WASHINGTON — Climate activists have been calling on the next president to declare a national climate emergency within his first 100 days in office. But the legality of such a move will be tested when the Supreme Court rules on whether President Donald Trump can proceed with his wall on the southern border.

So far, over 20 countries have declared climate emergencies, including Canada and the United Kingdom. Many of these declarations have come from their legislatures, akin to what Rep. Alexandria Ocasio-Cortez, D-N.Y., and Sen. Bernie Sanders, I-Vt., have included in their “Green New Deal.”

“The time is now for Congress to declare a climate emergency and swiftly mobilize federal resources” the two say in their climate emergency resolution, “to protect the interests of our nation and its people.”

The resolution had little, if any, chance of getting through Congress, with a Republican-controlled Senate. Instead, the resolution represented part of a larger effort by Democrats to test climate legislation so that if a Democrat wins the White House in November, they will already know what legislation has a chance of passing and what doesn’t. In this scenario, climate action could begin on day one of the next presidency.

A resolution like the Green New Deal would be able to influence what is on the table in a legislative session. Congress declaring a climate emergency would help in clarifying legislative priorities on the environment.

But the president declaring a national climate emergency is not so easy – it runs into constitutional questions governing the use of executive power.

That power is being tested on the border now – and the result could answer whether a national climate emergency declaration by the president is a realistic goal.

On Feb.15, 2019, Trump released Proclamation 9844, declaring a National Emergency Concerning the Southern Border of the United States. Under statutes including U.S .Code 1431-1435, declaring a national emergency allows more flexibility in the use of Defense Department funds.

For this emergency, Trump intends to put billions of newly flexible defense funds toward wall construction.

The declaration faced many legal challenges from the outset. But one, Trump v. Sierra Club, cuts to the question of executive power in national emergencies. The Sierra Club filed a lawsuit alleging one key requirement of a national emergency is that the threat be immediate and time-sensitive.

The lawsuit is on its way to the Supreme Court, and the court’s decision on the breadth of executive power will affect how other presidents approach their agendas. Sen. Marco Rubio, R-Fla., was one of the first public figures to point this out.

“If today, the national emergency is border security … tomorrow the national emergency might be climate change,” said Rubio in January 2019. At that point, the emergency declaration was a hypothetical, while wall funding was being used as a bargaining chip in budget negotiations.

Now, that hypothetical is closer to becoming a reality. In July 2019, the Supreme Court allowed the federal government to continue progress on building the wall while legal arguments played out in the courts.

University of California, Berkeey law professor Dan Farber said this decision provides insight into how justices are leaning on the case.

“[The Supreme Court] generally won’t intervene unless they think that in a case like this, the lower courts are off track,” said Farber. What is unclear, he said, is the justices’ reasoning.

“There seem to be big issues of standing in these cases,” he said, referring to the question of whether a group like Sierra Club, or any organization that isn’t part of the federal government, can challenge a national emergency declaration.

“I read the Supreme Court’s action as probably being based on those doubts,” said Farber. “So we don’t know one way or the other what they think about the emergency action.”

What the court’s decisions on the border wall so far do show, Farber said, is what the court is prone to consider in a case on executive emergency power. If the court is willing to allow the emergency because the Sierra Club does not have standing to sue, the question would change from whether a national emergency has to present an immediate danger, to who, if anyone, has the ability to challenge a presidential emergency declaration.

Farber said , he is concerned that if the president gets the go-ahead from the court this year, there would be a dramatic expansion of what the president can do unencumbered.

“I don’t think that should become the new normal, that we’re just going to do things by declaring emergencies and then presidents will jump in with both feet,” he said.

If the court does allow the wall emergency to move forward, what would a national emergency on climate change even look like? Federal code governing emergency procedures was written with the idea that an emergency would present a physical or medical threat to the population.

This means that a lot of the code allows the president to take direct control over federal operations and take actions, like increasing defense production or rolling back environmental regulations.

Because the code allows for executive control of defense functions, defining a climate emergency could allow for defense production systems to be converted to produce climate-sensitive products, like solar panels and batteries.

Expanding a national emergency would open up presidential power in all directions. “What about malnutrition among children?” Farber asked. “It seems to me there are a lot of social problems where you could make some kind of argument” for declaring a national emergency.

If the court were to rule that not only does Sierra Club have standing to sue, but the president cannot declare an emergency on an issue so broad, then a president looking to declare a climate emergency would be facing the same set of issues as Trump with the border wall.

In that instance, activists may be better served pursuing legislation like the Green New Deal. Most other countries that have declared climate emergencies have done so through their legislatures.

Farber also points out a difference between a border emergency and a climate emergency. “I think climate change is a stronger case…because you’ve already got pre-existing government statements that it involves national security,” he said.

In early 2019 the Pentagon released a report saying in part, “The effects of climate change are a national security issue.” The report explains that over 60% of military outposts surveyed are currently suffering increased natural disasters linked to a changing climate. A national security threat could put climate change in the realm of a more traditional national emergency.

“I think you can make an argument that although it’s a long-term problem, we have a decreasing window of opportunity to act that makes it urgent,” said Farber.

For now, construction of the border wall will proceed as lawsuits are fought in court over the emergency order. It is likely that at some point this year, the court will hear a case relating to the issue, which may, or may not, clarify the question of presidential power.

However, the idea of a national climate emergency will not necessarily die in the courts. If the president wins re-election in November, it is all but guaranteed a climate emergency will not be declared in the next four years. If a Democrat wins the White House, executive power could see a new use in the fight against climate change.

“That would be the trade off,” Farber said, referring to trading executive climate action for an expansion of presidential power that could be difficult to walk back. “Is this something we really want to establish as being the new normal?”