An artificial virus, deadly and highly contagious, with long incubation periods, forcing people to stay home and sending high CO2 emission industries crumbling into bankruptcy. Look what something as weak as COVID has done in a short amount of time. Also make another virus that can spread through high methane emitting livestock and kills them quickly, and anyone who eats an infected animal.

At the same time if you create a vaccine against the virus, you can sell it to powerful and wealthy individuals on the black market for vast sums of money. There might be riots, but why contain it? Let it spread through the schools and churches.