John Shepherd’s Commentary on Life, the Universe, and Everything

Removing carbon dioxide from the air…

Can’t we just remove carbon dioxide from the air to fix climate change? Not yet

John Shepherd
, University of Southampton

Originally published on “The Conversation” at

If we have put too much CO2 into the air, wouldn’t it make sense to find ways to remove it again? Well, yes: it would. But sadly it isn’t likely to be easy or cheap and, according to new research, it isn’t an adequate “solution” to the problems of climate change.

The possible “carbon removal” techniques are very diverse. They include growing trees on land or algae in the sea and capturing and burying some of the carbon they have taken from the atmosphere. There are also engineered solutions that “scrub” CO2 directly from the air, using chemical absorbents, and then recover, purify, compress and liquefy it, so that it can be buried deep underground. That sounds difficult and expensive, and at the moment, it is.

Both the UK Royal Society and the US National Research Council point out that doing it on a large enough scale to make a real difference would be hard. Nevertheless, a joint communiqué from UK learned societies recently argued that to limit global warming to 2℃ we are likely to need CO2 removal (CDR) rates in the latter part of this century that will exceed emissions at that time (“net negative emissions”). That will only be possible if we can deploy CDR technologies.

‘Negative emission’ technology comes in many forms.
Caldecott et al / SSEE

A new paper in Nature Communications shows just how big the required rates of removal actually are. Even under the IPCC’s most optimistic scenario of future CO2 emission levels (RCP2.6), in order to keep temperature rises below 2℃ we would have to remove from the atmosphere at least a few billion tons of carbon per year and maybe ten billion or more – depending on how well conventional mitigation goes.

We currently emit around eight billion tonnes of carbon per year, so the scale of the enterprise is massive: it’s comparable to the present global scale of mining and burning fossil fuels.

Carbon removal could potentially help to reduce problems such as ocean acidification. So a second paper in Nature Climate Change is also discouraging because it shows that even massive and sustained carbon removal at rates of five billion tonnes a year or more would not be enough to restore anything like pre-industrial conditions in the oceans, if mitigation efforts were to be relaxed.

Don’t give up

Does all this mean that carbon removal is a blind alley, and that further research is a waste of time (and money)? Well, no. But it is nothing like a magic bullet: this latest research should serve to prevent any unrealistic expectations that we could find a “solution” to climate change, or that carbon removal is any sort of alternative to reducing emissions.

Maintaining and increasing our efforts to reduce emissions is still the crucial top priority. But if we can develop removal methods that are safe and affordable, and that can be scaled up to remove a few billion tonnes per year, that would be useful even now, as it could augment those efforts to reduce CO2 emissions (which is not proving to be easy either).

In the longer term, once we have eliminated all the “easily” fixed sources of CO2 emissions, by generating more electricity from renewable sources and capturing carbon from power plants, we shall still be left with several intractable sources, including aviation and agriculture, that are exceedingly hard to abate.

It is then that we shall really need CO2 removal, to take from the air what cannot easily be prevented from reaching it. And beyond that, should we eventually decide that the level of CO2 in the air at which we have stabilised is too high for comfort, and should be reduced, carbon removal will be the only way to achieve that.

Massive scientific challenge

The low-tech biologically based removal methods are all going to be limited in their scale, not least by potential side-effects in the oceans and conflicts over alternative uses for any land required.

However several groups are working on promising methods for direct (physical and/or chemical) capture from the air, trying to reduce the energy, water and materials demands – and of course the costs – to acceptable levels.

Is this the future? This US firm plans to capture carbon dioxide directly from the atmosphere.
Carbon Engineering

In the longer term someone may find a suitable catalyst to accelerate the natural geochemical weathering processes that already remove CO2 from the air (but much too slowly to cope with man-made emissions). That would solve the CO2 disposal problem too, especially if we can avoid mining billions of tons of minerals to use as absorbent. But it’s likely to take several decades to get from the lab to industrial-scale deployment – and none of these technologies will be deployed in practice until we have established a price on carbon emissions that makes them commercially worthwhile.

Carbon removal is not a magic bullet, but it is still a vitally important technology that we shall almost certainly need eventually. We should be researching it steadily and seriously, because it is going to take time and a lot of effort to develop methods that are safe and affordable and can be deployed on a massive scale.

So we should continue to research removal, not as a possible quick fix, but as a vital tool for the end game. It’s a massive scientific and engineering challenge that really needs the sort of concerted effort that was devoted to going to the moon or building the Large Hadron Collider. And in my opinion it would be far more worthwhile.

The Conversation

John Shepherd is Professorial Research Fellow in Earth System Science at University of Southampton.

This article was originally published on The Conversation. Read the original article.

