In the next several days, the Biden administration is expected to announce plans across the economy to reduce America's greenhouse gas emissions dramatically by 2030.
The Biden administration's goal is to speed the process to avoid a climate tipping point that scientists warn is quickly approaching. If global warming continues at its current pace, rising seas and heavy rain will flood cities around the world, wildfires and hurricanes will become even more destructive, and many more plant and animal species will go extinct.
But reducing emissions, even sharply, will not immediately fix the problems up in Earth's atmosphere. It took decades for greenhouse gases to accumulate in the atmosphere and trap heat, and it will take centuries for those gases to dissipate once humans decide to stop pumping them into the air.
"When you emit carbon dioxide, the climate stays altered for a long time," says Solomon Hsiang, a climate scientist who is the co-director at the Climate Impact Lab at the University of California, Berkeley. "And so we kind of have to deal with that baggage no matter what."
Everyone alive will still need to adapt to a warming climate. Today's adults will be dealing with climate-driven extreme weather for decades to come. But if countries transform their economies to cut heat-trapping emissions sharply, today's kindergartners could inherit a safer world when they reach middle age.
"It's kind of like you're driving a giant train that's very heavy. You slam on the brakes. The train keeps going for a while," Hsiang explains. "There's some amount of heating that we would continue to experience," even with dramatic cuts to greenhouse gas emissions. It will take decades for forests, oceans and other natural systems to soak up all the excess greenhouse gases that have accumulated in the atmosphere.
Why greenhouse gases persist
The major greenhouse gases emitted by cars, trucks, factories, power plants and farms are carbon dioxide, methane and nitrous oxide. The gases make their way up into the upper part of the atmosphere and hang out there, trapping heat and causing global warming.
The good news is that carbon dioxide, methane and nitrous oxide don't live forever in the atmosphere. Eventually, they break down or are absorbed by plants, oceans, soil and rocks on Earth's surface.
But humans have been generating such enormous emissions that new greenhouse gases arrive in the atmosphere a lot more quickly than old emissions break down or are absorbed. That's why gases have been accumulating in the atmosphere extremely quickly, especially in the last 50 years.
Different greenhouse gases take different amounts of time to break down or be absorbed. Nitrous oxide lingers in the atmosphere for about 100 years. Carbon dioxide can persist for hundreds or even thousands of years.
"If we got rid of all emissions tomorrow, carbon dioxide would come down very, very slowly," says James Butler, director of global monitoring at the Earth System Research Laboratory at the National Oceanic and Atmospheric Administration. "It would take thousands of years to come out."
Then there's methane, the shape-shifter of greenhouse gases. Methane only accounts for a small portion of the greenhouse gases emitted each year. But once it's in the atmosphere, methane is extremely good at trapping heat from the sun, which makes it far more potent than carbon dioxide in the short term.
Luckily, methane breaks down relatively quickly. If humans stopped emitting greenhouse gases, it would only take about 100 years for the excess methane in the atmosphere to dissipate, Butler says.
Here's the problem: When methane breaks down, it can turn into carbon dioxide. Both gases have a carbon at their center: Methane has hydrogen atoms attached, and carbon dioxide has oxygen atoms attached. As methane breaks down, the hydrogen atoms get replaced by oxygen from the air. One greenhouse gas is replaced with another.
Living with heat
The White House plan's goal is to reduce fossil fuel use to slow down the atmospheric buildup of greenhouse gases and the projected rate of the Earth's warming. Electricity would come from renewable sources such as wind and solar, cars and trucks would run on electricity, and factories would capture their emissions before they get into the atmosphere. Trains and buses would be more efficient and ubiquitous. New buildings would require less energy to heat and cool.
Economists already see signs that the economy is beginning to shift to the next generation of cleaner, cheaper energy. The price of wind and solar energy has plummeted, and energy-efficient vehicles are increasingly popular.
But because greenhouse gases persist for so long in the atmosphere, scientists warn that reducing emissions will not be enough to protect everyone. Many people will also need help adapting to a hotter Earth.
"Imagine you're driving a train and you're heading right for a cliff," Hsiang says. If you hit the brakes too late, some of the cars will go over the edge.
The train is humanity. The cliff represents deadly heat waves, wildfires, floods and droughts. So far, the U.S. has been tapping the brakes. Now, the U.S. is pledging to press down more firmly and try to stop the train before the whole thing goes over the edge.
But the cliff is looming, Hsiang says. "If we don't slow things down quickly enough, there are large regions of the world, many different people, who will be heavily impacted by the changes that continue to unfold over the next century."
Those most at risk include poor people, people who farm, people who work outdoors and people who live in places that are already dangerously hot or prone to flooding. "Those are the people who get sent off the cliff," Hsiang says.
The Biden administration has acknowledged that reducing U.S. emissions isn't a complete response to climate change. The proposed infrastructure bill would include money to help American towns and cities avoid damage and health impacts from storms, fires and heat waves.
And other countries are also looking to the U.S. for help with adaptation. As a newly reinstated signatory to the Paris climate agreement, the U.S. is back on the hook for $2 billion that the Obama administration pledged to the Green Climate Fund.
The fund exists to help pay for climate-related projects in poorer countries that are already suffering the effects of global warming, including adaptation projects such as building sea walls and transitioning to drought-resistant crops.
Earlier this year, White House climate envoy John Kerry said the U.S. will "make good" on the Obama-era pledge but hasn't said whether it will increase contributions in the future given that it has emitted more cumulative greenhouse gases than any other country.