loose date not exactly etched in the climate’s schedule, as if we were all awaiting the opening pitch of the baseball season, and all the balls had been juiced.

It was crazy. Not because there is serious uncertainty in the science relating hurricane intensity and climate change (see the recent statement from a WMO meeting on tropical storms). But because even if future warming in the tropical Atlantic does increase the likelihood of more intense storms, there will still be weak hurricane seasons. Just like even if the planet warms by several degrees, there will still be cold days.

Everyone forgot the basic rule, the difference between weather and climate. You just can’t lean on an individual event, or an individual season, for proof of climate change. It is a house of cards. Your argument is doomed to collapse.

That doesn’t mean people should stop talking about hurricanes. It is important to continue to study and discuss the effect of climate change on hurricane intensity. Climate models can be used to investigate how warming could alter the probability of individual events like more powerful storms or storm seasons. Depending on those results, individual storms or storm seasons can continue to be legitimately seen as examples of events that some research suggests may be more common in the future.

But those working to promote concerns about climate change must not fall into trap of looking at individual storms or storm seasons as the smoking gun for climate change. It has emotional appeal - but it is fundamentally bad science. Hurricane Katrina and the 2005 Atlantic cyclone season did not end the “debate” about climate change, nor did the weak 2006 Atlantic cyclone season reignite the “debate”. Let’s not reduce it to that.

Read More...

Summary only...