Try a combination of destroying wetlands, laying huge swaths of impervious cover, and defoliating acres of natural areas. Bonus for doing so in the same order from upstream to down, in a single watershed.
Redistributing species doesn’t really change the environment. You’re just speeding up what might take a few million years, and life will adapt. You want a nice big meteorite. Find your asteroid. Build your own space program. Launch probes to the asteroid to divert it towards an impact. Sit back and watch the end of the Earth. Or at least terrestrial megafauna for smaller impacts.
You’re rich and industrious, bankroll somebody with an engaging combination of ignorance, arrogance, and bad hair, get him/her elected to high office, and see if he/she doesn’t destroy civilization and most of nature 1st chance he/she gets.
Be careful, though: if your efforts end up wiping out humanity, that can only be good for the environment. Find the right balance lest you at last defeat yourself.
You’re too late. This guy already won this race, with extra points because he actually meant only to do good.
Make a misleading movie about how carbon dioxide is destroying the planet and watch everyone destroy the world while attempting to save it.
I think the OP is looking for ideas based on actual reality, not some right-wing version of it.
In between evil schemes you can simply buy as much methane and then simply release it unburnt as it is a powerful greenhouse gas. Other substances probably have a higher ratio of cost to effect but people would wonder what you were doing with all that hexafluoroethane you were buying whereas methane would have plausible deniability.
Hard to compete with the “Gates of Hell.”
How about working your way into a position aboard an Oil tanker, hijacking and then sabotaging it?
What if you sailed a full oil tanker to the Galapagos islands and then beached it, how much damage would a big oil spill there do?
If you just set up a lab and start cranking out the CFCs, you could probably do enough damage to the ozone layer that LAWKI would basic be burnt off the planet. Something with slightly different chemistry would eventually replace it, though.
Develop ice-nine.
Locally, probably a lot…globally, nothing…the biggest oil tankers are 320,000 m3, the Pacific Ocean is estimated at 171,000,000 km3…so, one really big oil tanker represents about 2x10exp-12 of the volume of the Pacific…
No one mentioned setting wildfires yet? Seems to me like the most cost effective option.
At the risk of bumming everyone out, I do think politics is the way to go. A lot of nations are working to slow down climate change; bankrolling people who can undermine international agreements will be huge. The United States is already taken; consider duplicating the effect elsewhere.
When a butterfly flaps its wings, it can really make a mess of things in the future. Breed thousands of butterflies, and imagine the damage you could do.
An old essay by Uncle Al Shwartz points in a diabolical direction: In it he mentions that stoners working outside the law have raised the level of expressed THC in cannabis ten-fold in a matter of a few decades. He also observed that Kudzu is an invasive species in the United States that grows feet/day and that resinous expression is also a characteristic of Poison Ivy. His conclusion: A terrifying ecoterrorist bioweapon can be had by a bit of motivated minor tinkering to get Kudzu to express urushiol in high percent.
You would have a rapidly growing invasive species, unfriendly to mammals, that cannot be pulled out fast enough to keep it under control and cannot be burned out (vaporized urushiol is nobody’s friend), and won’t freeze out over the winter.