o ! r c i s a o ll R O G E R R E V E L L E C O M M E M O R AT I V E L E C T U R E r t e i c c t l e h i v e r a s b e di e s e t n p ir b u ub t i o li n o s h e f a d in n y p O c o ea r t n What Corals are Dying to Tell Us i o o n o g r a f t p h h y i s a , V r o

About CO and Ocean Acidification t lum i c

2 l e b e 20, N BY KEN CALDEIR A y p h o umb t o c o e p r 2, a q y m a

Remarks from the eighth annual Roger Revelle Commemorative Lecture. !is lecture was created by the Ocean Studies Board of the c h u in a r e t e National Academies in honor of Dr. Roger Revelle to highlight the important links between ocean sciences and public policy. !is year, , r r e l y j p o o s ur the speaker was Ken Caldeira, and the lecture was held on March 5, 2007, at the Baird Auditorium in the Smithsonian National Museum t in n g a , o of Natural History in Washington, D.C. l o r o f ! t h e O e r m c e e a a n n o s i g r Emissions of carbon dioxide are caus- extinction, marked the end of the dino- some types of phytoplankton and the s p a p e h r y m ing the oceans to become more acidic. saurs that walked on land. Every land foraminifera, unicellular marine amoe- S i o t t c e i e d o t

Recent experiments show that this ocean animal larger than a cat disappeared. bae (Figure 1). y . C nl y w o p

acidification threatens many ocean eco- This massive extinction event also Calcium carbonate is key to under- y i t r h t i g h h t 2007 b

systems. The ancient past may provide a affected the oceanic realm (Zachos et standing what happened at the e a p p cautionary tale, indicating what might al., 1989). Marine Plesiosaurs became Cretaceous-Tertiary extinction event. r o y v a ! l o happen if we change ocean chemistry extinct, as did many other marine organ- The word “Cretaceous” comes from the e O f ! c e e O too much too fast. isms. The pattern of marine extinctions Latin word creta, meaning chalk, a form a n o c e g a r a n

at the end of the Cretaceous tells a cau- of calcium carbonate. The famous white p o h g y r a S p o

AN EXTINCTION EVENT 65 tionary tale for the future. cliffs of Dover consist of chalk depos- h c y i e S t o y . A MILLION YEARS AGO Surface waters are the most produc- ited during the Cretaceous period in c i e ll r t y . i S Sixty-five million years ago, a comet tive areas of the ocean, the site where the form of billions of shells of micro- g e h n t s r d a e ll c slammed into the Yucatán Peninsula, nutrients and sunlight are converted into scopic organisms. These organisms were s e r o v r e r d. P just north of what is now the Mexican foodstuffs by the photosynthetic activity mostly coccolithophores, single-celled e s p e o r n m city of Merida (see Pope et al., 1998 for of marine algae, particularly microscopic algae enclosed within calcium carbonate d i e s n si c o e t a review). This comet, perhaps six miles phytoplankton. The most productive shells. Hence, the Cretaceous is named n i o: in s g r a f o@t in diameter, would have ejected enough areas are found in coastal waters, which for the chalky deposits produced by n t e d t o s

soot and dust into the atmosphere to serve as nursery areas for many marine abundant marine microorganisms with o c .o r g o o p y t blot out the sun for months or longer. species. In the open ocean, the produc- calcium carbonate shells. r ! h i e O The ensuing cold and darkness would tivity of the surface waters also supports But, when the comet slammed into s a r t c i c e l a e f have caused the widespread death of life in the depths—the remains of upper- Earth at the end of the Cretaceous, n o o g r u r a s p

plants and the animals that fed on them ocean organisms sink to the bottom everything changed. Just about every e in t h y S o e c

(Pope et al., 1997). This event, known and feed the deep-dwelling animals that marine species that built shells or skel- a i c e h t y in , P as the Cretaceous-Tertiary boundary never see sunlight. etons out of calcium carbonate became g a O B n d r o

