A group of U.S. governors, mayors, business leaders and academic institutions plan to submit an independent pledge to the United Nations promising to help meet the U.S.’s emissions reduction targets under the Paris climate accord, in a response to President Donald Trump’s decision to pull the country out of the 195-nation agreement.

The group is being organized and largely funded by former New York City mayor and billionaire philanthropist Michael Bloomberg, who also serves as a U.N. envoy on climate issues, according to a...