The oceans are not more acidic now

As claimed by the Guardian and carelessly re-Tweeted by the usual suspects:

Fiona Harvey’s article in the Guardian on 3 October 2013 Ocean acidification due to carbon emissions is at highest for 300m years misrepresents the scientific literature. This error has propagated across the Twittersphere. 

... Hönisch et al. 2012, cited by the State of the Oceans report, showed in Figure 4D that the ocean has been more acidic for most of the past 300 million years than it is now. The rate of acidfication may be faster now, but Hönisch’s graph has a resolution of 20 million years, so cannot address that question.

It is unfortunate that an environmental journalist should confuse the rate of acidification with levels of acidity, but appalling that this story was tweeted uncritically by Nature Geoscience and a number of climate scientists other influential accounts. This is not some esoteric area of climate science. It is well known that CO₂ was much higher during parts of the past 300 million years than it is today and therefore ocean surface pH would be expected to be lower. Why was Harvey’s assertion that “[the] oceans are more acidic now than they have been for at least 300m years” not challenged (as far as I can see) by anyone from the scientific establishment?

A little misrepresentation goes a long way in the world of climate scare-mongering.

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