Feature

Missing Chapters of the Climate Story
01.25.10
 
Glory beauty pass image The Glory mission will collect much-needed measurements of aerosols and the sun's total solar irradiance. Credit: NASA/Goddard Space Flight Center
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Greenhouse gases force the climate to warm, while aerosols force the climate to cool. Greenhouse gases warm the climate (red), while aerosols cool it (blue). The error bars (thin black lines attached to the colored bars) are larger for aerosols, showing that the level of scientific understanding is lower for them than for greenhouse gases. Credit: IPCC
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testing the deployment of one solar array Engineers test the deployment of Glory's solar arrays in a clean room. Credit: NASA's Goddard Space Flight Center
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Every second, millions of tons of hydrogen fuse into helium in the sun’s core as part of a massive chain of thermonuclear reactions that yield the energy equivalent of billions of exploding hydrogen bombs. This energy eventually makes its way to the sun’s surface and radiates outward in the form of light—some of it on a trajectory toward Earth.

However, only about half of the light headed toward Earth will thread its way through the atmosphere in such a way that it affects climate. About a third ricochets off the atmosphere, and the rest is temporarily absorbed, only to be radiated back out into space soon after.

Crucial among the gatekeepers are aerosols, tiny liquid and solid particles suspended in the atmosphere. Certain aerosols reflect radiation away from Earth, while others absorb it readily. Collectively, aerosol particles—which encompass everything from industrial pollution to sea salt to dust—play a key role in influencing Earth’s climate.

For climate scientists, aerosols pose a unique challenge. In comparison to greenhouse gases, they are extremely complex, short-lived, and dynamic. As a result, determining the best way to measure their abundance, quantify their behavior, and forecast their impact remains scientifically controversial.

It’s well-understood, for example, that greenhouse gases can trap heat in the atmosphere and change the balance between incoming and outgoing energy. Scientists call such a change in the Earth's energy budget a "forcing". Such forcings can push the climate toward either warming (a positive forcing) or cooling (a negative forcing).

In contrast, aerosols are considerably more complicated. While some aerosols, such as black carbon and dust, also absorb incoming sunlight and cause warming, others reflect incoming light and force the climate to cool.

Overall, aerosols are thought to have a net cooling effect on the Earth, but many questions about the elusive particles remain. Scientists, for example, are still working out the details of how aerosols are distributed globally, how they affect clouds, and how they absorb sunlight.

Such unresolved issues have caused the IPCC to list the level of scientific understanding about aerosols as "low." (Scientific understanding of greenhouse gases, in contrast, is listed as "high".) With the aim of reducing uncertainties associated with aerosols, both the U.S. Climate Change Science Program and the Intergovernmental Panel on Climate Change have identified aerosol research as a top priority.

The Glory satellite—which contains a sophisticated aerosol monitoring instrument that collects data at nine different wavelengths from the visible to short-wave infrared spectrum—will give scientists a much improved understanding of aerosols and how they affect climate. The instrument, NASA's first-ever space borne polarimeter, will make critical distinctions between natural and manmade aerosols possible. The information Glory's polarimeter provides will be used to refine global climate models and help scientists determine how the planet is responding to its changing climate.

Glory has another important climate monitoring objective: to maintain and improve a decades-long record of total solar irradiance (TSI), the amount of solar radiation striking the Earth’s upper atmosphere each second. Though considered constant in a broad sense, solar radiation actually fluctuates slightly as the sun cycles through periods of more and less intense activity approximately every eleven years.

While scientists have concluded that such variability isn’t substantial enough to cause the warming observed on Earth in recent decades, the sun provides the baseline of Earth’s climate over the long term. Accurate records of TSI will improve the accuracy of climate models and help scientists tease out the sun’s longer cyclical changes and how they may impact climate.
 
 
Adam Voiland
NASA Goddard Space Flight Center