Carbon Dioxide Levels Are Highest in Human History

The amount of planet-warming carbon dioxide in the atmosphere broke a record in May, continuing its relentless climb, scientists said Friday. It is now 50 percent higher than the preindustrial average, before humans began the widespread burning of oil, gas and coal in the late 19th century.

There is more carbon dioxide in the atmosphere now than at any time in at least 4 million years, National Oceanic and Atmospheric Administration officials said.