Under representative democracy, policies are not defined by voting. Representatives are voted in, to make the decision. They supposed to make decisions based on facts (including scientific facts) and interests of the constituents. In order to do that, institutions are created, such is bureaucracy, executive branch, committees, etc., those will employ scientists as needed. But a policy can not be made just by scientists. Climatologists can not make policy about climate change, for example, because those should rely on many aspects, including economics, security, international relationships and even internal politics (different states have different needs).