The series of warmest years began in 2015, when global temperatures rose 1°C above pre-industrial levels for the first time in modern history.
Now experts are predicting 2020 could see an average temperature up to 1.23°C higher than in the late 19th century.
So far 2016 has been the hottest year since records began in 1850, with an average worldwide temperature reaching 1.16°C above the baseline.
This is followed by 2017, 2015 and then 2018.
However, it’s thought 2019 is likely to move into second place after a series of temperature records were broken.
This summer was the northern hemisphere’s hottest in recent history, while Australia is currently undergoing a blistering heatwave that saw the country experience its highest ever temperature.
The effects of the El Niño weather system, which caused warming in the tropical Pacific, boosted the global temperature in 2016.
However, 2020 temperatures are expected to be similar or even higher without the impact of El Niño.
Human-induced climate change
Meteorologists say rising concentrations of greenhouse gases are pushing temperatures upwards.
Professor Adam Scaife is head of long-range prediction for the Met Office.
He said: “Natural events such as El Niño-induced warming in the Pacific influence the climate system, but in the absence of El Niño this forecast gives a clear picture of the strongest factor causing temperatures to rise - greenhouse gas emissions.
The latest forecast is based on the key drivers of the global climate, but doesn’t include unpredictable events such as a large volcanic eruption, which would cause a temporary cooling.
Dr Nick Dunstone, an expert in climate variability at the Met Office, said: “Although the Earth has warmed by about 1°C on average since pre-industrial times, this isn’t spread evenly over the surface of the globe as much of the warming is occurring in the Arctic and over land masses.”
The Met Office forecast for 2019’s global mean temperature, which was issued at the end of 2018, agrees closely with the latest observations so far this year.
Data from January to October shows the global mean temperature is around 1.11°C above 19th century levels.
Dr Doug Smith, Met Office research fellow, said: “The forecast for 2020 would place next year amongst the six warmest years on record, which would all have occurred since 2015.
“All of these years have been around 1°C warmer than the pre-industrial period.”
Environmental campaigners say the latest predictions illustrate the need for urgent action to tackle the climate crisis.
“How many more temperature records must be broken before polluting governments wake up to the scale of this emergency and act accordingly,” said Caroline Rance, of Friends of the Earth Scotland.
“Behind these alarming headlines lies the devastating reality that millions of people around the world are losing their homes, their livelihoods and ultimately their lives as the climate crisis intensifies.
“The opportunity to limit warming to the Paris Agreement goal of 1.5°C is closing quickly, but we do know what the solutions are. We must end the use of fossil fuels and deliver a just transition to a safe, renewable future for all.
“With the UN climate talks coming to Glasgow in November 2020, both the UK and Scottish governments have an even more important role to play in the coming year.
"It’s time for both governments to end unconditional support for the oil and gas industry, and instead focus on stepping up climate action and delivering a just transition for workers.”