People have said for decades that humanity would work together against a common threat. We have it: it is global warming.
Now consider it as an enemy. Know thy enemy said Sun-Tsu in the art of war. How do you get to know it better? Research, scientists, etc… are our scouts and spies in this war.
You also have to inform the general population on how to recognize the enemy. Historical example: If he says Heil Hitler, he’s probably a bad guy.
Once everybody knows we are at war, we can then fight. Everyone can resist individually. You can turn the enemies ‘weapons’ against them, by changing consumption, or try to sabotage these weapons by working with big corporations to change them. You can create new weapons by developing green businesses. You also need to get governments involved, to guide the people in times of war.