The 800 lb. Gorilla in the Ocean

When most of us think of climate change and global warming we think of the atmosphere –  warmest decades on record recently, atmospheric CO₂ at record levels and climbing, increased floods, droughts, hurricanes, melting glaciers, etc.  When we do speak about the oceans, it is typically about heat content, Arctic ice melt, and sea level rise. However, the 800 lb. gorilla in the room just might be ocean acidification.

The ocean has absorbed about half of all the anthropogenic (human) carbon emissions since the beginning of the Industrial Revolution (Doney, 2006).  The CO₂ that the oceans absorb is causing a lower pH which means the oceans are moving toward more acidic conditions.  The consequences could be dire.

Where Do Carbon Emissions End Up

The figure below (Ruddiman, 2008) shows where the carbon emissions end up. Fossil fuel burning and land clearance are sources of carbon while the oceans and land (primarily due to vegetation) are sinks for carbon. The measured amount of carbon in the atmosphere is only 55% of what is actually being emitted into the air.

Where does the carbon go?

According to the Global Carbon Project (2009):

The global oceanic CO₂ sink removed 26% of all CO₂ emissions for the period 2000-2008, equivalent to an average of 2.3 GtC per year. While the total amount of CO₂ being removed by the ocean is increasing, its efficiency (the fraction removed of the total emissions) appears to have been declining over the last two decades partially owing to the decline in efficiency of the Southern Ocean and North Atlantic Ocean where long term field observations and model results appear to be in agreement. In 2008, the oceans removed an amount of CO₂ slightly below average. 

The amount of CO₂ that is absorbed by the oceans is a function of water temperature, pressure, and salinity (salt content) known as the solubility pump and biological processes known as the biological pump.  Colder, saltier water absorbs more CO₂ while warmer, less salty water absorbs less CO₂.  There are various complex factors that control the biological pump and these are shown below.

Carbon pump

Careful measurements show that the bulk of the CO2 being absorbed by the oceans resides in the shallowest layers (Doney, 2006).

Oceanic CO2

 

Ocean Chemistry:

When carbon dioxide is absorbed by the oceans, it forms carbonic acid (H₂CO₃), the same weak acid found in carbonated beverages.   Carbonic acid then releases hydrogen ions (H⁺) into solution which lowers the pH of the water.  The full chemical reaction ends up leaving both bicarbonate ions (HCO3^–1) and, to a lesser extent, carbonate ions (CO3^–2).

The carbon chemical reaction

The normal pH of sea water in perfect conditions ranges between 8.0 and 8.3 (slightly alkaline).  The pH of the oceans has already decreased 0.1 from pre-Industrial values due to the absorption of anthropogenic CO₂ and the projections are for the pH to lower an additional 0.3 by 2100 if emissions go unabated (Ibid).

Although these changes appear to be small, these small pH changes may cause devastating effects on sea creatures that need carbonate ions to build their calcium carbonate shells.  The excess H⁺ ions combine with carbonate ions to make bicarbonate.  The result is less and less carbonate for shelled sea creatures.

The Shell Game:

Plankton is the base of the marine food chain.  Coccolithophorids, which are covered with small plates of calcium carbonate and are commonly found floating near the surface of the ocean (where they use the abundant sunlight for photosynthesis) are in danger with slightly lower pH values. Other potentially endangered planktonic organisms are foraminifera and pteropods.  These life forms constitute a major food source for fish and marine mammals, including some species of whales.  And, of course, humans and other animals rely on a robust marine food chain.

The abundance of commercially important shellfish species (i.e., clams, oysters, sea urchins) could also decline, which could have serious consequences for marine food resources (Cooley et al., 2009).

Shells and corals hurt by acidification

Corals are also endangered with lower pH.  Corals are essentially calcium carbonate skeletons of the creatures that live inside them.  Coral reefs are the most productive and biologically diverse ecosystems in the ocean.  Coralline algae (algae that also secrete calcium carbonate and often resemble corals) contribute to the calcification of many reefs, too. The Great Barrier Reef off the coast of Australia, for instance—the largest biological structure in the world—is simply the accumulation of generation after generation of coral and coralline algae. Less obvious examples occur deeper down in the sea, where cold-water coral communities dot continental margins and seamounts, forming important fish habitats (Doney, 2006).

The calcium carbonate in corals or in the shells of other marine creatures comes in two distinct mineral forms: calcite and aragonite. Aragonite and magnesium calcite are more soluble than normal calcite. Thus, corals and pteropods, which both produce aragonitic shells, and coralline algae, which manufacture magnesium calcite, may be especially susceptible to harm from ocean acidification (Ibid).

The saturation horizon is the level below which aragonite and calcite begin to dissolve.  The influx of carbon dioxide from the atmosphere has caused the saturation horizons for aragonite and calcite to shift closer to the surface by 50 to 200 meters compared with where they were positioned in the 1800s.  As the ocean becomes more and more acidic less and less of the sea will remain hospitable for calcifying organisms (Ibid).

The outlook for coral reefs is bleak.  For those precious ecosystems, ocean acidification is but one of many environmental stresses, an onslaught that includes greenhouse warming, local pollution, over-fishing and habitat destruction.  Many coral reefs are already in decline, and ocean acidification may push some over the edge into nonexistence (Ibid).

