As a climate scientist, I'm under pressure to be a political advocate. This comes mainly from environmentalists. Dan Cass, wind-farm director and solar advocate, preferred me not to waste my time debating "denialist morons" but to use political advocacy to "prevent climate catastrophe".

Jeremy Grantham, environmental philanthropist, urged climate scientists to sound a "more desperate note … Be arrested if necessary." A concerned member of the public judged my efforts at public engagement successful only if they showed "evidence of persuasion".

Others ask "what should we do?" At my Cheltenham Science Festival event Can we trust climate models? one of the audience asked what we thought of carbon taxes. I refused to answer, despite the chair's repeated requests and joke (patronisingly; his aim was to entertain) that I "shouldn't be embarrassed at my lack of knowledge".

Even some of my colleagues think I should be clearer about my political beliefs. In a Twitter debate last month Gavin Schmidt, climate scientist and blogger, argued we should state our preferences to avoid accusations of having a hidden agenda.

I believe advocacy by climate scientists has damaged trust in the science. We risk our credibility, our reputation for objectivity, if we are not absolutely neutral. At the very least, it leaves us open to criticism. I find much climate scepticism is driven by a belief that environmental activism has influenced how scientists gather and interpret evidence. So I've found my hardline approach successful in taking the politics and therefore – pun intended – the heat out of climate science discussions.

They call me an "honest broker", asking for "more Dr Edwards and fewer zealous advocates". Crucially, they say this even though my scientific views are absolutely mainstream.

But it's not just about improving trust. In this highly politicised arena, climate scientists have a moral obligation to strive for impartiality. We have a platform we must not abuse. For a start, we rarely have the necessary expertise. I absolutely disagree with Gavin that we likely know far more about the issues involved in making policy choices than [our] audience.

Even scientists who are experts – such as those studying the interactions between climate, economy and politics, with "integrated assessment models" – cannot speak for us because political decisions necessarily depend on values. There are many ways to try to minimise climate change (with mitigation or geoengineering) or its impacts (adaptation) and, given a pot of money, we must decide what we most want to protect. How do we weigh up economic growth against ecosystem change? Should we prioritise the lives and lifestyles of people today or in the future? Try to limit changes in temperature or rainfall? These questions cannot be answered with scientific evidence alone. To me, then, it is simple: scientists misuse their authority if they publicise their preferred policy options.

Some say it is safe to express our views with sufficient context: "this is just my personal opinion, but … " In my experience such caveats are ignored. Why else would we be asked "what should we do?" by the public or media, if not with an expectation of expertise, or the desire for data to replace a difficult decision? Rather than being incoherent – "I don't know much about policy, but I know what I like" – or dictatorial – "If I were to rule the world, I would do this" – we should have the courage and humility not to answer.

Others say it is simplistic and impossible to separate science from policy, or that all individuals are advocates. But there is a difference between giving an estimate of the consequences of a particular action and giving an opinion on how or whether to take that action; between risk assessment, estimating the probability of change and its effect on things we care about, and risk management, deciding how to reduce or live with that risk. A flood forecaster provides a map of the probability of flooding, but she does not decide what is an unacceptable level of risk, or how to spend the budget to reduce the risk (sea defences; regulation of building and insurance).

We must be vigilant against what Roger Pielke Jr in The Honest Broker calls "stealth issue advocacy": claiming we are talking about science when really we are advocating policy. This is clearly expressed by Robert T Lackey:

"Often I hear or read in scientific discourse words such as degradation, improvement, good, and poor. Such value-laden words should not be used to convey scientific information because they imply a preferred … state [or] class of policy options ... The appropriate science words are, for example, change, increase, or decrease." (Science, Scientists and Policy Advocacy)

I became a climate scientist because I've always cared about the environment, since a vivid school talk about the ozone layer (here, page 4) and the influence of my brother, who was green long before it was cool. But I care more about restoring trust in science than about calling people to action; more about improving public understanding of science so society can make better-informed decisions, than about making people's decisions for them. Science doesn't tell us the answer to our problems. Neither should scientists.

This post is part of a series on science and the green movement.



Tamsin Edwards is a climate scientist at the University of Bristol using computer models to study climate change and sea level. She blogs at All Models Are Wrong and on Twitter she is @flimsin

