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A measure of the build-up of warming in the biosphere -  as those gases trap more of the sun's radiation - is the record ocean heat levels for a second year in a row.

As the waters warm, the oceans are also expanding, lifting sea levels at an accelerating rate. Adding in the effect of melting ice sheets and glaciers, the global mean sea level last year reached a record 3.7 millimetres in 2018. That pace was quicker than the average 3.15mm annual increase during the 1993-2018 period.

For Australia, 2018 was the third hottest year on record for data going back to 1910. For day-time temperatures, last year was the second hottest, the Bureau of Meteorology said in January. 

The heat has continued into 2019, with Australia's summer the hottest on record by a substantial margin, particularly for maximum temperatures.

Much of the WMO report looks at the impact of extreme weather events, particularly for developing nations.

It noted that food security was undermined in many places, with world hunger resuming its climb "after a prolonged decline".

Up to September last year, more than two million people had been displaced by disasters "linked to weather and climate events", with drought, flood and storms including cyclones leading culprits.

Heatwaves were also raising "alarm bells for the public health community" as they are expected to worsen in intensity, duration and frequency as the planet continues to warm.

"Between 2000 and 2016, the number of people exposed to heatwaves was estimated to have increased by around 125 million, as the average length of individual heatwaves was 0.37 days longer, compared to the period between 1986 and 2008," the report said. "In 2015 alone, a record 175 million people were exposed to 627 heatwaves. "

Last year, 281 weather and climate events recorded by the Centre for Research on the Epidemiology of Disasters affected almost 62 million people. Of those, floods topped the peril list, affecting 35 million people.

The impacts were not restricted to poorer nations, though, with the US alone reporting 14 "billion dollar disasters" last year.

Major storms also included super typhoon Mangkhut, that affected more than 2.4 million people and killed at least 134 people, mainly in the Philippines, the report said.