Heating up militarily, not climactically. If there is a place World War 3 would start, it is the South China Sea.
For the last decade, China has slowly but inexorably been claiming more and more of the South China Sea, the body of ocean water bordered by China to the north, Vietnam to the west, the Philippines to the East, and Brunei and Indonesia to the South (see map above). It is estimated that about one third of global trade passes through the South China Sea (though by some estimates that figure could be closer to 20%).
China has claimed virtually the entire South China Sea within the so-called “nine dash line” (roughly equivalent to the area within the red line in the map above) as its territorial waters. Needless to say, this conflicts with claims by the other nations bordering the South China Sea. As part of its claims to the region, China has gone so far as to build at least seven new islands (information from 2015; likely more now) and fortified at least 27 other islands in international waters of the South China Sea. A 2016 arbitral decision has essentially dismissed China's claims to the extent they exceed what is allowed under the United Nations Convention on the Law of the Sea.
Since 2000, China has engaged in an unprecedented naval buildup focused primarily on the South China Sea, and now has the world's largest navy at 360+ ships. However, based on the types of ships it counts in its navy, it’s reach would not be limited to that body of water.
"Already commanding the world's largest naval force, the People's Republic of China is building modern surface combatants, submarines, aircraft carriers, fighter jets, amphibious assault ships, ballistic nuclear missile submarines, large coast guard cutters, and polar icebreakers at alarming speed."
Some of those will be the equal or better of anything the US or other naval powers can put in the water.
China’s shipbuilding has not slowed. Just three days ago, China commissioned three new vessels, a nuclear ballistic missile submarine, a destroyer, and an amphibious assault ship (essentially a helicopter carrier). Noteworthy is that the latter vessel is especially suited for island invasions. (Also noteworthy is that this last contention comes from the linked Global Times article; the Global Times is a Chinese news outlet owned by the People’s Daily, so essentially an official propaganda sheet.)
While conflicting claims to the South China Sea are being handled primarily through a war of words at the moment, other interested countries have begun ramping up their own military efforts. The US sent the aircraft carrier Theodore Roosevelt into the South China Seas two weeks ago, and the US and the Philippines planned to conduct joint naval drills at the same time, including the Philippines conducting Coast Guard drills. China sent its own aircraft carrier, the Liaoning, into the area in response. Vietnam, not a strong naval power, has built up a maritime militia of as many as 8,000 fishing boats and 70,000 persons to keep track of Chinese military vessels and fishing ships. Even Germany has sent one of its frigates into the South China Sea as a show of support for the US, Japan and Australia.
In a move that signals further increased tensions, Great Britain is sending a naval flotilla that includes its largest aircraft carrier, the HMS Queen Elizabeth, on an excursion to the Pacific, which will almost certainly include sailing through the South China Sea. This sailing has wider strategic implications, as well:
In Monday's carrier strike group announcement, the Defense Ministry said the deployment is aimed toward a deeper UK security role in the region, with exercises planned alongside India, Japan and South Korea as well as US forces in the region.
[The sailing] will also highlight one of Britain's oldest security relationships, the Five Powers Defense Agreement among Malaysia, Singapore, Australia, New Zealand and the UK. Exercise Bersama Lima will mark the 50th anniversary of the defense pact, the Defense Ministry said.
Overarching the South China Sea conflict is China’s more aggressive stance towards Taiwan, which lies at the northern end of the South China Sea. Taiwan is, of course, an independent country, but China has always viewed it as a breakaway province of China. China recently has been conducting naval sailings around Taiwan, regularly sending military flights into Taiwan’s Air Defense Interdiction Zone, and testing Taiwan's air defense radar's capabilities. China has even threatened Taiwan directly, stating two weeks ago that Taiwan "won't stand a chance" if it decides to invade. The chances of an invasion, and of Chinese success in doing so, are improving more and more as time goes by.
It seems obvious, therefore, that China’s rapid naval buildup is geared not just towards securing China’s asserted fishing and mineral rights in the South China Sea, but establishing a force projection capability that would thwart a US or US-led coalition response to a Chinese invasion of Taiwan.
I am reminded of the scene in Hunt For Red October in which the US Defense Secretary upbraids the Russian Ambassador, saying “It would be well for your government to consider that having your ships and ours, your aircraft and ours, in such proximity is inherently dangerous. Wars have begun that way, Mr. Ambassador.” It is possible we are seeing a real-life example of this quote unfolding in the South China Sea.