Clams, oysters, reef-building cor- either rare or extinct. Coral reefs disap- x 1931, e s e a r KEN CALDEIRA (kcaldeira@globalecology. als, and many other creatures in shal- peared from the fossil record for at least c h R . o R c e k stanford.edu) is a senior staff member low- and deep-water ecosystems build two million years. Nearly all plankton p v ub ill e li , MD 20849-1931, U c in the Department of Global Ecology, calcium carbonate shells and skeletons. with calcium carbonate shells became a t i o n Carnegie Institution of Washington, Calcium carbonate structures can also extinct, not to return for another half , s y s t e Stanford, CA, USA. be found in minute organisms, such as million years or so. Many species of mm a t i c r S A e p . r o du c t i 188 Oceanography Vol. 20, No. 2 o n ,

PELAGIC sulfate to high temperatures and pres- sures very rapidly, and this caused the release of sulfur dioxide and other sulfur compounds into the atmosphere. In the atmosphere, these compounds reacted with oxygen and water to form sulfuric acid. Even if as little as 5% of the sulfur released by the impact of the comet were A. Foraminifera B. Pteropod C. Coccolithophore to rain down on the ocean as sulfuric acid, it would still be enough to make BENTHIC upper-ocean waters corrosive to the cal- cium carbonate shells and skeletons of marine organisms. The pattern of extinction that occurred in the ocean at the Cretaceous- Tertiary boundary could be explained by the sequence of events that acidified D. Reef Coral E. Sea Urchin F. Deep Sea Coral the upper ocean. However, the amount of sulfur that rained down on the ocean Figure 1. Many marine organisms make shells or skeletons out of calcium carbonate. Upper row: would not have been enough to acidify pelagic organisms that live in the open ocean. Lower row: benthic animals that live in shallow habi- the entire ocean. Within a few years, the tats (D and E) or in deep waters (F). All of these organisms are threatened by ocean acidification, but the corals and pteropods appear to be most at risk. A. USGS; B. Victoria J. Fabry, California State acidic surface waters would have mixed University San, Marcos; C. Jeremy Young, "e Natural History Museum, London, UK; D-F. NOAA with the vast waters of the ocean depths, restoring the chemistry of the upper ocean to an environment suitable for mussels disappeared. About ten mil- cium carbonate to build its shell or corals, coccolithophores, and other car- lion years passed before marine ecosys- skeleton died off during the Cretaceous bonate-shelled creatures. But it was too tems re-evolved the wealth of carbon- extinction event. Organisms that lived late—they were already gone. ate species that were found before the in the deep ocean or did not need cal- Because the upper ocean acidification extinction event. On the other hand, cium carbonate had a much better lasted only a year or two, some corals diatoms—phytoplankton with shells chance of survival. managed to survive the harsh environ- made of silica—persisted through the The comet that struck the Yucatán mental change and prospered when the extinction event. Dinoflagellates—a kind blasted a giant crater, now hid- conditions improved. But, it took two of plankton that forms a plastic-like cyst den beneath more recent sediments million years for them to become abun- in sediments—also survived this event (Figure 2). The Yucatán Peninsula is dant enough to leave a trace in the fossil unharmed. In addition, many organ- basically a giant block of calcium car- record and ten million years before corals isms that lived on the deep seafloor bonate—formed from the skeletons of recovered to their pre-extinction levels seemed to have survived, even if they ancient marine organisms—but includ- of diversity. And, it took 500,000 years had shells or skeletons that were made ing significant amounts of calcium before new species of calcium-carbonate- of calcium carbonate. sulfate (the substance used in making producing plankton evolved from their What could explain this pattern of gypsum wallboard). shell-less cousins. Again, it took many extinction? Nearly every organism that When the comet blasted into the more millions of years to recover pre- lived in the upper ocean and used cal- Yucatán, the impact heated the calcium extinction levels of species diversity.

