Guy McPherson, retired professor of conservation biology from the University of Arizona, has been on a speaking tour of New Zealand this month peddling a bleak message: we're going to push the planet's climate system over the edge and we've only got a decade to live.
A prominent New Zealand climate scientist sees no basis for that claim and says such alarmism, which has already generated a slew of scary headlines, is counter-productive to the crucial effort of combating the worst potential effects of climate change while we still can.
Science reporter Jamie Morton talked to James Renwick, a professor of physical geography at Victoria University of Wellington who served as a lead author on the last two IPCC reports and recently co-hosted a Royal Society of New Zealand-sponsored series of public talks on climate change.
What do you make of his claims? Is he misrepresenting climate science?
Misrepresenting - I'm not sure if that's quite the right word.
I've read stuff on his website and I've had a look at some of the papers that he's written and a lot of what he says is quite right and mainstream.
Where we seem to part company is this idea that [humans will be wiped out] in the next 10 years.
I even saw one comment where he said, up to the second to the last day of the 10th year everything that will be fine, and then bang, on that last day, catastrophe.
What he's saying is there's going to be some kind of amazing rapid feedback that will suddenly kick the Earth into a totally different state and we will not be able to cope with it as a species and we'll all die.
And I just don't see where that comes from. There's no indication of that from the geological record.
I mean, sure, in the past at times, the Earth has been a lot warmer and a lot colder and with a very different climate state and all the rest of it but, as far as anybody knows, there's no mechanism to suddenly change the climate overnight.
Even in the space of 10 years, you'd be struggling to do much.
The dinosaurs were mostly wiped out by a big asteroid that hit the Earth, and yes, something like that would certainly change the climate overnight.
But just processes within the climate system itself would not lead to quick enough and catastrophic enough changes to destroy all human life on Earth.
As far as I'm aware that's just not possible.
Within 10 years, there's a very small chance we could see much more rapid loss of sea ice around the Arctic, for instance, and it's possible it could disappear in 10 years - it's pretty unlikely, but summer sea ice could be all gone in 10 years, let's say.
And let's imagine that temperatures could ramp up quite quickly. At worst, in 10 years, it might be another degree warmer, or something.
That would be a significant change in the climate - but it wouldn't destroy all life, by any means.
So we're not all going to die in 10 years?
As far as I know. [Laughs.]
Can we however clarify just what long-term effect climate change is going to have on the human race?
That's a hard question and it depends very much on how much more climate change we experience, and how different countries handle the effects.
The best-case scenario is that the Paris Agreement works and the countries of the world reduce emissions and we don't get more than another, say, half a degree of warming from where we're at today.
And therefore we don't get more than another metre of sea level rise from where we are today.
That would be nice but that would still mean displacement for probably millions of people and would mean large changes in the frequency of droughts, wildfires and all the rest of it.
So it would put more stress on global food supplies and water availability in a lot of places, which could possibly lead to famine in places or further regional conflict, like Syria.
Even that much could cause the premature deaths of hundreds of thousands to possibly a few million people.
The worst-case scenario is we don't do anything much at all and by the end of the century temperatures have gone up by another, say, four degrees, and we are on the way to 25m of sea level rise.
That is the sort of scenario - and even when you read IPCC reports it's discussed in muted terms - is usually associated with ideas around major failures in global food security and the break-down of the rule of law.
But even that wouldn't necessarily wipe out all human life on Earth.
Do you think McPherson has the qualification and expertise to be making such a claim?
As a few people have pointed out, the University of Arizona is an excellent institution, he's an emeritus professor and he's no faker - he knows a lot about ecology, biology and how the climate system works to a certain extent.
But it's his interpretation of things, isn't it?
I honestly don't know what his basis is for having this idea about there being 10 years until we all die.
Saying there's going to be some very rapid change... I haven't seen anything written by him, at least, that explains what that is or where the idea comes from.
But you just look at people like Dick Lindzen, the famous atmospheric physicist at MIT who is probably the most well-known climate denier in the climate dynamics community.
He has published all sorts of key papers over the years - and yet he thinks climate sensitivity is almost zero and climate change is not a problem.
So just because [McPherson] has been a top scholar at a good university for decades, doesn't mean he can't have funny ideas.
But are you concerned that many people will still take his claims as a given because he's an academic?
Maybe - [because he's] got some genuine scientific qualification and has studied aspects of it at least.
I mean, he's very reputable in most climate fields.
And this kind of thing, what surprises might be in store in the climate system, I would say nobody really knows - even guys like [German oceanographer and climatologist Professor] Stefan Rahmstorf who's at the forefront of understanding ocean circulation and possible shut-down of the thermohaline conveyor belt and so on.
There are various scenarios out there for what could happen in terms of methane release and other things.
But we honestly don't know what the climate system is capable of, really - and to the best of everyone's knowledge, the chances of those big changes happening in the next century are pretty small.
And even if they did happen, it would change the climate and it may accelerate warming and so on, but again, none of those things would wipe out life on Earth.
It just wouldn't.
We are a bit more resilient than that.
As a climate scientist, what do you feel undermines the field and public advocacy efforts more: climate denial or climate alarmism?
I'd say both play a role but climate apathy is probably the biggest issue.
If it's not your job, then why would you spend any time at all thinking about climate change?
You've got your family to feed and your life to live and the reality is that the climate on human time-scales is changing relatively slowly.
You get up in the morning, and the weather looks okay, and you think, so what?
Most people can go through their life like that - how do you motivate people in that situation?
That's why it's called a wicked problem and we've got this frog-in-the-pot scenario where we are all slowly cooking and we don't even notice.
This idea that giving people this kind of information - and especially trying to alarm people - is really counter-productive.
It just turns people off, and you've really got to hold their hands and persuade them that there are all these opportunities going and if we band together it can all be great stuff.
Despair, I suppose, is a really useless emotion and that's kind of what McPherson is getting at: he's saying, just love the one you're with right now, because we've only got 10 years to live, and there's nothing we can do, so give up.
He's basically telling people to give up and I really don't like that.
No matter how far the emissions go, there's always an opportunity to pull back and stop change, so we should never give up on what we can do.
Human activity is what's changing the climate so human activity can definitely stop the climate from changing - we've got all the power.
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