Global warming and global dimming

  • Jet planes produce trails known as “contrails”
  • Contrails can reflect sunlight back into space and cool the earth.
  • Contrails can blanket the earth at night and keep it warm
  • How do we know the net effect?
  • The green leaf can reverse everything

By Bill Butterworth

4 January 2015


photo (2)

Just after dawn, high “cirrus” clouds formed by the contrails of jet planes over-flying Heathrow

Some mornings when I walk the dog just after dawn, I can count 40 or more contrails (the exhaust trails of high-flying jet planes) over-flying Heathrow. Jets at cruising height on long haul flights are generally flown very high in the atmosphere because it is less energy consuming and lower cost. So far, so good. The problem is the contrail may vaporise quickly but it may, if the thin air is low enough in temperature (around minus 40 degrees C) form ice crystals and spread out forming thin, wispy “cirrus” clouds. We know that these high cirrus clouds can reflect sunlight back into space which implies that jet contrails could and do reduce global warming. Similarly, do they act as a global blanket and keep us warm at night? Now, this poses a question; will these clouds have any over-all effect on global warming? How can such a question be answered and the answer tested?

Well, it is difficult or even impossible to set up a model to test in the laboratory. However, every now and then, nature offers an opportunity to observe a real-life test. Such an opportunity occurred when the attack on the twin towers, on the infamous “9-11”, in New York occurred and the world stopped nearly all jet flying for three days. No contrails for three days.

The results of much research into this unique opportunity suggest that contrails can suppress both daytime highs (by reflecting sunlight back to space) and night time lows (by trapping radiated heat). So, they can be both cooling and warming clouds. Judging the net effect is not easy and involves some hopefully informed guesswork. Most of the judgment is that there is a slight warming effect over-all.

There is, of course, another effect and that is that these jet planes are also burning enormous amounts of aviation fuel and that means producing a lot of Carbon dioxide which has another effect on our atmosphere. If you are not a chemist, you can still follow the equations below. The molecules in aviation fuel each have between 10 and 15 atoms of Carbon. The equation, for simplicity, uses a molecule with 10 Carbon atoms in it. The sum of the atoms on the left, arithmetically matches the sum on the right; so the equation balances, nothing is lost and nothing is gained; it all just changes in form.

2C10H22 + 31O2 → 20 CO2 + 22 H2O

Or very roughly

1 tonne of aviation fuel burns 3.5 tonnes of Oxygen

to give 3 tonnes of Carbon dioxide and 1.5 tonne of water

A “Jumbo” jet will burn well over 100 tonnes of fuel on a long flight; that produces a lot of Carbon dioxide and a lot of water vapour. In a fill year, the global use of aviation fuel is around 1 billion litres, or 210 million Imperial gallons. Put it another way, the fuel used is about 300,000,000 tonnes pa, producing 900 million (yes 900,000,000) tonnes of Carbon dioxide. That must change something. However, in discussions about greenhouse gases and global warming, it is rare that anyone asks where the Oxygen in the Carbon dioxide comes from. Well, that global use of aviation fuel locks up 1.5 billion tonnes of Oxygen. Hey, that is the stuff we breathe!

Now, here we do have a known and proven answer. A green leaf takes water through its roots and Carbon dioxide out of the atmosphere in order to grow. When it does this, it gives us Oxygen back

6H2O + 6C02                   C6H12O6 +6O2

Or, in common language,

6 molecules of water, plus 6 of water, will be turned by a green leaf, into one molecule of sugar and 6 of Oxygen.

So, what farming can do, with the green leaf or algae, is to reverse the damage of burning fossilized fuels. See this blog, scroll down to 7 December 2014, “Reversing global warming” for farms that have done it – from waste and without mineral fertilisers.