Let's apply critical thinking to Warren's letter.  
He has never met a climate change denier. Is Warren's survey of one valid? No. It's anecdotal evidence. Strike 1
Earth has been hotter and colder as he states. Glaciers have grown and receded. Yet Warren ignores the rate. Climatic warming rose a total of 4 to 7 degrees Celsius over about 5,000 year but in the past century alone, the temperature has climbed 0.7 degrees Celsius, roughly 10x faster than the average rate of ice-age-recovery warming (NASA). The cause is primarily CO2 and methane. Warren is right about post hoc ergo propter hoc if it is used as a singular justification. But if we find after the fact the reasons for what came before that affect today, then this is evidence. Strike 2.
He says the "evidence is slight" that climate change is human caused and seems to think climate science is less rigorous than other branches. Isn't that post hoc ergo propter hoc to make that after-the-fact claim without evidence? Strike 3?