claim: Doubling the concentration of atmospheric CO2 from its pre-industrial level, in the absence of other forcings and feedbacks, would likely cause a warming of ~0.3°C to 1.1°C.
evidence: The letter goes on to warn of predicted impacts on the United States such as sea level rise and increases in extreme weather events, water scarcity, heat waves, wildfires, and the disturbance of biological systems.