I would argue that under the premise of the Geneva Conventions, and with the threat of global heating and climate change, a Planetary Emergency, no Nation State can be bar anyone from entering their country.
This blog contains articles and commentary on Climate Change / Global Warming. These changes will have an affect on the entire planet and all of us who reside therein. Life as we know it will change drastically. There is also the view that there is a high likelihood of climate change being a precursor of conflits triggered by resource shortges.