Science Bits

National Center for Supercomputing Applications, Pittsburgh Supercomputing Center, San Diego Supercomputer Center, NCAR, University of Illinois

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Planetary waves have a role in plankton production

In a paper published in Nature, Mete Uz, James Yoder, and Vladimir Osychny reported on their two-year analysis of satellite- derived data on chlorophyll and sea-surface height. The data show that oceanic planetary waves—which travel in a westward direction—are associated with 5–20% of the variability in chlorophyll concentration. The scientists theorize that disturbances in the ocean caused by these waves are bringing nutrients to surface waters on a global scale and affecting the production of phytoplankton.

Uz and his collaborators used ocean color data from NASA's SeaWiFS (Sea- viewing Wide Field-of-View Sensor) program and concentrated on the role of large planetary, or Rossby, waves in enhanced biological productivity. These waves can be hundreds of miles wide, but they are only a few centimeters in height, making them observable only through satellite imagery. A Rossby wave moves so slowly that it could take months or years to cross the ocean. As the wave passes, it causes water motion that pumps more nutrients from the depths to the surface waters of the ocean.

"For physical oceanography, this study presents one more tool with which to observe Rossby waves, especially their vertical dynamics," said Uz. "For biology, it illustrates for the first time a wave propagating through an ecosystem. This means that at the large spatial and temporal scales at which these waves operate, one cannot think of biology as a functional block separate from physics. If you want to guess what the productivity will be at a given location and time in the future, you cannot use a purely biological model. The coupling between physics and biology is important."

The study also adds indirectly to the scientific knowledge about the oceanic carbon cycle. Uz noted that variability in the oceans' biological productivity has a significant impact on projections of global climate change. Because we do not know enough about what controls productivity in the oceans, he said, "we are not in a position to make a reasonable guess about how the ocean will respond under different scenarios of global change. However, our research puts in place one more piece of the jigsaw puzzle of [the] oceanic carbon cycle."

University of Rhode Island

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Scientists watch dark side of the Moon to monitor Earth's climate

Earthshine is readily visible to the naked eye, most easily during a crescent moon. Leonardo da Vinci first explained the phenomenon, in which the Moon acts as a giant mirror, showing the sunlight reflected from Earth. The brightness of the earthshine thus measures the reflectance of Earth.

In the 1 May issue of Geophysical Research Letters, a team of scientists from the New Jersey Institute of Technology (NJIT) and the California Institute of Technology report that Earth's albedo is currently 0.297, with a margin of error of 0.005.

"Earth's climate is driven by the net sunlight that it absorbs," says Philip Goode, leader of the NJIT team and director of the Big Bear Solar Observatory. "We have found surprisingly large—up to 20%—seasonal variations in Earth's reflectance. Further, we have found a hint of a 2.5% decrease in Earth's albedo over the past five years." If Earth reflected even 1% less light, the effect would be significant enough to be a concern with regard to global warming.

In the early 20^th century, the French astronomer Andre-Louis Danjon undertook the first quantitative observations of earthshine. But the method lay dormant until Caltech professor Steven Koonin described its modern potential in a 1991 paper. The newly published data are the first that are precise and systematic enough to infer the relative health of Earth's climate.

The new albedo measurements are based on about 200 nights of observations of the dark side of the Moon at regular intervals over a recent two-year period, and another 70 nights during 1994–95. Using a six-inch refractor telescope and precise charge-coupled devices at Big Bear, the researchers measure the intensity of the earthshine. By simultaneously observing the bright "moonshine" from the crescent, they compensate for the effects of atmospheric scattering.

The study relies on averages over long periods, because the albedo changes substantially from night to night with changing weather, and even more dramatically from season to season with changing snow and ice cover. The locations of land masses also affect the albedo as Earth rotates on its axis.

It is significant that earthshine data suggest that the albedo has decreased slightly during the past five years, as the Sun's magnetic activity has climbed from solar minimum to maximum during that time. This supports the hypothesis that the Sun's magnetic field plays an indirect role in Earth's climate.

The study was funded by both NASA, beginning in 1998, and the Western Center for Global Environmental Change during 1994–95.

California Institute of Technology, New Jersey Institute of Technology

Edited by Carol Rasmussen, carolr@ucar.edu
Prepared for the Web by Jacque Marshall
Last revised: Thu Jun 21 18:56:13 MDT 2001