Notrdhaus’ model is good but imperfect. He freely admits in his papers his damage estimates are conservative (it ignores certain factors that are very difficult to quantify). Many argue he also doesn’t put high enough weighting on tail risks, which is a fair critique. So the optimal temperature is likely lower than that. BUT the reason I pointed to his slides was to point to his estimates for what carbon tax would be required to keep the temperature at various levels. So we’re aren’t even using the part of his model that you’re criticizing.

The slides you linked to are from Steven Keen (who is an economist, not a climate scientist), who is, with due respect, a crank that no one in economics takes seriously. His “projections” aren’t just on the harsh side, they’re downright insane. Let’s take one example: sea level rise of 20–40 meters. The IPCC (the gold standard of climate science) estimates sea level would rise between 0.61–1.1 meters. I would strongly advise against reading from Steven Keen; you aren’t getting an accurate picture of mainstream science on anything (economics, climate science, or otherwise). 4 degrees would be bad; likely not anywhere near that bad though.

So my reply is:

1. Between a Yale Nobel laureate and Steven Keen, I’ll go with Nordhaus
2. Yes, Nordhaus’ model likely underestimates climate damages though (which is evident in the survey I linked to; most economists do estimate higher damages)
3. But that wasn’t why I pointed to Nordhaus. I pointed to Nordhaus’ slides to show estimates of what carbon tax might be required to keep temperature at certain levels (like 2C for example)