At the stroke of midnight on Dec. 31, Earth finished up its hottest year in recorded history, scientists said on Friday. The previous hottest year was 2023. And the next one will be upon us before long: By continuing to burn huge amounts of coal, oil and gas, humankind has all but guaranteed it.

The planet’s record-high average temperature last year reflected the weekslong, 104-degree-Fahrenheit spring heat waves that shuttered schools in Bangladesh and India. It reflected the effects of the bathtub-warm ocean waters that supercharged hurricanes in the Gulf of Mexico and cyclones in the Philippines. And it reflected the roasting summer and fall conditions that primed Los Angeles this week for the most destructive wildfires in its history.

“We are facing a very new climate and new challenges, challenges that our society is not prepared for,” said Carlo Buontempo, director of the Copernicus Climate Change Service, the European Union monitoring agency.

But even within this progression of warmer years and ever-intensifying risks to homes, communities and the environment, 2024 stood out in another unwelcome way. According to Copernicus, it was the first year in which global temperatures averaged more than 1.5 degrees Celsius, or 2.7 degrees Fahrenheit, above those the planet experienced at the start of the industrial age.

For the past decade, the world has sought to avoid crossing this dangerous threshold. Nations enshrined the goal in the 2015 Paris agreement to fight climate change. “Keep 1.5 alive” was the mantra at United Nations summits.

Yet here we are. Global temperatures will fluctuate somewhat, as they always do, which is why scientists often look at warming averaged over longer periods, not just a single year.

But even by that standard, staying below 1.5 degrees looks increasingly unattainable, according to researchers who have run the numbers. Globally, despite hundreds of billions of dollars invested in clean-energy technologies, carbon dioxide emissions hit a record in 2024 and show no signs of dropping.

One recent study published in the journal Nature concluded that the absolute best humanity can now hope for is around 1.6 degrees of warming. To achieve it, nations would need to start slashing emissions at a pace that would strain political, social and economic feasibility.

But what if we’d started earlier?

“It was guaranteed we’d get to this point where the gap between reality and the trajectory we needed for 1.5 degrees was so big it was ridiculous,” said David Victor, a professor of public policy at the University of California, San Diego.

The question now is what, if anything, should replace 1.5 as a lodestar for nations’ climate aspirations.

“These top-level goals are at best a compass,” Dr. Victor said. “They’re a reminder that if we don’t do more, we’re in for significant climate impacts.”

The 1.5-degree threshold was never the difference between safety and ruin, between hope and despair. It was a number negotiated by governments trying to answer a big question: What’s the highest global temperature increase — and the associated level of dangers, whether heat waves or wildfires or melting glaciers — that our societies should strive to avoid?

The result, as codified in the Paris agreement, was that nations would aspire to hold warming to “well below” 2 degrees Celsius while “pursuing efforts” to limit it to 1.5 degrees.

Even at the time, some experts called the latter goal unrealistic, because it required such deep and rapid emissions cuts. Still, the United States, the European Union and other governments adopted it as a guidepost for climate policy.

Christoph Bertram, an associate research professor at the University of Maryland’s Center for Global Sustainability, said the urgency of the 1.5 target spurred companies of all kinds — automakers, cement manufacturers, electric utilities — to start thinking hard about what it would mean to zero out their emissions by midcentury. “I do think that has led to some serious action,” Dr. Bertram said.

But the high aspiration of the 1.5 target also exposed deep fault lines among nations.

China and India never backed the goal, since it required them to curb their use of coal, gas and oil at a pace they said would hamstring their development. Rich countries that were struggling to cut their own emissions began choking off funding in the developing world for fossil-fuel projects that were economically beneficial. Some low-income countries felt it was deeply unfair to ask them to sacrifice for the climate given that it was wealthy nations — and not them — that had produced most of the greenhouse gases now warming the world.

“The 1.5-degree target has created a lot of tension between rich and poor countries,” said Vijaya Ramachandran, director for energy and development at the Breakthrough Institute, an environmental research organization.

Costa Samaras, an environmental-engineering professor at Carnegie Mellon University, compared the warming goals to health officials’ guidelines on, say, cholesterol. “We don’t set health targets on what’s realistic or what’s possible,” Dr. Samaras said. “We say, ‘This is what’s good for you. This is how you’re going to not get sick.’”

