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," Kump said.
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, Kump added, is a positive feedback loop. More warming leads to less biological productivity, which leads to even less cloud cover and more warming.
"There is a lot of global warming potential in this feedback," he said.
Jury Still Out
Meinrat Andreae is a biogeochemist at the Max Planck Institute for Chemistry in Mainz, Germany.
He wrote a paper published last year in Science that estimated the pre-human concentration of cloud condensation nuclei over land and sea. That paper triggered Kump and Pollard to pose their theory that reduced particle production led to lower albedo and thus enhanced warming.
Andreae said the jury is still out on how biological production responds to warming but that the scenario presented by Kump and Pollard is plausible.
"The system is quite sensitive to that kind of change in [particulate] concentrations at the low background natural level that presumably would have existed at that time," he said.
But the study's implications for future warming are uncertain, he said.
In addition to carbon dioxide, for example, humans are filling the atmosphere with other pollutants such as aerosol particles.
Kump noted that climate models indicate that the warming of the past 30 years would have been much greater "had it not been for the presence of pollutant aerosols in the atmosphere."
Today, thanks to environmental laws like the Clean Air Act, he said, the pollutant aerosol load is dwindling and thus having less of a masking effect on global warming.
So scientists need to determine if biological production really does decrease under higher greenhouse gas concentrations.
"If there's a missing feedback that we can demonstrate was very important in establishing paleoclimates," he said, "I think that it's imperative we work that into the models."
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