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Climate 101 is a Mashable series that answers provoking and salient questions about Earth’s warming climate.

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Top U.S. earth scientists announced Thursday that 2021 was among the hottest years on record. 

Specifically, the average global surface temperature was the sixth warmest, according to both NASA and the National Oceanic and Atmospheric Administration (NOAA), making the last eight years the eight warmest in over 140 years of reliable record-keeping. Temperatures in 2021 were nearly 2 degrees Fahrenheit (about 1.1 Celsius) hotter than average temperatures in the late 19th century. Crucially, however, climate scientists emphasize it's the long-term temperature trend that really matters and best illustrates how global surface temperatures are changing, rather than what occurs during a particular year or group of years.

And the decades-long trend is unambiguous. Temperatures have been on an upward trajectory for nearly half a century. 

"It is the long-term effects on climate that we're really worried about," Zeke Hausfather, a climate scientist and researcher at the environmental science organization Berkeley Earth, told Mashable. "It is crystal clear that temperatures are going up, and they're going up quickly."

(Matching NASA and NOAA's temperature analysis, Berkeley Earth also independently found that 2021 was the sixth warmest on record.)

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Average global surface temperatures since 1880Credit: NASA

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Amid the rising global temperature trend there are small bumps, like little peaks and valleys. This is due to recurring, short-term climate patterns impacting the larger warming signal. The most influential of the patterns occur in the sprawling Pacific Ocean, which can see year-to-year periods of sea surface warming (El Niño) or cooling (La Niña). This temporarily pushes overall global temperatures up or down. That's why the decades-long story is crucial to watch. It cuts through the noise. 

"It is crystal clear that temperatures are going up, and they're going up quickly."

"We live on a dynamic planet with lots of daily, weekly, monthly, and annual fluctuations," emphasized Sarah Green, an environmental chemist at Michigan Technological University who had no involvement with the 2021 climate reports. "If you're looking for long-term changes, you have to average over the long term."

"The focus on short-term variability is not really helpful," agreed Hausfather.

In 2021, La Niña conditions in the Pacific Ocean had a cooling effect on Earth. But even so, the human impact on our climate remains outsized. To illustrate, 2021 makes 1998 look like an unusually cool year. But 1998 was "crazy warm" at the time, noted Hausfather, as the warming trend was enhanced by a potent El Niño event in the Pacific Ocean.

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Today's relentlessly rising temperatures are no surprise. Large-scale human activities, primarily the burning of fossil fuels refined from ancient, carbon-rich, decomposed creatures, have driven momentous changes in the atmosphere. For example, levels of the most important greenhouse gas, carbon dioxide, are now the highest they've been in some 3 million years, and are still rising. Each passing year, humanity emits prodigious amounts of carbon into the atmosphere.

With current carbon-cutting commitments from global nations, the world is on track to warm by some 2.7 C (nearly 5 F), which would have extreme, disastrous environmental consequences. Already, the consequences of warming are serious. For example:

• Extreme fires:Increased temperatures and dryness parch vegetation and allow wildfires to burn more rapidly, significantly contributing to unnatural infernosand extreme urban firestorms. ("It takes just a little bit of warming to lead to a lot more burning.")

• Severe deluges: A warmer climate allows the atmosphere to hold more water.  This boosts the odds for more severe and record-breaking deluges.

• Destabilized ice sheets: Warmer ocean waters have destabilized the Florida-sized Thwaites Glacier. It's receding back; if it collapses it can ultimately raise sea levels in the coming centuries by many feet.

• Ocean heating: The ocean absorbs over 90 percent of the heat humanity traps on Earth. That's a nearly unfathomable number. This portends continued sea level rise, great disruptions to animal life, and beyond. Ocean heat hit a record high in 2021.

• More vector-borne disease: As the climate warms, creatures that infect us with pathogens (vectors like mosquitoes and ticks) spread. 

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The impacts of climate change will only grow until nations drop carbon emissions to around zero. But with each passing year, efforts to limit warming to some 2 C (3.6 F) above 19th-century levels grow more daunting. The big solutions, however, like the vast expansion of powerful ocean wind farmsand electric vehicle adoption, are well-known. 

"The more you delay, the harder it is," said Green.