The U.S. military views climate change as a threat multiplier, one that is likely to worsen already existing weaknesses of government and poverty.

Why it matters: Internal and external climate displacement is already occurring. Depending how quickly and significantly temperatures rise, the specter of climate migration and refugee flows looms large by midcentury.

Rising seas: In 2017 alone, nearly 19 million new internal displacements were recorded in more than 130 countries worldwide, largely triggered by extreme weather events such as floods and tropical cyclones, according to the Internal Displacement Monitoring Center. That's more than were displaced due to armed conflict in the same year, and climate change is aggravating many of these extreme weather events.

Residents of small island nations like the Marshall Islands, Vanuatu and Kiribati are fleeing rising tides and powerful storms to seek better economic opportunities in the U.S. mainland and elsewhere. Other small island countries are wrestling with contingency plans should waters rise too high.

A recent U.N. climate report found that many low-lying island states will face an existential threat should global warming exceed 1.5°C, or 2.7°F, above preindustrial levels by 2100.

On our present course, we're headed for more than 3°C, or 5.4°F, above preindustrial levels by the end of the century. Such scenarios could even spur millions of Americans to migrate away from the coast, too.

Elsewhere, sea level rise and the monsoon season have pushed populations inland in heavily populated Bangladesh, as Rohingya refugees are forced to settle in marginal areas vulnerable to flooding.

Drought: In addition to sea level rise, water stress is the biggest concern of humanitarian groups and military planners.

By making droughts hotter and drier, the threat of drought induced migration and conflict is growing in areas that are vulnerable to social and military strife.

Climate studies have tied the Syrian Civil War's beginnings in part to a record drought that struck the Fertile Crescent in 2007-2010, which set in motion political events that set the country on a ruinous course.

That drought, climate models show, was made more likely and severe due to global warming.

Precise NASA satellite measurements of groundwater storage — which acts as a water savings bank for farmers — show water stress building in heavily populated and conflict-prone parts of the world: Places like the border region between Pakistan and India, the Fertile Crescent, and the Northwest Sahara Aquifer System, which provides water for Algeria, Tunisia and war-torn Libya.

The big picture: The locations most at risk from climate change-related stresses in the near-term includes heavily populated areas like India, Pakistan and Bangladesh, and extends around the world to parts of South America and Africa, encompassing well over a billion people.

The bottom line: No refugee crisis or migration flow is purely due to climate change. But already, global warming-related factors are playing a role in setting people into motion, both within countries and between them.