Keeping the Earth's temperature rise to 1.5 degrees Celsius means making rapid, unprecedented changes in the way people use energy to eat, travel and live or we risk even more extreme weather and loss of species, a U.N. report said on Monday.

Meeting 1.5C (2.7 degrees Fahrenheit) above pre-industrial levels, rather than the 2C target agreed at global climate talks in Paris in 2015, would have "clear benefits to people and natural ecosystems," the United Nations Intergovernmental Panel on Climate Change (IPCC) said on Monday.

Without real change, the world is not even on course to reach the 2C target, experts said.

U.N. World Meteorological Organization (WMO) secretary-general Petteri Taalas told reporters in Geneva: "There is clearly need for a much higher ambition level to reach even a 2 degrees target, we are moving more towards 3 to 5 (degrees) at the moment."

The past 18 years have been the warmest on record since the 1850s when measurements began, he said. Scientists attribute the temperature rises and extreme weather mainly to greenhouse gases including carbon dioxide from fossil fuels such as coal, oil and gas.

The IPCC report said at the current rate of warming, the world's temperatures would likely reach 1.5C between 2030 and 2052 after an increase of 1C above pre-industrial levels since the mid-1800s.

Meeting the 1.5C target would keep the global sea level rise 0.1 meter (3.9 inches) lower by 2100 than a 2C target, the report says. That could reduce flooding and give people on the world's coasts, islands and river deltas time to adapt.

The lower target would also reduce species loss and extinction and the impact on ecosystems, the report said.

"Even the scientists were surprised to see ... how much they could really differentiate and how great are the benefits of limiting global warming at 1.5 compared to 2," Thelma Krug, IPCC vice-chair, told Reuters.

The IPCC met last week in Incheon, South Korea to finalise the report, prepared at the request of governments in 2015 to assess the feasibility and importance of limiting warming to 1.5C.

The report is the main scientific guide for governments on how to implement the 2015 Paris Agreement, agreed by nearly 200 nations, and will be debated at the Katowice Climate Change Conference in Poland in December.

U.S. President Donald Trump withdrew the United States from the accord last year, invoking concerns for the U.S. economy, and has espoused pro-fossil fuel policies. But U.S. states led by California and many cities are living up to their commitments to cut greenhouse gas emissions, WMO's Taalas said.

"The USA is on the right track to reduce its emissions, you have already been reaching 50 percent of the pledges the Obama administration was having on the table as part of the Paris Agreement," he said, and cited companies such as electric vehicle maker Tesla as helping bringing about change.

To contain warming at 1.5C, man-made global net carbon dioxide (CO2) emissions would need to fall by about 45 percent by 2030 from 2010 levels and reach "net zero" by mid-century. Any additional emissions would require removing CO2 from the air.