Copenhagen, December 14—"The most important number in the world," Mohamed Nasheed, president of the Maldive Islands, told an audience of hundreds of climate activists in downtown Copenhagen, "is 350." Why 350? That's the threshold for parts per million of carbon dioxide in the atmosphere that will cause dangerous anthropogenic interference with the world's climate, according to Goddard Institute for Space Studies climatologist James Hansen. Hansen outlines his reasoning for a 350 parts per million (ppm) threshold in his new book, Storms of My Grandchildren: The Truth About the Coming Climate Catastrophe and Our Last Chance to Save Humanity.

In brief, Hansen argues that carbon dioxide above that level will so increase global temperatures that the melting of the vast ice sheets in Greenland and Antarctica will become inevitable. This could eventually mean sea level increases of a meter or more per decade. In addition, the temperature differences between polar seas cooled by melting ice and subtropical seas warmed by greenhouse gases will produce monster storms that could ravage the inland areas.

Hansen's troubling vision of a world climatically out of whack provoked environmentalist Bill McKibben to create a new global organization, 350.org. It launched a campaign to limiting the amount of carbon dioxide in the atmosphere to 350 ppm on October 24, with 5,200 events in 181 countries. Unfortunately, I missed most of Nasheed's speech today at the 350.org rally, which took place at the Klimaforum—a self-styled "alternative" or "people's" summit—due to some amazing United Nations organizational incompetence.

I spent the day waiting with thousands of others in subfreezing cold to try to get into the proper building to obtain credentials for the official United Nations Climate Change Conference. I clocked about five hours in line while my housemate, in town representing a Colorado NGO, waited 10.5 hours and was also turned away. The conference chaos makes one wonder how anyone expects the U.N. to run the world's climate if it can't manage a queue?

But even as reporters and advocates shivered outside, the show went on. There are various negotiating texts for possible political declarations that will be issued at the end of the 15th Conference of the Parties (COP-15) of the United Nations Framework Convention on Climate Change (UNFCCC). Nasheed asserted that he and many other developing nations are pushing to have the limit 350 ppm carbon dioxide included as an official goal for an eventual legally binding global treaty.

There is one particularly interesting wrinkle in this goal: It aims to reduce the amount of carbon dioxide to levels below what is already in the atmosphere. Pre-industrial levels were around 280 ppm, but today's atmosphere contains 387 ppm and is rising at about 2 ppm annually. Just to give you a sense of what achieving the 350 ppm goal would involve, Hansen argues that it requires the banning of all coal mining and burning in developed countries by 2020.

More from the Copenhagen climate change tomorrow, hopefully from somewhere indoors.

Ronald Bailey is Reason's science correspondent. He will be filing dispatches from Copenhagen all week.