There are a number of ways that climate change threatens the national security of the United States beyond the mere 'existential threat' of habitat destruction.

The first of these is that boundaries between nations and regions are in large part based on natural geography - difficult to pass geographical features like large bodies of water or rivers, think forests, faultlines, mountain ranges and deserts form natural buffer zones between populations. As these features change on a global scale: as rivers are redirected, water levels displace populations, as deserts expand and forests move these boundaries are shifted and so conflict can bubble between regions regarding their territory.

On a similar tact changes to the resource supply chain also causes territory disputes. In some places (e.g. Yemen), water shortages fuel conflict that can only get worse as water dries up. Resources move and ecological changes shift country's resource supplies. Large changes and migrations to game or fish populations force regions to renegotiate rights to territory for hunting and fishing.

Furthermore climate change makes some previously unavailable resources accessible. The elephant in the room here is the Arctic, which contains roughly 20% of the world's oil and huge amounts of minerals and other resources - as well as it being a natural and valuable geostrategic location for missile deployment and satellite communication.

Some months ago a new executive order was published for the purposes of developing a steering committee to unify agencies across the United States government to implement goals it describes as:

"... critical long-term strategic, ecological, cultural, and economic value, and it is imperative that we continue to protect our national interests in the region, which include: national defense; sovereign rights and responsibilities; maritime safety; energy and economic benefits; environmental stewardship; promotion of science and research; and preservation of the rights, freedoms, and uses of the sea as reflected in international law. ..."

The steering committee is responsible for a few things, and first:

"Sec. 3. Responsibilities of the Arctic Executive Steering Committee. The Steering Committee, in coordination with the heads of relevant agencies and under the direction of the Chair, shall:

(a) provide guidance and coordinate efforts to implement the priorities, objectives, activities, and responsibilities identified in National Security Presidential Directive 66/Homeland Security Presidential Directive 25, Arctic Region Policy, the National Strategy for the Arctic Region and its Implementation Plan, and related agency plans;

..."

https://www.whitehouse.gov/the-press-office/2015/01/21/execu...

In NSPD-66 the national security interests are enumerated as:

"The United States has broad and fundamental national security interests in the Arctic region and is prepared to operate either independently or in conjunction with other states to safeguard these interests. These interests include such matters as missile defense and early warning; deployment of sea and air systems for strategic sealift, strategic deterrence, maritime presence, and maritime security operations; and ensuring freedom of navigation and overflight.

The United States also has fundamental homeland security interests in preventing terrorist attacks and mitigating those criminal or hostile acts that could increase the United States vulnerability to terrorism in the Arctic region.

The Arctic region is primarily a maritime domain; as such, existing policies and authorities relating to maritime areas continue to apply, including those relating to law enforcement.[1] Human activity in the Arctic region is increasing and is projected to increase further in coming years. This requires the United States to assert a more active and influential national presence to protect its Arctic interests and to project sea power throughout the region.

The United States exercises authority in accordance with lawful claims of United States sovereignty, sovereign rights, and jurisdiction in the Arctic region, including sovereignty within the territorial sea, sovereign rights and jurisdiction within the United States exclusive economic zone and on the continental shelf, and appropriate control in the United States contiguous zone.

Freedom of the seas is a top national priority. The Northwest Passage is a strait used for international navigation, and the Northern Sea Route includes straits used for international navigation; the regime of transit passage applies to passage through those straits. Preserving the rights and duties relating to navigation and overflight in the Arctic region supports our ability to exercise these rights throughout the world, including through strategic straits."

http://fas.org/irp/offdocs/nspd/nspd-66.htm

This multinational bid, consisting of over 20 nations claiming parts of the arctic (Russia claiming about half of it) and 170 members on the international Arctic Council is bolstered by espionage, claim staking missions and cutthroat diplomacy:

http://foreignpolicy.com/2015/05/11/frozen-assets-arctic-esp...

The US and Canada have been increasing defense cooperation in the Arctic in anticipation of claims and challenges:

http://www.defense.gov/news/newsarticle.aspx?id=118768

But there have also been disputes with Canada about sovereignty:

http://www.theguardian.com/world/2014/sep/13/canada-uses-fra...

The US just held an emergancy artic council, under the guise of preserving the Artic for the sake of nature, and have been meeting bi- and multi-laterally with other artic nations to create as many national stakes to artic territory as they can (so that there can be political stalemate over the area).

Russia is now claiming approximately half of the arctic and has mobilized troops, drones and ballistic missile systems:

http://www.newsweek.com/.../what-russia-arctic-308941.html

http://www.theguardian.com/world/2014/oct/21/russia-arctic-m...

http://www.businessinsider.com/russia-is-militarizing-the-ar...

http://www.businessinsider.com/russias-arctic-pivot-is-a-mas...

http://www.zerohedge.com/news/2015-04-26/russia-deploys-tact...

The United State's world order is one where boundaries are not allowed to be changed. It has kept the Middle East, despite its high volatility, mostly static for the majority of a century. The current resource and land distribution of the world benefits the United States - in the sense that it is uniquely able to feel secure from the imperial ambitions of its neighbors (like the Britannica that bore it it is surrounded by weak neighbors and by geographical features that protect it). It is in the US national security interests that boundaries do not shift - both because it is a garunteur of static boundaries so that shifting boundaries undermine its legitimacy, and because it opens the opportunity for nations to become regional hegemons that can 'grow up' to play global power games against the US (I'm channeling the leaked DoD Wolfowitz Doctrine here).

The US does not want power projected or ballistic missile systems on its northern flank. In turn it enumerates 'strategic deterrence' as a national security issue in the Arctic: the US feels the need to deploy its own missile systems aimed at other nations as a retaliatory measure to deter possible future hostility.

Finally, the global ranging security pacts of the US will force it to intervene if conflict arises nearly anywhere over this changing world. If the world erupts into territory dispute caused by geographical changes and resource contention the US is obligated to be a part of that conflict.