The United States military has, for some decades, been planning for how to deal with the effects of climate change, which include the expected migration of huge numbers of people. Here’s one article:

https://www.defense.gov/News/News-Stories/Article/Article/31...

Two excerpts:

As regions become too hot for sustaining livelihoods or human habitation, climate change may spur migration. Tensions over water and other resources could lead to instability and conflict. Climate change is already making the Arctic more accessible with nations vying over resources and access. Due to warming temperatures in many regions, diseases and viruses are expected to migrate to new areas of the globe.

And:

The world’s largest naval base, Naval Station Norfolk, Virginia, is endangered by rising sea levels, Water in the bay has risen about a foot-and-a-half since the 1920s, and a normal high tide now comes close to lapping over containment walls. The Navy, the city and businesses that depend on the harbor must work to adapt to rising sea levels.