"One of the big harms climate change is going to do is killing people. So ... how bad is that?" It's a question only a philosopher - or a sociopath - would ask. But, like many such questions, once you stop sputtering with disbelief you realise it's harder to answer than you think.
"A[nother] problem that has been totally ignored up until now is what to think about population," continues Professor John Broome, an Oxford University philosopher who's recently been recruited by the world's governments to help them tackle climate change.
It's been seen as a significant change of direction for the IPCC, having a moral philosopher as a lead author (among hundreds of others) of next year's report on climate change mitigation. The Times newspaper interpreted Broome's job as being "to rein in the economists".
However Broome was an economist himself for 30 years. He veered towards philosophy because he believes - with many others - that most economic questions are, at heart, ethical ones: they ask us to examine what we value the most. Which seems especially true when considering climate change.
"If [climate change] gets fairly extreme it's going to cause a collapse of population because the world isn't going to be able to sustain its present population. What should we think about that? Is that a bad thing that the population collapses? That's never been thought about."
When I meet John Broome he starts by apologising for the weather. He lives down in Lyme Regis in Devon, from whose picturesque harbour the French Lieutenant's Woman stared into the foam-flecked waves. Today it's windy and cold, and rain spits through the sea mist.
He's worried, he says, about a south-westerly storm toppling an enormous neighbouring tree through his roof - when the weather looks dicey he and his wife sleep on the other side of the house.
But he didn't get into climate change research because of concerns about the future. It was because of wounded professional pride.
He had written a quick paper for Sir Nicholas Stern's 2006 review of the economics of climate change commissioned by the UK government. The paper dealt with a technical question: how can we apply economic values to different policies on climate change?
"[Economics] is about what you ought to do, how things ought to be," Broome says. "Some decision or other is good for some people and bad for others. And when you are adjudicating different people's interest: that's a moral question.
"The questions of climate change are obviously moral questions. Why should you put any effort into reducing climate change? It's not going to harm us very much - not me at any rate. The reason is, it'll do good for other people, people who haven't yet been born, poor people in the world who I don't know.
"It's quite difficult to figure out what we ought to do."
He put some of this thinking into the Stern report. But the response really riled him. Several American economists pooh-poohed his argument - since the 1980s, most North American economists have been suspicious of anything more than "the market knows best". They argued Stern's sums didn't add up if you applied the usual economists "discount" to the future costs of climate change. And they put it really nastily.
"The [Stern] review takes the lofty vantage point of the world social planner, perhaps stoking the dying embers of the British Empire," wrote Yale economist William Nordhaus. "I find the ethical reasoning ... largely irrelevant."
Broome was astonished by the aggressive response from North America. "I actually got rather angry," he says. "It's so obvious that these economists are applying ethical principles without noticing. Something needed to be done about it, they shouldn't get away with this. It was no longer a sideline, this is a challenge."
The line between economics and philosophy has always been thin - Adam Smith was a philosopher, and John Stuart Mill. In the 1960s, philosopher-economists asked questions like: how do you measure a poor person's wellbeing against a rich person's? What rate of growth is the best to balance future generations with present generations?
Until recently, the big questions in climate philosophy were on "burden sharing". These were questions of fairness and justice - if we make sacrifices to do something about climate change, how should they be divided among nations, and parts of society? Politicians were interested in this. And they were interested in blame, and cause: if Western economies were responsible for the rise in temperature to date, does that mean they should bear more of the burden for mitigating it in the future?
Broome is more interested in the fundamental question of "what should we do?" rather than "who should do it?"
And so he started writing on the topic, and about two years ago he was invited to write for the IPCC's next report on climate mitigation, due next year. Co-chairman Ottmar Edenhoffer is a fan of philosophy. "There is a whole space of morally legitimate standpoints with a view to climate change," he told Nature magazine.
"One might legitimately argue that the fight against global warming is as morally imperative as abolishing child labour or slavery.
One might argue - just as legitimately - that poverty and diseases in many parts of the world are more imminent problems that should be addressed first."
However, he adds, some perspectives cannot be tolerated. "Denying out-and-out that climate change is a problem to humanity, as some cynics do, is an unethical, unacceptable position."
Broome is not exactly full of praise for the IPCC. For example, he accuses Working Group I's new report of plucking a "one trillion tonnes" limit for carbon emissions out of nowhere. Technically, it's an estimate of how much carbon you emit while still having a two-thirds probability of not hitting a two-degree increase in temperature.
"It's based on two numbers which have just been pulled out of the air," he says.
But anyway, it's not the right way to think about things. There's "decision theory", which sounds like a good name for an 1980s electro-pop group, but is in fact the way philosophers deal with uncertainty.
"Thinking about the probability of going beyond some target is just not the right way to think.
"The IPCC has constantly concentrated on likelihood, on the probabilities of this happening and that happening. But what matters are not likelihoods."
He uses the example of a household fire extinguisher. You don't get one because of what's most likely to happen. You get one because of the dire consequences of what might happen.
Broome feels guilty about the amount of air travel his job requires. He resents having chapter and even section headings imposed on the authors by the IPCC. And the work "just goes on and on", he says.
"There are much better ways of doing good than this.
"The best way is to send your money to curing tuberculosis, which can save whole lives for a few hundred dollars."
But he believes solving the very real problem of climate change requires political action - and hopefully the IPCC reports will help bring that about. And within these boundaries, he's finding many interesting questions to ask.
Such as: should we value only what affects us as humans? Is there a value to nature in its own right? Is it a bad thing when a species goes extinct, or a unique ecology disappears?
And when it comes to humans: is there a value in a particular culture?
If the Inuit no longer hunt on the ice because of climate change (which is already happening), but instead visit supermarkets, have we lost something as a planet? Do cultures have value, over and above the wellbeing of the people in them?
Then there's the biggie: future value.
"There are going to be people who are going to be killed terribly as a result of climate change, through disease or starvation or war," says Broome.
How should we deal with that? Are people further away from us, in geography and time, less important to us? Should we care less? After all, we're fine with assuming we can care less about people on the other side of the world than our own family. Doesn't the same apply to the people of the future? Or people unborn, who may never be born if the climate changes?
The problem of future value is a vexed one, and it's fundamental to the philosophy of climate change.
Broome believes we should each offset our greenhouse emissions. He's worked this out from first principles. "You ought not to cause harm to people for your own benefit," he says. "I think you can avoid harming people by offsetting your emissions. This is not meant to be a solution to the problem. It's merely meant to be a way of dealing with the injustice that you do."
In our lifetime, each person's carbon emissions will harm humanity to the extent of half a year of life, "so that's a harm my emissions are doing", says Broome.
Of course, there's a philosophical objection to this. "[Carbon offsets] just happen to be something that we, the rich, can do at the moment," he says.
Take, for example, two factories pouring poison into a river. If one pays the other to stop pouring poison into the river, it has cancelled out its harm. But we still feel they are responsible for the poison.
It's a typical philosopher's paradox. And it's just the kind of question that Broome says we need to sink our teeth into if we're to decide how to tackle our changing climate.