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Type the words “climate change” into Google and you could get an unexpected result: advertisements that call global warming a hoax.

“Scientists blast climate alarm,” said one that appeared at the top of the search results page during a recent search, pointing to a website, DefyCCC, that asserted: “Nothing has been studied better and found more harmless than anthropogenic CO 2 release.”

Another ad proclaimed: “The Global Warming Hoax — Why the Science Isn’t Settled,” linking to a video containing unsupported assertions, including that there is no correlation between rising levels of greenhouse gases and higher global temperatures.

(In reality, the harmful effects of carbon dioxide emissions linked to human activity, like rising temperatures and melting sea ice, have been acknowledged by every major scientific organization in the world.)

America’s technology giants have come under fire for their role in the spread of fake news during the 2016 presidential campaign, prompting promises from Google and others to crack down on sites that spread disinformation.

Less scrutinized has been the way tech companies continue to provide a mass platform for the most extreme sites among those that use false or misleading science to reject the overwhelming scientific consensus on climate change. Google’s search page has become an especially contentious battleground between those who seek to educate the public on the established climate science and those who reject it.

Not everyone who uses Google will see climate denial ads in their search results. Google’s algorithms use search history and other data to tailor ads to the individual, something that is helping to create a highly partisan internet.

A recent search for “climate change” or “global warming” from a Google account linked to a New York Times climate reporter did not return any denial ads. The top results were ads from environmental groups like the Natural Resources Defense Council and the Environmental Defense Fund.

But when the same reporter searched for those terms using private browsing mode, which helps mask identity information from Google’s algorithms, the ad for DefyCCC popped up.

“These are the info wars,” said Robert J. Brulle, a Drexel University professor of sociology and environmental science who has studied climate advocacy and misinformation. “It’s becoming harder and harder for the individual to find unbiased information that they can trust, because there’s so much other material trying to crowd that space.”

After being contacted by The New York Times in mid-December, Google said it had removed an ad from its climate search results, though it declined to identify which one. An ad from DefyCCC was still turning up at the top of searches days later. As of Wednesday, no ads at all were turning up for Times reporters and editors running these searches.

The climate denialist ads are an example of how contrarian groups can use the internet’s largest automated advertising systems to their advantage, gaming the system to find a mass platform for false or misleading claims.

Google allows companies to bid on search terms, and displays paid content at the top of its search results in the same blue font used for unpaid content. (For example, a candy maker might bid on the term “Christmas candy” so that its ads pop up when someone searches for those words.) Google identifies ads in its search results with an icon below the link.