It won’t work up here in White River Junction, cuz we’re sufficiently above sea level, but here’s my thinking for activists nearer to the coasts: paint lines on buildings showing where sea level will be in 50 or 100 years. Isn’t much of lower Manhattan projected to be in permanent gurgle mode? Slap some paint at the new high-and-dry mark. Green paint cuz it’s a “green” issue? Blue paint to indicate water? Stenciled paint so it clearly indicates what it means? (As in, “this is how high the oceans will be in 50 years if we don’t stop global warming now!”) Or have that kind of text printed on “police line” yellow tape and stick it up instead of paint. (That has the advantage of reducing the chances of being considered a criminal vandal. Anyway, you get the idea.
And remember, Uncle JTE says, “don’t break any laws!” (Including the uncodified ethical laws that say, “whenever possible, you must act to reduce suffering in the face of impending disaster.”)