0122 GMT April 21, 2018
The conference opened on Monday hours after the United Nations issued its 2018 World Water Development Report warning that about 3.6 billion people, or half the world's population, already live in areas where water can be scarce at least one month a year.
That could rise to 5.7 billion people by 2050, the report said.
"There is simply no time to lose," Temer said in opening remarks at the 8th World Water Forum which runs all week in the Brazilian capital.
Under the slogan "sharing water," the forum brings together 15 heads of state and government, 300 mayors and dozens of experts. An estimated 40,000 people are expected to attend, organizers say.
Participants were meeting against the backdrop of the drama in Cape Town, which until earlier this month was projected to run out of water as early as July, forcing the closing of household taps and extreme rationing.
That crisis has now eased, with the local government saying that a campaign to bring 60 percent reduction in consumption has done enough to avert the shut-off.
"This is the consensus," Temer said. "Life on earth is threatened if we don't respect nature's limits."