Temperatures in the first two months of 2016 followed a year that broke "all previous records by a wide margin," the UN's weather agency said.
The WMO pointed to record 2015 sea surface temperatures, unabated sea-level rise, shrinking sea ice and extreme weather events around the world.
"The alarming rate of change we are now witnessing in our climate as a result of greenhouse gas emissions is unprecedented in modern records," the WMO's new chief, Petteri Taalas, said in a statement.
Dave Carlson, head of the WMO-co-sponsored World Climate Research Programme, said the rising temperatures this year were especially alarming.
"The startlingly high temperatures so far in 2016 have sent shockwaves around the climate science community," Carlson said in the statement.
WMO confirmed findings by the National Oceanic and Atmospheric Administration (NOAA) last Thursday.
The US agency determined that last month was the warmest February since modern records began, with an average temperature that was 1.21 degrees Celsius (2.18 degrees Fahrenheit) bove the 20th-century average.
The hike in temperatures during the first two months of the year was especially felt in the far north, with the extent of sea ice in the Arctic at a satellite-record low in February, the agency said.
Carbon-dioxide concentrations in the atmosphere also crossed the threshold of 400 parts per million (ppm) during the first two months of the year, WMO said.
In 2014, CO2 levels had already risen to 397.7 ppm, which was 143 percent of levels prior to 1750, considered the start of the industrial era.
Monday's report came against a backdrop of the Paris climate talks in December. UN members enshrined a goal of limiting global warming to "well below" 2 C above pre-industrial levels, with a more ambitiouis target of 1.5 C if possible.
But Taalas warned that the planet is already about halfway to the 2 C milestone, with no sign of slowing down.
"Our planet is sending a powerful message to world leaders to sign and implement the Paris Agreement ... now before we pass the point of no return," he said.
"Today, the Earth is already one degree Celsius hotter than at the start of the 20th century," Talaas said, warning that "national climate change plans adopted so far may not be enough to avoid a temperature rise of 3 C.
He stressed though that "we can avert the worst-case scenarios with urgent and far-reaching measures to cut carbon dioxide emissions."