“If anyone proposes using trees to undo climate change, they need to realise that country-sized facilities are required.  I don’t see how it could ever work.”  Sustainable Energy – Without The Hot Air, David Mackay 2009

It is amazing how often even well-educated people seem to believe what they want to believe.   I assume I do the same and just fail to notice.

In the context of climate change, the propensity for self-delusion is fuelled by a reluctance to accept that one’s own lifestyle is irremediably unsustainable.   A rich variety of slippery rationalizations readily occur, consciously or unconsciously, to an intellect that wants a guilt-free existence.  Here are a few:

• It is the fault of others.  We highlight the damage done by ‘them’ and downplay the harm done by ‘us’: people who travel hardly at all blame those who travel a lot; flat-dwellers and down-sizers blame inhabitants of detached houses; vegetarians blame meat-eaters; to those without children it is the fault of those with families.
• Technology will save us.  We will invent new sources of delicious steak-like protein without the methane emissions; and light planes that do not need much fuel; houses that miraculously heat and light themselves without using energy; and stuff made without burning fossil fuels.
• It is fine for me to have a larger carbon footprint than others because I deserve it – I have earned the right to my lifestyle through sheer hard work.  And as long as others, who are less deserving, have a relatively small footprint, there is no problem with sustainability.
• Life is intrinsically unfair and hey I am just one of the lucky ones, so get over it.
• Climate change is not such a big deal anyway.  Why get het up about an extra degree of two?  Of course our lifestyle is sustainable: we just need to adapt a little bit in the future.

Biofuels: flying made green

Recently, I was reminded of the second of these rationalizations; of how even scientifically trained people, including me, are susceptible to scientific ‘solutions’ to global warming and a little unwilling to test them for flaws.  I was talking to someone about reducing pollution in China.  His expertise was in better forms of transport.  This being something of a hobby horse of mine, I remarked that I was looking forward to the day when the carbon emissions from flying could be controlled.  Someone else, hearing this, said this could be done using biofuels, upon which note a brief and entirely amicable argument arose.

The idea is simple and not new.  As they grow, trees consume carbon dioxide, or “CO2”, via the process of photosynthesis, and thereby counteract the emissions of CO2 from the combustion of fossil fuels.  The rates need to match though: it is no good planting saplings that will take 20 years to grow and claiming that over the 20-year period, the saplings will consume as much CO2 as was released into the atmosphere during a particular flight – unless you plan on waiting for 20 years until your next flight.  But in theory at least, we can plant enough trees such that the aggregate rate at which they absorb CO2 equals the rate at which we emit it into the atmosphere through our flying habit.

The difficulty is that the amount of land which we would need to devote to biofuels in order to make this work would entail drastic social change, in particular a reduction in farming and changes to the national diet.  Any of us can work this out with a few rough calculations; the calculations are not difficult and no-one needs to be an expert.  Here is a ballpark analysis:

Firstly, there are around 63 million people living in Great Britain (plus 2 million people in Northern Ireland whom we are going to ignore).  The size of Great Britain is 209331 square kilometres, according to Google, which should be trustworthy on this basic point.  So that’s about 3300 square metres of land per person, a patch of 58 metres by 58 metres, i.e. you could distribute everyone spatially in a simple grid such that each person is about 58 metres from the next closest persons to the East, West, North and South; and proceeding thus you would cover our island evenly with its human population.

Secondly, the average emissions from flying are 100 grams of CO2 per passenger per kilometre flown.  That is from current government statistics (for instance, try this link – https://www.gov.uk/government/publications/greenhouse-gas-reporting-conversion-factors-2016).  For economy passengers the figure is a bit less (their seats are closer together), for business class the figure is more – and there is some variation between short haul and long haul flights – but for the purposes of a rough calculation, this average is about right.  So, for instance, if you were to make a return trip from London Heathrow to New York and back, each way on a plane carrying 400 passengers, the total CO2 emitted by the flights would be around 400 times 100 times 11170 (the round trip distance in kilometres, again according to Google) equalling 446,800,000 grams of CO2, or equivantly about 447 tonnes of CO2, of which your individual share would be 1.1 tonnes.

The CO2 equivalent, or CO2e, impact of flying is worse than this due to various effects in the stratosphere, including the production of ‘contrails’, the vapour trails visible from the ground when planes fly overhead, which act a bit like cloud cover or a blanket and stop heat from the surface radiating into space.  To account for this additional damage, it is customary to use what is known as a “radiative forcing” factor that converts from CO2 to CO2e for airplanes.  The government source quoted above uses a factor of 1.89, i.e. the CO2e figure is about double the CO2 figure, in other words flying is about twice as bad in terms of the greenhouse effect as you would conclude if you merely focussed on the quantity of CO2 emissions.  But according to my companion at the recent social event, there is some evidence the contrails are not so bad when biofuels are consumed instead of jet kerosene.  Let us be generous and assume that the additional impact of these stratospheric effects is zero when biofuels are used.

Now let’s suppose that people in Britain make, on average, one such transatlantic trip per annum, from Heathrow to New York and back: in other words, that this equates to the average amount of flying done.  Some people fly every week on business; others fly two or three times a year on holiday; others do not fly at all.  As an average it may be a little high, but flying is becoming more popular and common all the time.  (About a year ago, I did a more thorough piece of research and concluded that the average per capita CO2e impact of flying by the British was 1.0 tonnes per annum.  That was based on standard jet kerosene and used the government’s figure for radiative forcing.  This simpler calculation seems to be in the ‘right ballpark’.)

The final piece of the jigsaw is knowing how quickly trees absorb CO2 as they grow.  According to David Mackay (2009) in his book entitled Sustainable Energy – Without The Hot Air, “the best plants in Europe capture carbon at a rate of roughly 10 tons of dry wood per hectare per year – equivalent to about 15 tons of CO2 per hectare per year”.  According to the Forestry Commission in September 2014, the sequestration rate (as it is known) can be as high as 11 tonnes of CO2 per hectare per year (see https://www.forestry.gov.uk/forestry/infd-889hsz).  Let’s use Mackay’s figure, which is higher, in order to give carbon capture by tree growing all the help that we can.

We can now estimate the amount of the British landscape that would need to be devoted to tree growing in order to offset in full the CO2 emissions from the flying that we do.  In total, this calculation suggests that we emit just over 70 million tonnes of CO2 per annum from flying (that’s 1.1 tonnes per person multiplied by the British population).  Dividing by Mackay’s figure for the capture rate of growing trees, we need 4.69 million hectares, or 46914 square kilometres.  Using the above figure for the size of Great Britain, that’s around 22% of the total area of our island.  So if we devote about one fifth of our land to growing trees for the purpose, we can have an independent British offset to the global warming caused by our flying habit, assuming we power our planes with no-contrail-producing biofuels.

To set that in context, the Forestry Commission in 2017 estimates that 13% of the land area of the United Kingdom is covered in woodland (I cannot quickly find a figure for GB, which is presumably a slightly lower percentage because Northern Ireland is relatively sparsely populated).  Of course none of that is devoted to offsetting carbon emissions from airplanes.  Just how realistic would it be to increase the percentage to 35% (13% plus 22%) in order to be able to claim that Britain’s use of biofuels in aircraft is self-sustaining with respect to global warming?  What happens to our farmland, or our remaining wilderness, and what about the impact on town planning, roads and railways and so forth?

It is a delusion that planting trees and using biofuels will solve the climate impact of plane travel – unless we borrow someone else’s country.  It is the sort of delusion that occurs easily to a mind eager for solutions, and is reluctantly accepted as such only after a bit of careful but entirely accessible analysis.