Activists held events around the world yesterday to mark the number they say the world needs to reach to prevent disastrous climate change: 350.

The number represents 350 parts per million of carbon dioxide emissions in the atmosphere that some scientists say is the safe upper limit. The atmosphere currently reaches about 390 parts per million, according to research by NASA climate scientist James Hanse cited by 350.org.

Hundreds of events highlighted the number in different ways.

In what 350.org founder Bill McKibben called a global game of Scrabble, groups in Australia, Ecuador, India, the United Kingdom, the U.S. and Denmark each spelled out one of the numbers in 350. Hundreds gathered in New York City’s Times Square and watched slideshows of the other events on giant screens.

McKibben, an environmentalist and author of “The End of Nature,” said the day was unique because it emphasized the science behind a politically complicated topic.

“It was ordinary people rallying around a scientific data point,” McKibben said. “Nothing like that has ever happened before.”