A UN agency says 2018 is on track to be the fourth hottest since records began.
The World Meteorological Organisation says the 20 warmest years on record have all been in the past 22 years - while the top four have been in the past four years.
In the year to October, the global average temperature was almost 1°C above the 'pre-industrial baseline'.
According to the WMO, it's among the 'tell-tale signs' of climate change - along with rising sea levels, melting glaciers and extreme weather events.WMO Secretary-General Petteri Taalas has warned that the world is not on track to meet climate change targets - meaning that we'll struggle to rein in temperature increases.
He said: "Greenhouse gas concentrations are once again at record levels and if the current trend continues we may see temperature increases 3-5°C by the end of the century. If we exploit all known fossil fuel resources, the temperature rise will be considerably higher.
"It is worth repeating once again that we are the first generation to fully understand climate change and the last generation to be able to do something about it."
WMO Deputy Secretary-General Elena Manaenkova added that "every fraction of a degree of warming makes a difference" - in areas such as people's access to food and fresh water, the often catastrophic impact on wildlife, or the survival of coral reefs in the world's oceans.
Last week, the weather and climate agency revealed that the levels of greenhouse gases in the atmosphere reached another record high last year.
Earlier this week, the UN warned that countries around the world will have to triple their efforts to cut greenhouse gases to have any chance of preventing a climate change catastrophe.