Record warm oceans make it likely 2014 will be the hottest year. Photo: Kate Geraghty

Ongoing record warmth in the world's oceans has increased the likelihood that 2014 will be declared the hottest year since reliable data began more than a century ago, US and Japanese agencies say.

The warmth comes as conditions in the Pacific remain conducive to an El Nino event forming in coming months, Australia's Bureau of Meteorology said.

Surface temperatures have exceeded El Nino threshold levels for several weeks, and the bureau estimates there is a greater than 70 per cent chance of such an event soon.

The first 11 months of the year were the warmest on record, with combined global land and sea-surface temperatures running 1.22 degrees above the 20th-century average, the US National Oceanic and Atmospheric Administration said.

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This year will be the hottest on record – eclipsing 2005 and 2010 – provided December is at least 0.76 degrees above average, NOAA said.

Year-to-date global temperatures have 2014 as warmest on record.

While land temperatures were less anomalously warm last month – coming in at the 13 hottest – the fact that the oceans remain exceptionally warm suggests a sudden cooling is unlikely.

The combined land and sea temperatures made last month the seventh warmest November, NOAA and the Japan Meteorological Agency said.

The eastern Pacific is among the ocean regions reporting record warmth, a sign often associated with an El Nino.

NOAA said there remained a two-in-three chance that an El Nino event will be present during the southern summer and will last into the autumn.

How December would have to cool off markedly for 2014 not to be hottest calendar year.

Any El Nino event, which typically brings drier and hotter than normal weather to most of Australia, is likely to be a relatively moderate one, the bureau said.

Last month was Australia's warmest November for a second year in a row, countering cooler-than-usual conditions over most of North America.

After the first 11 months of this year, Australia's mean temperatures were vying with those of 1998 as the third warmest year in records going back to 1910. 

Last year was easily Australia's warmest calendar year, with 2005 in second place, the Bureau of Meteorology said.