A paradox about the climate of the early Earth that has beenplaguing scientists for nearly 50 years may have a new solution.

The so-called 'young'sun paradox ? first proposed by Carl Sagan and his colleague George Mullen in1972 ? refers to the fact that the Earth had liquid oceans for the first halfof its more than 4-billion-year existence despite the fact that the sun waslikely only 70 percent as bright in its youth as it is now.

A lower solar luminosity should have left Earth's oceansfrozen over, but there is ample evidence in the Earth's geological record thatthere was liquid water ? and life ? on the planet at the time.

Over the past few decades, scientists have proposed severalpossible mechanisms that may have kept the Earth toasty enough to keep waterfrom freezing during our planet's early history ? a period of time called theArchaean. But just when scientists think they have the paradox solved, otherresearchers come up with alternate explanations or reasons why a previousproposal doesn't work.

"It keeps resurfacing," said atmospheric scientistJim Kasting of Penn State University, who put forward his own explanation for theyoung sun paradox in 1980s and '90s. That explanation involved a greenhousegas effect that would have kept the planet warm ? similar to thehuman-driven effect that is warming the Earth today. The early greenhouse,first proposed by other scientists in the 1970s, would have been on a muchlarger scale than current climate warming, with theoretical calculationssuggesting that about 30 percent of the Earth's atmosphere at the time consistedof carbon dioxide. For comparison, today, Earth's atmosphere is about 0.038percent carbon dioxide.

A powerful greenhouse effect on the early Earth is "theobvious solution" to the paradox, said Minik Rosing of the University ofCopenhagen in Denmark. Rosing and his colleagues have offered up a new explanationfor the seeming paradox that is detailed in the April 1 issue of the journalNature.

Carbon dioxide constraints

To see what carbon dioxide (CO 2 )concentrations might have actually been in the Archaean, Rosing and his teamanalyzed samples of 3.8-billion-year-old mountain rock from the world's oldest ?sedimentaryrock, called Isua, in western Greenland.

The samples contain features called banded iron formations(BIFs) that formed in abundance when the Earth was young, but not since. TheseBIFs contain certain iron-rich minerals that give clues as to the atmosphericenvironment in which they formed.

"The analyses of the CO2 content in the atmosphere,which can be deduced from the age-old rock, show that the atmosphere at thetime contained a maximum of one part per thousand of this greenhouse gas. Thiswas three to four times more than the atmosphere's CO2 content today. However,not anywhere in the range of the 30 percent share in early Earth history whichhas hitherto been the theoretical calculation," Rosing said.

So Rosing and his colleagues looked at another avenue thatcould explain the paradox.

All about albedo

One of the factors that partly determines the temperature ofthe Earth is the amount of incoming sunlight the Earth's surface and atmospherereflect back to space, called the planet's albedo.Different types of surfaces reflect or absorb different amounts of light ? forexample, ice is highly reflective, while the open ocean is highly absorptive.

Rosing and his team looked at two possible influences on theearly Earth's albedo: the amount of land on the planet's surface and the amountof cloud cover in the atmosphere.

Geologists haven't yet determined when the Earth'scontinents first formed, but radioactive tracers in the hot rock of the Earth'smantle can help determine the rate at which the crust of the planet formed, hintingat how much land was exposed above the oceans.

Rosing and his colleagues suggest that there was lesscontinental area on the early Earth, and because oceans are more absorptive ofsunlight than land, the Earth's albedo would have been slight lower, meaningthe Earth's surface would have absorbed slightly more sunlight than it doestoday.

A bigger effect might have been the thinner cloud cover ofthe early Earth, which could have allowed more sunlight through the atmosphereto reach the surface.

"The reason for the lack of cloud [cover] back inEarth's childhood can be explained by the process by which clouds form,"Rosing said.

The droplets of water that make up clouds form by glommingon to tiny particles, called cloud condensation nuclei, many of which arechemical substances produced by algae and plants, which weren't present on theEarth at that time.

Rosing and his team came to this conclusion by observingareas of the present-day ocean that have very little biological activity andthin cloud cover, which "shows that the clouds are different in suchplaces" and therefore were likely the same for the early Earth.

Any clouds that did form would have had larger drops ? ashappens when cloud condensation nuclei are in low supply ?? which are moretransparent to sunlight and so would have allowed more through to reach theEarth's surface, keeping it warm.

So the combination of less continental area and anatmosphere more transparent to sunlight could explain why the Earth didn'tfreeze over, despite the smaller amount of sunlight.

But this explanation may not settle the paradox for allscientists who have looked into the problem.

Potential controversy

Kasting, who wrote an accompanying editorial piece to thenew study also appearing in Nature, had several critiques of the explanation tothe paradox.

The part of the study he found most interesting was theanalysis of the BIFs to determine the amount of carbon dioxide in the ancientatmosphere.

"But I think that's going to be controversial," Kastingtold SPACE.com, as other researchers have looked at the same rock and come tothe completely opposite conclusion about the carbon dioxide content, suggestingthat it contained substantially more than Rosing and his team concluded.

To figure out the issue once and for all, geochemists needto come up with a model that explains how the BIFs formed, something that hasbeen missing from the equation up until now.

Kasting also wasn't sure that thinner cloud layer could explainthe paradox.

"I'm not that sold on the cloud-feedbackmechanism," he said. In part this is because the temperature that thethinner clouds would boost Earth up to isn't as warm as scientists think theEarth was during the Archaean, he said. "It just barely gets you up to thefreezing point."

Rosing counters though that not all scientists agree withthe evidence that has been used to suggest that the early Earth was a very warmplace.

So while the new research provides a plausible explanationfor what kept the early Earth from freezing over, the paradox isn't likely tobe declared solved anytime soon.

"We keep solving it, and someone comes along and tellsyou that you haven't solved it right," Kasting said. Still, other studiesare already in the works with other possible explanations for the young sunparadox, he added.