SAO PAULO (AP) — Just days ahead of the U.N. climate conference to be held in Brazil, a group of 300 mayors gathered Monday in Rio de Janeiro to pledge coordinated climate action and address rising heat that is hurting many of their residents.

The summit was organized by C40, a network of mayors from big cities that has pushed to be included in decisions on how to combat global warming and adapt to its effects.

Selwyn Hart, the U.N. special adviser and assistant secretary-general for climate change, said at the opening that mayors are on the front lines of the climate crisis.

“In the midst of all the geopolitical tensions and divisions, it is truly amazing and inspirational to see what is happening in this room and on the ground in your cities,” Hart said, adding that local leaders are needed “more than ever” as the world enters the second decade of implementing the 2015 Paris Agreement.

The Paris Agreement aims to keep average global temperature from rising beyond 2 degrees Celsius (3.6 degrees), and ideally limit it to 1.5 degrees Celsius (2.7 degrees Fahrenheit), compared to the 1850s. To do that, the agreement says nations must slash planet-warming pollution that results when coal, oil and gas are burned.

Ana Toni, Brazil’s climate change chief and CEO of COP30, said meeting those goals would only be possible by engaging mayors. “It is you, mayors, who have to make very hard decisions in daily life, together with people,” Toni said.

The U.S. decision to withdraw from the Paris Agreement and set back its climate goals seemed to loom large at the mayors’ summit.

London Mayor Sadiq Khan said mayors have “long been climate doers, while too many nations and states have been the climate delayers or, indeed, climate deniers.” Now, he added, the challenge has moved beyond a battle against climate denial.

“Now, it is an existential fight between the climate wreckers and climate defenders,” Khan said. “Among the wreckers is the president of the United States of America, someone who stood up at the U.N. only a matter of weeks ago and called the climate crisis a scam.”

Phoenix Mayor Kate Gallego drew applause from the audience after saying she was bringing “good news from the United States” and asked fellow American city leaders to raise their hands.

“We have 50 cities from the U.S. that are here, all committed to ambitious climate action. So, while our national government goes backward, these cities go forward,” Gallego said.

Gallego addressed how extreme heat is affecting her city, which has repeatedly broken temperature records in recent years. “This year, we hit 118 degrees — nearly 48 Celsius — not once, but twice,” the mayor said.

She then introduced the city’s plan to plant trees and install shade structures, develop pilot cool-surface technology, and offer heat-relief training to first responders, such as paramedics handling heat emergencies.

Scientists say that 4 billion people, about half the world’s population, experienced at least one extra month of extreme heat because of human-caused climate change from May 2024 to May 2025.

The extreme heat caused illness, death, crop losses and strained energy and health care systems, according to the analysis from World Weather Attribution, Climate Central and the Red Cross. Yvonne Aki-Sawyerr, C40 co-chair and former mayor of Freetown, Sierra Leone’s capital, said that extreme heat stands now as the deadliest weather related disaster and contributed to an estimated 489,000 deaths each year.

“And the economic consequences are staggering. Global extreme heat is projected to cause $2.4 trillion dollars of loss productity by 2030, as it becomes too dangerous for working outdoors,” she said, mentioning businesses like construction and agriculture.

On Monday, the C40 mayors launched a global coalition to prepare cities for a hotter future through an effort called the Cool Cities Accelerator.

A coalition of 33 cities — including Austin, Texas; Boston, Buenos Aires, Argentina, Freetown, Sierra Leone, London, Nairobi, Kenya; Phoenix, Paris and Singapore — has pledged to collaborate, share best practices and report progress on emergency measures such as strengthening early warning systems and ensuring access to cooling projects.

Within five years, cities aim to improve building standards, expand urban tree cover and shade, and future-proof critical infrastructure.

The Accelerator has implementation support from ClimateWorks Foundation, Robert Wood Johnson Foundation, Z Zurich Foundation and Danish Ministry of Foreign Affairs.

The Rockefeller Foundation is also contributing just under $1 million to help cities launch the effort.

Cities will report back every two years on progress toward protecting residents from heat, said Emilia Carrera, the foundation’s director of health. Key metrics will include the establishment of cooling centers, the design of cooler urban spaces and updated building codes.

“Mayors see these challenges very closely,” Carrera said. “They have a fresher perspective and an ability to respond more quickly.”

Hannah Machado, an urbanist and climate researcher in the City +2°C Program at the Center for Urban Studies at Insper Research Institute in Sao Paulo, said cities have a long way to go to adapt to the changing climate.

"Especially because the effects of extreme climate events are felt in cities,” she said.

Cities, however, have reduced emissions faster than national governments, according to the C40 group. The group's cities said they have cut emissions five times faster than the global average.

“In this year of major geopolitical shifts, that’s an understatement, city leadership has become even more important,” said Catherine McKenna, Canada's former climate minister who also served at the U.N. as the chair of the Net-Zero Emissions Commitments.

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