MadSci Network: Astronomy
Query:

Re: Can we create an artificial solar eclipse to cool the earth?

Date: Tue Feb 20 12:01:37 2007
Posted By: James Holliday, Grad student, Physics Department, University of California, Davis
Area of science: Astronomy
ID: 1171316282.As
Message:

Fernando,

For the sake of others reading this thread, let me describe what we mean by global warning. Global warming is defined as the gradual increase in planet-wide temperatures. A panel convened by the United States National Research Council (my country's premier science policy body) in June 2006 voiced a "high level of confidence" that Earth is the hottest it has been in at least 400 years, and possibly even the last 2,000 years. Studies indicate that the average global surface temperature has increased by approximately 0.3-0.6C over the last century. This is the largest increase in surface temperature in the last 1,000 years. Furthermore, scientists are predicting an even greater increase over this century.

Many scientists believe that average global temperatures may increase by an additional 1.4-5.8C by the end of the 21st century. These numbers may sound small, but they can trigger significant changes in climate. For example, the difference between global temperatures during an Ice Age and an ice-free period is only about 5C. Besides resulting in more hot days, an increase in temperatures may also lead to changes in precipitation and weather patterns. Warmer ocean water may result in more intense and frequent tropical storms and hurricanes. Sea levels are also expected to increase by 0.09 - 0.88 meters in the next century, mainly from melting glaciers and expanding seawater. These changes will likely affect wildlife and species that cannot survive in warmer environments, causing them to become extinct. Finally, because of global warming our own health is at stake since global warming may result in the spreading of certain diseases such as malaria, the flooding of major cities, a greater risk of heat stroke for individuals, and poorer air quality.

So what can we do to help fight global warming? Some scientists actually are looking at ways to decrease the amount of solar radiation that reaches the earth. Most of these ideas are currently only computer simulations, but some are being tested today.

One popular idea is to try shading the planet with an orbiting array of mirrors. The US National Academy of Sciences found that 55,000 orbiting mirrors would reflect enough sunlight to counter about half the doubling of carbon dioxide. Each mirror, however, must be approximately 100 square km in size if they are to be built on earth. Larger mirrors would require a manufacturing plant on the Moon. This limitation, along with the price of putting the mirrors into orbit, makes it a prohibitive choice.

A second idea is a giant sunshade. A sunshade is a planetary scale parasol. Megastructure sunshades in orbit have been proposed by science fiction writers and more recently are becoming serious engineering proposals, both for Earth and for terraforming Venus. In theory, a sunshade with a diameter of 210 km can be orbited at an altitude of 16,093 to 24,140 km. This would put it in a solar-synchronous orbit. The sunshade could then produce a continuous solar eclipse which would circle Earth's tropic zone continuously. In theory, the sunshade would then reduce the solar input to Earth by an amount sufficient to reverse the recent global warming trends and cancel the effect of green house gases put in Earth's atmosphere by humans. Unfortunately, the weight of such a sunshade and the price of putting it into orbit, makes this this choice prohibitive as well. Carbon nanotubes, however, may eventually reduce the mass of a viable sunshade considerably and make possible a space elevator to move the sunshade materials to orbit.

A third idea is one called the "human-volcano" approach. This approach is fully earth based, and is therefor much more cost efficient. Inspired by studies of the Mt Pinatubo eruption of 1991 and the cooling effect of its sulphur plume, some scientists suggests that naval guns shoot sulphur pellets into the air to increase Earth's albedo, or reflectivity. This would have the same effect of a sunshade but at much less the price. One of the problems of putting sulphate particles in the stratosphere, however, is that it would destroy the ozone layer. Unless a work-around can be found, sulphur pellets would solve the global warming problem and doom mankind in the process.

It's important to note that the biggest problem with any of these approaches is that once we've figured out a way to build and put the shields in orbit, we must then figure out how to get the balance right. Current computer models are not up to the task of predicting the consequences of large-scale plans such as Earth shades. It's hard to predict the side effects of reducing the amount of solar light that hits the Earth, though it is probably easier (relatively) to fix unexpected problems by adding or removing solar shields than it is to affect the atmosphere in other ways.

Overall, protecting against incoming radiation is costly, difficult to implement, and only addresses a small portion of the overall problem. Global warming is largely attributed to the increase of greenhouse gases in the Earth's upper atmosphere caused by human burning of fossil fuels, industrial, farming, and deforestation activities. These greenhouse gases reabsorb heat reflected from the Earth's surface, thus trapping the heat in our atmosphere. This natural process is essential for life on Earth because it plays an important role in regulating the Earth's temperature. However, over the last several hundred years, humans have been artificially increasing the concentration of these gases, mainly carbon dioxide and methane in the Earth's atmosphere. These gases build up and prevent additional thermal radiation from leaving the Earth, thereby trapping excess heat.

That's not to say there's no contribution from solar radiation. Unfortunately, some uncertainty remains about the role of natural variations in causing climate change. Solar variability certainly plays a minor role, but it looks like at most less than a quarter of the recent variations can be attributed to the Sun. During the initial discovery period of global warming, the magnitude of the influence of increased activity on the Sun was not well determined.

Solar irradiance changes have been measured reliably by satellites for only 30 years. These precise observations show changes of a few tenths of a percent that depend on the level of activity in the 11-year solar cycle. Changes over longer periods must be inferred from other sources. Estimates of earlier variations are important for calibrating the climate models. While a component of recent global warming may have been caused by the increased solar activity of the last solar cycle, that component was very small compared to the effects of additional greenhouse gases. According to a NASA Goddard Institute for Space Studies (GISS) press release, "...the solar increases do not have the ability to cause large global temperature increases...greenhouse gases are indeed playing the dominant role..." The Sun is once again less bright as we approach solar minimum, yet global warming continues.

In order to really fight global warming, to help protect the polar ice-caps from melting, we should focus our attention first here on earth.

For more information, check out the following links:

I hope this has helped.

Sincerely,

James R Holliday


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