Fighting global warming with mirrors in space or by seeding clouds would cool the Earth, but the side effects could outweigh the benefits, researchers have concluded.
It’s not that floating giant mirrors in space or boosting cloud growth in terrestrial skies to reflect heat from the Sun away from the Earth would fail to reduce temperatures. Both could do so, and by several degrees.
But, space mirrors could reduce rainfall considerably in certain regions, while cloud seeding might, in the worst case, actually add to the warming it is meant to mitigate.
The globe has already warmed by about 0.8 degrees Celsius in the years since the Industrial Revolution, and a rise of 2 degrees Celsius is widely viewed as a threshold to dangerous changes such as more floods, heatwaves and rising sea levels.
Some scientists believe humans might be able to intervene to turn the temperature down, but there are significant risks attached.
‘Geoengineering raises vital questions about global governance: is any nation or group of nations allowed to intentionally modify climate on a global scale?’ said the steering committee of IMPLICC, an EU-funded project bringing together researchers from France, Germany and Norway in a report. ‘Many of the side effects of such modifications cannot be predicted with any degree of certainty.’
‘The effectiveness of these techniques are unclear, and they might have undesirable side effects for crucial parts of the Earth system and humankind.’
Hauke Schmidt, Hamburg’s Max Planck Institute for Meteorology
Over the past five years, the IMPLICC team examined the effects, and side-effects, of dimming the sunlight reaching the Earth so as to tinker with its heat balance, and they have concluded that it’s a high-risk business.
Their message to policy-makers is simple: be very, very careful before you try any of this.
‘When climate change becomes more evident than today, and geoengineering is more seriously considered, it must be remembered that the effectiveness of these techniques are unclear, and they might have undesirable side effects for crucial parts of the Earth system and humankind,’ said Hauke Schmidt, deputy director of the department 'Atmosphere in the Earth System' at Hamburg’s Max Planck Institute for Meteorology, one of the institutions in IMPLICC.
Cloud land, but not cuckoo
Not that the science or the technologies behind ‘solar radiation management’ (SRM) are simple. They exist in the same realm as towing icebergs to supply drinking water, or lassoing asteroids to mine their precious minerals - amazing, yet in the realm of science-fact rather than purely science-fiction.
Of the three techniques scientists judge most suitable for turning down the Earth’s thermostat, the easiest one, in economic terms, is to manipulate marine clouds. Spraying sea water into the atmosphere above the ocean can spur the formation of numerous small, bright clouds at 1 km elevation that readily reflect sunlight back into space.
The idea relies on the so-called Twomey effect, which says that hiking water-droplet concentration within a cloud raises the overall surface area of the droplets, thereby boosting the cloud’s reflectivity.
So, the idea is to have an entire fleet of unmanned ships, driven by the wind, sailing around spraying sea water up in the air. Salt particles in the water act as nuclei when they reach the clouds above, making them bigger and more reflective.
The immediate effect of this seeding would be a local cooling of the sea surface. So the technique could be targeted at vulnerable regions, such as coral reefs or diminishing ice sheets. Especially in the tropics, and off the west coasts of continents, climate impacts can be significant.
If done at sufficient magnitude, says the IMPLICC team, the method could limit temperature increases to 0.7-1.2 degrees Celsius during the 2020-2070 period, instead of the several-degree jump projected under one of the main scenarios of global warming postulated by the Intergovernmental Panel on Climate Change (IPCC).
But, there is a problem. The size and amount of water droplets have to be just so. If too much is sprayed, the clouds become supersaturated and less reflective. If particles are too large or too small the cooling effect can fail to happen or even be reversed into warming.
The IPCC will present key findings from its latest report into climate change as part of a climate conference at the European Commission in Brussels in November.
When a volcano is not available
The second approach to shading the Earth is to promote clouds 15-50 km above ground in the stratosphere. It is much higher than conventional cloud seeding that typically happens in the lower atmosphere and uses silver iodide or dry ice. For stratospheric seeding, sulphate particles - 'aerosol' - are injected into the upper atmosphere where they remain for months or even years.
The eruption of Philippine volcano Mount Pinatubo in 1991 showed that the technique works: Earth’s surface temperatures fell in its ash-strewn wake by up to 0.5 degrees Celsius. But, of course there are no push-button volcanoes, so a man-made programme would involve launching sulphate from high-altitude airplanes or balloons, or firing it into the sky from modified artillery cannons.
Impact of climate engineering on precipitation (in mm/yr), as averaged from the results of four different Earth system models. The impact is calculated as the difference between a climate under quadrupled CO2 concentration, compensated for by a reduction of solar radiation and a preindustrial control climate. © Schmidt, H., K. Alterskjær, D. Bou Karam, O. Boucher, A. Jones, J. E. Kristjánsson, U. Niemeier et al. ‘Solar irradiance reduction to counteract radiative forcing from a quadrupling of CO2: climate responses simulated by four Earth system models.’ Earth System Dynamics 3 (2012): 63-78.The overall thermal impact of the exercise, says IMPLICC, is about the same as that of salt seeding, but again, there are problems. First is a quandary about whether the sulphate should be sulphuric acid or sulphur dioxide. Studies by the University of Potsdam suggest that the two would create significantly different cloud effects – precise differences still need to be clarified experimentally. Then there is the ozone layer that protects us from harmful UV-radiation. IMPLICC estimates that the amount of sulphate injections required to block carbon-driven warming could diminish stratospheric ozone by 5-10 %.
The IMPLICC team did not want to be drawn into speculation of how ozone depletion might cause cancer or even blindness, but they note that the depletion would be ‘large enough to be of concern, especially over populated regions near the poles’.
The final geoengineering method is most fantastic – and most expensive. At a cost running into trillions of Euros, giant mirrors could be flown into stationary orbit (at a Lagrangian point between Earth and the Sun) that would bounce the most direct light rays somewhere other than into the Earth’s atmosphere. Like cloud seeding, IMPLICC says, mirrors would work, keeping Earth’s temperatures several degrees lower than if no action were taken.
But, the project team warns, using the man-made reflectors would ‘create an entirely new climate’. Unfortunately, that would be one much drier, with rainfall cut by more than 10 % in northern Eurasia as well as parts of North and South America. Also, more of the cooling would be directed at the tropics, while temperature reductions would be less pronounced in higher latitudes.
So, the risks of geoengineering might outweigh its rewards. And even those might be distributed very unevenly.
‘The “techno fix” of geoengineering can bring greater dangers of unintended or unadvertised consequences that we cannot afford to suffer,’ says executive director Pat Mooney of the environmental-activist ETC Group, who reviewed the IMPLICC study.
Instead of learning the hard way, it might be better to keep geoengineering in the closet.
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