Oceanography June 2007 189 A In round numbers, these sources add up to about a half a billion tons of carbon dioxide per year. This carbon dioxide accumulates in the atmosphere, traps heat, and prevents Earth from sinking into a deep freeze. But, if carbon dioxide kept accumulating in the atmosphere, Earth would become an oven because it would retain too much heat. Figure 2. !e extinction of the dinosaurs is linked B to the impact of a comet 65 million years ago. On Earth, silicates, such as granites !e comet hit Earth near the northwest tip of and basalts, are the most common type the Yucatán Peninsula (A) and formed a crater (B), named Chicxulub, that is roughly 180 km in of rock. Most of Earth’s deep interior is diameter. !e image in B is a horizontal gradient made up of silicate rocks. On the surface, map of the Bouguer gravity anomaly over the Chicxulub crater. !e Yucatán coast is shown as a these rocks begin to dissolve as they are white line. A striking series of concentric features rained upon or come into contact with reveals the location of the crater. A. NASA; B. Alan groundwater in the presence of carbon Hildebrand, Mark Pilkington, and Martin Connors dioxide. This process is known as chemi- cal weathering. When silicate rocks dis- solve, they release calcium and other minerals into waters that eventually pour into the ocean. Release of calcium through chemical weathering of silicate rocks followed by the production of cal- cium carbonate shells and skeletons by CARBON DIOXIDE, OCEAN acid (Doney, 2006). In the late nine- marine organisms has removed carbon CHEMISTRY, AND MARINE LIFE teenth century, it was common to refer dioxide from the atmosphere for most

So, what is the relevance of this ancient to CO2 as “carbonic acid” because this of Earth’s history. The balance between extinction event for our modern marine reaction took place so readily. Could geologic degassing of carbon dioxide and environment? The burning of fossil fuels the “carbonic acid” that we are releasing the weathering and subsequent deposi- produces carbon dioxide, a greenhouse into the atmosphere from the burning tion of carbonate shells and skeletons is gas that causes atmospheric warm- of fossil fuels have the same effect on the what has kept Earth’s stable over ing and drives . An early marine environment that sulfuric acid billions of years. paper describing this effect was writ- had 65 million years ago at the end of In the Cretaceous, 100 million years ten in 1896 by the Swedish chemist the Cretaceous period? ago, it is thought that volcanoes were Svante Arrhenius. The title of the paper To answer this question, we need to releasing carbon dioxide perhaps twice was, “On the Influence of Carbonic understand something about Earth’s as rapidly as they do today. As a result, Acid in the Air upon the Temperature long-term . Carbon dioxide Earth warmed up, and in the warm, of the Ground.” Why did Svante is naturally supplied to the atmosphere, moist environment, silicate rocks dis- Arrhenius refer to carbonic acid rather ocean, and biosphere by various geologic solved about twice as rapidly, releas- than carbon dioxide? sources, such as volcanoes, terrestrial ing twice as much calcium and driving Carbon dioxide dissolved in water hydrothermal vents (such as those at the formation (and sedimentation) of forms carbonic acid, much in the same Yellowstone National Park), and under- calcareous marine shells and skeletons. way that sulfur oxides become sulfuric sea mid-ocean ridge hydrothermal vents. In short, the carbon cycle ran about

190 Oceanography Vol. 20, No. 2 Figure 3. Over the past few decades, atmospheric CO2 has been changing about one hundred times more rapidly than it has changed

over the past several hundred thousand years. !ese graphs show rates of atmospheric CO2 change in (left) the glacial-interglacial period (thousands of years before present) and (right) in recent time (1950s to present). !e ocean’s natural chemical buffering system is unable to keep up with these rates of change. Data from (left) Petit et al. (1999), and (right) Keeling and Whorf (2005)

twice as fast as it was naturally running are increasing carbon dioxide by nearly acid chemistry. One thing that all acids up until recent times, but the system a factor of 100—a whopping 10,000% have in common is that they increase the remained in balance. increase in carbon dioxide emissions. activity of hydrogen ions in water—in Through fossil-fuel use, we are now This rate is too high to be balanced by essence, acids increase the effective con- emitting 25 billion tons of carbon diox- natural weathering processes (Caldeira centration of hydrogen ions in water. ide into the atmosphere each year. This and Wickett, 2003). Now, let’s think about a coral polyp is nearly 100 times greater than the The average person in the world or marine alga trying to assemble a natural emissions of carbon dioxide today releases close to 25 pounds (more skeleton or shell out of calcium carbon- from volcanoes and other natural geo- than 10 kg) of carbon dioxide into ate, which is the product of a positively logic sources (Figure 3). In other words, the atmosphere each day. The average charged calcium ion, Ca2+, and a nega- 2- if we were to cut carbon dioxide emis- American emits carbon dioxide about tively charged carbonate ion, CO3 . To sions to just 2% of current emissions, five times more rapidly than the average form calcium carbonate, the coral must this would still be enough to double the citizen of the world—over 120 lbs per be able to absorb both of these ions natural sources of carbon dioxide. Just American per day. from seawater. The addition of carbonic

