I think "plastic in the ocean" is more easily imaginable. Until very recently climate change was pretty abstract and in many ways it still is, even though we see the effects.

Not to be forgotten: Obviously the two are related, the petrochemical industry is a huge contributor to greenhouse gas emissions and ultimately a part of the oil industry. If people cut down unnecessary plastic use because they're worried about plastic particles in the ecosystem, they're also doing something good for the climate.