Humans emit carbon dioxide into the atmosphere primarily through burning fossil fuels and through other activities. Plants take up carbon dioxide as they grow and store it in their tissues, and the oceans can also absorb it from the atmosphere. This latter process has its own drawback: Carbon dioxide drawn into the oceans causes them to acidify. Ocean acidification is seen as a serious threat to the health of ocean ecosystems.

Earlier work has suggested that these natural gas-sucking processes might be falling behind, leading to a faster-than-expected rise in atmospheric carbon dioxide. The new results suggest this hasn’t happened yet, rather they show these carbon-dioxide sinks continue to pull about half of people’s carbon dioxide out of the atmosphere.

“Since we don’t know why or where this process is happening, we cannot count on it,” Tans said in a statement.