A well-known gay rights lawyer and environmental advocate set himself on fire in protest against climate change.
Buckel had also reportedly sent the note to multiple news outlets, including The New York Times, warning that people were dying early deaths as a result of breathing in bad air.
Mr Buckel wanted his death to lead to increased action, according to the suicide note.
As a senior attorney with LGBT civil rights organization, Lambda Legal defense, Buckel was a lead lawyer in a 2000 lawsuit on behalf of transgender 'Boys Don't Cry' rape-murder victim Brandon Teena, helping the family recover additional damages from neglectful Nebraska law enforcement.
He was also the strategist behind same-sex marriage cases in New Jersey and Iowa, and helped set a precedent that United States schools have a duty to prevent anti-gay bullying.
Buckel's body was already completely burned by the time firefighters arrived, according to the New York Post.
The New York Times said it received an emailed copy of the note, which also said, "Most humans on the planet now breathe air made unhealthy by fossil fuels, and many die early deaths as a result - my early death by fossil fuel reflects what we are doing to ourselves".
"This is a tremendous loss for our Lambda Legal family, but also for the entire movement for social justice", Taylor said.
"My name is David Buckel and I just killed myself by fire as a protest suicide", the hand-written message read.
David Buckel killed himself by lighting himself on fire Saturday in Prospect Park in Brooklyn.
'A lifetime of service may best be preserved by giving a life...
"Honorable goal in life invites honorable objective in death", Buckel wrote.
In a statement, the organization said he had won several key court victories including in the Lewis v Harris case, in which the New Jersey Supreme Court recognized the harm done to same-sex couples and their children from being excluded from the rights and responsibilities of marriage. He also worked with the NYC Compost Project as an ecological activist.
Camilla Taylor, Lambda Legal's acting legal director, said in a statement that Buckel was a "brilliant legal visionary" who was universally kind to all he worked with.
'Here is a hope that giving a life might bring some attention to the need for expanded actions, and help others give a voice to our home, and Earth is heard'. But he was best known as a champion of gay rights.
Susan Sommer, a former attorney for Lambda Legal and now the general counsel for the Mayor's Office of Criminal Justice, also reacted to Buckel's death. "It took us a little while to process it", she said.
- First Rohingya family returns to Myanmar after conflicts
- Amazon.Com INC (AMZN) Stake Lifted by L & S Advisors Inc
- U.S. would not attack Syria over gas attack allegations: Syrian lawmaker
- Russia accuses UK of staging 'fake' chemical attack in Syria
- Real Madrid - Bayern: No "fear clause" for James
- Max Verstappen admits Sebastian Vettel collision was his fault
- Ramirez homers in victorious return to Red Sox lineup
- Has silence of Prime Minister Narendra Modi become deafening?
- No matter how influential Unnao accused is, he won't be spared: Yogi
- Philadelphia Eagles' Daryl Worley arrested, report says; team releases statement