Boston — Ever since scientists first speculated about the ``greenhouse effect,'' the idea has seemed plausible, if alarming. The theory goes like this: Fumes from the engines of modern industry are causing a buildup of the carbon-dioxide gas enveloping the planet. As a result, the sun's energy is being increasingly trapped in Earth's atmosphere. The phenomenon heralds profound climatic changes, scientists reason, as world temperatures begin to rise.
The problem is, no one can say for sure whether or when the greenhouse effect will happen as predicted. And while similar phenomena may have occurred during earlier periods of Earth's history, until now scientists have been unable to say when.
Under a tract of ocean stretching from Tahiti to the Panama Canal, however, two geologists from the University of Michigan say they may have found the long-sought-after historical precedent for the greenhouse effect. If their case holds up under scrutiny from the scientific world, it will be the first solid evidence of a previous global temperature rise caused by a buildup in carbon-dioxide levels. The discovery may lead to more precise predictions about any future greenhouse warming cycle.
As announced today in the journal Science, the discovery came from laboratory analyses of core samples gathered from crust of the East Pacific Rise, an underwater mountain range running from the Gulf of California through the southeast Pacific Ocean at a point of ``sea-floor spreading'' -- where hot magma from Earth's underlying mantle pushes through the crust to the surface, cools, and forms a new sea floor.
Michigan researchers Robert Owen and David Rea surmised from the samples that during an early segment of the Eocene Epoch 50 million years ago, unusually vigorous geologic activity dominated the chemical changes in the world's oceans. That, in turn, led to a increase in worldwide carbon dioxide (CO2) levels, higher atmospheric temperatures, and an altered climate.
The Eocene period was one of great change. It was an epoch with a muggy climate that saw the extinction of many types of mammals and the rise of others, like rodents and whales. While debate continues over what combination of factors -- weather and other conditions -- affected such changes, Dr. Rea says that the link between geologic activity and an increase in CO2 levels is clear. ``Our calculations fit right in with the geologic record,'' he says.
Many other periods in Earth's history witnessed warm climatic spells. The first half of the Mesozoic era, for example, was characterized by a steamy, swampy climate where dinosaurs flourished. While high CO2 levels and volcanic activity have been invoked as causes, such explanations are speculative.
``There has been lots of geologic data to say that it's been very hot or very cold,'' explains Rea. ``But this is the first time a specific cause has been linked with a specific climatic effect.''
The work involves the science of plate tectonics, the study of Earth's crust and the forces that cause changes in it. Earth's outer shell is paved with eight large plates and several smaller ones, which over time joust for position. In the short run, they cause earthquakes. On a grander scale, these maneuverings can lead to major rearrangements in the plate boundaries about every 15 million years. When that happens, vast amounts of heat and minerals are spewed into the sea. The iron, calcium, silica, and roughly 30 other elements that seep out from the mantle during all this hydrothermal activity cause vast changes in ocean chemistry and worldwide climate.
Calcium, for example, goes through a complex sequence of chemical reactions in order to help produce CO2 in the ocean. Under ``normal'' conditions, the oceans contribute about 20 percent of the atmosphere's CO2. But a major tectonic rearrangement can cause four times the amount of such minerals as calcium into the ocean environment. That, the Michigan team calculates, means the amount of atmospheric CO2 would double.
Rea says the virtue of this study is that it provides a hypothesis that can be readily tested. Says Rea: ``The next logical step would be to look at other Eocene sites'' -- places that underwent similar geologic activity at the same time -- to see if the same mineral concentrations found in the East Pacific Rise samples are found elsewhere. Then, further studies would be needed to see if the Michigan hypothesis applied to other geologic time frames.
If they do, this discovery could go a long way toward refining the young field of paleoclimatology -- the study of how past climate changes occured -- in order to determine how future changes might take place.
``In a football game, you guess who will win based on their past record, not on the color of their uniforms,'' observes Rea. ``Why should climatology be any different?''
The samples were collected during a two-month cruise in early 1983 by the world-roving drill ship Glomar Challenger as part of the Deep Sea Drilling Program. During the 15-year, internationally sponsored project, the Challenger made 96 voyages and bored ocean-floor samples at over 600 sites.
A successor project, the Ocean Drilling Program, is scheduled to get under way this month.