John Holdren, US President Barack Obama's science adviser, talks to Nature.

On access to Obama: "I don't have to ask anybody's permission to see the president, except the president."

On cap-and-trade costs: "The actual numbers are far from devastating. If we had a price of $100 a ton on carbon, the effect on a gallon of gasoline would be about 30 cents. We all know that the price of gasoline goes up and down by more than that in a week."

On climate legislation: "If we go to Copenhagen without a climate policy in place, the freedom of our negotiators to negotiate anything meaningful is very limited."

On nuclear proliferation: "I think we ultimately ought to put all uranium enrichment and fuel reprocessing, if any is done, under multinational control."