Ocean Acidification:The Other Carbon Problem

Spring 2012, Lecture 12

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Carbonic Acid• Carbon dioxide combines with water in the atmosphere

to form carbonic acido CO2 + H2O → H2CO3

• It can also dissolve in ocean water directly from the atmosphere

• Carbonic acid is present in all carbonated beverages – cola, beer, champagne…

• It is a weak acid, but is produced in enormous quantities

• Rainwater is naturally acidic because it contains some carbonic acid

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Acidity Scale

• Natural rainwater has a pH of around 5.5-5.6, but this varies from place to place

• pH is defined as:o pH = - log [H+] , where [H+] is the hydrogen ion

concentration3

• pH.swf• Acid is usually measured on

a scale called the pH scale, which ranges from 0 (highly acidic) to 7 (neutral) to 14 (highly alkaline), as the figure shows

Addition of Carbonic Acid to the Ocean

• The increase in carbon dioxide levels in the atmosphere also means that more carbonic acid is added to the ocean

• Carbonic acid is more soluble in cold water than warm, so more carbon dioxide is added to polar oceans

• That is gradually reducing the pH of the ocean, with several unfavorable consequences

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CO2 Enters the Ocean

• Part 1: Half of the CO2 introduced into the atmosphere since the start of the Industrial Revolution has been absorbed by the ocean

• Part 2: It combines to form carbonic acid

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Carbonate Reduction

• Part 3: Carbonic acid raises the hydrogen ion concentration and reduces the carbonate ion (CO3

2-)

• Organisms need carbonate ion to form shell material, so this hinders that activity

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• pH values have been reduced by more than 0.1 pH units since the start of the industrial revolution

• By the end of the 21st century, it is estimated pH will fall another 0.3-0.4 units

Continuing Damage

• Carbon dioxide already in the atmosphere will ensure that this problem will continue to grow worse for at least a century and probably longer

• As more carbon dioxide is added to the atmosphere, the effects will increase

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Effects on Life

• As waters become more acidic, organisms like plankton, coral, and mollusks will have trouble building and maintaining internal skeletons or exoskeletons

• This will also affect the ocean food chain

• Plankton are the base of the food chain so any reduction in planktonic abundance will deplete available food for almost all higher organisms

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Global Variation in Marine pH

• pH varies from region to region

• Areas with lower pH values (more acidic) are often the result of the upwelling of cold, deep, CO2 rich waters

• The west coast of Africa and, especially, of South America show this effect

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Warm-Water Coral Reefs

• Calcification rates of warm-water corals may drop by 50-60%

• This will have adverse effects on the structure of the reefoGrowth depends on the corals growing faster than

the reef is erodedoWeak structures will suffer more erosion from

storms and large waves

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Cold-Water Coral Reefs

• These reefs provide vital habit for commercially important fish specieso They often serve as a “nursery” providing habitat and

protection for young fish until they grow large enough to have a chance to survive in open waters

• Conservative estimates are that 50-70% could find themselves under threat by the end of the century

• Some scientists say the end will come much quicker

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Plankton

• The term “Plankton” covers a variety of species

• Some are unaffected by increased acidity

• Others, such as the coccolithophores (single-celled algae) show marked decreases in calcification rates when exposed to acidic waters

• Plankton are also adversely affected by increasing water temperature

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Plankton and Water Temperature

• An article in the July 29, 2010 issue of Nature by Boyce et al. claimed that plankton biomass has been declining at a rate 1% of the global ∼median per year

• This work was based on satellite data, as well as observations of ocean transparency from Secchi dish observations dating from 1899

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The Importance of Plankton

• Plankton account for approximately half the production of organic matter on Earth

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Plankton Biomass as a Climate Feedback

• A reduction in plankton abundance thus affects the entire marine food chain

• It also reduces the oceans ability to absorb carbon dioxide, since the plankton use carbon dioxide to create organic matter, and excrete oxygen as a bye-product

• This creates another positive feedback for climate change

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Geoengineering Climate Change Using Plankton

• One group has proposed artificially bringing cold, nutrient rich waters to the surface in order to increase plankton production and reduce carbon dioxide in the atmosphere

• The video on the next slide discusses this idea

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“The Power of Plankton”

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Invertebrates

• Mollusks, such as mussels and clams, appear to suffer from thinning of their shells o Juveniles are more susceptible than adultso This could cause population depletion

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Sea grasses

• Some work indicates that sea grasses grow better in high CO2 waters

• Grass offers a valuable feeding and spawning site for a variety of specieso Some of these are commercially valuable fish

• More research will be needed to determine overall affect on the marine food chain

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Effect on the Carbon Cycle

• Exchange of carbon dioxide between the atmosphere and the ocean has been a sink for CO2, reducing the amount of CO2 in the air

• It is estimated that 50% of global CO2 currently enter the oceans

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• The oceans absorb carbon in two main ways - physically and biologically

Physical Absorption

• CO2 dissolves into cold ocean water near the poles

• It is carried to the deep ocean by sinking currents,

• The deep ocean residence time is hundreds of years

• Thermal mixing eventually returns the water to the surface, and the ocean emits carbon dioxide into the atmosphere in tropical regions

• This natural carbon pump moves carbon from the atmosphere into the sea for storage

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Biological Absorption

• The biological absorption of CO2 involves phytoplanktono They use sunlight, water and dissolved CO2 to

produce carbohydrates and oxygen

• When the plankton, or the sea animals that eat the plankton die, they sink to the ocean floor

• A small percentage of the carbon in the creatures' remains is eventually buried and stored in the sediment

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Feedback

• Ocean acidification will likely reduce plankton blooms in the ocean

• This will result in less CO2 absorption in the ocean, with more CO2 remaining in the atmosphere, creating a positive feedback

• This will warm the oceans further, possibly leading to even less absorption

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Heat Transfer in a Cold Ocean

• When the oceans are cold, the upper layers are relatively dense

• It is easier to mix surficial layers with underlying waters

• Mixing transfers both chemicals and heat

• This means dissolved CO2 and heat will be more easily distributed through a large volume of ocean

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Heat Transfer in a Warmer Ocean

• When the ocean surface warms, it becomes less dense

• The density difference between surficial and deeper waters is greater

• This impedes mixing, which means dissolved CO2 and heat will be trapped in the surface layer

• The ocean will gradually become stratified

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Thermal Stratification

• If the ocean is warm enough, it may begin to behave like many lakes

• Layers of different temperature will form, creating a thermally stratified ocean

• If the temperature differences are sufficient, thermal stratification can be stable over long periods

• Stratification inhibits mixing, and may finally cut off mixing entirely

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Effects of Stratification

• Lack of mixing prevents upwelling of cold, nutrient rich waters, which will lower the biologic productivity of the ocean

• Deeper waters will have less oxygen, and portion may be anoixic

• Stratification will inhibit CO2 absorption from the atmosphere even further, leading to greater warming

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Mixing Times for Heat

• When the earth is cooling, the oceans mix easily, and heat, or lack thereof, is quickly distributed through the ocean

• When the earth is warming, the oceans will mix with increasing difficulty, and heat is concentrated in the surface layers

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Ocean Acidification Video

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• Dr. Graham Phillips of the Australian Broadcasting Corporation (ABC) interviews Dr. Will Howard and Dr. Charlie Vernon.

NOAA Ocean

Acidification

• Dr. Jane Lubchenco has been the administrator of NOAA since March 20, 2009

• Her specialty is marine ecology

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