She said that she had been following news of the fires that have engulfed the Amazon rainforest with great concern.
She said the fires which broke out during her crossing were "a clear sign we need to stop destroying nature."
Having used wind-power to make her way across the North Atlantic, she crossed the sea without a single drop of fossil fuel. However, she sacrificed efficiency for the cause.
Her boat was slowed by bad weather and slow progress at sea. Arriving in New York on Wednesday, she is a little behind schedule but still in time to participate in the climate summit.
The UN event will be held on the 23 September.
Thunberg's ship the Malizia II is a high-speed, 18m racing yacht which has water-turbine electric generators and solar panels which have allowed her to keep charge and contact with the outside world.
She was accompanied by her father, Svante, who crewed the ship.
The Malizia II is built for high-speed racing, and there is little space for luxury. There was no toilet — just a bucket — or a fixed shower aboard, and only a small gas cooker to heat up freeze-dried food.
The sailboat's electronics are powered by solar panels and underwater turbines, allowing her to keep up her twitter correspondence.
When asked what her message was for the president of the United States, another prominent twitter user, she told reporters.
"My message for him is just: listen to the science," she said. "And he obviously doesn't do that. So as I always say to this question: If no one has been able to convince him about the climate crisis, the urgency, then why should I be able to do that? So I am just going to focus on spreading awareness."
She was met by a fleet of sailboats from the United Nations. There were 17 boats organised to represent each of the 17 goals about to be set out at the climate summit.
Flight shame: Seglastolthet
The sixteen-year-old is known for organising school strikes and introducing the word "Flygskam" to the English language.
Thunberg popularised the Swedish movement that translates as "flight shame", encouraging young Europeans to boycott air travel for rail.
The two-week journey aboard the Malizia II was Greta's first attempt at long-haul anti-air travel.
However, this kind of endeavour is not feasible for everyone everywhere to return to a speed of travel pre-1903.
When calculating if the "flight shame" phenomenon could ever "take off" in New Zealand, the figures do not look promising.
Travel by rail and sail between Sweden and France - a favourite destination for tourists - is roughly 14 hours.
From New Zealand it's at least a month, probably on a cargo ship.
