UN warns world must unite to tackle top issues

What's more, climate change is a threat multiplier, meaning that it makes things like land degradation, deforestation and biodiversity loss worse, said Katharine Hayhoe, a climate scientist at Texas Tech University and chief scientist at the Nature Conservancy, who wasn't involved in the report.

- 

"If we don't fix climate change, were not going to be able to fix these other issues too," Hayhoe said.

Among other challenges: Up to 40% of land area globally is degraded; more than 1 million plant and animal species face extinction; and pollution contributes to an estimated 9 million deaths a year.

Adopting a comprehensive approach would be expensive, scientists acknowledge, but costs far less than the harms that otherwise could result.