Well, if you can't argue from reality, I guess you have to beat them with their own delusions?

I make this argument to leftists all the time: in Canada, because of the population distribution and climate, we have close to the highest per capita carbon emissions in the world. Every person imported to Canada will produce more CO2 here than they would elsewhere. Therefore, immigration should be severely limited.

Of course, that ignores that our natural forests absorb ten times the emissions we put out every year, but so does the government...