"Extraordinary claims require extraordinary evidence."
A quip popularly attributed to Carl Sagan.
You know what Carl Sagan had to say about climate change?
"For our own world the peril is more subtle. Since this series [Cosmos] was first broadcast the dangers of the increasing greenhouse effect have become much more clear. We burn fossil fuels like coal, and gas, and petroleum putting more carbon dioxide into the atmosphere and thereby heating the earth. The hellish conditions on Venus are a reminder that this is serious business. Computer models that successfully explain the climates of other planets predict the deaths of forests, parched crop lands, the flooding of coastal cities, environmental refugees; wide spread disasters in the next century, unless we change our ways. What do we have to do? Four things:
(1) Much more efficient use of fossil fuels. Why not cars that get 70 miles-per-gallon instead of 25?
(2) Research and development on safe alternative energy sources, especially solar power.
(3) Reforestation on a grand scale.
and (4) Helping to bring the billion poorest people on the planet to self-sufficiency, which is the key step in curbing world population growth.
Every one of these steps makes sense apart from greenhouse warming! Now, no one has proposed that the trouble with Venus is that there once was Venusians who drove fuel inefficient cars, but our nearest neighbour nevertheless is a stark warning on the possible fate of an earth-like world."
~Carl Sagan, Cosmos (episode 4: Heaven and Hell (update - 10 years later))
Dr. Sagan clearly believed that the "extraordinary claims" of climate science were backed up by extraordinary evidence.