In December 2019, an "unexplained pneumonia" began to sicken people in China. The culprit, a microbial parasite now known as the new coronavirus, would soon flip 2020 upside down. The pandemic has likely become a defining period of the century — and the outbreak is not nearly over.

While the virus circulates, sickens, and kills, natural events, climate-fueled disasters, and human achievement continue apace. Here are some of the momentous moments of the scientific world, so far, in 2020.

1. The pandemic

Significant things to know about the coronavirus pandemic:

• It has killed over 119,000 Americans (as of June 22), even with extreme social distancing measures in many states.

• It's part of a trend of new, emerging human diseases coming from animals

• It's in some ways unprecedented. "This is like the 1918 flu and the Great Depression at the same time," a labor historian told Mashable.

• It's starkly different than other recent infectious disease outbreaks like SARS or the H1N1 flu.

• There's definitive evidence that big spreading events happen indoors — at restaurants, workplaces, and other gatherings — where there's close contact with others (though the outdoors aren't nearly risk-free)

• Disease experts expect infected people to gain immunity to the coronavirus — but it's uncertain for how long.

• Getting tested for coronavirus isn't scary

• Wear a mask when you can't social distance. Masking reduces the number of virus particles you exhale.

• Medical historians speak about a bitter reality: The pandemic has long been expected. It’s totally predictable. And another outbreak is inevitable. 

2. Warmest January on record

In the 141-year global temperature record, January 2020 was the hottest January ever recorded, according to the National Oceanic and Atmospheric Administration (NOAA).

January's record came on the heels of a profoundly warm 2019, the second hottest year on record.

3. Second warmest February on record

Only February 2016 was warmer than February 2020.

"The 10 warmest Februarys have occurred since 1998," wrote NOAA.

4. Second hottest March on record

In 141-years of modern record-keeping, March 2020 was the second warmest March ever recorded, only eclipsed by March 2016.

2016 ended up being the warmest year on record, and 2020 has a good shot of being one of the warmest years on record, too.

5. Second warmest April on record

April 2020 wasn't just the second-hottest April on record.

"Ocean temperatures were historically hot," wrote NOAA. "It was the highest April ocean temperature since global records began in 1880."

Oceans, the true keeper of climate change, have been absorbing almost unfathomable amounts of heat since around 1990.

6. Hottest May on record

The European Union's Climate Change Service reportedthat May 2020 was the warmest May in its records.

Though a few regions like the Eastern U.S. and Eastern Europe had below-average temperatures, most of the planet experienced warmer than average temperatures.

7. Disastrous dam failure in Michigan

After getting deluged with unusually extreme rains for two straight days, two critical dams in Michigan failed in May. The failures led to unprecedented flooding in Midland County, and the evacuation of towns submerged in floodwaters. 

See the precise moment of the first dam failure here.

"This is unlike anything we’ve seen in Midland County," Michigan Governor Gretchen Whitmer said in a statement.

Heavier rains are a well-understood consequence of a relentlessly warming planet. This is especially the case in the Midwest. There, the most extreme rain events increased precipitation by a whopping 37 percent between 1958 and 2012.

8. Australian bushfire smoke travels around the world

Australia's historic bushfires, burning into early 2020, released profound amounts of smoke into the atmosphere — both visible and invisible.

The smoke circled the globe.

Most of the bushlands burned, on the order of 90 percent, combusted into carbon dioxide, a potent greenhouse gas.

9. The Arctic's ozone hole closed

You're probably familiar with the infamous ozone hole over Antarctica, caused by damaging and now-illegal chemicals.

But during March and April this year there was a notable zone of depleted ozone — which protects life from the sun's ultraviolet radiation — over the Arctic, too. It closed in April, though it wasn't nearly as robust as the annual Antarctic ozone hole.

Read about why it opened, and closed, here.

10. Zombie fires could be awakening in the Arctic

Some wildfires survive underground during the winter and then reemerge the following spring, as documented in places like Alaska. They're called "overwintering," "holdover," or "zombie" fires, and they may have now awoken in the Arctic Circle — a fast-warming region that experienced unprecedented fires in 2019.

Read about zombie fires — and the significant risk they pose to the climate — here.

11. SpaceX successfully launches astronauts to the space station

After successfully launching NASA astronauts Doug Hurley and Bob Behnken into space aboard a Falcon 9 rocket on May 30, SpaceX's Crew Dragon craft later safely docked with the International Space Station, safely delivering the astronauts.

It was the first-ever launch of astronauts into Earth's orbit by a private company, signaling a burgeoning, new era in spaceflight.

12. U.S. Megadrought

Much of Western America is mired in a historically unprecedented drought lasting some two decades, though there have been wet spells within the persistent dry period. But this isn't a normal drought. Previous research has suggested the Southwest might be in a bonafide megadrought— a fuzzy term referencing the most severe and enduring of droughts over the last millennium.

Now, a study published in April in the journal Scienceprovides evidence that this parched period (covering nine U.S. states from Oregon down to California and New Mexico) is among the worst droughts to hit the region in some 1,200 years — and the relentlessly warming climate is a major reason why.

"This current drought is on par with the megadroughts of the Medieval Era," said Benjamin Cook, a research scientist at Columbia University’s Lamont-Doherty Earth Observatory and an author of the study.

Read about it here.

13. Giant hornets

The term "murder hornet" skyrocketed into popularity in April after the New York Timespublished a viral story about the arrival of an invasive insect species in Washington state ("murder hornet" is in the headline). Mashable, like many other outlets, used the term, too. But entomologists say that's an irresponsible name for the species, Vespa mandarinia, even if it's the largest hornet in the world at some two inches in length.

"It's a ridiculous name," said Akito Kawahara, an entomologist at the Florida Museum of Natural History who researches the evolution and diversity of insects. "I think it's totally misleading."

"Insects already have a bad perception," he added

• Learn more about why 'murder hornet' is a terrible name.

• Read about how the hornets came to the U.S.

14. Atmospheric CO2 hit a record high in 2020

The pandemic couldn't thwart the relentlessly rising carbon dioxide in Earth's atmosphere.

Scripps Institution of Oceanography, which collects daily measurements of atmospheric CO2 atop Hawaii's Mauna Loa, announced in early June that CO2 levels reached a record high in May 2020 (atmospheric CO2 hits its annual high point each May). The research institute measured an average of just over 417 parts per million, or ppm, last month, likely the highest amount in millions of years.

Read more about it here.