The remains of David Buckel, 60, were found by civilians passing through the NY park, and police say he was pronounced dead at around 6:30 a.m EST.
Buckel noted in his letter that he was fortunate to have "good health to the final moment", and added that he hoped his death would lead to increased action on environmental concerns.
Buckel was the lead attorney in a lawsuit involving Brandon Teena, a transgender man who was murdered in Nebraska. The case inspired the Oscar-winning 1999 movie "Boys Don't Cry", directed by Kimberly Pierce and featuring Hilary Swank as Teena.
Buckel was marriage project director for Lambda Legal, and in that position he persuaded the organization to file a suit for marriage rights in Iowa in 2005, "at a time when only MA permitted same-sex couples to marry", noted a statement released by Camilla Taylor, the group's director of constitutional litigation and acting legal director.
"My early death by fossil fuel reflects what we are doing to ourselves", Buckel wrote.
Camilla Taylor - Lambda Legal's director of constitutional litigation and acting legal director - said in a statement: "The news of David's death is heartbreaking".
More recently, Buckel worked as an urban gardener and ecologist with the Brooklyn Botanic Garden, helping run what he called the largest composting program in the country to use only renewable sources of energy.
"Pollution ravages our planet, oozing inhabitability via air, soil, water and weather", Buckel wrote the Times in an email.
A county sheriff was found negligent in the killing for his failure to protect Teena from his rapists - who returned and killed him once he went to authorities.
"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.
Sommer added: 'He was a very smart and methodical lawyer.
An NYPD spokesperson confirmed the suicide of David Buckel, 60, but declined to comment further. David helped create Lambda Legal's focus on LGBT youth.