Extreme heat is one of the most deadly problems from climate change even though it receives less attention than other knock-on effects like hurricanes and flooding, two of the world's leading humanitarian organizations warned Thursday.

The year 2023 was the hottest on record, with rising temperatures affecting the most vulnerable populations in particular -- the elderly, outdoor workers and those without access to cooling systems such as air conditioners.

The Red Cross and the US Agency for International Development delivered their warnings against the "invisible killer" of extreme heat at a virtual summit, on the heels of the United States exiting its warmest-ever winter on record.

"We are calling on governments, civil societies, young people and all the stakeholders to take concrete steps around the globe to help prepare countries and communities for extreme heat," said Jagan Chapagain, secretary general for the International Federation of Red Cross and Red Crescent Societies.

USAID chief Samantha Power warned that in the United States, "heat is already deadlier than hurricanes, floods and tornadoes combined."

"We are calling on development agencies, philanthropies and other donors to recognize the threat that extreme heat poses to humanity, and to put resources towards helping communities withstand that threat," she said.

Highlighting ongoing efforts addressing extreme temperatures, Power said USAID was supporting a program to build "heat resilient schools" in Jordan, using "passive heating and cooling systems, thermal insulation, double glazed windows and air conditioning."

Climate change's effects aren't limited to already hot places like the Middle East: in Europe, the fastest-warming continent in the world, more than 60,000 people were estimated to have died in heat waves in 2022, noted US climate envoy John Podesta.

"Climate information and services including early warnings can save lives and assets," he added. "But one-third of the world's population doesn't have access to this life-saving information."

Other efforts include those in Freetown, the capital of Sierra Leone, where nearly a million trees have been planted since 2020.

"But we mustn't allow this conversation to let anyone off the hook when it comes to reducing emissions," Freetown Mayor Yvonne Aki-Sawyerr said.