> Through this decade growth has averaged 3.1%/year. So every year we have 1.031 times as much stuff produced as the previous year. 1.031^20 is 1.84150671005791. So in 20 years I'd expect us to see 84% growth in production. If we multiply that by 0.98 to account for the cost of mitigating global warming, we wind up with about 1.80467657585675 times as much production as today instead. When I take a 20th root of that I get 1.02995907626323. So we lost about 0.001% of growth/year, compounding annually. That is about 3 percent of annual growth lost.

WP says that 2008 GDP (GWP, apparently) was 61 trillion. Growing at 3.1% a year, that means 2009 growth ought to be 1.891 trillion dollars. If we immediately spend 1 trillion on carbon mitigation, then growth was only .891 trillion dollars, or 1.4% of 61 trillion (for a loss/difference of 1.7% annually).

Following your assumption that we lose 1.7% of world growth to carbon mitigation, then our compound interest formula is FV = P (1 + r)^Y, or FV = 1(1 + 0.014)^20 (20 years, whatever GWP currently is = 1, the 1.4% becomes 0.014). I get 1.3205629241491976 in ghci.

But I might've messed up, so I double-checked in http://www.moneychimp.com/calculator/compound_interest_calcu... (principal = 1, no annual addition beyond the interest, 20 years to grow, 1.4% interest, compounded annually). It says 1.32. (Switching 3.1 for 1.4 gives me ~1.84 as it should in both ghci and the calculator.)

184% vs 132% is the difference between 112 trillion and 80 trillion (61 * 1.84 & 61 * 1.32). So global warming mitigation efforts will result in an economy producing 32 trillion dollars less each year than it otherwise would. To make that worthwhile would take some pretty serious damages from global warming.

I'm not entirely sure where your error is, but I'm suspicious of your multiplying GWP-20-years-from-now by 0.98.

> Those figures say that in the middle of the projections for where we are a century from now, crop yields are likely to decrease. Furthermore we're not just talking higher temperatures. We're also talking changing weather patterns, which likely means increased drought in most mid-latitude areas. You might want to ask some of the farmers in California how drought is helping their crop yields these days, if you can find any in business.

Future decreases are of less interest because they have to be discounted for being, well, future. As for California, I have heard that the West periodically undergoes prolonged drought (such as with the Pueblo Indians?), and the past few centuries are unusually wet, so who knows how much global warming is actually hurting those farmers?

> I think you're underestimating the economic and lasting effects of war.

Perhaps. But growth is far more dependent on people than exogenous events like war. Note that Korea and Japan are some of the largest economies in the world, despite minimal resources and having both been devastated in war, and both only growing since the '40s or '50s. Is their growth in part due to the war itself and the destruction & rebuilding? "War is the father of all things".

(A further point, I have seen Iran's centuries long decline in the second millennium as due to Hulagu Khan's invasion in the 1200s. Japan & Korea could bounce back stronger than ever within a few decades, yet Iran couldn't recover for centuries? Further evidence that the effects of war are not obviously negative.)

> If you generate the power on the grid with solar and wind, then replace cars with electric vehicles, it is amazing how much air pollution goes away.

Sure, maybe cars will go all-electric. But why would the grid switch to solar and wind? It's barely doing so right now, even with generous subsidies and the threat of oil shocks. The probability of switching both grids & cars is necessarily lower than the probability of either... The best way to fight air pollution will not always coincide with reducing greenhouse gas emissions.

> Well your comment about not valuing millions of poor people very much speaks volumes about your ethical system.

It speaks more about my Malthusian leanings. Why should I wish to badly damage the industrialized world's growth for the sake of some poor country like Maldives, which with its population growth of 5.57% will see its population double in less than 15 years? They're going to screw themselves over - those islands can only support so much - and the longer they can keep the musical chairs going, the worse it will be at the end. If global warming ends the game early, it wouldn't bother me. (The lower the limit, the higher per capita can be.)