There are obvious and (to some people) surprising similarities between global climate change and global inequality. Both are obviously global problems. Neither can be solved by a single country, group or individual. In both cases, there are significant externalities and consequently coordination problems. Both issues are even formally linked (that is, not only conceptually): elasticity of carbon emissions with respect to real income is around 1. This means not only that if person’s (or country’s) income increases by 10%, emissions tend to increase at the same rate, but that the distribution of emitters mimics the distribution of income. Since in the global income distribution the top decile receives at least 50% of global income, it is also responsible for at least one-half of all emissions.
People don't want to leave their homeland, they are forced to. Migration is a "global good" that is only good in the abstraction of reducing income inequality, at the intangible cost of disrupting families and cultural ties.
Hans Rosling ('Factfulness') used to say global population would plateau at about 11 billion as progress moves through all nations. It looks like increasing decline in fertility rates may force greater division between Haves and Havenots, a phenomenon aggravated by warming. More pressure on developed nations to accept more involuntary & undocumented migrants can only increase internal antagonisms, between citizens who have restricted their own family sizes and Others who favour larger numbers of children. In some countries these internal tensions will be most obviously between Whites and Non-Whites. In some instances it may be too late to use democratic means for your suggestion of temporary citizenship for immigrants.