Karlén is a retired professor of the Department of Physical Geography and Quaternary Geology from Stockholm University and a member of the Royal Swedish Academy of Sciences. In 2010, he predicted that natural climate changes, caused to a large degree by the sun's activity, would more likely make the climate colder than warmer in the next decades. 

He is part of the climate realists non-profit association that promotes a rational climate and energy policy. This group supports the claim that the scientific basis for climate policy is insufficient to justify the disruptive transformation of society that is now underway. Misguided measures without a secure scientific basis have negative consequences for the environment and the economy and do not have the intended effect.

“Newspapers should think about the damage they are doing to many persons, particularly young kids, by spreading the exaggerated views of a human impact on climate. As far as I can see, the IPCC 'Global Temperature' is wrong. Temperature is fluctuating but it is still cooler than in the 1930s and 1940s. It will take about 800 years before the water level has increased by one meter.”