Three days before the beginning of a critical international climate conference in Paris, one of the world's most famous climate scientists, James Hansen, has written a withering criticism of President Obama's approach.

The Paris meeting will be attended by the heads of state of more than 130 countries, including Obama. Heading in, the United States has adopted a policy of calling for each country to set limits on carbon dioxide emissions, and will push for the adoption of technology to capture and store carbon dioxide. That approach, Hansen wrote in a new letter posted on his web site, "is so gross, it is best described as unadulterated 100 percent pure bullshit."

In his "communication" published on Friday, Hansen argued that world leaders are eager to avoid the embarrassment of the last major climate meeting in Copenhagen in 2009, which was largely ineffectual. This time, world leaders will reach a deal, Hansen says, and pat themselves on the back. This deal will likely include pledges to cut emissions by 2025. For example, the United States is expected to aim for cuts of 25 percent based on 2005 carbon levels.

"Watch what happens in Paris carefully to see if all that the leaders do is sign off on the pap that UN bureaucrats are putting together, indulgences and promises to reduce future emissions, and then clap each other on the back and declare success," Hansen writes. "In that case President Obama will have sold our children, and theirs, down the river."

However, Hansen says, that approach will fail as it has in the past. The only way to meaningfully address carbon emissions is to put a tax on carbon-based fuels, and to refund the entire amount as a dividend to consumers. The net effect of this will be to spur research and development of renewable fuels, Hansen believes. The document also makes clear that Hansen believes that conservatives in the US and elsewhere would eventually be receptive a tax-based approach.

An atmospheric physicist, Hansen became a leading voice among scientists in the 1980s and 1990s, raising public and political concern about climate change. From 1981 to 2013, he was the head of the NASA Goddard Institute for Space Studies in New York City. During that time, while he continued to do scientific research, he became more involved in advocacy, and further removed from US climate policy decisions.