According to Freeman Dyson, an emeritus professor of physics at the Institute for Advanced Study, the debate about global warming has become too narrow and opinions have become too entrenched. Relying on a computer model designed by the Yale University economist William D. Nordhaus, Dyson compared the effectiveness and economic feasibility of various options for addressing climate change--from ambitious proposals like those championed by Al Gore and enshrined in the Kyoto Protocol, which call on developed countries to restrict emissions, to a "low-cost backstop" policy that assumes technology capable of removing carbon dioxide from the atmosphere will soon become available. In his conclusion, Dyson dismissed the more-ambitious approaches as "disastrously expensive" and advocated the "low-cost backstop" alternative. Dyson's much-discussed quasi-contrarian approach to global warming has been mocked by some critics as overly optimistic, while others have found merit in his nuanced approach to environmental security.