WASHINGTON — In November 1979, a little over a week after student militants seized control of the U.S. Embassy in Tehran and took 52 American citizens hostage, President Jimmy Carter issued Executive Order 12170 declaring a national emergency against Iran.

President Biden speaks in the Roosevelt Room of the White House on Friday. Evan Vucci/Associated Press

That order remains in effect today, renewed most recently in the weeks before last Thanksgiving by President Biden, who noted then that “our relations with Iran have not yet normalized.”

The Biden administration’s declaration Aug. 4 of a public health emergency on monkeypox frees up federal money and resources to fight a virus that has already infected more than 10,000 people in the United States. But public health emergencies expire every 90 days, unless extended by the Department of Health and Human Services.

Those are different from national emergency declarations, which give presidents broad leeway to make policy and tap federal funds without congressional approval. That’s what activists have clamored for to better fight climate change, but Biden has held off despite energy shortages in much of the world and high gasoline prices at home.

“This is actually the true test of whether President Biden takes the climate crisis seriously,” Karen Orenstein, climate director of Friends of the Earth. “There could not be a more crucial move.”

Presidents have declared 76 national emergencies in the last nearly five decades, and 42 remain in effect, according to a list compiled by the Brennan Center for Justice at New York University Law School.

Biden has recently declared emergencies related to hostage-taking and detained U.S. nationals abroad, while extending one on Mali. He’s also issued them on Myanmar and Afghanistan and authorizing sanctions on Russia, Ethiopia and individuals linked to the global illicit drug trade.

Such declarations stem mostly from the National Emergencies Act of 1976, which came after President Richard Nixon issued a series of them, including on currency restrictions and a national postal strike.

The law requires that those declarations automatically end after a year, unless the president orders a renewal. Congress can also end emergencies, but doing so effectively requires a veto-proof two-thirds vote, which has never happened.

“The origin of the law was clearly an attempt to set limits on presidential power,” said Chris Edelson, author of “Emergency Presidential Power: From the Drafting of the Constitution to the War on Terror.” “Before the actions passed, presidents could declare emergencies and no one really knew what it meant. And they stood for decades.”

An emergency declared in 1950 by President Harry S. Truman to combat communism globally in the context of the Korean War was still in effect in the 1970s, before the law.

Emergencies set since it took effect have similar, extended shelf lives, though. President George W. Bush’s emergency three days after the Sept. 11 attacks still stands. President Donald Trump declared COVID-19 a national emergency in 2020 and Biden has extended it through at least February 2023.

Only once has Congress even discussed thwarting emergency declarations, Edelson said. That was in 2019, when 12 Senate Republicans joined Democrats to block Trump’s efforts to declare one on the U.S.-Mexico border and put $6 billion-plus from the military and other federal funds toward building a wall. Trump used a veto to preserve his border emergency declaration until Biden nixed it upon taking office.

Sen. Marco Rubio, R-Fla., warned during the wall funding fight that allowing Trump to what he wanted might let future Democratic presidents to do similar on the climate. Trump used a veto to keep his border emergency declaration in place until Biden nixed it upon taking office.

“It sets long-term precedents,” Rubio told CNBC in 2019. “Tomorrow, the national security emergency might be climate change, so let’s seize fossil fuel plants or something.”

That prediction hasn’t yet proved prescient. Biden said last month that climate change “is an emergency” but didn’t issue a declaration, which would have let him take major actions meant to reduce greenhouse gas emissions, including limiting offshore drilling and U.S. oil exports.

With Congress approving nearly $375 billion over a decade in climate change-fighting strategies as part of a larger budget package, political pressure on him to do so may dissipate.

The White House hasn’t said whether declaring a national climate emergency is now off the table. But it’s hard to imagine the administration imposing limits on oil and production after prices at the pump hit record highs. They have since fallen steady throughout the summer – a fact the White House has trumpeted.

Still, declaring a national climate emergency could let Biden move to fundamentally remake the U.S. economy in a greener way, a pledge that was a centerpiece of his 2020 presidential campaign. The president also has promised to slash the nation’s carbon emissions in half by 2030 – a goal the budget package’s climate provisions aren’t enough to meet.

“Now more than ever we need to declare a climate emergency,” said Cassidy DiPaola, a spokesperson for the Stop the Oil Profiteering campaign. She said the budget measure, known as the “Inflation Reduction Act,” is “totally packed with handouts to the fossil fuel industry.”

“Our messaging to Biden is saying, ’Hey, you need to fix what the IRA left out and what the IRA sacrificed,” said DiPaola, who added that of the measure, “This is Congress that passed IRA. President Biden has still made all of these climate commitments.”

Delaying a national climate emergency declaration even this long, however, may undermine the core argument that a crisis is at hand.

“The real indicator that this doesn’t really meet the definition of an emergency as intended by the act – even though it’s not clearly defined – is that he waited,” said Edelson, who is also a professor of government at American University in Washington, about climate concerns. “If it’s a real emergency, you act right away.”

Adam Green, co-founder of the Progressive Change Campaign Committee, countered that conservatives on the Supreme Court and in Congress have repeatedly defied popular opinion on top issues — underscoring Biden’s need to act unilaterally.

“Everyone grants the president can declare an emergency if there is an individual fire or hurricane. But when the entire planet is suffering heatwaves, unprecedented fires are rampant, and oceans are on the verge of flooding American cities, the president can’t declare that an existential climate emergency?,” Green asked. “He clearly has the power and his grandchildren are depending on him to use it.”