Earth has a climate conundrum: the amount of carbon dioxide released globally each year has, for the most part, stopped increasing.

• But for some reason, CO2 has continued to build up in the atmosphere at a fast pace, reports the New York Times. In 2015 and 2016, amounts of excess CO2 in the air increased at a faster rate than any previous year. It slowed in 2017, but the rate of increase is still above average.
• What's happening: Unclear. Although nations are still pumping out high levels of CO2 and greenhouse gases, it's less than in previous years. Some researchers think that something could be happening to the carbon "sinks" like the ocean and forest, which help store excess carbon and curb warming.
• Why it matters: If carbon sinks are in trouble, levels of atmospheric greenhouse gases could continue to rise, even as emissions rates continue to drop.