"You are choosing the path focused on corporate greed," climate scientist James Hansen has told carbon traders in a open letter which he and climate activists attempted to deliver to a carbon trading conference in New York today.

In below-freezing temperatures, climate change campaigners gathered at midday at the Irish Hunger Memorial in Vesey Park, near the Embassy Suites Hotel where the conference is being held, to hear Hansen read parts of his open letter. Tomorrow there will be another demonstration at the same spot, at which an unconfirmed number of activists have pledged to commit acts of nonviolent civil disobedience.

Hansen's letter advocates using the fee-and-dividend approach to reducing carbon emissions, rather than cap-and-trade. Fee-and-dividend is a "transparent, honest approach that benefits the public", he says, in contrast to cap-and-trade, which "is a hidden tax … because cap-and-trade increases the cost of energy for the public, as utilities and other industries purchase the right to pollute with one hand, adding it to fuel prices, while with the other hand they take back most of the permit revenues from the government. Costs and profits of the trading infrastructure are also added to the public's energy bill."

"The public must understand the difference between cap-and-trade and fee-and-dividend," states Hansen, head of the Nasa Goddard Institute for Space Studies in New York. Otherwise, "the present administration may jam down the public's throat just such an approach, which, it can be shown, is not a solution at all."

The other speakers present, who included the Harlem community organiser Cecil Corbin-Mark of We Act for Environmental Justice, Charles Komanoff of the Carbon Tax Centre, and Father Paul Mayer of the Climate Crisis Coalition, spoke about the inequity of a cap-and-trade system. One of the organisers, Brian Tokar, said: "Carbon trading is unfair, it's unethical, and it just doesn't work."

A carbon trading system depends on allocating a market price to carbon emissions, and either hands out, auctions or sells carbon permits to industry sectors. A fee-and-dividend system imposes a fee on the initial sale of a fossil fuel which is then redistributed to the public; the rising cost of carbon-intensive products would, it is hoped, encourage families to keep their carbon footprints low.

The civil disobedience planned for tomorrow outside the conference is part of the growing climate change activist movement in the US. Tokar told the Guardian: "In the last few months there's been a real rising of awareness about climate change issues in the US. For a long time they were seen as kind of abstract, something that the scientists were talking about, but now, in the months leading up to Copenhagen, people fully realised that these were issues that are affecting vulnerable people around the world in the shape of floods and droughts.

"Back in November we had the day of Climate Justice Action on 30 November on the 10th anniversary of the Seattle day of action. People blockaded the Chicago Climate Exchange, and blockaded a shipment of components for a new coal burning power plant, and protested outside the Bank of America in San Francisco. Activists are definitely beginning to focus on climate change in a way that they weren't a couple of years ago."