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No 53 - July 2007

More on cooling the Earth

In Journals 48 and 51, I wrote about proposals to combat global warming by reducing insolation, so giving us more time to try to control greenhouse gas emissions.

I am not alone in believing that radiative forcing from the greenhouse gases emitted as a result of burning fossil fuels, and the associated positive feedbacks, will continue unabated, and/or because it is already too late to stop the forcing or because sufficient action will not be taken in time. Therefore, it is urgent to find other ways to cool the Earth before the results of 'global heating' (James Lovelock's appropriate term) lead to the collapse of civilisation.

Evidently, some scientists agree and have suggested various ways to cool the planet. Many of them met in November 2006 at a seminar organised by NASA and the Carnegie Institution at its Stanford (CA) station. A British newspaper described two of the geo-engineering methods discussed, a giant mirror in space and reflective dust pumped into the atmosphere, 'as a last-ditch way to halt global warming' (1). It reported that the US lobbied for such a strategy to be recommended by the Intergovernmental Panel on Climate Change (IPCC). The US wanted recognition of direct cooling methods. The US response stated: "Modifying solar radiance may be an important strategy if mitigation of emissions fails. Doing the R&D to estimate the consequences of applying such a strategy is important insurance that should be taken out. This is a very important possibility that should be considered."

The article noted that scientists have previously estimated that reflecting less than 1 % of sunlight back into space could compensate for the warming generated by all greenhouse gases emitted since the industrial revolution. Besides a giant space mirror, possible techniques include launching thousands of tiny, shiny balloons, or pumping microscopic sulphate droplets into the high atmosphere to mimic the cooling effects of a volcanic eruption. In reply, the IPCC said that such ideas were "speculative, uncosted and with potential unknown side effects". (2) (This view has been repeated in the IPCC's Fourth Assessment Report Climate Change 2007: Mitigation of Climate Change - Summary for Policymakers. Evidently, the US view did not prevail.)

A recent newspaper article (3) reviewed more of the proposed methods and an earlier TV programme featured various proposals for countering global warming (4). One of the methods was Prof. Roger Angel's idea for a shield at the Earth-Sun L1 point. However, instead of one giant shield, he proposed the deployment of millions of small glass discs each about 60 cm in diameter and weighing only 1 g (5).

The US scientists might have had in mind a suggestion by Curtis Struck, a scientist at Iowa State University in Ames. He has suggested reducing insolation by obscuring the Sun with a cloud of dust to orbit around the Earth in the orbit of the Moon at the L4 and L5 points where it is thought the Kordylewski clouds already exist. Either this dust would come from a comet somehow dragged into these positions or from Moon dust fired from the Moon's surface by a mass driver. This would provide Earth shading for decades at least, so giving time for reductions in greenhouse gas emissions to take effect. To sceptics who point out that the shadow of the Moon on the surface of the Earth, at times of solar eclipse, is very small, he pointed out that if the Moon were three times its size, subtending 1.5°, the whole Earth would be in its shadow when the Sun Earth and Moon are all lined up.*) If, as he claims, the proposed dust clouds (roughly spherical and homogeneous) subtended about 10°, they would shade the Earth. (6)

Struck does note some drawbacks, such as the brightening of the night sky as sunlight is reflected off the clouds, an increase in micrometeorites and the devastation of ground and orbit-based astronomy in many wavebands.

Notes and references:

*) Prof. Struck agrees that this is an error and that the Moon would have to be 4.68 times bigger. This note is added with his agreement.

  1. David Adam: 'US answer to global warming: smoke and giant mirrors. Washington urges scientists to develop ways to reflect sunlight as "insurance"', The Guardian, 27 January 2007.
  2. Climate Change 2001: Mitigation (section 4.7) in IPCC Third Assessment Report: Climate Change 2001.
  3. Jonathan Leake: 'Sounds crazy but it may save the planet', The Sunday Times, 18 March 2007.
  4. 'Five ways to save the world', BBC2, 19 February 2007.
  5. See:
  6. Curtis Struck: 'The Feasibility of Shading the Greenhouse with Dust Clouds at the Stable Lunar Lagrange Points'. J. of the British Interplanetary Society, Vol. 60, pp. 82-89 (March 2007).

Steuart Campbell


Cover page

Solar observing

More on cooling the Earth

Minor planet (7170) Livesey

Recent observations


Two BAA meetings in Scotland

Glen Lyon weekend 16-18 March 2007

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