2% of current carbon dioxide emissions, The ocean today is absorbing about acid to seawater (through CO2 in the if sustained over a long period of time, 40 lbs (nearly 20 kg) of carbon diox- atmosphere) produces excess hydro- 2- would be enough to produce the warmer ide each day for each American. With gen ions that react with CO3 to form - climate experienced on our planet carbon dioxide pollution occurring at HCO3 , thereby decreasing the availabil- - 100 million years ago. this scale, there are bound to be con- ity of CO3 to corals and other calcifying In contrast to current trends, at 2% sequences. Even the vast ocean cannot organisms (Figure 4). of today’s rate, carbon dioxide would accommodate this scale of pollution, and Currently, the ocean is absorb- gradually build up in the atmosphere. As yet there is barely any appreciation of the ing carbon dioxide at about one-third the climate warmed, weathering would impacts of continued fossil fuel combus- the rate that Americans are producing release more silica and calcium and sup- tion on the health of the ocean. it—the remainder is accumulating in ply the rapid growth of corals and other To fully understand the consequences the atmosphere. Although the surface calcifying marine organisms—it would of excess carbon dioxide emissions ocean absorbs carbon dioxide quickly, it be like another Cretaceous. However, we requires an understanding of carbonic will take about a thousand years to mix

Oceanography June 2007 191 carbon dioxide released today through- skeletons of marine organisms will actu- picked up within the past ten years as a out the deep ocean because the upper ally start to dissolve. If current carbon result of a number of experiments on ocean mixes slowly with the deep ocean dioxide emission trends continue, shells marine organisms and several modeling (Caldeira and Wickett, 2003, 2005; Orr of marine organisms will start dissolv- studies that predict future ocean carbon et al., 2005). With continued emissions ing in some parts of the ocean sometime dioxide concentrations. However, as a over the next century or two, the sur- around mid-century. field of research, this work is still in its face waters will continue to acidify, and infancy. The following is a very subjec- even the deep waters of the ocean will be RESEARCH INTO BIOLOGICAL tive short history that mentions just a affected (Figure 5). EFFECTS OF OCEAN few names. I apologize in advance to At today’s concentrations, the addi- ACIDIFICATION those who made important contribu- tional carbon dioxide simply makes it While a few lone scientists were con- tions that I fail to mention. more difficult for many marine organ- cerned some decades ago about the In the late 1980s, Brad Opdyke, then isms to survive. At higher concentra- chemical effects of carbon dioxide on working at the University of Michigan, tions, the calcium carbonate shells and the marine environment, interest really inferred from the geologic record that the chemical condition of the ocean con- trols the rate of production of carbon- ate shells and skeletons in the marine Figure 4. Addition of CO 2 environment over geologic time scales. to water makes the water O O H O C O H C more acidic. When CO2 O H Several researchers, including Chris O H dissolves in water, it forms Langdon from the University of Miami carbonic acid, releasing a CO + H O ‡ HCO - + H+ hydrogen ion into solu- 2 2 3 and Jean-Pierre Gattuso (Gattuso et tion. !e extra hydrogen al., 1998) in Villefranche, France, per- ion (acidic component) formed experiments on tropical corals can then cause calcium O O O O carbonate to dissolve and C C that showed coral growth rates slow- H inhibit the formation of O H Ca Ca O ing dramatically as CO2 concentrations new calcium carbonate increased. Joanie Kleypas, at the National structures. + 2+ - CaCO3 + H ‡ Ca + HCO3 Center for Atmospheric Research in Boulder, Colorado, led a ground-break- ing study (which included Opdyke,

