____________________________________ The Trudeau government has vowed to adopt a decision-making style that requires two things many will find uncomfortable, perhaps disturbing.To be truly based on transparency and scientific evidence, the process must allow open debate by politicians.  And it must allow them to change their positions when science provides new and convincing data.Both aspects, however, are likely to expose politicians to criticisms of weakness, flip-flopping and division in their ranks.  Here is how this could play out in the areas of environmental and health protection.The first critical headline writes itself: “Backbenchers Oppose PM on (Fill in the Blank.)”Any issue worth its salt has at least two legitimate points of view.  While everyone is familiar with open debate and disagreement on economic, social or foreign policy, there is not a long tradition of political debate over science.  Is science really so different?Well, yes it is.  The scientific community has protocols for finding answers to difficult questions based on research and the peer review of studies.  The problem for politicians is that those answers are often found only after years of open scientific debate.It took more than a decade to reach a consensus on the science of acid rain and to convince both the Canadian and U.S. governments to reduce emissions.  It took more than a decade for scientists to prove that some chemicals were depleting the ozone layer and for the international community to commit to eliminating their production.  During these extended periods of debate there were voices calling for prompt action and others challenging the need for any action at all.While scientists pursue their own style of truth-finding, other parties such as industry and environmental advocates also engage in the discussion.  All sides can be convincing.In such circumstances, politicians in the same party may hold differing views on whether the science is strong enough for the government to compel action.  They may debate in public, running counter to expectations of party discipline and message control.It bears emphasizing that expressing different views on the strength of the scientific case for a decision is a positive contribution to the public debate.  But using science-based decisions as wedge issues, amplifying sectoral or regional concerns for partisan benefit, is not.Some debates last for decades.  Scientists are vigorously debating the extent of health and environmental damage caused by a group of chemicals called endocrine disruptors.  Many studies done by academic scientists have produced evidence that these chemicals interfere with the endocrine system ─ the hormones that govern growth, reproduction and other essential functions.  You may have read stories about low sperm counts and eggs growing in the bellies of male fish.There are hundreds of chemicals suspected of having endocrine disrupting properties and the tests are very difficult to conduct.  The debate has been ongoing for more than 20 years and is far from settled.  Meanwhile a lot of research is being done and each new study gets headlines.It is the job of governments to decide if and when there is enough science to warrant action, to take a position in the face of scientific uncertainty and to make an informed and reasoned judgement call.  But the research and scientific debate will continue and scientific conclusions will change.Another kind of headline is likely: “Government Flip-Flops on safety of (Fill in the Blank)”Few things stick to politicians more than the charge of “flip-flopping”.  It implies weakness, inconsistency or bad judgement.  New research findings, however, can change old conclusions and politicians should change their positions in accord with new scientific consensus.In one recent example, after years of warning of the dangers of too much cholesterol, over-seers of U.S. dietary guidelines lifted the recommended limit on eggs.  Officials explained that while a healthy diet is important to reduce cardiovascular disease, they no longer believe there is a strong link between dietary cholesterol and blood cholesterol.  Looking farther back, margarine was once touted as the healthy alternative to butter; then came the discovery of harm from trans fats in margarine.Most agree that our consumption of salt is too high but there is a healthy debate (pun intended) over how much it should be reduced.In the scientific community, tests are constantly being developed that have the potential to alter past conclusions on safety.  Some nine year ago, for example, an internationally-recognized test for harm done by prenatal and childhood exposure to neurotoxins first became available — well after hundreds of chemicals had been screened and declared safe.  Neurotoxins harm the brain and can permanently lower IQs.  Children are especially vulnerable.In the political arena, elected leaders can only base their decisions on information available at the time.  Further findings can indicate greater harm, or less harm, than previously thought.  In such cases, an evidence-driven government must change its actions to suit.To cite another recent example: The effects of a changing climate are now being felt ─ well before the 2-degree increase in average global temperature previously considered to present an acceptable risk.  In addition, over the past few years new and cost-effective renewable energy technologies have become available.As a result, governments at the climate change conference in Paris last year set a lower target of 1.5 degrees for global temperature increase.  The increased sense of urgency, combined with new electricity storage technologies and lower costs for solar and wind energy, justify changes in policies.  Governments around the world are strengthening their actions to reduce emissions.Open, vigorous debate on scientific findings and taking new positions when new science dictates are at the core of evidence-based decision-making.  For too long, both have been seen as signs of political weakness. This needs to change.Marjory Loveys spent ten years as a senior policy advisor in the Office of Prime Minister Jean Chretien.  To view other articles in this series, click here.