(NEXSTAR) – 2021 was the sixth warmest year on record, the National Oceanic and Atmospheric Administration and NASA announced in a joint press conference Thursday. Global surface temperatures were just a bit warmer in 2021 than in 2018, NOAA said, but were actually cooler than the year before.

“2021 was noticeably cooler than 2020,” said Gavin Schmidt, the climate scientist who heads NASA’s temperature team.

One reason, as counterintuitive as it may be, is that 2021 didn’t have as many countries on lockdown or under stay-at-home orders.

“The impacts of lockdowns were seen very, very clearly in the concentration of pollutants,” said Schmidt. “Where those lockdowns happened first – you saw it in China, then in Northern Europe and then in the U.S. – we saw a very rapid cleaning of the atmosphere.”

Those types of particles that disappeared from the air, mainly because of a dramatic drop in transportation emissions, are reflective particles. They generally reflect the sun’s heat away from the surface, keeping temperatures cooler on land.

“Take them away, and you have a slight warming effect,” Schmidt said. “As lockdowns lifted and activity resumed, especially in transportation, we have seen aerosols have picked up again as have carbon dioxide emissions.”

The slight cooling effect caused by these reflective pollutants returning to the atmosphere is far outweighed in the long-run by the main drivers of global warming. As the global economy reopened and transportation resumed, we started burning more fossil fuels again, releasing carbon dioxide that causes the planet to heat up.

There was also an estimated 6% drop in global carbon dioxide emissions associated with the 2020 shutdowns, he said, but “we’re putting so much into the atmosphere, those changes lag slightly and are less visible.”

The scientists also said a La Nina pattern — natural cooling of parts of the central Pacific that changes weather patterns globally and brings chilly deep ocean water to the surface — dampened global temperatures in 2021 (just as its flip side, El Nino, boosted them in 2016).

“La Nina tends to cool global temperatures slightly because its a big chunk of the earth’s surface” that’s cooler than average, said Russell Vose, chief of climate monitoring at NOAA. That cooling effect may continue into 2022 as we’re still in a La Nina winter.

Still, they said 2021 was the hottest La Nina year on record.

Even with the slight cooling from 2020 to 2021, the longterm trajectory clearly shows the planet warming up.

“So it’s not quite as headline-dominating as being the warmest on record, but give it another few years and we’ll see another one of those” records, said climate scientist Zeke Hausfather of the Berkeley Earth monitoring group. “It’s the long-term trend, and it’s an indomitable march upward.”

Schmidt said “the long-term trend is very, very clear. And it’s because of us. And it’s not going to go away until we stop increasing the amount of carbon dioxide in the atmosphere.”

And despite the slight cooling off between 2020 and 2021, climate change still contributed to extreme weather events that caused billions of dollars in damage and took at least 680 lives last year, Vose said. That's the highest death toll in a decade, he said.

The Associated Press contributed to this report.