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.