What we know — and what we don’t — about global warming #Auspol 

Editor’s Note: Last year was officially the hottest year on record, according to NASA and the National Oceanic and Atmospheric Administration. Yesterday, the American Meteorological Society delivered more bad news in a report on the state of climate in 2014. Greenhouse gases continued to climb, sea surface temperatures and the global sea level hit a record high and the number of tropical cyclones increased.
Gernot Wagner and Martin L. Weitzman, co-authors of “Climate Shock,” argue that we should insure ourselves against climate change. With a 10 percent chance of temperatures rising 11 degrees Fahrenheit or more and the catastrophic damages that could occur as a result, why wouldn’t we? Below, they lay out their case for pricing carbon dioxide pollution and discuss the economic consequences of a warming planet. Watch Thursday’s Making Sen$e segment for more on the subject.
— Kristen Doerer, Making Sen$e Editor

Two quick questions:
Do you think climate change is an urgent problem?
Do you think getting the world off fossil fuels is difficult?
This is how our book “Climate Shock” begins.
In fact, it’s not our quiz. Robert Socolow from Princeton has posed versions of these questions for a while. The result is usually the same: most people answer “Yes” to one or the other question, but not to both. You are either one or the other: an “environmentalist” or perhaps, a self-described “realist.”
Such answers are somewhat understandable, especially when looking at the polarized politics around global warming. They are also both wrong. Climate change is incredibly urgent and difficult to solve.
What we know is bad
Last time concentrations of carbon dioxide were as high as they are today — 400 parts per million — we had sea levels that were between 20 to at least 66 feet higher than today.
It doesn’t take much to imagine what another foot or two will do. And sea levels at least 20 feet above where they are today? That’s largely outside our imagination.
This won’t happen overnight. Sea levels will rise over decades, centuries and perhaps even millennia. That’s precisely what makes climate change such an immense challenge. It’s more long-term, more global, more irreversible and also more uncertain than most other problems facing us. The combination of all of these things make climate change uniquely problematic.
What we don’t know makes it potentially much worse
Climate change is beset with deep-seated uncertainties on top of deep-seated uncertainties on top of still more deep-seated uncertainties. And that’s just if you consider the links between carbon dioxide concentrations in the atmosphere, eventual temperature increases and economic damages.
Increasing concentrations of carbon dioxide are bound to lead to an increase in temperatures. That much is clear. The question is how much.

The parameter that gives us the answer to this all-important question is “climate sensitivity.” That describes what happens to eventual global average temperatures as concentrations of carbon dioxide in the atmosphere double. Nailing down that parameter has been an epic challenge.

Ever since the late 1970s, we’ve had estimates hovering at around 5.5 degrees Fahrenheit. In fact, the “likely” range is around 5.5 degrees plus-minus almost three degrees.
What’s worrisome here is that since the late 1970s that range hasn’t narrowed. In the past 35 years, we’ve seen dramatic improvements in many aspects of climate science, but the all-important link between concentrations and temperatures is still the same.
What’s more worrisome still is that we can’t be sure we won’t end up outside the range. The Intergovernmental Panel on Climate Change calls the range “likely.” So by definition, anything outside it is “unlikely.” But that doesn’t make it zero probability.
In fact, we have around a 10 percent chance that eventual global average temperature increases will exceed 11 degrees Fahrenheit, given where the world is heading in terms of carbon dioxide emissions. That’s huge, to put it mildly, both in probability and in temperature increases.
  

We take out car, fire and property insurances for much lower probabilities. Here we are talking about the whole planet, and we haven’t shown willingness to insure ourselves. Meanwhile, we can, in fact, look at 11 degrees Fahrenheit and liken it to the planet ‘burning’. Think of it as your body temperature: 98.6 degrees Fahrenheit is normal. Anything above 99.5 degrees Fahrenheit is a fever. Above 104 degrees Fahrenheit is life-threatening. Above 109.4 degrees Fahrenheit and you are dead or at least unconscious.
In planetary dimensions, warming of 3.6 degrees Fahrenheit is so bad as to have been enshrined as a political threshold not to be crossed. Going to 11 degrees Fahrenheit is so far outside the realm of anything imaginable, we can simply call it a planetary catastrophe. It would surely be a planet none of us would recognize. Go back to sea levels somewhere between 20 and at least 66 feet higher than today, at today’s concentrations of carbon dioxide. How much worse can it get?
Do we know for sure that we are facing a 1-in-10 chance unless the world changes its course? No, we don’t, and we can’t. One thing though is clear: because the extreme downside is so threatening, the burden of proof ought to be on those who argue that these extreme scenarios don’t matter and that any possible damages are low. So how then can we guide policy with all this talk about “not knowing”?

Press link for more: Gernot Wagner & Martin L. Weitzman | pbs.org

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