Climate change is the world’s greatest environmental threat. It is also increasingly understood as a threat to domestic and international peace and security – recognized by the Department of Defense as a “threat multiplier,” by Secretary of State John Kerry as “perhaps the world’s most fearsome weapon of mass destruction,” and by President Obama, in an address to graduates of the United States Military Academy, as “a creeping national security crisis.” The Supreme Court’s temporary blocking of the Clean Power Plan highlights the Federal-State divide over how to address climate change, but because of its national security dimension, climate change also raises unique separation of powers issues between the president and Congress with regard to how the military can respond.

While the president and Congress have certain constitutional authorities to address national security issues, the precise contours of their overlapping powers are unclear in the context of climate change, which presents both challenges to infrastructure and potentially more missions abroad.

As the nation’s commander in chief, the president has the constitutional authority to repel sudden attacks and take care that laws are faithfully executed. Congress has the power to provide for the common defense, declare war, and fund the military. Yet modern realities have given these responsibilities different meanings. Congress has not declared war in more than 70 years, and the president now commands a vast standing army and a global military presence.

There are few judicial decisions delineating the scope of power the commander in chief can employ in the face of Congressional support, inaction, or objection, but they are significant. The starting point for any analysis is Justice Robert H. Jackson’s concurrence in Youngstown Sheet & Tube Co. v. Sawyer, known as the Steel Seizure case, a monumental Supreme Court decision at the height of the Korean War when President Truman, acting pursuant to his power as commander in chief, ordered the secretary of commerce to take possession of the nation’s steel mills.

Writing in agreement with the majority, Jackson ruled that the president’s power is at its highest when acting pursuant to an express or implied authorization from Congress. Secondly, the president can only rely on his or her independent powers in the absence of either Congressional grant or denial of authority. Finally, the president’s power is at its “lowest ebb” when taking measures incompatible with the expressed or implied will of Congress. Significantly, Justice Jackson indulged the president wide latitude as commander in chief when taking action against the outside world, but granted no such indulgence when turned to purely domestic matters. The analysis found in Steel Seizure is highly relevant to any discussion of the president’s power to combat climate change today, both at home and abroad, and will only increase in importance.

Domestic Skirmishes

President Obama has begun to independently prepare for the impacts of climate change at home. The president has developed a Climate Action Plan and issued executive orders requiring each federal agency to identify critical infrastructure at risk to climate change. The president has also mandated federal agencies, including the Department of Defense, take steps to reduce their greenhouse gas emissions and create “climate roadmaps.” Aligned with this goal, the Navy recently launched the Great Green Fleet as part of its renewed focus on alternative energy and improved operational efficiency.

In some cases, these actions have drawn sharp objections from Congress, and most recently, the Supreme Court. For example, the House drafted a resolution in 2014 that prohibited federal funding to implement climate change assessments and reports for the military, and Secretary of the Navy Ray Mabus’ ambitious alternative energy programs have drawn the ire of some members over costs.

At risk of being lost amidst these political skirmishes is a real need to respond. Climate change threatens domestic military installations, particularly near the coast and in low-lying areas. This will undoubtedly require significant investment in physical infrastructure that can better resist sea-level rise and storm surge. For example, Norfolk Naval Station is the largest naval base in the world and located in an area that is particularly vulnerable to sea-level rise. Senior leaders have also recommended that climate change be a crucial consideration for future Base Realignment and Closure recommendations, an already fraught process given the possibility of certain areas losing the economic boost military bases can provide.

International Peace and Security

The president’s powers to address climate change are comparably greater when turned to humanitarian assistance and disaster relief missions abroad. This is due, in large part, to historical precedence characterized by executive initiative, Congressional acquiescence, and judicial deference.

Overseas, climate change is expected to increase the demand for international humanitarian assistance and disaster response – actions normally undertaken with significant military involvement. Climate change may act as a “threat accelerant” or “threat multiplier,” increasing instability as changing weather patterns disrupt livelihoods and rising seas threaten small-island states. Climate change is also melting polar ice caps, opening Arctic sea lanes and creating a rush for natural resources. For these reasons, climate change is increasingly understood as a novel and pernicious “threat” to international peace and security. In response, today’s military is beginning to prepare by seeking out novel partnerships and establishing climate change task forces.

In his or her capacity as commander in chief, the president not only has the authority, but also the obligation, to protect the nation from all threats, including those posed by climate change. Complicating matters from a legal standpoint, however, is that international humanitarian assistance and disaster response missions are outside the traditional international justifications for the use of military force premised on self-defense envisioned by the UN Charter. Hence, a host nation must provide its clear consent prior to the U.S. military assisting in any international humanitarian assistance or disaster response mission.

The UN Security Council’s definition of a threat to international peace and security continues to evolve, however, and may expand to one day encompass threats related to climate change. This would create a broader international legal basis to potentially address climate problems via the military. The recently signed Paris agreement, indeed, adopted the “threat” vernacular, acknowledging that climate change is not just an environmental problem but presents “an urgent and potentially irreversible threat to human societies.”

The Paris Agreement

The Paris Agreement is being celebrated as a historic step toward reducing greenhouse gas emissions. From a military perspective, it is noteworthy that, unlike the Kyoto Protocol, it does not provide an opt-out provision that would exempt armed forces from reporting or cutting emissions. In fact, it doesn’t specifically address military compliance at all, creating an open question regarding how they will be factored into future reporting and reduction efforts. This is perhaps most significant for the United States military, the world’s largest military and an enormous energy consumer. Some estimate the Department of Defense is responsible for as much as 80 percent of the federal government’s greenhouse gas emissions.

Internationally, the Paris Agreement has the status of a treaty. Within the United States, however, it does not appear that Congress will have a role in providing advice and consent because it was signed pursuant to existing legal authorities. Of course, Congress could pass domestic legislation that exempts the military from meeting the Paris emission goals. But the president could fight this, arguing that as commander in chief, he or she has discretion to include the military within any obligations from the Paris Agreement.

The military’s status under the Paris Agreement is but another twist in this complicated separation of powers dance. Climate change will continue to test the issue at home and abroad. Specifically, the president has less authority to unilaterally protect domestic military infrastructure in the face of Congressional intransigence, but has comparably greater authority as commander in chief to respond to climate-induced events abroad. Indeed, the president’s constitutional powers of the “sword” must ultimately be reconciled with Congress’s constitutional powers of the “purse.” Time will tell how this contest is ultimately reconciled.