Climate change skeptics are lashing out at President Obama for preparing to ratify an "illegal" climate deal before attending the Group of 20 Summit in China next week.

"It is an absolute disgrace that President Obama would assume powers found nowhere in our Constitution to sign a treaty that has never been considered by our Congress — which would reject it if given the opportunity," said Jay Lehr, science director with the outspoken free-market group Heartland Institute.

"To take such a hollow and illegal step as he ends his presidency should tarnish his legacy forever as a man who thought himself king, not president."

It is widely anticipated that Obama will meet with Chinese President Xi Jinping ahead of the G20 Summit of industrialized nations on Sept. 4 to ratify the climate change deal together.

The deal was hashed out in Paris in December by nearly 200 countries that agreed to cut greenhouse gas emissions to combat global warming and climate change. Although many countries, including the U.S., signed the Paris deal earlier this year in New York, many have not taken the formal step of "joining" the agreement, which means ratifying it for it to go into effect.

The deal was crafted in such a way so as to avoid congressional approval. That has stoked the ire of many Republicans, who argue that it should be ratified by Congress as other international accords have been.

Some have sought to undermine the deal through the budget process by attempting to defund a $100 billion-per-year Green Climate Fund. The fund would be used to assist poorer nations with adapting to climate change.

Heartland is raising many of the criticisms while reiterating its well-established position that climate change is a made-up issue that does not exist.

"The treaty was a sham from the beginning, which would foist more and more economic harm on the poorest of the poor around the world," Lehr said Wednesday. "Vast resources have been spent on a problem — human-caused catastrophic climate change — that does not exist.

Climate change has "no scientific validity but continues to rob more than a billion people in the world who have no electric power or access to adequate sanitation and water supply," he said. "Human-caused global warming is the biggest fraud ever perpetrated on society."

The institute also poked at the fact that Obama is expected to join with the president of a totalitarian dictatorship.

"The irony is overwhelming as President Obama heads to China, a country with an unelected dictator, to unilaterally 'ratify' the Paris climate treaty through executive order, even though the U.S. Constitution expressly states the president does not have this power, and that all treaties must be ratified by the Senate," said Institute research fellow Isaac Orr.

"President Obama's belief he can do whatever he wants because he's the president, Senate be damned, is un-American and dangerous," Orr said.

"The climate photo-op with President Obama and President Xi Jinping will be all show, no go," said H. Sterling Burnett, another energy research fellow with the group.

Chinese media in recent days have reported that the meeting to ratify the agreement could come as soon as Friday, but U.S. officials would not confirm those reports, the Washington Examiner reported.

White House climate adviser Brian Deese said climate change would be a top focus for the G20 Summit and Obama is committed to joining the Paris climate accord this year.