Some records we’d prefer not to be broken. Carbon-dioxide levels in the atmosphere surpassed 400 parts per million for the first time since the U.S. government began recording the concentration of the gas in the air, and likely the first time in millions of years. High levels of carbon dioxide traps heat in the atmosphere and increases global temperatures. The last time it reached 400 parts per million was in the epoch called the Pliocene at least 3 million years ago, when sea levels may have been as much as 60 to 80 feet higher than they are today. For all of human civilization, carbon-dioxide levels have hovered around 280 parts per million.