Quite what tips a population from apathy to frenzied action is often mysterious. Why does drink-driving engender fraternity one minute, and revulsion the next, while a quarter of UK citizens still smoke tobacco, despite 40 years of solid evidence?

In general, changing behaviours which are immediately pleasurable, but which are associated with some indeterminate personal risk at some unspecified time, are very hard to shift: unprotected sex, indulgence in excess alcohol, use of tobacco, or overconsumption of high-calorie foods are all classic examples. To change such personal behaviours requires a number of factors: the use of a trusted vector (the advice of a doctor, being trusted, increases abstinence from smoking), a solid and clearly communicated evidence base, persistent and consistent communication (which is why one-off campaigns fail), emotional engagement (‘this matters to you and those you love’), empowerment to act, a roadmap to do so, and support in taking such action. Big tilts in population behaviours, however, happen much more rapidly, when a given way of behaving becomes ‘accepted as the standard’.

Climate change threatens not only our health as individuals, but also our very survival. And it does so imminently. In recent years, the alarm has been raised by a vast array of diverse medical bodies, including the American Academy of Pediatrics, The American College of Preventative Medicine, the American Medical Association, The American Public Health Association, the Australian Medical Association, the World Federation of Public Health Associations, and the World Health Organization itself.

An increasing urgency in the message reflects the alarming nature of the latest data, with a recent University College London (UCL)/Lancet Commission describing climate change as the greatest global health threat of the 21st Century,1 a message recently reinforced by the presidents of 17 international medical academies.2

In this issue of the New Zealand Medical Journal, Scott Metcalfe and colleagues3 add their voice. As the authors describe, climate change threatens our health in many ways: through direct effects of heat; injury from storm and floods; changes in disease vectors; flooding; drought; crop failure; economic collapse; and poverty. Together, these drive mass migration and war—for which the departments of defense of most nations are documented to be preparing. Were that not enough, human activity appears to have initiated a mass-extinction event some 10,000 as great as any on the fossil record, and to which climate change is likely to contribute substantially in the future.4

But what should be done, and how fast? The authors offer a detailed summary of previously accepted targets, which suggest that greenhouse gas emissions should be halved in the next 11 years. They also rightly draw attention to the fact that such recommendations are based on data which has already been superseded. Indeed, the ‘worst case’ emissions scenarios were already being significantly exceeded in the first 7 years of this century.5 The total atmospheric greenhouse gas burden which can be tolerated is probably a deal smaller than previously recognised, and is being fast approached.6,7 The measured impacts of climate change on nearly every physical measure (such as polar ice loss) has far exceeded that projected. And recent evidence suggests that, for any given level of greenhouse gas emissions, global, and regional temperature rises may be far greater than was thought: without drastic action, polar temperatures may easily rise by upwards of 16ºC in coming decades.8

The case, as made by Metcalfe and colleagues, is thus well grounded and compelling. We must act, act aggressively, and act now. As they state, there really is no effective way in which the world’s ecosystems can adapt to change on this scale, and no way in which humans can adequately react to such change when it happens. We must be proactive—and in a rational way. Whilst economists and technologists talk about ‘what can be done given current fiscal or technical boundaries’, we must indeed set and meet the targets which the science dictates.

So what can we do? It is clear that no one solution exists, and none are likely to be simple or painless. Firstly, we must all act on a personal level. Whilst our own small savings may in themselves be insignificant, they have greater power than we might think—changing the behaviours of those around us, and altering the behaviours of those companies (and their investors) from whom we buy or no longer buy.

Thereafter, perhaps we should think again about how we change health-damaging behaviours in those around us. As doctors, we can be the trusted vector who carries the message. We must communicate a clear and urgent message through every means open to us. We must engage at a personal level (‘this matters to you and your children’) and help people to act. Only then will population behaviour change. And only then will politicians and business feel that they have our permission to change.

The tragedy is that we have so little time available to us. From 7–18 December 2009, World leaders will meet to decide on emissions targets for the coming years. As leading international physicians recently noted, “There is a real danger that politicians [at Copenhagen] will be indecisive, especially in such turbulent economic times as these. Should their response be weak, the results for international health could be catastrophic.”

We must all act now to ensure that there is a deal, and that it is meaningful rather than fanciful. At present, voiced aspirations for large targets for 2050, or small ones for 2020, are nothing more than dangerous hot air.

A weak deal will represent not an historic international agreement, but a suicide pact. Now is the time for us all to act. If not us, who? If not now, when?

Author information: Hugh Montgomery, Director, UCL Institute for Human Health and Performance, University College London (UCL) Archway Campus, London, UK

Correspondence: Hugh Montgomery, Director, UCL Institute for Human Health and Performance, 2nd Floor, Charterhouse Building, UCL Archway Campus, Highgate Hill, Archway, London N19 5LW, UK. Email: h.montgomery@ucl.ac.uk