Module 1: Carbon, Climate, Chemistry
In order to understand how ocean acidification works, we first need to understand some of the basic processes that make our planet habitable and how different elements cycle through all life on Earth.
The Greenhouse Effect and Carbon in the Atmosphere
The greenhouse effect is a process that occurs when gasses in the Earth's atmosphere trap the sun's heat. During the day, the sun shines through the atmosphere, warming the Earth's surface. At night, the Earth's surface cools by radiating (releasing) heat back into the air. But some of that heat is trapped by the gasses in the atmosphere. Like a heat-trapping blanket, this process is what prevents the Earth from being too cold and barren to sustain life. Without the greenhouse effect of our atmosphere, life on our planet would not be possible.
The greenhouse effect is generated by five primary gasses in our atmosphere: water vapor (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and ozone (O3). The more of these gasses that accumulate in Earth's atmosphere, the more heat is trapped, causing temperatures to rise worldwide. The current acceleration in the greenhouse effect that is responsible for causing climate change is caused by carbon dioxide and methane.
Watch this short video produced by the U.S. Environmental Protection Agency for a quick refresher on the greenhouse effect.
The Human Cause
Combustion of fossil fuels takes carbon dioxide from where it was held in organic compounds deep in ground and chemically releases it into the atmosphere. Since we first started using fossil fuel-powered machinery and industry at the beginning of the Industrial Revolution in the 1800s, carbon dioxide has been accumulating in the atmosphere and causing the greenhouse effect to warm our planet.
Human activities have raised the atmosphere’s carbon dioxide content by 50% in less than 200 years. (source)
Climate change is a global issue with varying regional and local impacts. The Arctic is being affected by climate change more than most places on Earth.
The Carbon Cycle
Carbon is the chemical backbone of all life on Earth. When new life is formed, it is carbon that forms key molecules like proteins and DNA. Carbon is an essential component of our atmosphere in the form of carbon dioxide (CO2). Despite the different ways carbon is used, no new carbon is being formed - all of the carbon currently on Earth has been here since its beginning. Instead, we rely on the carbon cycle, nature's way of reusing carbon atoms. In the carbon cycle, carbon atoms travel from the atmosphere into organisms in the Earth and then back into the atmosphere over and over again. Most carbon is stored in rocks and sediments, while the rest is stored in the ocean, atmosphere, and living organisms. These are the reservoirs, or “sinks”, where carbon may reside for a long time. The ocean is a giant carbon sink that absorbs carbon from the atmosphere. Marine organisms, from marsh plants to fish and from seaweed to birds, also absorb and release carbon through living and dying. Over millions of years, dead organisms break down and become fossil fuels: coal, oil, and natural gas. When humans burn these fuels for energy, vast amounts of carbon dioxide that were once locked away in long-term storage are rapidly released back into the atmosphere. This excess carbon dioxide changes our climate by increasing global temperatures, causing ocean acidification, and disrupting the planet’s ecosystems. (source)
This brief video by the EPA makes it easy to understand this concept through an animated tour of the carbon cycle.
The Planet's Carbon Sponge
When excess carbon is released into the atmosphere, not all of it remains there long-term. Instead, scientists now know that about a quarter (25%) of human carbon dioxide emissions have been absorbed by the oceans. Monitoring shows that burning fossil fuels has caused unprecedented changes to ocean chemistry due to ocean uptake of millions more tons of CO2 each year. In this way, the ocean acts as a carbon dioxide sponge.
Watch the first minute (60 seconds) of this video to learn more.
What is Ocean Acidification?
Ocean acidification is a change in the chemistry of ocean water that occurs when carbon dioxide (CO2) from the atmosphere is absorbed into the ocean in a chemical process that increases the amount of hydrogen in ocean water, making it more acidic. When carbon dioxide (CO2) reacts with water molecules (H2O), it forms carbonic acid (H2CO3). This acid compound then breaks down into a single hydrogen ion (H+) and bicarbonate (HCO3-).
In chemical terms, ocean acidification is described like this:
CO2 + H2O → H2CO3 → (H+) + (HCO3-)
Watch this fun animation by The University of Plymouth to easily visualize and understand the process of ocean acidification.
The amount of hydrogen ions in the water determines the pH of the ocean (the “H” in “pH” represents hydrogen!). Adding more hydrogen ions to seawater makes it less basic and more acidic - hence the link between pH and the term “ocean acidification".
The pH scale runs from 0 to 14, with 7 being a neutral pH. Anything higher than 7 is basic and anything lower than 7 is acidic. The ocean’s average pH is now around 8.1, which is slightly basic. As the ocean continues to absorb more CO2, excess hydrogen ions will continue to accumulate, causing pH to decrease and the ocean to become more acidic.
These extra hydrogen ions (H+) cause trouble in the marine environment. Many shell-forming organisms rely on carbon in the form of carbonate (CO32-) in order to make their shells. You will learn more about how shells are formed in Module 4. Like the University of Plymouth video showed, ocean acidification causes carbon to be in a form that is inaccessible to shell-building organisms:
As carbonic acid breaks apart into hydrogen ions (H+) and bicarbonate ions (HCO3-), the hydrogen ions then bond with carbonate ions (CO32-) in the water, making them less available to shell builders. If the concentration of carbonate ions becomes too low, organisms can have trouble building or maintaining their shells. At extreme levels, shells and skeletons can even begin to dissolve.
A pteropod shell is shown dissolving over time in seawater with a lower pH. When carbon dioxide is absorbed by the ocean from the atmosphere, the chemistry of the seawater is changed. (Image credit: NOAA)
What is Aragonite Saturation?
Aragonite is a form of calcium carbonate that marine organisms like coral, plankton, and shellfish use to build their shells and skeletons. Aragonite saturation tells us how much of this mineral is available in seawater. Ocean acidification lowers the amount of this building block in the ocean. The more carbon dioxide that is dissolved in the ocean, the lower the aragonite saturation state. As aragonite saturation goes down, it gets harder for shelled organisms and corals to build their shells and skeletons.
Aragonite saturation is represented by the symbol Ωar. If you’re interested in learning more about the chemical reactions that describe aragonite saturation state, download this handy guide from the University of Hawaiʻi at Mānoa.
*You will learn more about aragonite and how shell-builders use it in Module 4.
As excess carbon dioxide is absorbed into the oceans, it is starting to have profound effects on marine life, from oysters to tiny snails at the base of the food chain. Watch this brief video to learn more.
Review
As you've learned in this module, ocean acidification is a measurable, global phenomenon. As humans continue to emit carbon dioxide and other greenhouse gases by burning of fossil fuels, excess carbon accumulates in the atmosphere. This atmospheric carbon reacts with seawater, causing ocean acidification. A main issue of ocean acidification is the way it changes ocean chemistry: carbonate becomes much less abundant as the ocean absorbs more of the carbon dioxide that humans are pumping into the atmosphere. Ocean acidification is expected to continue and to accelerate as humans continue to burn fossil fuels.
Now that you have a basic understanding of the chemical processes that cause ocean acidification, move to the next module to explore why OA is affecting Alaska more intensely than other places.