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Some folks propose to deal with global warming by putting up a big umbrella. Calculations show that a parasol just (!) 800 kilometers in radius, placed in space where Earth's and Sun's gravity balance, could reflect enough sunlight to offset the global warming caused by a doubling of carbon dioxide in the atmosphere.

This doubling is expected to raise Earth's surface temperature by several degrees Celsius, in the fastest warm-up since modern humans began tromping the planet 10,000 years ago. The obvious preventative would be to burn less fossil fuel. But that does not seem to interest governments.

So scientists are considering more far-fetched options to prevent a global roasting.

Like the giant parasol.

3 maps show vegetation growing more intensely with increase in carbon dioxide. The geoengineered map, showing effects of the sun shade, barely changes the effect doubling carbon-dioxide.
Annual production of biomass per square meter a) today, b) after carbon dioxide doubles, and c) with doubled CO2 and a space-based sun shade. According to the model, the sun shade barely reduces plant productivity. Is this good? Information and maps from Lawrence Livermore National Laboratory.

But even if such a huge umbrella could be launched and maintained in space, would the idea make sense? Or would the umbrella reduce sunlight, photosynthesis, plant growth and plant uptake of carbon dioxide? Would the ironic result be more global warming?

No, according to a computer model published in November. In fact, net primary production (NPP) -- the tonnage of plant biomass produced each year -- would rise by 72 percent with a parasol, almost as much as without it. "Critics suggest that 'turning down the sun' could harm terrestrial ecosystems that depend on light for photosynthesis, but this new work shows that a change in solar flux to stabilize climate would have little effect on the terrestrial biosphere," wrote Ken Caldeira, a researcher with the Lawrence Livermore National Laboratory, in a press release describing the research.

We'll return to that idea in a moment, but first, how was the research done?

The Lawrence Livermore researchers linked a global circulation model to software that simulates plant growth. They then balanced the warming caused by a doubled level of carbon dioxide by erecting a virtual shield that blocked 1.8 percent of incoming sunlight.

  Surface Temperature Change (degrees C) Plant biomas (NPP in billions tons carbon per year)
Control 0.0 56.4
Doubled 2.42 100.1
Geoengineered 0.14 97.1

Table data: Govindasamy et al.

Despite the reduction, plant productivity barely changed. "It went against our working hypothesis," says Bala Govindasamy, an atmospheric scientists at Lawrence Livermore's climate and carbon cycle modeling group, and chief researcher on the study. "Photosynthesis is affected by sunlight, water and CO2, and ... if there is a reduction in sunlight, I was expecting a significant reduction in photosynthesis... but we did not see that for 1.8 percent reduction in sunlight. In this model, sunlight had a very weak influence on the biosphere compared to the increase in CO2."

From the standpoint of controlling global warming, higher plant productivity is a boon. Photosynthesis removes carbon dioxide from the atmosphere and stores it in plants, thus serving as an important control on global warming caused by burning stuff.

The ol' warning label
As carbon dioxide increases, plants will grow faster. Would a giant sun shade slow their growth? No single computer model can be an exhaustive description of reality, or a perfect prediction of future conditions, and the Lawrence Livermore effort is no exception. For one thing, the model did not consider plants in the ocean, which are about equal to land plants in terms of primary productivity. For another, the model concerned only light, temperature, water, and carbon dioxide, while ignoring limitations due to the availability of nitrogen and other important plant nutrients. "The story is not just about CO2; lots of minerals are needed for photosynthesis," Govindasamy acknowledges.

Still, Lawrence Livermore billed the study as good news, under this headline: ("Livermore Researchers Determine Biosphere Unaffected by Geoengineering Schemes"). Indeed, the press release quoted Govindasamy as saying, ""Our models show plant life getting a big boost from the carbon dioxide fertilization when atmospheric CO2 levels are doubled due to anthropogenic fossil fuel emissions. We noticed that in a CO2-enriched world, the terrestrial biosphere was largely unaffected by decreases in surface solar radiation by a couple of percentage points through various geoengineering schemes."

But from an ecological standpoint, such a drastic increase in primary production would almost inevitably cause major changes in species numbers and locations. While farm productivity might rise, the change would probably favor easily adapted invasive species, promoting rapid change in ecosystems.

A giant sun shield would do little to slow the increase in plant growth caused by a doubling of carbon dioxide. Is that a good thing?
A political economy
The issue of global warming, like all major scientific problems, has technological, political and economic dimensions. It's not just the problem of coming to terms with the overuse of fossil fuels, but also the enormous complexity of the parasol solution. "Geoengineering would need an international consensus, it would be very hard to achieve," says Govindasamy. "It's not a one-time effort, the shield would have to be maintained for a long period. It would really be an international effort, and I think it will be very hard to achieve. My own view is that prevention is better than cure, it's better not to burn fossil fuels."


But here's a final thought. Climate, as Govindasamy notes, "is a nonlinear system." Translated: surprises are predictable. What cannot be predicted is whether the surprises will consist of a change in ocean currents that freezes Europe, a massive release of carbon dioxide stored in Arctic permafrost that accelerates global warming, or perhaps a massive melting of Antarctic ice.

You burns your gasoline, and nature makes her choice.
-- David Tenenbaum illustration of an Earth with a face on it


Impact of Geoengineering Schemes on the Terrestrial Biosphere, Bala Govindasamy et al, Geophysical Research Letters, 26 Nov. 2002.


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