The four hottest years on record were 2015, 2016, 2017 and 2018, according to government institutions including NOAA and NASA.

The much-anticipated assessment by the UN's Intergovernmental Panel on Climate Change paints a bleak picture of humanity's odds of averting a potentially catastrophic rise in global temperatures. That increase leaves the world at greater risk of sea level rise, drought, extreme weather events and species extinction.

However, it would take an effort the likes of which the planet has never seen.

The nations of the world have a narrow path to preventing global temperatures from overshooting the most ambitious target in the Paris Agreement on managing climate change, a Nobel Peace Prize-winning United Nations panel said in a new report.

Climate scientists gathered in Incheon, South Korea this month to assess the world's odds of achieving the tougher of two temperature targets in the Paris Agreement.

The agreement aims to mobilize nations to take action to prevent global temperatures from rising by 2 degrees Celsius above pre-industrial levels by 2100. But it also calls on countries to pursue measures that would cap that rise to 1.5 degrees Celsius.

The difference of half a degree could make a huge difference. In the 1.5 degrees scenario, IPCC scientists forecast 70-90 percent of the world's coral reefs would be lost. If temperatures rise by 2 degrees Celsius, coral reefs would be nearly wiped out across the planet.

"Every extra bit of warming matters, especially since warming of 1.5ºC or higher increases the risk associated with long -lasting or irreversible changes, such as the loss of some ecosystems," said Hans-Otto Portner, who co-chairs IPCC's working group on the impacts of climate change.

IPCC said it would take "rapid and far-reaching" transitions in energy, industry and transportation to keep temperature rise below 1.5 degrees Celsius.

"Limiting warming to 1.5ºC is possible within the laws of chemistry and physics but doing so would require unprecedented changes," said Jim Skea, co-chair of the IPCC working group that focuses on mitigating climate change.

Those efforts would have to drive down emissions of carbon dioxide, the primary cause of climate change, by about 45 percent from 2010 levels by 2030.

By 2050, humans would have to ensure they reach "net zero" carbon emissions, meaning any carbon emitted is offset by measures that remove the greenhouse gas from the atmosphere.