All those insults and changes resulting from climate disruption add up quickly: $15 billion for Midwest farmers staring at a year of crop loss and rebuilding as the Mississippi River floods; 600 deaths and 1,000 hospitalizations as a heat wave bakes Chicago; and $147 million gone as Alaska’s king crab fishery succumbs to acidification and changing prey/predator structures.

The list touches virtually every human endeavor – forestry, health, tourism, energy production, city planning, agriculture, commerce, even culture.

The total cost of climate change seems impossible to pin down, given the uncertainties. But an assortment of climate researchers and economists are now chasing after that sum, attempting to arrive at a bottom line.

In February an inter-agency workgroup released the administration’s best guess of what each ton of carbon dioxide dumped in the atmosphere costs society: $21, plus or minus, or roughly $121 billion worth of damages annually as a result of U.S. carbon dioxide emissions.

Until this summer, the exercise was mostly academic. No more. The death of cap-and-trade and the shift in Congress following the mid-term elections means that bottom line has the potential to shape U.S. climate policy for the foreseeable future.

As federal agencies like the U.S. Environmental Protection Agency take the lead to limit emissions, their proposals must balance the cost of new restrictions against benefit of avoiding climate change impacts.

That’s because the costs of averting global change are also steep: Trillions of dollars to rework energy infrastructure, change habits, upgrade housing stock and capture and sequester planet-warming emissions, just to start.

The lower the estimated cost of disruption – known as the “social cost of carbon” – the less action the Obama Administration can justify. And several economists and scientists fear that the Administration has low-balled the figure, handicapping its ability to curb emissions.

“It’s like a volume dial on regulation,” said Kristen Sheeran, executive director for the Economics for Equity and the Environment Network. “The higher the social cost of carbon, the more stringent those regulations can be.”

“My fear is that they’re going right back to the exact same models that have provided a lot of grist for the justification of inaction in the first place.”

Those models provide the best information to date, said Richard Tol, a research professor at the Economic and Social Research Institute in Dublin, Ireland, who developed one of the three impact models used by the federal government.

More research will help narrow the range of uncertainty, he said, but the numbers the federal government has picked reflect established, peer-reviewed science. “The number is not precise, but it’s not a crazy number.”

The $21 figure came from an inter-agency effort consisting of representatives from 12 federal agencies. It has become the default value for government benefit-cost analyses; efforts by individual agencies to assess the value of emissions reductions have largely ceased. In recent months the Department of Energy has cited that figure to assess the impact of new air conditioning efficiency standards, and the EPA cites it in the agency’s analysis of greenhouse gas emissions from light trucks, among others.

But the figure is low, critics say – a danger that will become apparent as the administration tries to justify additional and more ambitious mitigation efforts. If capturing carbon emissions from coal-fired power plants – a technology that does not exist commercially today – ends up costing only $30 per ton of carbon dioxide yet the benefit of keeping each ton out of the atmosphere is worth just $21, carbon sequestration would fail the benefit-cost test.

“If $21 a ton actually drove policy, where would we end up? Well, we’d end up with a whole lot more warming,” said a former EPA official who declined to be identified because of his continuing work with federal agencies on the effort. “$21 a ton doesn’t really justify much.”