The hottest year on record so far has been 1998, when a strong El Nino, a warming event in the Pacific, pushed up temperatures, the BBC reported.
This year saw a weaker El Nino, and scientists say that should have meant substantially lower temperatures than 1998, but that has not been the case.
Temperatures in the last 12 months are the warmest recorded by NASA and the second highest in the United Kingdom's data set, the U.K. Met Office said.
"It's a sign that we've got man-made global warming," Vicky Pope, head of climate science advice at the Met Office, says.
Although the rate of warming has slowed over the last decade, the Met Office says that is due to natural variability, not because man-made warming has stopped.
Pope says she expects warming to increase in the next few years.
"The long-term warming trend is 0.16 C (0.3 F)," she says. "In the last 10 years the rate decreased to between 0.05 C (0.09 F) and 0.13 C (0.23 F)."
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