How much Carbon can we (and the land) handle?

13 February 2012

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© National Geographic

What’s the problem?

As carbon dioxide levels in the atmosphere soar towards levels defined by scientists as ‘dangerous’ it has become apparent that traditional mitigation approaches might not be enough to ensure a stable climate and healthy oceans over this century. Increasingly, the concept of negative emissions—activities that go beyond ‘zero carbon’ by actively removing carbon dioxide from the atmosphere and storing it permanently on a globally significant scale—has become integral to strategies which aim to ensure that humans avoid causing dangerous climate change.

Some such methods aim to ‘lock away’ carbon in the terrestrial biosphere including in plants, soil and deep underground. Following the recent seminar series hosted by the Oxford Geoengineering Programme (OGP) on the topics of afforestation, biochar and bio-energy with carbon capture and sequestration (BECCS), Nigel Moore, assistant with the OGP, describes the predicament caused by the scale of the carbon problem.

Some key questions

First, the obvious question. Why can we call planting trees, biochar (‘charring’ biomass and burying it to store the carbon in the ground), and bio-energy production with carbon sequestration, “geoengineering”? The answer is the scale at which they are being discussed. If these techniques are going to have a significant impact on carbon dioxide concentrations in the atmosphere they will have to be carried out at a massive, global scale. Human activities on such scales will always have side-effects and trade-offs that must be studied in advance. This is even true if we are talking about large scale afforestation, as pointed out by Abigail Swann in of the University of Washington in Wednesday’s seminar. Covering large areas of the land with trees changes the colour of the land, impacting how much energy is absorbed from incoming sunlight in potentially significant ways (think of how much hotter it is in the sun when wearing a black as opposed to a white shirt.) Large scale afforestation has been also predicted by models to impact weather patterns and regional climates significantly.

Thus, even seemingly benign approaches to lowering the human carbon footprint take on a new form when we try to use them at a large enough scale to make a significant impact on carbon dioxide levels. For example, the idea of using agricultural residues as a feedstock for BECCS seems like a great use of an otherwise ‘wasted’ resource. But if humans are to use enough to make a difference this means using nearly all of it, bringing about questions regarding soil health and the role that these residues play in providing minerals to and preventing the erosion of the agricultural soils that feed humanity.

Some key challenges

There are inherent limitations on how much carbon land-based approaches can, or should, be utilized to sequester. The photosynthetic capacity of the planet is by no means infinite. According to Professor Tim Lenton of the University of Exeter it would take about 15% of the land on Earth to countervail the ~10Pg of carbon emitted by humans every year. This means growing biomass, burning it and sequestering the carbon for an area equal to the extent of agriculture today. It is important to remember that by burning hydrocarbons we are releasing the carbon locked away in millions of years of plant material. Expecting existing plant life to be able to uptake all of this is simply not possible.

There are also significant land use issues. The planet is getting full and competition with agriculture and biodiversity make any approaches, such as BECCS, potential contributors to biodiversity loss and higher food prices if employed on a large enough scale. Social acceptability can also be an issue. Transformation of familiar landscapes into biomass monocultures may fall prey to ‘NIMBYism’, as have so many other environmental initiatives.

Terrestrial carbon removal approaches must also pass the test of reliably storing carbon over the long term. This is something that—like the ‘other’ kind of geoengineering, solar radiation management—cannot be field tested because of the spatial and temporal scale of proposed deployment. But imagine the consequences if an effort like this failed to reliably store years worth of carbon emissions as originally hypothesized? What if biochar in soil doesn’t stay stable for as long as we believe? What if we plant vast forests which are promptly wiped out by climate change already been committed to by past emissions? What if underground carbon sequestration doesn’t work for technological, ecological, or even social reasons? What might this do to traditional mitigation negotiations, or to market-based carbon mechanisms?

Scale is the most impressive variable in this equation. An undertaking of the 10Pg Carbon per year scale would appropriate something like 1/6 of the current photosynthetic capacity of the planet and would be a multi-decade project comparable in extent to global agriculture today. Many of these proposals would provide no, or minimal economic incentives to undertake if we don’t have a high carbon price. So how on earth might they get financed?

So, what do we do?

It may seem wise to forget about projects like this, or at least to focus energy elsewhere. This however cannot be the response given. It is the scale of the carbon problem itself that is far more impressive and demands that we look everywhere for solutions—at all of the options, no matter how difficult, contentious, or complex they may seem. Negative emissions techniques would be hugely beneficial if we can get them to be effective, safe and economically deployable. An interdisciplinary research effort into these techniques is urgently needed, in large part to explore their potential ‘whole-of-system’ costs.

The path of extensive hydrocarbon development and concurrent carbon emissions increases is untenable without reliable negative emissions technologies to counter those emissions. The choice humans face is between developing increasingly expensive hydrocarbons and technologies tocapture and sequester their carbon or deploy renewable energy technologies en masse instead. We know how much it costs to develop the fossil energy, we know how much it costs to develop the renewable energy, but we have no idea how much it will cost to do carbon dioxide removal, nor do we have a reliable carbon price. Without any idea of these two variables how can we expect our policy makers, who heavily rely on cost-benefit analysis, to make intelligent decisions regarding energy policy?

Nigel used to be Governance Project Manager at the Oxford Geoengineering Programme and is now Research Fellow at the IASS.

This opinion piece reflects the views of the author, and does not necessarily reflect the position of the Oxford Martin School or the University of Oxford. Any errors or omissions are those of the author.