In 2009, the Obama administration brought 12 federal agencies together to hash out a best estimate of the social cost of carbon. They based their answer on three widely used academic models that tried to quantify the harm to things like human health and coastal property as heat waves increased and sea-levels rose.

These models produce a wide range of estimates. But in 2016, the Obama administration recommended that federal agencies use a central estimate that, in current dollars, pegged the damage from a ton of carbon dioxide emitted in 2020 at around $50 per ton, rising over time.

At the time, some researchers and environmentalists criticized the Obama number for being incomplete. It did not, for example, fully account for many plausible climate impacts like damage from increased wildfires or the loss of diverse ecosystems. In one survey of climate economists from 2015, 51 percent of respondents said the number was probably too low. Only 9 percent said it was probably too high.

The Trump administration’s new numbers

When President Trump came to office, he disbanded the interagency team working on this issue. Instead, the E.P.A. under Scott Pruitt came up with its own “interim” estimate of the social cost of carbon.

First, the E.P.A. took the Obama-era models and focused solely on damages that occurred within the borders of the United States, rather than looking at harm to other countries as well. That change alone reduced the social cost of carbon estimate to around $7 per ton.

The reasoning was simple: If Americans are paying the cost of these rules to mitigate climate change, then only benefits that accrue to Americans themselves should be counted.

Critics counter that this approach overlooks two key points. First, many of those climate impacts in other countries could indirectly hurt the United States, say, by roiling the global economy or by increasing the number of refugees. Second, climate change can only be solved by all countries working together. If the United States refused to consider the impacts of its emissions on other countries, other countries could respond in kind.