The journey was often rough, but Greta Thunberg has sailed into New York City waters.

The 16-year-old Swedish teenager -- who over the last year has emerged as one of the warming planet's foremost climate activists -- arrived in the Big Apple on Wednesday afternoon, following a two-week trip across the stormy Atlantic Ocean.

Thunberg is in town to participate in September's U.N. Climate Action Summit. To avoid the prodigious carbon emissions created by air travel, she chose a wind-powered journey across the Atlantic aboard the high-tech racing sailboat the Malizia II.

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The choppy seafaring journey was not luxurious. The team often documented their trip, sometimes as waves crashed over the relatively small boat, from the cramped quarters of the spartan cabin, a place with no refrigerator or showers.

But, for Thunberg, that was preferable to a five-hour flight. "Someone flying from London to New York and back generates roughly the same level of emissions as the average person in the EU does by heating their home for a whole year," the European Commission notes.  

Thunberg, of a generation that will experience the worsening consequences of relentlessy rising global temperatures, advocates for global society to limit Earth's warming this century to 1.5 degrees Celsius (2.7 degrees Fahrenheit) above pre-Industrial Revolution levels.

“The science is clear," Thunberg said in a statement before her Atlantic journey. "We must start bending the emissions curve steeply downwards no later than 2020, if we still are to have a chance of staying below a 1.5 [Celsius] degrees of global temperature rise."

This will be a challenging -- if not nearly impossible -- goal to achieve. But even if society misses that ambitious objective, the message from climate scientists and activists alike is largely the same: Carbon emissions must come down rapidly.

"The choices we make now will make a difference," Joe Shea, a glacier researcher at the University of Northern British Columbia, told Mashable this summer. "But we need to start mitigating [carbon] 20 years ago."

Earth's atmospheric carbon dioxide emissions are now skyrocketing. CO2 levels haven't been this high in at least800,000 years — though more likely millions of years. What's more, carbon levels are now rising at rates that are unprecedented in both the geologic and historic record.