Prince of Tides

Party planners may have one more thing to worry about in planning their event. To the list of volcanoes, sunspots and other phenomena known to affect the weather, scientists have now added another potential element: ocean plankton.

Though tiny, these ocean-floating organisms may influence weather patterns all over the world, particularly in the tropics. Dr. Hezi Gildor of the Institute’s Environmental Sciences and Energy Research Department revealed this potential link through computer models he designed to examine how nature’s complex web of interacting elements determines global climates.

Plankton drift with ocean currents. They range in size from the microscopic to the barely visible, but they are so abundant their populations can be tracked from orbiting satellites. NASA tracks plankton because they make up the bottommost levels of the marine food chain, and the health of the entire ocean may depend on them.

One of Gildor’s models involves two major groups of plankton: plant-like phytoplankton, which, like their rooted cousins, take up sunlight, carbon dioxide and nutrients and convert them into sugars using chlorophyll; and zooplankton, animal-like organisms that live off the phytoplankton. Under certain conditions, the predator population grows at the expense of its prey until the latter’s dwindling amounts can no longer sustain it. At this point, the zooplankton population drops and its prey (phytoplankton) bounces back, and so on. These repeating cycles, or predator-prey oscillations, can be described mathematically.

Oscillation patterns are seen in global climate systems as well. The western Pacific Ocean is a case in point. The amount of rainfall in this tropical region swings through a cycle every 40-50 days, and the temperature of the surface water beneath oscillates in a more or less corresponding cycle. (Interestingly, even small rises in this region’s oceanic surface temperature can affect weather all across the globe, leading, through a complicated set of interactions, to such far-flung climatic phenomena as rain in India or floods in South America.)

Gildor and colleagues at Columbia University in New York wondered whether these two cycles - of oceanic temperature levels and plankton populations - might somehow be connected.

Their key clue was the phytoplankton’s chlorophyll. Built to absorb light, chlorophyll can block a portion of the warming sunlight that penetrates the ocean’s surface. When conditions are right, plankton congregations can be so dense they effectively shade the water below. Therefore, changes in phytoplankton numbers could affect sea water temperatures.

To test their theory, the team put together a complex simulation based on existing models of three dynamic systems: the atmosphere, ocean water and plankton. They then ran the model to simulate ten months of weather over the tropical Pacific, alternately with and without the plankton component, to see if there were any differences between the two situations.

Their study suggested that the plankton cycle interacts with changing atmospheric conditions, such as cloud formation. Clouds disrupt the normal flow of energy from the sun into the water and from the water back out toward space. As a result, cloud formation affects weather stability along a simple scale: When the level of cloud interference in the atmosphere is low, weather patterns tend to be stable (characterized by un-changing rainfall levels), whereas a high level of cloud interference is characterized by increased instability, in which the system swings between periods of heavy rainfall and clear skies.

But put the phytoplankton into this equation and the scales shift even further. Gildor showed that at the mid-cloud range, where the weather is usually stable, the presence of phytoplankton (due to the natural “ups” of its population cycle) affects the system, driving it toward increased instability. Moreover, as the level of cloud interference rises into the realm of instability, the plankton further influence rainfall patterns, significantly cutting the transition period from clear skies to rain. “It turns out that not only the flap of a butterfly’s wings in Brazil can set off a tornado in Texas, but plankton in the Western Pacific can cause rain in India,” says Gildor.

Cracking the Ice Age

In other research, Gildor applies computer models to examine the history of ice ages on Earth. In the “Sea-Ice-Switch” model, developed together with Prof. Eli Tziperman of the same department, ice forming on the ocean’s surface was found to play a major role in regulating the switch from climatic heating to cooling and back.

Such models are judged by how well they explain existing climate records. Gildor and Tziperman have successfully used the model to explain the mechanism that makes ice sheets advance and retreat; why recent ice ages took place in cycles of 100,000 years, whereas over a million years ago the cycles lasted only 41,000 years; and why CO₂ levels in the atmosphere decreased as the ice advanced.

Dr. Gildor’s research is supported by the Sussman Family Center for the Study of Environmental Sciences and the Sir Charles Clore Prize - the Clore Foundation.