On April 22, scientists from around the world will mark Earth Day by participating in an unprecedented “March for Science.” The aim of the march will be to “celebrate and defend science at all levels – from local schools to federal agencies.” For the rest of the world, it is important to understand why the usually sedate community of scientists will be leaving their labs and offices to take to the streets in a global demonstration of concern.

The answer was signalled in November 2016, when Oxford Dictionaries named “post-truth” its “Word of the Year.” In an era in which “objective facts are less influential in shaping public opinion than appeals to emotion and personal belief,” scientists like us cannot afford to stay silent any longer. So we will be marching to return scientific “certainty” to its rightful place in public debate.

“Post-truth” describes well a year in which disregard for facts became a pervasive feature in world politics. As a candidate, US President Donald Trump denied the overwhelming evidence for climate change, endorsed the discredited claim that vaccinations caused autism, and asserted that compact fluorescent light bulbs can cause cancer.

But Trump does not have a monopoly on post-truth politicking. Policymakers in the US and Europe have trafficked in equally outrageous “expert views” on the consequences of their opponents’ positions on topics ranging from genetically modified foods to nuclear energy to Brexit. Recent social media attacks on a measles-rubella vaccination campaign even surfaced in India, fueling a mix of conspiracy theories, safety concerns, and questions of motivation – and demonstrating the extent to which lives can be imperiled when facts are ignored.

Earlier warnings, such as Ralph Keyes’ 2004 book The Post-Truth Era: Dishonesty and Deception in Contemporary Life, attracted little attention from the science community. That’s because we’d heard it all before; “post-truth” responses to “objective facts” are as old as science itself. An early example was the persistent belief in a flat earth, a view maintained for centuries after the ancient Greeks had accumulated clear evidence to the contrary. In some places, the denial and invective hurled at Darwin and his theory of evolution in the nineteenth century continue to this day. “Don’t confuse me with the facts,” goes an old joke capturing the post-truth sensibility: “my mind is made up.”

But now we have arrived at a watershed moment, when this sensibility has entered the political mainstream, influencing policies that will profoundly affect the health and wellbeing of the planet and its inhabitants. Those who regard the scientific method – the systematic observation, measurement, and hypothesis testing that has underpinned humans’ apprehension of ourselves and the world for centuries – as a core value of society must step forward to defend its central role in guiding public debate and decision-making.

To be persuasive, however, we scientists must put our own house in order, by avoiding behavior that can fuel post-truth rhetoric. Lapses in ethical standards give ammunition to the enemies of science. When published findings are based on falsified data or deliberately misleading conclusions, the credibility of every scientist suffers. Peer review must be rigorous and always strive to detect and snuff out shoddy or deceptive work.

Equally important, researchers must do a better job explaining what scientific “certainty” means, helping the public and policymakers to distinguish between proven hypotheses and unverified theories. They must show how alternate models are tested against all available evidence under controlled conditions, yielding observations that can be repeated – and measurements that can be reproduced – by other researchers. Conclusions that are not derived from such carefully controlled observations must remain conjecture.

Those engaged in science urgently need to develop and implement more effective strategies to communicate scientific advances and discoveries that affect society and the environment. A central focus of this effort should be to explain and defend the methods and rigor of the underlying process of evidence collection and validation. Simply put, a higher level of science literacy among the public, the media, and especially among policymakers is essential to recognizing and rejecting unreasoned attempts to discredit science and scientists.

In his 1946 book The Discovery of India, India’s first prime minister, Jawaharlal Nehru, advocated the development of a “scientific temper” – the adoption of the scientific method as a way of life. To defeat the post-truth threat, that temper is needed now more than ever. On April 22, let’s defend it with passion.