Figure 5. Acidification will Langdon, and Gattuso) combining continue even if CO2 emis- model predictions of future ocean chem- sions are curtailed. !ree istry with results coming out of labora- graphs illustrate anthropo- tory experiments on corals. This study genic CO2 emissions (top), atmospheric CO2 concentra- concluded that coral growth would be tions (middle), and ocean severely impeded later this century. pH (bottom). Even if emis- sions are curtailed over the Although the Kleypas et al. (1999) next 250 years, the ocean study was alarming, the situation on a will continue to acidify natural tropical reef is likely to be even as it absorbs more of the atmospheric CO2, reaching more disturbing. Laboratory experi- levels that have not been ments examining the effect of high CO experienced by marine eco- 2 systems for many tens of on coral growth did not account for pre- millions of years. dation on corals. In the real world, corals need to grow rapidly enough to keep up

192 Oceanography Vol. 20, No. 2 with the animals that prey on them, a process called bioerosion. Furthermore, sea grasses and other organisms compete for the same space that the corals occupy. For corals to persist in the wild, they must outgrow their predators and their competitors (Figure 6). Another approach is to evaluate the environmental conditions where corals live today. Tropical corals live in shal- low waters (they don’t grow below about 150-m water depth) where the calcium carbonate building blocks are readily available. As suggested by Brad Opdyke Figure 6. Effect of predation on corals. In the natural world, corals need to grow rapidly enough nearly 20 years ago, the availability of to withstand predation by fish and other organisms and to compete for space with algae and these building blocks largely determines sea grasses. Ocean acidification reduces the ability of corals to compete ecologically. Hence, the decline of coral reefs could be more rapid than predicted based on laboratory studies of the whether the corals can grow rapidly effects of acidification. Frank and Joyce Burek enough to survive bioerosion and eco- logical competition. Carbon dioxide dissolves more readily in cold waters, leading to higher concen- dioxide emissions, and that would allow time of a coral polyp. trations of carbonic acid in the surface corals to survive. Second, corals might And it’s not just corals that are vul- waters of polar and subpolar regions. be better able to withstand bioerosion nerable. Vicky Fabry at the California Coral reefs are not found in these colder- and maintain the edge over competitors State University at San Marcos has been water environments. We can character- more effectively than appears at present. studying sea snails, called pteropods, that ize the chemistry of the waters in which (No evidence has been presented in the live in the open waters of the Southern shallow coral reefs are found today and lab or field suggesting that corals might Ocean (Fabry, 1990). These snails may contrast it with conditions expected later be able to undergo evolutionary adapta- consume up to half of the algal growth this century under the current trend in tion to survive a more acidified ocean.) in that area, forming a critical compo- carbon dioxide emissions (Figure 7). In Therefore, there is at least a reasonable nent of the Southern Ocean food web several decades, where will the ocean expectation that if current carbon diox- that eventually feeds the whales and have chemical conditions similar to ide emission trends continue, corals will other top predators in the ecosystem. those where corals are found growing not survive this century. Fabry’s experiments indicate that the today? The answer is, “Nowhere.” Today’s carbon dioxide emissions may shells of these snails will dissolve later Does this mean that we will do to be worse for the ocean than the sulfuric this century under current projections of the ocean over the course of this cen- acid rain that fell 65 million years ago. carbon dioxide emissions. tury what a comet did to the ocean After the comet hit the Yucatán, ocean Ulf Riebesell at the Leibniz Institute over the course of a single afternoon? acidification only lasted at most a year or of Marine Sciences in Kiel, Germany, Will shallow-water corals go extinct two until the surface waters were diluted has performed experiments that sub- this century, perhaps to re-evolve in by mixing with deeper ocean waters. In ject plankton to different carbon diox- several million years? contrast, the ocean acidification event of ide conditions (Riebesell et al., 2000; Nobody can say for sure. First, we the present is likely to last many thou- Riebesell, 2004). These experiments could dramatically reduce our carbon sands of years, much longer than the life- indicate that plankton are less sensitive

Oceanography June 2007 193 8.4 7 pre-industrialpre-industrial 4 8.3 6 year 22000000 8.2 pre-industrial 5

