The 60-year-old set himself on fire in the busy NY park, leaving a suicide night behind to explain his actions.
"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", the note read, according to the New York Times which received an emailed copy.
His suicide note expressed the hope that his death would serve as a call to action.
Buckel was the lead attorney in a lawsuit that involved Brandon Teena, a transgender man who was murdered in Nebraska.
In another pivotal case, Buckel won a New Jersey Supreme Court ruling in 2006 acknowledging that gay couples are entitled to the same legal rights and financial benefits as heterosexual married couples.
In the note, Buckel referenced the Tibetan monk protesters who set themselves on fire in protest the Chinese occupation in Tibet, arguably an image that has captured resistance and protests more powerfully than any other.
"I am David Buckel and I just killed myself by fire as a protest suicide", he wrote.
'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'.
Police pronounced Buckel dead at 6.30am.
"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 said that privilege comes with the suffering of others.
'There's no denying that sticking with renewable resources means a lot of elbow grease with pitchforks and shovels, ' he wrote in a 2016 article on the Botanic Garden Web site.
"He deserves tremendous thanks for recognizing this was in many ways at the heart of what it meant to be gay for many Americans and making it a priority", Susan Sommer, a former attorney at Lambda Legal, told the Times. "He knew his craft and his trade and was strategic in how to build the blocks toward a sweeping victory".
"The news of David's death is heartbreaking", she said in her statement. And he spearheaded a federal case where a court ruled schools are obligated to prevent the bullying of gay students, Camilla Taylor, acting legal director at Lambda Legal, told the Times.