A 16-year-old Swedish climate activist has landed in New York after crossing the Atlantic Ocean on a zero-emissions sailboat to attend a conference on global warming.

On Wednesday before dawn, Greta Thunberg tweeted, "Land!! The lights of Long Island and New York City ahead."

She and a sailing crew encountered rough seas on their two-week journey that delayed their arrival for a day, until the Malizia II was able to anchor opposite the famous beach resort of Coney Island in Brooklyn.

Thunberg set foot on land at approximately 4 p.m. ET after she and her crew were escorted into a lower Manhattan marina.

"It is insane that a 16-year-old would have to cross the Atlantic ocean to make a stand," Thunberg said at a brief press conference with reporters and activists.

"We need to stand together … and support each other and to take action, because otherwise it might be too late. So let's not wait any longer, let's do it now."

Thunberg described the trip as "surprisingly good" and said she "did not feel seasick once."

According to Boris Herrmann, the captain of Malizia II, they spent roughly 330 hours at sea, covering a distance of 3,711 nautical miles (6873 kilometres).

The teenager had refused to fly to New York to avoid a plane's fossil-fuel emissions. Action against climate change has been a theme of protests she has led in Sweden that inspired student strikes in about 100 cities worldwide.

Thunberg is set to speak at the United Nations Climate Action Summit next month. During today's press conference, she confirmed a coming trip zero-emissions trip to Chile, utilizing "a lot of trains, busses, and probably even sailing as well."

We have anchored off Coney Island - clearing customs and immigration. We will come ashore at North Cove Marina earliest 14:45 tide allowing. <a href="https://t.co/t6yy5z2asp">pic.twitter.com/t6yy5z2asp</a> —@GretaThunberg

She left Plymouth, England on Aug. 14, bound for New York, along with her father, Svante Thunberg, documentary filmmaker Nathan Grossman, skipper Boris Herrmann and co-skipper Pierre Casiraghi — grandson of Grace Kelly of Monaco.

They travelled in a high-tech, but decidedly low-comfort 18-metre sailboat, outfitted with solar panels and underwater turbines to generate electricity.

Thunberg became a European celebrity last year when she refused to go to school in the weeks before Sweden's general election to highlight the impact of climate change.

She continued her school strike on Fridays after the election, spurring thousands of young people to follow suit. Since then, she has met the Pope, spoken at Davos and attended anti-coal protests in Germany.

She is now taking a year off school to pursue her activism.