“If we were going to say, ‘Well, 1.5 is likely out of the question, let’s put it to 1.75,’ it gives people a false sense of assurance that 1.5 was not that important,” said Dr. Samaras, who helped shape U.S. climate policy from 2021 to 2024 in the White House Office of Science and Technology Policy. “It’s hugely important.”

Scientists convened by the United Nations have concluded that restricting warming to 1.5 degrees instead of 2 would spare tens of millions of people from being exposed to life-threatening heat waves, water shortages and coastal flooding. It might mean the difference between a world that has coral reefs and Arctic sea ice in the summer, and one that doesn’t.

Each tiny increment of additional warming, whether it’s 1.6 degrees versus 1.5, or 1.7 versus 1.6, increases the risks. “Even if the world overshoots 1.5 degrees, and the chances of this happening are increasing every day, we must keep striving” to bring emissions to zero as soon as possible, said Inger Anderson, the executive director of the United Nations Environment Program.

Officially, the sun has not yet set on the 1.5 target. The Paris agreement remains in force, even as President-elect Donald J. Trump vows to withdraw the United States from it for a second time. At U.N. climate negotiations, talk of 1.5 has become more muted compared with years past. But it has hardly gone away.

“With appropriate measures, 1.5 Celsius is still achievable,” Cedric Schuster, the minister of natural resources and environment for the Pacific island nation of Samoa, said at last year’s summit in Azerbaijan. Countries should “rise to the occasion with new, highly ambitious” policies, he said.

To Dr. Victor of U.C. San Diego, it is strange but all too predictable that governments keep speaking this way about what appears to be an unachievable aim. “No major political leader who wants to be taken seriously on climate wants to stick their neck out and say, ‘1.5 degrees isn’t feasible. Let’s talk about more realistic goals,’” he said.

Still, the world will eventually need to have that discussion, Dr. Victor said. And it’s unclear how it will go.

“It could be constructive, where we start asking, ‘How much warming are we really in for? And how do we deal with that?’” he said. “Or it could look very toxic, with a bunch of political finger pointing.”

You don’t have to live close to a wildfire to be affected by its smoke. With severe winds fanning the fires in and around Pacific Palisades, the Pasadena foothills and Simi Valley, huge swaths of the Southland are contending with dangerous air quality.

Wildfire smoke can irritate your eyes, nose, throat and lungs. The soot may contain all kinds of dangerous pollutants, including some that may cause cancer. The tiniest particles in smoke can travel deep into your lungs or even enter your bloodstream.

Conditions like these aren’t good for anyone, but they’re particularly bad for people in vulnerable groups, including children, those with asthma or other respiratory conditions, people with heart disease and those who are pregnant.

Here’s what you should know to keep yourself safe.

Stay indoors

Minimize your exposure to unhealthy air by staying inside and keeping your doors and windows shut.

If you have a central heating and air conditioning system, you can keep your indoor air clean by turning it on and keeping it running. Make sure the fresh-air intake is closed so that you’re not drawing in outdoor air.

Keep your pets inside

They shouldn’t breathe the unhealthy air either.

Check your air filters

Clean filters work better than dirty ones, and high-efficiency filters work better than regular ones. The California Air Resources Board and the South Coast Air Quality Management District recommend filters with a MERV rating of 13 or higher.

You might consider using portable high-efficiency air cleaner in a room where you spend the most time. The U.S. Environmental Protection Agency has information about them here, and CARB has a list of certified cleaning devices here.

Don’t pollute your indoor air

That means no burning candles or incense. If your power is out and you need to see in the dark, you’re much better off with a flashlight or headlamp.

If you’re cold, bundle up. This is not the time to start a cozy fire in the fireplace. Don’t use a gas stove or wood-fired appliances, since these will make your indoor air quality worse, not better, the AQMD says.

The CDC also advises against vacuuming, since it can stir up dust and release fine particles into the air.

Take care when cleaning up

You don’t want your skin to come into contact with wildfire ash. That means you should wear long sleeves, pants, gloves, socks and shoes. The AQMD even wants you to wear goggles.

If you’re sweeping up ash outdoors, get a hose and mist it with water first. That will keep it from flying up in the air as you move it around. Once the ash is wet, sweep it up gently with a broom or mop. Bag it up in a plastic bag and throw it away.

It’s a good idea to wash your vehicles and outdoor toys if they’re covered in ash. Try not to send ashy water into storm drains. Direct the dirty water into ground areas instead, the AQMD advises.

Those with lung or heart problems should avoid clean-up activities.

