It depends how you celebrate. If you just want to make a day dedicated to enjoying nature and G-d's creations, it's okay. But if you worship nature, it's idolatry. Also, there is a difference in rejecting global warming and rejecting Science. Global warming in junk science. If people want to combat real global warming, then they should advocated bombing the Iranian nuclear reactors.
The Left is just trying to hijack Science just like they hijacked many other things as well as words (peace, gay, etc...). I remember Earth Day in 1990 at Fairmount Park. I was in 2nd Grade and I was interested in things like recycling and nature. I remember I won the Student of the Month Award that April and I think Student of the Week for being interested in Earth Day.
In 2010, I did my own Earth Day celebration inspired by religious things such as Birkat HaChama the year earlier. I read about the Creation of the Earth in Genesis 1, ate Earth foods with the brachot (Adamah, HaEtz, and HaMotzi) and then I walked to the stream near me and observed the different Creations. But I thank G-d for creating Earth and all of its nature. I also read the Third Day of Creation once on Arbor Day, saw the trees, and ate fruit (HaEtz).
But the Bolsheviks have different intentions for Earth Day.