Climate change is on a lot of people's minds during this summer of extreme heat and lethal wildfires across the Northern Hemisphere.

People see its devastating effects not as an abstract possibility in some hypothetical future, but unfolding in front of their own eyes. Scientists make ever more dire projections about the decades ahead if substantial reductions are not made to carbon emissions.

The British Columbia government declared a provincial state of emergency to support the response to the more than 500 wildfires burning across the province. (Darryl Dyck/Canadian Press)

That paradox may be due, in part, to the large numbers of people who simply don't believe that climate change is real in the first place.

There are also countless people who think vaccines do more harm than good, that Barack Obama was born in Kenya or even that the Holocaust never happened.

Keith Kahn-Harris has a theory about why so many people reject scientific consensus and documented historical fact. He calls it denialism and argues that it is making the practice of politics and public policy more difficult and divisive.

Keith Kahn-Harris is a lecturer at Birkbeck College, University of London and at Leo Baeck College, also in London, England. He is also an associate fellow at the Institute for Jewish Policy Research. His latest book, coming out next month, is called Denial: The Unspeakable Truth.

Click 'listen' at the top of the page to hear the full interview.