Sunday, 7 August 2011

Carbon offsetting - does it really work?

My beloved asked me about carbon offsetting for airplane flights (maybe he's hankering after a long haul holiday journey after several years of no-fly policy). Does tree planting really work? he asked, and how much does it really cost to sequester carbon in this way? Do the carbon offsetting companies charge a fair price? So I did some research and calculations: how much does carbon cost?  What are the risks? How much land does it take to offset one airplane flight? What about other ways to sequester carbon like biochar?

There are many ways to price carbon. For example, the UK government sets a buy-out price for energy companies renewables obligation certificates - if they don't buy enough from renewable energy suppliers and need to top up. The ROC price for 2010/11 is £37/MWh (1). Working from carbon emissions for electricity at 545g CO2/kWh this is £68/tonne CO2. On the other hand I can buy carbon offsets from at about £7.50/tonne. Hmm.

Carbon offsetting doesn't mean planting trees

To my beloved's surprise, carbon offsetting projects like those that Climate Care support are mostly nothing to do with trees. You don't have to sequester carbon to offset your emissions - you can pay someone else to emit less than they would otherwise have done. The projects I looked at (not just from Climate Care) come broadly into these categories:
  • Renewable energy projects, mostly wind, hydro and biomass in developing countries.
  • Energy efficiency projects, such as promoting use of energy efficient stoves, or installing waste heat recovery systems at industrial plant.
  • Methane gas capture - for example capping landfill sites or installing composting facilities to divert biomass from landfill.
  • Avoiding deforestation, for example by teaching sustainable agroforestry techniques and funding police patrols.
  • Aforestation or reforestation  i.e. planting trees to make new forest, often after it has been cleared.
There are several standards authorities (here is a list) which certify the projects including checking, as far as is possible that the carbon savings are real and:
  • additional (i.e. they would not have happened anyway)
  • don't leak - i.e. they don't simply displace emissions somewhere else, such as by forest creation in one place causing forest clearance somewhere else.

What about planting trees

All of these projects are good things and need funding but maybe you don't think that avoiding emissions is the same as actually sequestering carbon. Does tree planting work? Forests do store more carbon than other forms of vegetation for the same land. My book discusses how much carbon is released when you clear forest, based on the above ground biomass. However, this doesn't necessarily mean that you sequester that much carbon when it regrows. Criticisms of tree planting for sequestration include:
  • Its hard to know how much biomass will be stored, and how long it will take (but savings from other projects aren't instantaneous either). It depends on which species are planted, how well they are suited to the conditions, and how well they are looked after
  • The effects of climate change might not be very good for your trees either e.g. too much or too little rain, nasty storms ...
  • Will your trees last for ever? Who can say what will happen in 100 years time. Will someone cut down your trees for fuel or to make room for something else? What about natural disasters like storms and forest fire?
The UK Forestry commission has published a voluntary code that describes how to assess and manage forestry projects for carbon sequestration including risk management (2). For example periodic clear felling is allowed as a strategy for fire management and the overall carbon storage claim has to take this into account when declaring the total carbon storage.

Other ways to store Carbon

There are at least three other ways to sequester carbon:
  • Bury plastic bags in landfill (plastic contains a lot of carbon and it is largely inert underground) -  but we are fast running out of landfill sites.
  • Geological sequestration - but oil companies pump CO2 into old wells mainly to get more oil and gas out, so that isn't great for reducing emissions whereas the salt dome sequestration projects are still at the prototype stage. There is active research going on to ensure the carbon doesn't leak out again (3). Anyway these projects aren't supported by small companies like Climate Care.
  • In soil, as inorganic carbon or charcoal, also known as biochar.


Biochar is basically fine charcoal made by heating organic waste with limited oxygen. It stays in the soil much longer than organic carbon (typically hundreds or thousands of years instead up one or two decades), because it isn't biologically active. However, it is an effective soil improver (though not necessarily for all soil types) and it helps increase water holding capacity which improves resistance to drought. Biochar can make poor soils much more fertile. Also, biochar can reduce emissions of other greenhouse gases from the soil such as methane and nitrous oxide. (4)

Biochar isn't yet accepted as a means of carbon sequestration for accounting purposes, and there are concerns. For example there is the issue of where the carbon comes from: if crop wastes are converted to biochar rather than being ploughed as is, then the level of organic carbon in the soil could drop too far. However, at least biochar sequestration can be combined with agriculture and doesn't need dedicated land. Arguably the waste being used for biochar could better be used as a biofuel - but the same argument applies to the wood sequestering carbon in a forest.

The technical potential for carbon sequestration through biochar has been estimated to be 12% of global CO2 emissions, with no forest clearance or impact on food production. (The biochar would be mostly from crop residues and wastes plus biomass grown on land which has been abandoned due to poor fertility) (5).

How much land does it take to offset one plane trip

The carbon capacity of soil is huge. The famous terra preta soil contained 50t/ha in the top metre. That is only a little less than the 60-70t/ha carbon in above ground biomass for temperate continental (e.g. European) forest (6). One return flight to New York emits 1 tonne of CO2, say 2 tonnes allowing for the uncertainties in high altitude chemistry. That is 545kg carbon so potentially 90 m2 of European forest or 100 m2 of biochar.

So what is the answer?

I am inclined towards biochar over tree planting because it is good for agriculture and doesn't need dedicated land. However, neither strategy gives truly permanent sequestration; for that you need to put the carbon back into geological storage, where the fossil fuel came from. Anything which reduces fossil fuel consumption has to be a good idea. Whether you do that yourself or pay someone else to do it is up to you.

(1) RO Buyout price 2010-2011 (Ofgem)
(2) Woodland Carbon Code -the basics (Forestry Commission)
(3) Fossil Energy: carbon sequestration in geologic formations (US Department of Energy)
(4) Biochar Application to Soils: a critical review (JRC European Commission) 
(5) Sustainable biochar to mitigate global climate change (Nature)
(6) Guidelines for National Greenhouse Gas Inventories Volume 4 Agriculture, Forestry and Other Land Use (IPCC)

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