Dimming the Sun

There was a recent article in the Science & Technology section of the Daily Mail that said there was a plan to spray millions of tonnes of chalk into the upper atmosphere and a feasibility project was going to find out the effects of distributing 2Kg of chalk at a height of 12 miles above the earth’s surface.

There are a number of problems with this:

1. The energy involved. Balloons work on the principle of reducing the average density of the balloon so that it is less than the volume of air it would replace. When a balloon is dead on the ground an average one would weigh say about 400kg. The air then needs to be heated so the average density reduces below the lift required for the whole construction at which balancing point it would weigh approximately 3.5 tonnes, so that it could rise at a rate of about 300 metres per minute. The fuel to last an hour rising in the air would probably be about 180 litres of propane. It would probably have an effective cargo mass of 600kg. At this equivalence, say you needed 3.5 million tonnes to work, if all the mass were chalk dust then it would work out to a requirement of say 100 million tonnes of fuel to do it. Propane is C₃H₈ so has a molecule weight of about 44. When the hydrogen is swapped for oxygen the CO₂ comes to the same value, so it would produce at least an extra 100 million tonnes of CO₂.

• The scale. It’s easy lifting 2kg to 12 miles, not so easy lifting the millions of tonnes required to 12 miles. The 2010 eruption of Eyjafjallajökull sent 100 million cubic metres of tephra into the atmosphere around the right height and had a density of around 3.5kg per cubic metre, so at least 350,000 tonnes and had little temporary effect on the earth’s climate, certainly no permanent effect. So a moderate temporary effect may result from 10 times this amount. The word is temporary. To have real lasting effect you need super volcano levels of alteration. But get it wrong and there is also the potential for catastrophic overcompensation that could result in an ice age.

• Durability and time. Massively large balloons probably costing around £2 million each could probably do a couple of trips a day, the trouble with drift and rotation meaning that there would need to be constant crews retrieving and re-launching them, so say you had 1,000 of them, you could probably raise 2 x 1,000 x 6,000kg a day, so the 3,5 million tonnes would take a year to do with effects dissipating over the year.

The idea of blocking the sun is not necessarily without merit, but other things that have recently occurred may give an idea for this. When there was the 9/11 attack in the US the air temperature had a dip as a result of the removal of aircraft contrails above the US. During the day the contrails reduced the temperature of the land as more of it was reflected away, but during low light and dusk the opposite effect was observed, with the contrails reflecting escaping heat back to the ground. It would be less efficient, but contrails have a limited duration, so if aircraft were produced that caused contrails were used in the day and flying lower so they didn’t cause contrails at night an overall reduction may occur.

The other possibility is that Light from the sun is not just is a straight line, it occurs and maximises at a focal point. If a barrier was placed at 1.37 million kilometres from earth, such as a solar sail in a synchronous powered orbit, a significant part of the suns disk could be obscured.