Regarding CO2, warmist Marshall Shepherd in 2011 claimed that "only since the industrial revolution have values moved beyond the “natural bound” of 270 to 280 ppm"

Carbon, climate change, and controversy

[Marshall Shepherd, 2011] Carbon dioxide levels do show natural variability, but only since the industrial revolution have values moved beyond the “natural bound” of 270 to 280 ppm. Current levels are approximately 390 ppm.
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Humans may not perceive that average temperatures are warmer in 2100, but they will notice when coastal cities are facing sea level rise, days exceeding 100 degrees Fahrenheit increase, food prices fluctuate because of changing crop yields or livestock productivity, or diseases appear in new latitudes because of a more favorable climate.

The apparent controversy over climate change is rooted in a campaign by special interests to create reasonable doubt (e.g., tobacco industry and smoking) or basic lack of understanding of simple climate, science, and statistical principles.

Anybody see a “natural bound” of 270 to 280 ppm here?