Any Good News?

Phytoplankton need dissolved CO₂ and nitrogen (N₂) to thrive.  These organisms may benefit from increased levels of those dissolved gas.  Some plants such as sea grasses may also thrive due to CO₂ fertilization.  Fu et al. (2008) note that several studies (Hutchins et al., 2007; Levitan et al., 2007; Ramos et al., 2007) have shown significant increases in N₂ fixation and photosynthesis in response to elevated CO₂.  They conclude that anthropogenic CO₂ enrichment could substantially increase global oceanic N₂ and CO₂ fixation which would increase certain types of phytoplankton but only if there is enough iron (Fe) availability. 

However, according to David Hutchins (2010), Professor of Marine Environmental Biology, University of Southern California, USA, “Some of the experiments that have been done so far suggest that the likely new dominant phytoplankton species in the future acidified ocean may be less able to support the productive food chains that we presently rely on to support healthy ocean ecosystems and fisheries resources.”

Iron and the Carbon Sink:

Sunda (2010) and Shi et al. (2010) have noted that the acidification of the oceans may decrease the biological availability of iron which would reduce the ability of phytoplankton such as diatoms to absorb CO₂.  These organisms play an important role in the ocean carbon sink.  Shi et al. found that the projected pH decrease by the year 2100 would reduce iron uptake by diatoms by 10% to 20%.  In addition, warmer ocean temperatures due to global warming do not allow as much dissolved CO₂ gas to be present.  According to the Global Carbon Budget (2008), “While the total amount of CO₂ being removed by the ocean is increasing, its efficiency (the fraction removed of the total emissions) appears to have been declining over the last two decades…””  The oceans are already struggling to keep up with human emissions.

The figure below shows representative examples of impacts of ocean acidification on major groups of marine biota derived from experimental manipulation studies.  The response curves on the right indicate four cases: (a) linear negative, (b) linear positive, (c) level, and (d) nonlinear parabolic responses to increasing levels of seawater pCO2 for each of the groups  (adapted from Doney et al., 2009).

Impact on several major organisms

 

Marine Oxygen Holes:

Hoffman and Schellnhuber (2009) in their paper, Oceanic acidification affects marine carbon pump and triggers extended marine oxygen holes, modeled ocean acidification into the future assuming a business as usual CO₂ emission scenario.  Their research shows that by the year 2200 with atmospheric CO₂ levels reaching 1750 ppm, the sea-surface pH value drops by >0.7 units on global average, inhibiting the growth of marine calcifying organisms.  They also found that as organic matter is oxidized in shallow waters when the carbon pump weakens, oxygen holes (hypoxic or dead zones) start to expand considerably in the oceans in the future—with potentially harmful impacts on a variety of marine ecosystems. 

Future ocean dead zones

I am an optimist so I think that society will realize the consequences of increased CO₂ well before reaching 1750 ppm CO₂ however, the image above is a sobering reminder of what might happen in a worst-case scenario future.

The Unknowns:

When one reads Impacts of Ocean Acidification on Coral Reefs and Other Marine Calcifiers: A Guide for Future Research, (Kleypas et al., 2006) it becomes quite clear that the impact of ocean acidification on marine life is uncertain because it is essentially a fledgling field.  It was not part of the IPCC TAR (2001) however the IPCC Fourth Assessment Report on Climate Change (2007) stated in the Summary for Policy Makers, “The progressive acidification of the oceans due to increasing atmospheric carbon dioxide is expected to have negative impacts on marine shell-forming organisms (e.g. corals) and their dependent species.” Ocean acidification and its impact will be seriously addressed by the Fifth Assessment Report of the IPCC to be released in 2013.

The Knowns:

According to Iglesias-Rodriguez, Doney, Widdicombe, Barry, Caldeira, & Hall-Spencer (2010): “The rate of human-driven ocean acidification is about 100 times faster in the surface ocean than that experienced by marine ecosystems globally for tens of millions of years.”

According to Barry, Schmidt, & Caldeira (2010): “Other than at times of the great mass extinctions, there is no evidence in the geologic record for sustained rates of change in atmospheric CO₂ that have been as great or greater than today’s. Even during extreme ocean chemistry changes in geological history— for example, during the Paleocene/Eocene thermal maximum (PETM) 55 million years ago when carbonate minerals dissolved in most of the deep and intermediate ocean—these changes probably happened over several thousands of years.  In general, ocean life recovers from extinction episodes by adaptation and evolution of new species, but this takes roughly 10 million years to achieve pre-extinction levels of biodiversity.”

Although much is unknown, it seems clear that humans are altering the oceans in a manner unique to their history and to the history of the marine life within.  Just as the grand experiment we are undertaking with the climate, we are doing the same with the oceans.  The results of both of these experiments will likely be harmful to life on this planet.

I see a large gorilla in the ocean that cannot be ignored.

 

Helpful Resources:

Frequently asked questions about ocean acidification

The Dangers of Ocean Acidification

Ocean Acidification: A Critical Emerging Problem for the Ocean Sciences

PMEL Ocean Acidification Home Page

US Interagency Report on Impacts of Ocean Acidification

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