(Bloomberg) -- It's no longer a question of whether 2016 will be the hottest on record, but by how much.

The El Niño warming pattern in the Pacific Ocean is over, but unprecedented heat remains across the planet. Last month was the hottest May in 137 years of record keeping, according to new reports from NASA and the National Oceanic and Atmospheric Administration (NOAA). In an age of rising temperatures, monthly heat records have become all too common: May was the 13th consecutive month to set a new record, according to NOAA data released on Wednesday.

Extremes of recent months

The extremes of recent months are such that we're not even halfway into 2016 and there's already a greater than 99% likelihood that this year will be the hottest on record, according to Gavin Schmidt, who directs NASA's Goddard Institute for Space Studies. NASA and NOAA maintain independent records of the Earth's temperatures, but they both agree that last month was a scorcher.

If NASA's Schmidt is right, 2016 will be the the third consecutive year to set a new global heat record—the first time that's ever happened. So far, 15 of the hottest 16 years ever measured have come in the 21st century. Results from the world’s chief monitoring agencies vary slightly. The Japan Meteorological Agency said last month was the second-hottest May, not the hottest. Nevertheless, all agree that the extremes of 2016 are unrivaled in the modern climate record.

Planet warming rapidly

Some of this is still the result of last year's monster El Niño, which releases heat into the atmosphere that typically lingers for months after the underlying conditions subside. Now that El Niño has finally come to an end, it may soon shift to a cooling La Niña this summer, according to NOAA's Climate Prediction Center. The agency gives a 75% chance of a La Niña pattern developing in 2016.