In its latest State of the Climate overview, the World Meteorological Organisation (WMO) reaffirmed that the last four years had been the hottest on record – figures previously announced in provisional drafts of the flagship report.
Last year also saw new heat records for the ocean's upper 2 000 metres, but data for that range only goes back to 2005.
About 93% of excess heat – trapped around the Earth by greenhouse gases that come from the burning of fossil fuels – accumulates in the world's oceans.
Some models predict the temperature of the top 2 000 metres of world's oceans will rise nearly 0.8°C by the end of the century if nothing is done to reduce greenhouse gas emissions, according to Science.
The thermal expansion – water swelling as it warms – is expected to raise sea levels 30cm, above any sea level rise from melting glaciers and ice sheets, according to the research published in Science.