3 Ω

8.1 year 2000 Ar

4 agonite H 8 B1 p Calcite A1BA 2

Ω 3 7.9 B2 B1 A1B 2 A2A 7.8 A2 1 B2 7.7 1

7.6 0 0 60°S 30°S EQ 30°N 60°N 60°S 30°S EQ 30°N 60°N Latitude Latitude

Figure 7. Ocean pH has already decreased by 0.1 unit and may decrease another 0.2 to 0.4 by the end of this century. Left panel: Ocean pH (a measure of acidity) for different latitude bands in preindustrial times, year 2000, and four different scenarios for year 2100. Right panel: Saturation state for two common calcium carbonate minerals, aragonite (e.g., corals, pearls) and calcite (most planktonic organisms). !e vertical pink band shows the latitude band where corals commonly grew in preindustrial times (year 1750). !e horizontal green band shows that corals are naturally found where the aragonite saturation state is above 3.5. By year 2000, most corals may already have been living in a marginal environment for calcification. All of the projections for year 2100 suggest that ocean pH will fall below what corals have been able to adapt to over the many millions of years of their existence.

to higher CO2 than corals. However, if acidification on ocean ecosystems will be the open ocean.

CO2 concentrations continue to increase, wide ranging, complex, and difficult to One could speculate that ocean acidi- plankton also will be unable to construct predict—to date, only a few individual fication will affect some ocean ecosys- calcium carbonate shells (Figure 8). species have been studied. tems in a way that might be analogous Almost all of these experiments were to cutting down a rain forest. Loss of a performed in laboratory environments, CONCLUSIONS rain forest means biodiversity is lost. But, not in the field. The few field experi- We know that we are toying with the invasive, rapidly growing generalists— ments took place in small enclosures chemistry of the ocean in a profound weedy plants—invade and grow. What and lasted only a few weeks. Nobody way (Raven et al., 2005; Kolbert, 2006; was once a complex, diverse, ecosystem has done the type of long-term experi- Ruttiman, 2006). We do not know with full of delicate ecological relationships ments necessary to understand how car- certainty what the consequences might that have developed over the course of bon dioxide may affect the reproductive be. Experiments to date have shown that evolution, are replaced with simple eco- cycles of marine organisms. For example, the chemical changes now occurring systems lacking the beauty and diversity adult fish may be relatively insensitive in the ocean are likely to be harmful to and perhaps the resilience of the com- to CO2 changes. Except for the gills, the organisms that build their shells or skele- plex ecosystems they replaced. internal tissues of fish are essentially iso- tons out of calcium carbonate. It is likely In the ocean, the combination of lated from ocean chemistry. However, that a continuation of current trends nutrients and light will continue to sup- fish eggs and fish larvae may be far more in carbon dioxide emission will lead port living systems. Life is far too robust sensitive to changes in ocean chemis- to the extinction of corals, which have to be eliminated by a single threat from try. Even if the fish itself is insensitive been around for many millions of years, human activities. But, it may be well to changes in ocean chemistry, its prey and may lead to the extinction of other within our capability to reduce the com- may be affected, thus influencing the marine species. We do not know what plex natural ocean environment to the growth of the fish. The consequences of the effects will be on the ecosystems of marine equivalent of weeds.

194 Oceanography Vol. 20, No. 2 coral calcification. Global and Planetary Change 18:37–46. Keeling, C.D., and T.P. Whorf. 2005. Atmospheric

CO2 records from sites in the SIO air sampling network. In: Trends: A Compendium of Data on Global Change. U.S. Department of Energy, Oak Ridge National Laboratory, Carbon Dioxide Information Analysis Center, Oak Ridge, TN. Kleypas, J.A., R.W. Buddemeier, D. Archer, J.-P. Gattuso, C. Langdon, and B.N. Opdyke. 1999. Geochemical consequences of increased atmo-

