A new study has suggested that during the age of dinosaurs about 100 million years ago, a warming spell caused cloud cover to drastically decrease, which helped to drive temperatures even higher.
According to a report in National Geographic News, average tropical temperatures during that era of the Cretaceous exceeded 100 degrees Fahrenheit (38 degrees Celsius), and the Polar regions were in the 50-degree-Fahrenheit (10-degree-Celsius) range.
In fact, atmospheric levels of the greenhouse gas carbon dioxide were four times higher then than they are today, scientists estimated.
"While high, though, that doesn't seem to be sufficient to get the type of warmth that the temperature data suggest," said Lee Kump, a geologist at Pennsylvania State University.
Now, Kump and colleague David Pollard have suggested that a reduction in cloud cover provided the temperature boost.
The researchers suggested that the reduction in Cretaceous cloud cover stemmed from a drop in cloud condensation nuclei, tiny particles around which water droplets form.
Today about half of such nuclei are human-produced pollutants. During the Cretaceous, the particles were derived mostly from plant material.
According to some climate models, warm global temperatures during the Cretaceous caused a decrease in plant growth. This was especially true in the oceans, because fewer nutrients were able to rise up to feed algae in the warm surface waters.
If the models are correct, this also reduced particulate matter in the atmosphere derived from plants and marine algae. Less particulate matter means fewer reflective cloud droplets.
"So the clouds wouldn't be as bright," said Kump.
Less brightness means less of the sun's energy is reflected back into space. The reflection is known as the albedo effect.
Starting with this premise, Kump and Pollard plugged various scenarios of reduced nuclei into a climate model to determine if they could account for all of the Cretaceous warming.
In addition to less brightness, they found the reduced particles led to bigger water droplets. Since droplets fall as rain when they reach a certain size, this also caused a reduction in cloud cover.
The scenario that best accounted for the warmth had cloud cover shrinking from 64 percent of the atmosphere to 55 percent.
"The system is a positive feedback loop. More warming leads to less biological productivity, which leads to even less cloud cover and more warming," said Kump.
"There is a lot of global warming potential in this feedback," he added.