When we think about the risks to a business's viability, or the threats to a nation's security, we normally ask the question 'What is the worst that could happen?'

Having identified the greatest risks, we can then decide how much effort to spend on reducing or avoiding them. Climate change, surely, should be no different. How else should a head of government decide how much effort to spend on reducing global emissions of greenhouse gases, other than by considering the worst case consequences of failing to do so?

Here is another reason for taking seriously worst case scenarios. Some have argued the rational response to climate change is to focus on adaptation. It is too difficult and uncertain, some claim, to anticipate the effects of climate change. Better then to wait and see what climate change serves up to us, and respond then as best we can. But this assumes that impacts are manageable. What if the limits to adaptation are exceeded?

Over the last year, I have worked with scientists and experts in risk from India, the UK, the US, and China to try to understand those worst case consequences a bit more clearly. The work was led by Sir David King, the UK Foreign Secretary's Special Representative for Climate Change, and was officially launched at the London Stock Exchange earlier this week.

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To give one example: we all know that as the world becomes warmer, heat waves will become more extreme. The worst case scenario is that heat waves become so extreme that they are fatal for anyone without air conditioning, even people resting in the shade. Such extreme conditions do not apply now, anywhere.

But there is a substantial risk (10-20 per cent per year) that people in hot parts of the world will be exposed to fatal heat, under a business as usual greenhouse emissions scenario, before the end of this century. We project that northern India, southern China and the south-east of the US would be affected in this way. If emissions continue on the current track, by the middle of next century we estimate it is more likely than not than an individual in northern India will experience fatal heat each year.

The direct consequences of such extreme heat would be mass fatalities anywhere that the whole population did not have access to reliable, twenty-four-hour air conditioning. The indirect consequences of such an event happening on a regular basis could be an even greater concern. People would be likely to migrate from hot and underdeveloped regions in huge numbers. Not only would the risks of state failure rise, but states themselves might have a strong incentive to find new territory elsewhere.

How likely are we to reach such high degrees of warming? This depends on two uncertain factors: the future trajectory of global greenhouse gas emissions, and the response of the climate. What we know about both these factors gives us cause for concern. Economic growth, only moderate levels of political effort, and a lack of investment in energy technologies all seem likely to keep global emissions on an upward path for some time. On such a path, high degrees of warming are a substantial risk.

Sometimes in the climate change debate, analysis such as this is met with accusations of 'alarmism'. We need to recognize how unusual this is. A country's national security adviser would rarely be criticized for considering the worst case military, intelligence or terrorist threats to the national interest. Similarly, an insurance firm would not be faulted for assessing the worst cases risks to its ability to continue as a going concern - on the contrary, it is obliged by regulation to do exactly that.

We live in a highly inter-connected world. New Zealand is a long way from the tropics, but that does not insulate our country from ecological and political crises. We must treat climate change as a threat to our national security not because it is a problem the military can solve (it is not), but because a focus on what we want to happen, or even what is most likely, would be dangerously complacent. Only if we focus on understanding what is the worst that could happen will we be able to make a well-informed decision about how hard we should try to avoid it.

Alistair Woodward is a professor at the University of Auckland, specialising in epidemiology, biostatistics and population health.