Over one hundred concerned scientists and economists urged President Obama and Secretary of State John Kerry to say no to the Keystone XL pipeline in a letter sent to the White House and State Department on Monday.

In the letter, the scientists outline the dangers of extracting what has been called the world’s dirtiest fuel within Canada’s tar sands fields—in particular, the drastic consequences of burning the carbon intensive reserves for the world’s present and future climate crisis.

“As scientists and economists, we are concerned about climate change and its impacts,” the letter states. “We urge you to reject the Keystone XL tar sands oil pipeline as a project that will contribute to climate change at a time when we should be doing all we can to put clean energy alternatives in place.”

As the letter’s authors point out, fuels produced from tar sands produce the most greenhouse gas emissions over their lifecycle than any other fuel produced from conventional oil. The development of the Keystone XL pipeline, which will run from Canada to Gulf Coast refineries for tar sands export all over the world, would cause “sizeable expansion” of tar sands production and thus a drastic increase in this greenhouse gas pollution.

Former NASA Scientist James Hanson called this scenario “game over” for the climate. 

The letter continues: