This is an old argument with unclear answers, but third world countries are a thing for many reasons and you can't dismiss their woes. Excusing it with "we were also poor once" is nonsense.
A couple centuries of colonialism which reinforces local corruption while keeping much of the population in poverty probably has something to do with it. I also think it's a false comparison to describe "us" (I'm guessing you mean Western Europe) as "dirt poor until very recently".
In any event, I see Canada taking the lead in accepting climate refugees when they start appearing in larger numbers. I also wonder at what point countries with aging workforces like Japan will start to take in larger numbers of refugees as a demographic necessity. Unfortunately I have a hard time seeing countries with such strong fossil fuel industries (e.g., America, Australia) allowing discussion on a public issue that legitimizes climate change so I don't expect them to make serious long-term sustained efforts.
