NASA JPL | 09 Aug 2010
Gaze up at a cloud-filled sky, and you may spot the white, fluffy shape of a dragon, fish or elephant. Looking at the same sky, Graeme Stephens sees a different vision -- a possible future for Earth's climate.
Stephens, a professor at Colorado State University in Ft. Collins, is principal investigator of NASA's CloudSat mission, launched in 2006 to improve our understanding of the role clouds play in our complicated climate system. Stephens says that as Earth's global temperature continues to rise, water vapor -- the most abundant greenhouse gas on Earth, which traps heat much as carbon dioxide does -- will continue to build, with uncertain results.
"We're seeing that now," Stephens said. "We just don't know what this will mean for how clouds might change, and for Earth's temperature and climate. Although a small change of clouds--for example, more low clouds--in the right direction would mitigate the effects of increased carbon dioxide, a small change of clouds in a different direction--for example, more high clouds--would amplify the warming caused by increasing carbon dioxide."
Calculating the balance between the cooling or warming effect of clouds and the warming effect of greenhouse gases is a complex problem for researchers, given their current understanding of clouds on Earth. And it's just one of many questions Stephens and fellow scientists are working to address with observations from CloudSat, an experimental satellite built and managed by NASA's Jet Propulsion Laboratory, Pasadena, Calif. CloudSat's goal is to learn about clouds and their effect on climate by studying them from space.
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It's difficult to say what our world would be like if there were no clouds. But, says Stephens, "It's certain that our world without clouds would be nothing like what we know today."
Mars: A World Without Clouds (Mostly)
In fact, it might be much like Mars, says JPL planetary scientist David Kass. The Red Planet today has relatively few clouds compared to Earth. That's because the Martian atmosphere contains less than a tenth of a percent of the amount of water vapor found in Earth's atmosphere. Without much water vapor, and with temperatures averaging 80 degrees Celsius (144 degrees Fahrenheit) colder than on Earth, only thin ice clouds form. They tend to look like a thinner version of Earth's wispy cirrus clouds.
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Venus: A Greenhouse Girl Gone Wild
If Mars is what an Earth without many clouds might look like, then Venus shows what our world might look like with far more.
Venus' skies are stuffed with brilliant white clouds that stretch around the entire planet without a single break. As a result, they -- and other molecules in the atmosphere -- reflect more than 80 percent of the sun's light back out into space. For many years, planetary scientists thought this would keep the surface of Venus relatively cool. Yet when the Russian probe Venera 4 landed on the Venusian surface in 1967, it measured a temperature of 482 degrees Celsius (900 degrees Fahrenheit). That's hot enough to melt lead.
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Titan: Partly Cloudy, With a Chance of Methane Rain
There is a middle ground between Mars' relatively clear skies and Venus' cloud-choked heavens. Scattered clouds float above the icy surface and liquid lakes of Titan, the largest of Saturn's many moons. These clouds, which are made mostly of methane, punctuate the sky more in the winter than in the summer, just like clouds on Earth. By trapping in the little heat that makes it through Titan's upper level of thicker atmospheric clouds, the scattered clouds warm the surface to a frigid minus 183 degrees Celsius (minus 297 degrees Fahrenheit) on average, keeping the moon's methane lakes and rivers liquid.
NASA's Cassini-Huygens spacecraft studies Titan and its climate, in part to learn more about how cloud cover and other variables affect climate.
CloudSat: Revealing the Inner Secrets of Earth's Clouds
So what have the first four years of CloudSat operations taught us about our mysterious friends in the sky? Stephens says the mission has already yielded a number of important findings.
Among the highlights, the satellite has gathered the first statistics on global vertical cloud structure, including overlapping clouds, to create three-dimensional maps of Earth's cloud cover. It measured the percentage of clouds giving off rain at any given time (13 percent) to better understand how efficiently clouds convert condensed water into rain. It has monitored nighttime storms at Earth's poles from space for the first time. And it has revealed connections between storms at the poles and very high clouds that help create ozone.
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By viewing this complete picture of how clouds operate both inside and out for the first time, and monitoring it on a global scale, CloudSat is offering climatologists the data they need to create better models of Earth's climate -- and help predict what the surface of our planet will probably look like in the future.
So could Earth ultimately turn into a steady inferno like Venus or a fluctuating icebox like Mars? Fortunately, says Stephens, data from CloudSat and other sources show that Earth's clouds are not about to shrink drastically or engulf our skies anytime soon.
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CloudSat Mission Pages
CSU CloudSat Home
Click to play embedded YouTube video.
