Ocean acidification will make global warming even worse

Most of the focus and the words spilled on climate change tend to focus on the effects of increasing temperatures, changes in precipitation, and sea level rise. One issue that largely gets mentioned in passing or ignored altogether is ocean acidification.

Much like the way that the oceans have recently absorbed most of the heat trapped on Earth, our oceans have taken up roughly 25-50% of the CO2 which humans have released since 1850. Because carbon dioxide is soluble in water, it dissolves and bonds with the oxygen molecule and one of the hydrogen molecules to form carbonic acid (HCO3). As a result, the IPCC noted in its Fourth Assessment Report (AR4) that the pH level of the ocean has dropped by at least 0.1 units. Under a high emissions scenario (SRES A2), Feeley, Doney and Cooley (2009) project that the pH level of the ocean will fall from a pre-industrial level of 8.2 to roughly 7.8, equivalent to a 150% increase in acidity (PDF).

Graph of Ocean pH projections

The range of potential pH levels for the world’s oceans through 2100, based on the various IPCC emissions scenarios. Even under the best case scenario, which is becoming increasingly unlikely by the day, pH will fall below 8 for the first time in at least 800,000 years (courtesy of the IPCC).

We already know that ocean acidification will wreak havoc on ocean ecosystems by degrading coral reefs and dissolving aquatic creatures with calcium carbonite shells. These outcomes, in turn, will likely increase food insecurity for the more than one billion people who rely on fish for their primary source of protein. In other words, it’s going to suck – a lot.

image of dissolving aquatic snail shell

Side-by-side images of the aquatic snail Limacina helicina antarctica. The image on the left shows an intact snail shell, while the image on the right shows the same species in a severe state of dissolution from ocean acidification. Shelled aquatic organisms which live in the Southern Ocean, like Limicina helicina antarctica, are uniquely susceptible to acidification (courtesy of the British Antarctic Survey).

But thanks to a new article published this week (paywalled) in Nature Climate Change, we know that ocean acidification may produce another drastic outcome – amplifying global warming. According to the study, seawater that is more acidic is less saturated with dimethylsulphide (DMS), a compound of sulfur that is a byproduct of phytoplankton production. As the authors note (emphasis mine):

[It has been] observed that DMS, a by-product of phytoplankton production, showed significantly lower concentrations in water with low pH. When DMS is emitted to the atmosphere its oxidation products include gas-phase sulphuric acid, which can condense onto aerosol particles or nucleate to form new particles, impacting cloud condensation nuclei that, in turn, change cloud albedo and longevity. As oceanic DMS emissions constitute the largest natural source of atmospheric sulphur, changes in DMS could affect the radiative balance and alter the heat budget of the atmosphere.

Using data from a mecosm study conducted in 2010 off the coast of Svalbard, Norway, the researchers attempted to analyze the relationship between ocean acidity and DMS levels and the associated impact upon radiative forcing in the Earth’s atmosphere. In their analysis, they project that DMS production will decrease 26% by 2100, leading to a drop in DMS emissions of roughly 12-24%.

Because DMS emissions can alter cloud dynamics, the authors project that ocean acidification will lead to an additional radiative forcing of 0.40 watts per square meter (remember that burning fossil fuels since the dawn of the Industrial Revolution has so far led to an additional 1.6 W m−2). They conclude (emphasis mine):

We find that even in a future CO2 emission scenario as moderate as the IPCC SRES A1B, pH changes in sea water are large enough to significantly reduce marine DMS emissions by the end of the twenty-first century, causing an additional radiative forcing of 0.40 W m−2. This would be tantamount to a 10% additional increase of the radiative forcing estimated for a doubling of CO2.

As the authors note, ocean acidification may increase global temperatures by an additional 10%, equivalent to perhaps 0.2-0.3C by 2100. This study presents another important piece in the equation for estimating the potential scope and scale of the consequences of our meddling with our Earth’s climate. The climate is one of the most complicated phenomenon in the universe, and our best scientists are only beginning to understand some of its most nuanced facets. For hundreds of years, we have been blindly pulling levers and turning knobs on the machine that controls the habitability of our planet, blissfully ignorant of the implications. We are the guinea pigs in a global experiment of our own making, and the time to stop is now.

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