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Fishery Reform: letter to Nature

I sent a letter to Nature entitled Fishery Reform: an Economic Problem, which they published on 28 July 2011. I have posted copies of the original  version (with references) and the shorter published version on my Fisheries page, here.

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On Climate “Sceptics”… or Dissenters

People who wish to question the validity of climate science usually call themselves “sceptics”. Scientists object to this, since all good scientists are perennially sceptical (indeed, the motto of the Royal Society is Nullius in Verba i.e. “take no-one’s word for it”). The so-called “sceptics” object to being called “deniers”, although that is usually what they do (i.e. deny the validity of statements, despite the evidence that supports them). Perhaps this is understandable, given the association of the word with the unpleasant business of holocaust denial. To avoid accusations of bad faith, I therefore commend the term “dissenters” as preferable. It is not pejorative, and also accurately describes what they do, i.e. dissent from the consensus of mainstream scientific opinion.

By the way, there is an excellent iPhone app called Skeptical Science that has good answers to lots of controversial questions on climate science. It’s a nice complement to the excellent RealClimate web-site.

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Ocean Acidification

Letter to The Times (8 Nov 2010) about Matt Ridley’s article on Ocean Acidification


Matt Ridley should brush up his basic chemistry & biology before he criticises scientists who worry about ocean acidification.
First, any shift of acidity is still important even if it is (just) within natural ranges; and in the future it will be much larger than that.
Second, it has been known for some time that some organisms may make shells starting from bicarbonate ions.  But there is abundant evidence that dissolution of their shells is controlled by the carbonate ion concentration, so decreases in carbonate means a problem for retention of shells.
Third, Ridley’s claim of “no significant mean effect” of predicted future CO2 levels is only true because that is an average over several processes and many species – among which there will certainly be winners and losers.
Overall, decrease in calcification rate means a change in the ecosystem structure, and probably its function too, with unpredictable consequences.  We entirely agree that that the jury is out on how damaging acidification will be, but the scientific community is right to be concerned to research this “other CO2 problem”.

Professor John Shepherd FRS, University of Southampton
Professor John Raven FRS, University of Dundee
Professor Andrew Watson FRS, University of East Anglia

See also the more detailed responses from others here and here

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Transparency and trust in Climate Science

Revised and extended version of a letter to The Guardian, (Monday 8 February 2010)

Congratulations to Fred Pearce for his balanced and thoughtful articles last week on the climate email affair, and commiserations to Simon Jenkins and Simon Hoggart for having lost the plot so completely. Those who refuse to accept something despite a mass of evidence for it (like climate change and evolution) go far beyond genuine and constructive scepticism. We should call them dissenters, at least, if the term “deniers” is considered to be too insulting. And those who believe in something without a shred of evidence for it (like homeopathy or astrology) can only be called credulous. Scientists, who may spend decades making observations, and using evidence to test their theories, try hard not to fall into either of these categories. If our cars or our bodies need to be fixed, we seek out, trust and take advice only from those who are trained and qualified to do the job. Why should we not do the same when it is the future of the world in which we live that is at stake ?

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Low-carbon Energy research

Response to an enquiry arising from the BBC “Start The Week” programme on 28 Dec 2009

On 28 Dec 2009, someone wrote to me

“Firstly I assume that you are the John Shepherd who featured on Start the Week today, 28th December ’09.

In it you mentioned that there are numerous worthy research programmes looking into methods of mitigating climate change, but that these are mostly underfunded.

Would it be possible to publicise a list of these, with a short description of what they are doing and what they hope to achieve, plus some contact details etc.? This might enable concerned citizens and the growing number of groups dedicated to this issue – and who now see that politics is probably unable to address the problem of climate change adequately – to consider fundraising for such programmes.

There are many organisations supported by voluntary public donation – but it is a prerequisite that donors must be convinced of a cause’s worth. I sense that tackling climate change is a cause to which many people who currently feel helpless might gladly contribut

I am a member of ‘Sustainable Frome’ which is only one of a number of local groups resolved to helping to tackle climate change. Were a list of appropriate research projects to be forthcoming I would be happy to publicise it around our group and around other groups with which we have contacts.”

To which I replied…

Yes, I am that person. The ideas I was referring to are summarised in another Royal Society report, on the Low Carbon Future, which you can get at

PDFs of the slides of most of the presentations to the meeting are also available at:

Unfortunately this sort of research is very expensive: you’d be looking for hundreds of millions p.a., which is an awful lot of fundraising… As the chair of our local action group (Hyde & District Climate Change Forum) I think we can be more effective at the bottom up stuff (encouraging people to conserve energy). We are affiliated to the New Forest Transitions Movement which does a lot of good things: are you part of that?

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