While the first two presidential debates have weighed heavily on questions about Donald Trump’s relationships with women and the contents of Hillary Clinton’s emails, the conversation has for the most part steered clear of a topic that President Barack Obama has called the greatest threat to future generations: climate change.

So far, the issue has tallied up two mentions. First, during the first debate when Clinton brought it up and accused Trump of calling the phenomenon a hoax (Trump denied the claim on stage but had actually said that in the past). The second mention came in the second debate when red sweater-adorned Ken Bone asked about coal policies. Moderators, notably, haven’t touched the subject yet.

Climate scientists and activists have expressed dismay at the lack of debate on the issue.

“I’ve been shocked at the lack of questions on climate change, it really is fiddling while the world burns,” Kerry Emanuel, a leading climate scientist at the Massachusetts Institute of Technology, told the Guardian. “This is the great issue of our time and we are skirting around it. I’m just baffled by it.”

The final showdown between Trump and Clinton was scheduled to be hosted by Fox News’ Chris Wallace at 9 p.m. EDT. Wallace announced the topics he planned on touching upon last week: debt and entitlements, immigration, the economy, the Supreme Court, foreign hot spots and the candidates’ fitness to be president.

“Let’s be clear: a last minute question about energy doesn’t do justice to the threat of the climate crisis,” May Boeve, executive director of the climate group 350 Action, said after the second debate, referencing Bone’s last minute question that evening, the Guardian reports.

Environmental activists, concerned that the issue will be ignored Wednesday by the moderator, are urging the candidates to bring up climate change on their own.

“Both Secretary Clinton and Donald Trump should take every opportunity during tonight’s debate to tell the American people how they plan to tackle climate change,  one of the greatest global crises facing the human race. While climate change is a catalyst to all of the proposed topics from the economy to immigration, solutions to the crisis can also help improve those issues,” Annie Leonard, the executive director of Greenpeace USA, said in a statement Wednesday.

Clinton has been the only candidate to put any sort of emphasis on combatting climate change during her campaign. Her website lists a variety of strategies her administration would take to tackle the challenge, including a $60 billion clean energy challenge partnership with state and local governments, investments in clean energy infrastructure and an expansion of pollution and efficiency standards that Obama has championed in office.

Trump, on the other hand, has ridiculed the issue. While 97 percent of climate scientists agree that climate change is a real and growing threat, the Republican presidential nominee has called the issue a hoax invented by the Chinese to hurt American industry and manufacturing. He has even welcomed the phenomenon, tweeting last year that we “could use a big dose of global warming” when a cold front hit the Northeast United States.