The disappearing white reflective surface of sea ice that bounces warming sunlight harmlessly back into space.
The retreating white of Arctic snow cover on land -- which has the same role in reflecting sunlight.
The thawing Arctic permafrost -- frozen or frigid ground -- which holds vast natural stores of greenhouse gases, CO2 and methane which will be released by warming, and in turn cause even more thawing of the permafrost.
These Arctic systems that reflect light and trap greenhouse gases are generally described by climate scientists as "the planet's northern air conditioner." Their diminishment bodes more global warming.
Yohe points out that adding to the various uncertainties about these areas of economic activity are the other basic unpredictable elements of climate change -- especially how humans will affect it.
Economists, like the computer experts who try to "model" how fast and far the temperature will climb over the coming decades, often point out that amid their swarming clouds of data, the single biggest unknown is "what will humans do?"
How much greenhouse emission will humanity finally agree on cutting -- and then actually succeed in cutting?
Nor do scientists know precisely how much warming will result from a given increase in CO2 ( though they do have a general range of estimates and seem to be improving their ability to measure the atmosphere's "sensitivity" to CO2.)
Also unknown is how much money humanity will end up spending to adapt to the warming it cannot prevent.
Given all these unknowns, any discussion of just how much humanity will have to pay in dollars for a melting Arctic quickly gets down "in the weeds" of economics arcana.
But as demonstrated by the work of Yohe and his colleagues around the world -- including the economists and scientists who authored this Pew study -- humanity now has no choice but to face the potentially massive cost in dollars of having gone so long putting no cost on the invisible emissions of CO2.