Discard spoiled food…

If you lost power for a significant length of time, the food in your refrigerator or freezer may be spoiled.

Food kept in a fridge should stay safe for up to four hours if you’ve kept the door closed. If you’ve been without power for longer than that, you’ll need to toss all perishable items, including meat, poultry, fish, eggs, milk and cut fruits and vegetables. Anything with “an unusual smell, color, or texture” should be thrown out as well, according to the U.S. Centers for Disease and Control Prevention.

Refrigerated medicines should be OK unless the power was out for more than a day. Check the label to make sure.

…even if it was in the freezer

Your freezer may be in better shape, especially if it’s well-stocked. Items in a full freezer may be safe for up to 48 hours if it’s been kept shut, and a half-full freezer may be OK for up to 24 hours. (The frozen items help keep each other cold, so the more the better.)

If items have remained below 40 degrees Fahrenheit (4 degrees Celsius) or you can still see ice crystals in them, they may be OK to use or refreeze, according to the federal government’s food safety website.

Ice cream and frozen yogurt should be thrown out if the power goes out for any amount of time. Meat, poultry, seafood, eggs, milk and most other dairy products need to go if they were exposed to temperatures above 40 degrees F for two hours or longer. The same goes for frozen meals, casseroles, soups, stews and cakes, pies and pastries with custard or cheese fillings.

Fruit and fruit juices that have started to thaw can be refrozen unless they’ve started to get moldy, slimy or smell like yeast. Vegetables and vegetable juices should be discarded if they’ve been above 40 degrees F for six hours or more, even if they look and smell fine.

Breakfast items like waffles and bagels can be refrozen, as can breads, rolls, muffins and other baked goods without custard fillings.

Consider alternative shelter

If you’ve done everything you can but your eyes are still watering, you can’t stop coughing, or you just don’t feel well, seek alternative shelter where the air quality is better.

Hold off on vigorous exercise

Doing anything that would cause you to breathe in more deeply is a bad idea right now.

Mask up outdoors

If you need to be outside for an extended time, be sure to wear a high-quality mask. A surgical mask or cloth mask won’t cut it — health authorities agree that you should reach for an N95 or P-100 respirator with a tight seal.

Are young children at greater risk of wildfire smoke?

Very young children are especially vulnerable to the effects of wildfire smoke because their lungs are still rapidly developing. And because they breathe much faster than adults, they are taking in more toxic particulate matter relative to their tiny bodies, which can trigger inflammation, coughing and wheezing.

Any kind of air pollution can be dangerous to young children, but wildfire smoke is about 10 times as toxic for children compared to air pollution from burning fossil fuels, said Dr. Lisa Patel, clinical associate professor of pediatrics at Stanford Children’s Health. Young children with preexisting respiratory problems like asthma are at even greater risk.

Patel advises parents to keep their young children indoors as much as possible, create a safe room in their home with an air purifier, and try to avoid using gas stoves to avoid polluting the indoor air.

Children over the age of 2 should also wear a well-fitting KN95 mask if they will be outdoors for a long period of time. Infants and toddlers younger than that don’t need to mask up because it can be a suffocation risk, Patel said.

What are the risks for pregnant people?

Pregnant people should also take extra precautions around wildfire smoke, which can cross the placenta and affect a developing fetus. Studies have found that exposure to wildfire smoke during pregnancy can increase the risk of premature birth and low birth weight. Researchers have also linked the toxic chemicals in smoke with maternal health complications including hypertension and preeclampsia.

What about other high-risk populations?

Certain chronic diseases including asthma, chronic obstructive pulmonary disease or other respiratory conditions can also make you particularly vulnerable to wildfire smoke. People with heart disease, diabetes and chronic kidney disease should take extra care to breathe clean air, the CDC says. The tiny particles in wildfire smoke can aggravate existing health problems, and may make heart attacks or strokes more likely, CARB warns.

Get ready for the next emergency

Living in Southern California means another wildfire is coming sooner or later. To prepare for the bad air, you can:

• Stock up on disposable respirators, like N95 or P-100s.
• Have clean filters ready for your A/C system and change them out when things get smoky.
• Know how to check the air quality where you live and work. The AQMD has an interactive map that’s updated hourly. Just type in an address and it will zoom in on the location. You can also sign up to get air quality alerts by email or on your smartphone.
• Know where your fire extinguisher is and keep it handy.
• If you have a heart or lung condition, keep at least five days’ worth of medication on hand.

Times staff writer Karen Garcia contributed to this report.