As the decade comes to a close, the world's top climate scientists warned Tuesday that policymakers' continued failure to curb the warming of the planet could lead to a global temperature increase of 5° Celsius by the end of the century and put the world "nowhere near on track" to avoid the worst impacts of the climate crisis.

2019 is expected to be the second or third warmest year ever recorded, according to the World Meteorological Organization (WMO), while the past five-year and ten-year periods are "almost certain to be the highest on record."

2019 concludes a decade of exceptional global heat, retreating ice and record sea levels. Average temps for 10-year (2010-2019) period set to be highest on record. 2019 is on course to be 2nd or 3rd warmest year on record. #StateofClimate #COP25 #ClimateAction pic.twitter.com/zeFW3ZnyMu — WMO | OMM (@WMO) December 3, 2019

The organization's report on the State of the Global Climate shows that carbon dioxide levels in the atmosphere hit record levels in 2018 and then rose again in 2019, with average global temperatures this past year rising to 1.1° Celsius (1.98° Fahrenheit) above pre-industrial levels.

The WMO repeated urgent calls by thousands of climate experts as well global campaigners for concrete action by world leaders to end the fossil fuel extraction that is contributing to carbon emissions and the warming of the globe.

"If we do not take urgent climate action now, then we are heading for a temperature increase of more than 3°C by the end of the century, with ever more harmful impacts on human wellbeing," said WMO Secretary-General Petteri Taalas.

"We are nowhere near on track to meet the Paris Agreement target" of a 1.5°C rise, Taalas added.

The report was released as the United Nations Framework Convention on Climate Change (COP25) began in Madrid and campaigners staged demonstrations to demand alternatives to market-driven approaches to the climate crisis.

The European Union declared a climate emergency last week, but world leaders have continued to allow big polluters to extract fossil fuels and contribute to carbon emissions that have led to record ocean temperatures, high ocean acidity, and rapidly melting sea ice in Antarctica and the Arctic.

"If we do not take urgent climate action now, then we are heading for a temperature increase of more than 3°C by the end of the century, with ever more harmful impacts on human wellbeing."

—Petteri Taalas, WMO Secretary-General

"If we wanted to reach a 1.5° increase we would need to bend emissions and at the moment countries haven't been following on their Paris pledges," Taalas told reporters in Madrid on Tuesday.

A 4° temperature rise, according to the Potsdam Institute for Climate Impact Research in Germany, would lead to the disappearance of many of the world's rainforests and the ice at the North and South Poles, as well as rising sea levels flooding even non-coastal regions.

The WMO report also warned about numerous effects on human health if global temperatures are allowed to rise above 1.5° Celsius, including increased transmission of illnesses like Dengue virus, which about half the world is now at risk for amid warmer weather; more frequent exposure of vulnerable populations to extreme heatwaves; and the continued rise of global hunger, which affected 820 million people in 2018.

"It's shocking how much climate change in 2019 has already lead to lives lost, poor health, food insecurity, and displaced populations," Dr. Joanna House of the Cabot Institute of the Environment at University of Bristol said of the report's findings. "Even as a climate scientist who knows the evidence and the projections, I find this deeply upsetting. What is more shocking is how long very little has been done about this. We have the information, the solutions, what we need now is urgent action."

Climate scientists praised the WMO for conveying the urgency of the crisis.

"These impacts are real and happening now and place huge pressures on communities and countries—put simply, these impacts make for a more unstable world," said Grant Allen, professor of atmospheric physics at the University of Manchester in England.

"It is already well over time to rapidly restructure our economies away from greenhouse gas emitting industries and practices," he added, "and adapt and mitigate for current and future change already locked into the climate system."