Over the next week or so, Illinois legislators will be voting on a comprehensive energy and climate policy. One of the most important aspects of the bill is that it will be the last chance to keep all of Illinois’ nuclear plants open.

If no action is taken, two of the state’s six plants will close this year, and two more plants may later follow. Once these plants close, they will never reopen.

The environmental and economic impacts of closing those plants cannot be overstated. The state’s nuclear plants provide about 84% of the state’s pollution- and carbon-free electricity.

If the four threatened plants close, it will more than offset all the progress Illinois has made to date on renewable energy, in terms of climate. Even with an aggressive renewables buildout, it will take over a decade to just get back to where we were in terms of emissions.

If we want to make progress on global warming it’s imperative that renewable generation be used to replace fossil sources, as opposed to other non-emitting sources like nuclear. We can’t afford to tread water.

When they close, nuclear plants are almost always mainly replaced by fossil generation (over the short-to-mid-term, at least). An example of this just occurred in New York. When the Indian Point nuclear plant shutdown, it was almost entirely replaced by two new gas plants and by increased utilization of old, dirty gas plants in the NYC area. The result will be increased air pollution impacts, in disadvantaged communities and elsewhere.

Closure of Illinois nuclear plants is expected to have similar impacts, i.e., increased air pollution that will significantly impact public health. According to the Clean Air Task Force, closure of the four at-risk Illinois plants would result in 1,200 to 2,700 premature deaths, and $10 to $24 billion in monetized damages from increased air pollution, over the next ten years.

Keeping existing nuclear plants open is significantly less expensive than building and operating new renewable generation to replace them. Hence, the subsidies needed to keep the four struggling nuclear plants open are far lower than those given to renewable sources.

Therefore, an effort to replace the state’s nuclear plants with (intermittent) renewable sources would result in higher power costs, as well as reduced grid stability, without reducing emissions.

Even without efforts to replace nuclear plants with new renewable generation, keeping existing nuclear plants open reduces power costs by maintaining an ample supply of power. Illinois power rates decreased after the passage of a 2016 bill which kept two of the state’s nuclear plants running. An analysis by the Brattle Group predicts that keeping the four currently-at-risk nuclear plants open would also reduce power costs, by $438 million per year.

In addition to increased air pollution, CO2 emissions and power costs, closure of nuclear plants has a devastating local economic impact, due to loss of a large number of high paying jobs and loss of local tax base. The result is local unemployment or emigration, higher local taxes, and reduced local government services.

Fortunately, the governor’s office has recognized the need to keep the state’s nuclear plants open and has expressed support for providing small subsidies to do so. However, the subsidy levels proposed by the governor are far too low to prevent the plants’ closure.

The current negotiation process needs to yield an adequate level of support. The economic and environmental benefits of keeping the state’s nuclear plants open will be well worth it.

James Hopf is a retired senior nuclear engineer and policy lead at Generation Atomic, a non-profit organization that advocates for low carbon, environmentally friendly energy.