Global Warming is Really Just a Risk Mitigation Exercise

There are four broad outcomes depending on whether we act to mitigate climate change, and whether it is as bad (or worse than) what the models predict in terms of temperature increase by 2100 and the associated likely impacts of a +4-8C world environment:

1. We do nothing. Global warming is not a big deal.
2. We do nothing. Global warming is as bad or worse than feared.
3. We act. Global warming is not a big deal.
4. We act. Global warming is as bad or worse than feareded.

Key here is the only "variable" we as humans control is whether we act.  We can (and have) studied the hell out of the science to determine the truth of the second part, but the only choice we face is whether we (collectively as a species & planetary civilization) act decisively enough to significantly reduce emissions versus the status quo emissions track.

What are the consequences of these four paths?

1. Outcome: Ideal.  No economic loss to an economically forced conversion to a carbon free energy system, no major harms from greenhouse gas emissions.
2. Outcome: Catastrophic.  Billions probably die. At the outlier of possibilites, humans actually go extinct. At minimum, huge economic & social impacts, particularly when considering the secondary effects like wars & unrest from water & food shortages.  Millions of people die and millions of species go extinct.
3. Outcome: Ok. Some unnecessary economic loss (Stern put it at 1% of global GDP in that 2005 UK study). Plus side: We move to a renewable (cleaner) energy system. No more oil spills, black lung, flammable fracking tapwater, Asian brown cloud, and the host of non-climate related ills from the fossil energy system.
4. Outcome: Ok. Some economic loss (Stern's 1%) compared to #1, but this is illusionary since if #4 is true then global warming is a big problem and outcome #1 was impossible anyway, so this is also "ideal."  Depending on how well we acted, the various harms of global warming are mitigated to some degree so hurt less (or perhaps not at all).  Same benefits of #3 for moving to a renewable energy system.

So you, as emperor of the world for a day get to make 1 decision, act or don't act on global warming.  Down one path ("don't act" - #1 and #2) you get one good outcome and one very bad one.  Down another path (act) you get two pretty ok outcomes.  As a risk mitigation exercise, the prudent choice is clear.  It's not so different as the choice you make to buy insurance on your home. If nothing ever bad ever happens to your home, that money is wasted!  Yet if it does, you're glad to be covered.

It's not even as if you have to wait for the year 2100 or something, if we act on global warming, enact carbon taxes and do a "Manhattan project" for clean energy, and then uncover strong evidence the climate problem is far less serious than feared, the carbon taxes can be repealed and the clean energy projects shuttered.  The acts we take on climate change are reversible if we really don't like the outcomes.  They're human laws & treaties. The consequences of climate change itself are physics and short of some much much riskier geoengineering projects, there's very little we can do to stop it past a certain point.
 
Am I understating that 1% cost figure to mitigate it?  Sure, maybe.  But we currently spend something like 2.5% of global GDP on military expenditures and few seem to feel this deadweight spending is some kind of global economic disaster choking off human potential.  During WWII and the heights of the Cold War, military spending (particularly in the combatent nations) was well into the double digits.  We have proven our societies can withstand relatively long spurts of much higher economic committment to shared goals without great economic loss.