Low levels of chlorine on planet's surface have long puzzled scientists. (p. 14)

Methane, ozone and other short-lived pollutants have a big impact on ocean heights, simulation finds. (p. 20)

Some scientists have proposed designating a new geological epoch, the Anthropocene, that would cover the period since humans became the predominant environmental force on the planet. But when would you have it begin? Some geologists argue that the Anthropocene began with the Industrial Revolution, when fossil fuel consumption started influencing climate. Others point back several thousand years earlier to the onset of agriculture, when humans cleared swaths of forest to make way for neat little rows of cultivated crops. But the roots of the Anthropocene may stretch even farther, deep into the...