Now University of Washington researchers, using evidence from fossilized raindrop impressions from 2.7 billion years ago to deduce atmospheric pressure at the time, have demonstrated that an abundance of greenhouse gases most likely caused the warm temperatures.
The new work allowed the scientists to determine limits of ancient air pressure by comparing raindrop impressions from today with the fossilized impressions from a time when there were no plants or animals on Earth but the planet was teeming with microbes.
The sizes of raindrop impressions depend on raindrop velocity, the atmospheric pressure and the composition of material into which the raindrops fall.
Buick and Harnmeijer poured latex over the raindrop impressions in fossilized volcanic ash found in South Africa, then returned the latex peels to Seattle, where high-precision laser scanning produced detailed measurements.
Meanwhile, to get a measure of raindrop impressions under present-day atmospheric pressure, Som and Polivka used a pipette to release varying sizes of water drops down an open stairwell into recent volcanic ash collected from Hawaii and Iceland, held in a tray nearly 90 feet below.