Climate change is accelerating, with carbon dioxide levels increasing, sea levels rising and ice sheets melting faster than ever, experts have warned.

The warning from the World Meteorological Organisation forms part of a "united in science" review for a UN climate action summit at which countries are being urged to increase their ambition to tackle emissions.

The WMO has published a report showing climate change and its impacts over the past five years between 2015 and 2019, which shows it was the hottest five-year period on record.

The world has warmed by 1.1C since pre-industrial times, and by 0.2C just compared to the previous five year period 2011-2015, the report showed.

Climate change impacts are hitting harder & sooner than predicted.



New #UnitedinScience report underlines gap between #ClimateAction targets & reality, and shows urgency of the transformation urgently needed to protect our planet. https://t.co/DPKoUqqPnI pic.twitter.com/YukQDJf0lN — United Nations (@UN) September 22, 2019

And with levels of carbon dioxide and other greenhouse gases rising more quickly than before, to new highs in the atmosphere, further warming is already locked in, the WMO warned.

Carbon dioxide concentrations in the atmosphere increased at a higher rate between 2015-2019 than in the previous five years, and are on track to reach a record 410 parts per million in 2019, data indicate.

Deadly heatwaves, bearing the hallmark of climate change and causing record high temperatures, devastating hurricanes and cyclones, and severe wildfires which release more carbon have all gripped the planet in the past five years.

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Sea levels have been rising by an average of 5mm a year in the past five years, compared to 3.2mm a year on average since 1993, with much of the rise coming from glaciers and ice sheets that are melting ever more quickly.

The Greenland ice sheet has witnessed a considerable acceleration in ice loss since the turn of the millennium while the amount of ice being lost annually from Antarctica in the past decade has increased by at least sixfold compared to the 1980s.

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Arctic sea ice has seen record low coverage in winter between 2015 and 2018, the WMO said.

The organisation's secretary-general Petteri Taalas, who is co-chair of the science advisory group of the UN climate summit, said: "Climate change causes and impacts are increasing rather than slowing down.

"Sea level rise has accelerated and we are concerned that an abrupt decline in the Antarctic and Greenland ice sheets, which will exacerbate future rise.

"As we have seen this year with tragic effect in the Bahamas and Mozambique, sea level rise and intense tropical storms led to humanitarian and economic catastrophes."

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The challenges "are immense", he said, and warned there was a growing need to adapt to the changing climate as well as reducing greenhouse gas emissions, particularly from energy production, industry and transport.

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The report has been released to inform the climate action summit convened by UN Secretary-General Antonio Guterres, and to urge countries to up their climate efforts.

Under the international Paris Agreement, countries committed to curbing temperature rises to "well below" 2C and pursuing efforts to limit increases to 1.5C above pre-industrial levels to avoid the worst impacts.

But current commitments put the world on track for around 3C of warming.

The report comes after millions of people led by children on strike from school took to the streets around the world calling for urgent climate action, including hundreds of thousands in the UK.