Why Earth was a hothouse 100 million years ago. Geography, CO2 levels may explain prehistoric swelter

IF you could visit Earth as it was 100 million years ago, you wouldn't recognize it. At that time our now-temperate planet was a hothouse world of dense jungle and Sahara-like desert overrun by dinosaurs. This period, the Cretaceous, has long fascinated scientist and layman alike. For Eric J. Barron the lure of this distant age is its dramatically different weather. Dr. Barron, a geophysicist with the National Center for Atmospheric Research (NCAR), has conducted the first quantitative studies of the Cretaceous climate. In particular, he has been exploring the relationship between the geography and the climate of this unusual period.

The geography of the Cretaceous is scarcely recognizable to today's observer. The majestic Himalayas had yet to form. The rugged Rockies consisted of hills and active volcanoes. Even the outline of the continents was unfamiliar. Because the slow but inexorable process of continental drift has had considerable effect in the intervening eons, land masses then were in different, generally more equatorial positions. South America and Africa were joined. The oceans were higher, so large parts of North America, Africa, and Europe were submerged.

Geologists have proposed that the altered position and topography of the continents account for the sweltering Cretaceous climate. With lower elevations and less land in northern regions to accumulate snow and ice, the entire globe should have been warmer, the argument goes. But a lingering question has been whether these geographic differences fully explain such a radical climatic departure. During this era, the general measure of climate appears to have been over 10 degrees F. hotter than now. This means, for example, that the weather in Greenland was Floridian, as attested by alligator-like fossils unearthed there. Evidence also suggests that the deep oceans were then about 20 degrees F. warmer than today. At the poles, no evidence of permanent icecaps has been found.

In February's Journal of Geophysical Research, Barron and his colleague, Warren M. Washington, report the results of an elaborate effort to account for the Cretaceous climate by its geography. One of NCAR's sophisticated computer models of Earth's atmosphere was adapted for this purpose. The scientists gradually altered the model's geography until it duplicated that of the earlier age. First, they removed all the electronic world's permanent ice cover, and this warmed the climate by 1.4 degrees. Bull-dozing down the mountains added another 2 degrees. Shifting the razed continents into their previous positions boosted the climate 5.6 degrees more. Raising the sea level, on the other hand, dropped the temperature average by 0.2 of a degree. And restoring the ancient elevations chilled things by 2 degrees more.

The net effect of these changes was a 6.8 degree F. increase in the global average surface temperature, significantly less than was thought to have been the case. ``You can sure change the climate by changing the geography. But the tricky part is that you can never make it warm enough,'' Barron summarizes.

Either current climate models have some serious flaws, or something in addition to what is already hypothesized was happening, the scientist has concluded. The added factor, he suggests, just might be high levels of carbon dioxide (CO2).

In the last year, several geological papers have proposed Cretaceous CO2 levels 2 to 15 times higher than today's.

Measurements show that the rate of sea-floor spread during this period was double that of current levels. So the planet's crust was being created and destroyed at an elevated rate. Old crust is destroyed when it sinks back into the bowels of the planet. And this process -- ``subduction'' -- is linked to volcanic activity. Thus, an increase in crustal destruction may have been accompanied by increased volcanic activity. And vulcanism is a major source of carbon dioxide, a gas that, like the glass in a greenhouse, traps heat in the atmosphere. So high CO2 levels could have boosted average temperatures well above that determined by geographic factors alone.

High CO2 levels may also explain the era's prodigious biological productivity. Most of the world's oil comes from Cretaceous rock formations. Petroleum derives from partially decomposed organic matter, and high carbon dioxide levels accelerate plant growth.

The creation of oil deposits requires a very specific set of conditions. Remarkably, Barron's work appears to be helpful in this regard as well. With his computer model, the scientist has identified a number of areas as likely sites for ocean upwelling during the Cretaceous. These are areas where the action of wind and water produce rising currents that are nutrient-rich and highly productive. Such intense biological activity also depletes oxygen from the water, and anoxia is necessary for hydrocarbon preservation. So ancient upwelling zones are potentially fertile fields for oil exploration. Barron's work associates Cretaceous upwelling zones with many known oil-producing areas, and also suggests similar zones in some relatively untapped areas like Antarctica.