PARIS: A decade-long surge of the potent greenhouse gas methane threatens to make the fight against global warming even harder, top researchers warned on Monday. “Additional attention is urgently needed to quantify and reduce methane emissions,” they wrote in the Environmental Research Letters journal, summarizing the findings of a consortium of 81 scientists. After rising slowly from 2000 to 2006, the concentration of methane in the air climbed 10 times more quickly the following decade, according to that study, which was published in the peer-reviewed Earth System Science Data. The unexpected — and largely unexplained — increase was especially sharp in 2014 and 2015. “Keeping global warming below two degrees Celsius [3.6 degrees Fahrenheit] is already a challenging target,” they said, referring to the goal set in the 196-nation Paris climate pact, which entered into force last month.