Warmist professor Dieter Helm claims that 400ppm CO2 is a critical threshold; he also correctly points out that it takes a lot of energy to dig out coal and transport it, but he forgets to mention the large payback for that energy expenditure

Forget Kyoto: Putting a Tax on Carbon Consumption by Dieter Helm: Yale Environment 360

The critical 400 ppm global threshold [What, specifically, is "critical" about 400ppm?] will shortly be crossed...it takes a lot of energy to dig out the coal and transport it to power stations