Collins Dictionary, publishing dictionaries now for two centuries, announced its 2019 "word of the year" on Thursday.

It's a term whose usage increased by a whopping 100-fold between 2018 and 2019, according to Collins' lexicographers, and since 2013 has been employed four times as much.

In mid-September 2019, Google Trends shows searches for the term skyrocketed to their highest levels ever. The word, or term, is "climate strike."

And it's quite relevant. Earth's climate has experienced 18 of the 19 warmest years on record since 2000, and in 2019 scientists recorded the hottest month in 140 years of reliable record-keeping.

Climate strikes and protests, stoked by teenage climate activist Greta Thunberg, have ensued.

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"This was the year Greta Thunberg became a superstar, spreading her call for climate action around the world — whether through appearances at international gatherings, or by popularising the tactic of striking to draw attention to the cause," Collins wrote online.

Some 7.6 million people participated in youth-led climate strikes and protests in the last full week of September, according to Global Climate Strike, the organization planning or monitoring the protests. People filled major streets in New York City, Mexico, and Islamabad. The movement even had participants in Antarctica.

The crux of the strikes — which Collins formally defines as "a protest demanding action on climate change" — calls for nations to curb Earth's human-caused warming at temperatures that would limit the worst consequences of climate change, according to climate scientists, geologists, and atmospheric scientists.

SEE ALSO:Worst reasons for Trump to quit the Paris climate pact, unranked

This means keeping Earth's warming at least2 degrees Celsius (3.6 degrees Fahrenheit) below pre-Industrial Revolution temperatures, or ideally below 1.5 C. Humans have already heated Earth by over 1 C, and this warming will certainly increase through the century as heat-trapping carbon dioxide continues to saturate the atmosphere. Though how much warming will occur is the multi-trillion-dollar question.

The relentless warming trend is driven by a clear physical reality: Atmospheric concentrations of carbon dioxide — the world's most important greenhouse gas — are at their highest levels in at least800,000 years. The pace at which these CO2 levels are rising is breaking records, too. Paleoclimatologists found that carbon dioxide concentrations are rising at rates that are unprecedented in both the historic and geologicrecord. 

Climate strikes broadly aim to inspire unprecedented government efforts to slash carbon emissions and transform how civilization produces energy for industry, electricity, and travel.

"Limiting warming to 1.5 C is possible within the laws of chemistry and physics but doing so would require unprecedented changes,” Jim Skea, a leading UN climate scientist, said in 2018.