Environmental groups on Thursday sent an open letter to President Barack Obama warning him that the controversial Transatlantic Trade and Investment Partnership (TTIP) threatens to upend his climate legacy.

The letter (pdf), released ahead of the upcoming Group of 20 (G20) summit in Hangzhou, China, calls on Obama to protect the 2008 G20 commitment to phase out "inefficient fossil fuel subsidies that encourage wasteful consumption" by opposing the TTIP.

If approved, the U.S.-European Union trade deal would allow G20 counties to "maintain fossil fuel subsidies for 'economic' or 'security of supply' reasons," and would "encourage industry self-regulation rather than actually requiring firms to increase energy efficiency," the letter states.

"Instead of undermining important climate agreements and goals, trade deals should move us towards the end of fossil fuel subsidies that is needed to avoid the worst impacts of climate change," it continues.

The signatories to the letter include the Center for Biological Diversity, the Center for International Environmental Law, Friends of the Earth, Greenpeace, and the Institute for Policy Studies.

"On the heels of the Paris climate agreement, it is clear that we must act now if we hope to avoid the worst impacts of climate change, and that to meet this generational challenge, we must boldly restrict fossil fuel emissions. To that end, we urge you to safeguard your climate legacy, publicly reject the above TTIP proposals, and make clear that we cannot afford trade policies that undercut climate action," the letter concludes.

Ilana Solomon, director of the Responsible Trade Program at the Sierra Club, which also signed the letter, said in a press release, "Eliminating fossil fuel subsidies would prevent billions from pouring into the pockets of Big Coal, Oil, and Gas each year, and corporate trade deals can't stand in the way. Instead, corporate trade deals like TTIP must be replaced with a new model of trade that supports, not hinders, climate progress."

Added Bill Snape, senior counsel at the Center for Biological Diversity, "Current trade deals give big oil and gas corporations a second and third bite of the apple to avoid democratic accountability. The only way forward is to stop the massive giveaways to the Exxons of the world."