Science file

This is a view of the Great Sand Sea of Egypt from the Gilf Kebir Plateau in the Sahara desert.

By Becky Oskin
LiveScience

From lakes and grasslands with hippos and giraffes to a vast desert, North Africa's sudden geographical transformation 5,000 years ago was one of the planet's most dramatic climate shifts.

The transformation took place nearly simultaneously across the continent's northern half, a new study finds. The results will appear in an upcoming issue of the journal Earth and Planetary Science Letters.

The findings come from analyses of dust blown west from Africa and dropped into the Atlantic Ocean. Researchers sifted through 30,000 years of dust and ocean bottom muck retrieved with ocean drilling ships. The changing levels of windblown dust in the ocean sediments provide scientists with clues to Africa's climate and how it has changed over time. Simply put, a lot of dust means drier conditions and less dust means a wetter environment.

The wet period, called the African Humid Period, started and ended suddenly, confirming previous studies by other groups, the sediments revealed. However, toward the Humid Period's end about 6,000 years ago, the dust was at about 20 percent of today's level, far less dusty than previous estimates, the study found.

The study may give scientists a better understanding of how changing dust levels relate to climate by providing inputs for climate models, David McGee, an MIT paleoclimatologist and lead study author, said in a statement. Sahara desert dust dominates modern-day ocean sediments off the African coast, and it can travel in the atmosphere all the way to North America.

McGee and his colleagues are now testing whether the dust measurements can resolve a long-standing problem: the inability of climate models to reproduce the magnitude of wet conditions in North Africa 6,000 years ago.

Email Becky Oskin or follow her @beckyoskin. Follow us @OAPlanet, Facebook or Google+. Original article on LiveScience's OurAmazingPlanet.

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Reuters

Rubbish is piled up in between parked cars in downtown Mexico City, Dec. 26. After city authorities shut down the Bordo Poniente landfill, one of the largest dumps in the world, garbage has started to accumulate and trucks have been slower to pick it up, according to local media.

Mexico City’s largest landfill shut down on Monday, part of a planned shift to recycle more of the city’s garbage, but the green effort left piles of trash across the city. With locals complaining, garbage truck drivers counter that they’re unable to move as much trash as before since they’re having to drive farther to get rid of it.

The new system requires drivers to haul their trash 3 to 4 hours away from downtown, whereas previously it only took an hour. “The trucks take a while to get there,” driver Joel Gara Murillo told the city’s Canal 11 TV station.

On top of that, long lines have formed at the new transfer stations while the drivers and station workers get used to the new system.

Read more about the landfill project.

Marco Ugarte / AP

A woman covers her face as she walks past piled up garbage that accumulated over the Christmas weekend in front of the Monument to Benito Juarez, one of Mexico's most important statesmen, in downtown Mexico City, Dec. 26. Garbage disposal workers complain that since last week's official closing of the Bordo Poniente city dump,one of the world's largest, they are backed up trying to get rid of the garbage.

Reuters

Rubbish is piled up next to the monument of Mexico's late President Benito Juarez in Mexico City, Dec. 26.