(5) Current rates of ocean acidification are unprecedented.

The IPCC report says:

The pH of seawater has decreased by 0.1 since the beginning of the industrial era, corresponding to a 26% increase in hydrogen ion concentration. […] It is virtually certain that the increased storage of carbon by the ocean will increase acidification in the future, continuing the observed trends of the past decades. […] Estimates of future atmospheric and oceanic carbon dioxide concentrations indicate that, by the end of this century, the average surface ocean pH could be lower than it has been for more than 50 million years.

(Fig SPM.7c) CMIP5 multi-model simulated time series from 1950 to 2100 for global mean ocean surface pH. Time series of projections and a measure of uncertainty (shading) are shown for scenarios RCP2.6 (blue) and RCP8.5 (red). Black (grey shading) is the modelled historical evolution using historical reconstructed forcings. [The numbers indicate the number of models used in each ensemble.]

Note that this doesn’t mean the ocean will become acid. The ocean has always been slightly alkaline—well above the neutral value of pH 7. So “acidification” refers to a drop in pH, rather than a drop below pH 7. As this continues, the ocean becomes steadily less alkaline. Unfortunately, as the pH drops, the ocean stops being supersaturated for calcium carbonate. If it’s no longer supersaturated, anything made of calcium carbonate starts dissolving. Corals and shellfish can no longer form. If you kill these off, the entire ocean food chain is affected. Here’s what the IPCC report says:

Surface waters are projected to become seasonally corrosive to aragonite in parts of the Arctic and in some coastal upwelling systems within a decade, and in parts of the Southern Ocean within 1–3 decades in most scenarios. Aragonite, a less stable form of calcium carbonate, undersaturation becomes widespread in these regions at atmospheric CO2 levels of 500–600 ppm.

You can download all of Climate Change 2013: The Physical Science Basis here. Click below to read any part of this series:

1. The warming is unequivocal.
2. Humans caused the majority of it.
3. The warming is largely irreversible.
4. Most of the heat is going into the oceans.
5. Current rates of ocean acidification are unprecedented.
6. We have to choose which future we want very soon.
7. To stay below 2°C of warming, the world must become carbon negative.
8. To stay below 2°C of warming, most fossil fuels must stay buried in the ground.

Climate Change 2013: The Physical Science Basis is also available chapter by chapter here: