By: James McCarthy and Ken Kimmell, Union of Concerned Scientists 

Our organization, the Union of Concerned Scientists, has filed a brief in the case challenging the Environmental Protection Agency’s Clean Power Plan, and we eagerly await a hearing today before the U.S. Court of Appeals.

Why has a group of scientists chosen to get involved in a lawsuit? Because science confirms that the stakes are enormous.

In this lawsuit, which is likely ultimately to be resolved by the U.S. Supreme Court, fossil fuel companies and a number of states are trying to undo the Clean Power Plan, the first national policy to reduce carbon emissions from power plants, the single largest source of U.S. global warming pollution.

Rigorous scientific study indisputably tells us that disruptive climate change is primarily driven by the carbon dioxide we are releasing into the atmosphere by burning coal, oil and natural gas. It also tells us that, because the world has waited so long to address this problem, missing any opportunity now to reduce emissions will make it increasingly difficult to reduce them sufficiently to stave off the worst consequences of climate change.

Proof of the threat is literally all around us as the world increasingly experiences harmful and costly effects of climate change, including the increased frequency and severity of heat waves and highly destructive downpours because warmer air retains more water vapor.

July was the hottest month on record since global record keeping began in the late 1800s, and we are on track for 2016 to set another record for the warmest year. This summer, residents of three states — Louisiana, Maryland and West Virginia — had to cope with massive deluges, any one of which would have been expected to occur only once in a 500-year period. These unusual extreme events and the associated tragic loss of life, livelihoods and property are clearly marked with the fingerprints of global warming.

Following the lead of the United States and China, nearly all of the world’s nations took a historic step late last year by approving the Paris Climate Agreement, which is a triumph of multilateral diplomacy and represents the best chance society has ever had to address this grave existential risk. The agreement aims to stabilize the climate by holding the increase in global average temperatures to well below 2 degrees Celsius. To accomplish that, all countries must first meet the emission reduction goals they have already pledged as part of the agreement and then raise their level of ambition with new pledges every five years.

The United States played an indispensable role in forging the Paris Agreement by pledging to reduce its own emissions 26 percent to 28 percent below 2005 levels by 2025. The Clean Power Plan serves as the backbone of our nation’s 2025 pledge, and with its success the United States and other nations will be emboldened to make further commitments for 2030.

The Paris Agreement depends most of all on trust among world leaders and their citizens that each country will do its share to reduce emissions. No country will go it alone.

That’s why this court case is so important. U.S. leadership was an essential ingredient in the Paris accord, and other nations are watching to see what we will do. A fully implemented Clean Power Plan will demonstrate our resolve to meet our commitments and thereby encourage other countries to implement their plans as well.

There may never be another opportunity as promising as this one to avoid the worst effects of climate change.

That’s why a group of scientists felt compelled to raise its collective voice at this time.