spheric CO2 on coral reefs. Science 284:118–120. Kolbert, E. 2006. The darkening sea: Carbon emissions and the ocean. The New Yorker. 20 November 2006:66–75. Orr, J.C., V.J. Fabry, O. Aumont, L. Bopp1, S.C. Doney, R.A. Feely, A. Gnanadesikan, N. Gruber, A. Ishida, F. Joos, and others. 2005. Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms. Nature 437:681–686. Petit, J.R., J. Jouzel, D. Raynaud, N.I. Barkov, J.M. Barnola, I. Basile, M. Bender, J. Chappellaz, J. Davis, G. Delaygue, and others. 1999. Climate and atmospheric history of the past 420,000 Figure 8. Calcification depends on the concentration of carbonate ions. !is graph summa- years from the Vostok Ice Core, Antarctica. rizes data from laboratory experiments. As CO2 concentration increases, the carbonate ion Nature 399:429–436. 2- concentration (CO3 ) decreases, resulting in slower growth. Corals (gray symbols) appear Pope, K.O., K.H. Baines, A.C. Ocampo, and B.A. to be more sensitive to decreasing carbonate than foraminifera (blue and red symbols), but Ivanov. 1997. Energy, volatile production, and

even foraminifera will be affected at high CO2 levels. Chris Landon, University of Miami climatic effects of the Chicxulub Cretaceous/ Tertiary impact. Journal of Geophysical Research-Planets 102(E9):21,645–21,664. Pope, K.O., S.L. D’Hondt, and C.R. Marshall. 1998. Meteorite impact and the mass extinction of Preservation of our valued marine work, and the National Academies’ species at the Cretaceous/Tertiary boundary. ecosystems could also be within our Ocean Studies Board for honor- Proceedings of the National Academy of Sciences of the of America 95:11,028– capability. But, it might take reductions ing me with the invitation to be the 11,029. doi:10.1073/pnas.95.19.11028. in carbon dioxide emissions to very 2007 Revelle Lecturer. Raven, J.A., K. Caldeira, H. Elderfield, O. Hoegh- low levels, reductions to levels that may Guldberg, P. Liss, U. Riebesell, J. Shepherd, C. Turley, A. Watson, R. Heap, and others. be lower than if global warming were REFERENCES 2005. Ocean Acidification Due to Increasing our only concern. Recall that we are Caldeira, K., and M.E. Wickett. 2003. Atmospheric Carbon Dioxide. Royal Society, Anthropogenic carbon and ocean pH. Nature London, UK, 68 pp. Available online at: now emitting carbon dioxide at a rate 425:365. http://www.royalsoc.ac.uk/displaypagedoc. that exceeds the natural emission rate Caldeira, K., and M.E. Wickett. 2005. Ocean model asp?id=13314 (accessed April 18, 2007). predictions of chemistry changes from carbon Riebesell, U. 2004. Effects of CO enrichment on by a factor of nearly 100, so that even 2 dioxide emissions to the atmosphere and ocean. marine phytoplankton. Journal of Oceanography a 98% reduction in our emissions will Journal of Geophysical Research (Oceans) 110: 60:719–729. more than double the natural carbon C09S04, doi:10.1029/2004JC002671. Riebesell, U., I. Zondervan, B. Rost, P.D. Tortell, Doney, S.C. 2006. The dangers of ocean acidifica- R.E. Zeebe, and F.M. Morel. 2000. Reduced dioxide emission rate. tion. Scientific American 294:58–65. calcification of marine plankton in response Fabry, V.J. 1990. Shell growth-rates of pteropod and to increased atmospheric CO2. Nature ACKNOWLEDGEMENTS heteropod mollusks and aragonite production 407:364–367. in the open ocean: Implications for the marine Ruttiman, J. 2006. Sick seas. Nature 442:978–980. I would like to thank the Carnegie carbonate system. Journal of Marine Research Zachos, J.C., M.A. Authur, and W.E. Dean. 1989. Institution for providing me with a 48(1):209–222. Geochemical evidence for suppression of Gattuso, J.-P., M. Frankignoulle, I. Bourge, S. pelagic marine productivity at the Cretaceous/ wonderful place to work, the Moore Romaine, and R.W. Buddemeier. 1998. Effect Tertiary boundary. Nature 337:61–64, Foundation for helping to support my of calcium carbonate saturation of seawater on doi:10.1038/337061a0.

Oceanography June 2007 195