Here are some things that they do know.

They know that the science is far from settled. They know that sun spot activity is the best guide for future temperature. They know that temperatures over the past ten years or so show a cooling trend despite higher carbon levels.  They know that water vapor and methane are far more capable greenhouse gases than is carbon dioxide.  They know that a large number of the weather stations themselves charged with measuring temperatures are not kept in proper isolation and that a number of the coldest measuring stations of the old Soviet Union are completely off line. They know that overall ice levels on the poles are increasing.  They know that the Earth has a long history of heating and cooling.  They know that carbon levels trail temperature increases rather than leading them. They know that todays carbon levels in the atmosphere are far below what they have been during a majority of the Earths existence.  They even know that other planets in the solar system mirrored our warming over the past 50 years despite an absence of the internal combustion engine.  Blah, blah, blah.