The past decade is almost certain to be the hottest on record, weather experts warn, painting a bleak picture of vanishing sea ice, devastating heatwaves and encroaching seas in a report launched at a climate summit in Spain.

An annual assessment of the Earth's climate by the Geneva-based World Meteorological Organisation (WMO) underscored the stakes at two weeks of talks aimed at shoring up the 2015 Paris Agreement to avert catastrophic global warming.

"Heatwaves and floods which used to be 'once-in-a-century' events are becoming more regular occurrences," WMO Secretary-General Petteri Taalas said in a statement.

"Countries ranging from the Bahamas to Japan to Mozambique suffered the effect of devastating tropical cyclones. Wildfires swept through the Arctic and Australia," he said.

"Heatwaves and floods which used to be 'once-in-a-century' events are becoming more regular occurrences,"

Among the report's findings:

• Average temperatures for the five-year (2015-2019) and 10-year (2010-2019) periods are almost certain to be the highest on record.
• 2019 is on course to be the second- or third-warmest year on record.
• Sea water is 26 percent more acidic than at the start of the industrial era, degrading marine ecosystems.
• Arctic sea-ice neared record lows in September and October, and Antarctica also saw record low ice several times this year.
• Climate change is a key driver of a recent rise in global hunger after a decade of steady declines, with more than 820 million people suffering from hunger in 2018.
• Weather disasters displaced millions of people this year and affected rainfall patterns from India to northern Russia and the central United States, and many other regions.

The report also noted that surges in sea temperatures known as "marine heatwaves" which devastate underwater life had become more common.

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The report said the concentration of CO2 in the atmosphere hit a record level of 407.8 parts per million in 2018 and continued to rise in 2019. Opening the climate summit on Monday, United Nations Secretary-General Antonio Guterres had warned that 400 parts per million had once been considered an "unthinkable" tipping point.

A drumbeat of dire reports from climate scientists in the past year has fuelled environmental activism, prompted some companies to commit to slashing emissions and raised concerns among investors about the stability of asset prices.

Nevertheless, delegates in Madrid are facing an uphill battle to persuade major emitters to embrace the kind of radical change needed to shift the Earth's climate system onto a more habitable trajectory.