The Oceans and Global Warming

Likely the biggest threats that global warming will face humanity with are regional droughts and ocean acidification. I want to begin a discussion on the latter, by offering an introduction to the process of ocean acidification and its role in global warming.

Imagine that you’ve taken a drive across the farm in your pickup truck. A CO2 molecule is emitted from the truck. That molecule lingers on some oak tree leaves overnight. When the sun comes out the next morning, as part of the process of photosynthesis, that carbon dioxide molecule is absorbed by the tree. The molecule is now part of the carbon in the oak tree.

Later that night when it’s cold, that molecule is released back into the atmosphere as part of plant respiration. The molecule makes its way through the air to the surface of the ocean. It dissolves and reacts to all the salts in the ocean waters. Here a diatom phytoplankton ingests the CO2 making the molecule part of the ocean’s plant-life through the process of marine photosynthesis.

One of a few things can happen at this point. A copepod might come along and gobble up the phytoplankton and in the process of digestion can turn that CO2 molecule into something to be used elsewhere in earth’s ecology; or it could just release the molecule as a sort of fecal pellet that just sinks to the ocean floor where it will eventually become a fossil fuel. Of course, the CO2 molecule could be re-admitted into earth’s carbon cycle in the form of a carbon atom.

There’s about 750 gigatons of carbon in the atmosphere (most in the form of CO2), and every year humans release upwards of another 5.5 gigatons. The ocean contains about 38,000 gigatons of carbon in its waters and dissolves about 40% of the human emissions. But the oceans are slow to mix. CO2 has to be absorbed at the surface and that surface water has to be mixed with all the other water in the ocean. It takes hundreds and hundreds of years for this mixing process to occur.

Ions in sea water resist the carbon that’s trying to enter the oceans in something called the carbonate buffer system, which tries to keep the pH levels balanced and hence resists the CO2 coming in. Because of this buffer, it takes about 8 to 10 CO2 molecules forcing their way into the ocean for every one (1) molecule that’s absorbed by the ocean. It takes a long time for lots of carbon to be absorbed into the oceans, even hundreds of years. Once the CO2 molecule dissolves, it forms a carbonic acid that reacts with the ocean alkalinity (the ability for water to neutralize the carbonic acid), as the ocean tries to restore its pH balance. If alkalinity drops the pH levels of water drops too and the oceans become more acidic (that’s why people use pH and alkalinity levels to measure acidity, though they are not the same thing).

The surface and low-latitude coast lines are the most acidic parts of the oceans (with some exceptions) because that’s where CO2 is exchanged with the atmosphere and the biosphere. This is why many marine eco-systems, like coral reefs and systems the fishing industries are dependent on, are under immediate threat of ocean acidification. A more detailed exploration of the state of our oceans, and how we know what we know, is on the very near horizon.

A recent announcement finds that CO2 emissions increased at a rate of 3.4% per year from 2000 to 2008, in contrast to 1% each year in the previous decade, comes from scientists of the Global Carbon Project report. The scientists attribute this rise to the increase in production and trade of manufactured products, especially from emerging economies, slow shifts from oil to coal, and the planet’s slowing capacity to absorb CO2. This is an understandable assumption in light of the knowledge that annual rates of increase in carbon dioxide emissions from fossil fuels has more than tripled in this decade, compared to the 1990s.

Of course, it’s important to note that as global temperatures rise the ocean releases a lot of CO2 into the atmosphere. When that extra carbon dioxide is released it will, in turn, increase the greenhouse effect, which then causes even more CO2 to be released, and so on. This is what’s called a positive feedback loop. When you combine the outgassing of the oceans with human emissions and our penchant for destroying forests, which both releases CO2 and removes plants that would otherwise absorb CO2, the end effect would predictably be a major rise in CO2 and a slowing of the ocean’s ability to absorb it.

The fact that it takes so long for CO2 to mix in the oceans is an indicator of the lag time between CO2 and actual rise in temperatures, given the oceans dominant role in controlling earth’s climate. A lot of people say that CO2 drives temperatures. A lot of people say that temperatures drive CO2 (some even say that CO2 doesn’t effect temperatures – which is ridiculous). The fact is, it’s both.

When we factor in Milankovitch cycles, which is a measurement of ice ages followed by warm periods, we see that temperature leads CO2 by about 800 or so years. However, given the human influence on earth’s ecological balances, I’m inclined to think that relationship becomes more indefinite. Basing projections for the future on Milankovitch cycles would seem unwise. Discounting their lessons seems equally unwise.

Even in the face of so much gloom and uncertainty, there is a reason to be hopeful. Scientists and engineers are making ever greater strides at the capacity for actually removing CO2 from the atmosphere. What sort of innovations the future holds will be the subject of my next entry.