In a suicide note found nearby, Mr Buckel wrote that he had immolated himself using fossil fuel to symbolise what he said was the damage human beings were doing to the Earth.
A jogger who stumbled across the remains and shared photographs of them on Twitter before they were covered up by police said they were "freaked out" by the discovery.
Buckel, 60, was a supporter of LGBT rights and also the lead attorney in Brandon vs County of Richardson, in which a Nebraska county sheriff was found guilty of failing to protect Brandon Teena, a transgender man who was murdered.
David Buckel's suicide notes were left inside an envelope labeled "for the police" placed inside a garbage bag left inside a shopping cart near the body. In it he urged the world's residents to protect the planet, The New York Times reported.
Buckel wrote that pollution was ravaging the planet and hoped his death would serve as a symbol. Honorable goal in life invites honorable purchase in death. "This is a tremendous loss for our Lambda Legal family, but also for the entire movement for social justice", the group said in a statement.
'A lifetime of service may best be preserved by giving a life...
In his note, Buckel compared his macabre demise with the suicides of those who set themselves afire to protest China's occupation of Tibet. The case inspired the Oscar-winning 1999 movie "Boys Don't Cry", directed by Kimberly Pierce and featuring Hilary Swank as Teena.
"It's a very important case, not only within Nebraska but nationally", Buckel said after the $80,000 judgment against the city, according to the University of Nebraska-Lincoln's student newspaper the Daily Nebraskan.
He was also a well-known ecological activist, working with the NYC Compost Project, funded by the city Sanitation Department and based at the Brooklyn Botanic Garden.
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. David helped create Lambda Legal's focus on LGBT youth.
"Our thoughts and condolences go out to all those who loved David", Taylor concluded".
Buckel's work included cases for Lambda Legal, the country's largest and oldest LGBTQ legal organisation.