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Scepticism is healthy. Denial is not. But for us to know whether we fall in the "accept" or "doubt" camps on climate change we need to understand some of the science behind it.

For many, that's about as easy and fun as nutting out theories around dark matter or quantum mechanics, so we need someone to do our homework for us.

Which is where Mark Lynas comes in. The Oxford-based science writer has trawled through tens of thousands of peer-reviewed reports and come out the other end with Six Degrees, a rather frightening account of what's going to happen if the world does heat up. Mark Broatch threw some of the sceptics' biggest bogeys at him and asked him if our long white cloud will stay in place.

Your book says that in order to be 75% certain of a temperature rise staying below two degrees, global greenhouse gas emissions need to peak within eight years, then fall steeply. How likely is that scenario?

I'd be the first to admit that it's highly unlikely, given the obvious inertia of modern-day consumer capitalism, and the rapid industrialization process going on in the developing world. But nor is it impossible, and in any case I wanted to present the reader with the truth as I see it, not focus only on what is "politically realistic" or some such. Even if we miss that particular target, all is not necessarily lost - we simply have less and less chance of staying below two degrees.

Six Degrees could be seen as apocalyptic. Do you have any doubt in the computer models?

Of course computer models are not infallible, though nowadays they are very accurate in representing past climate changes, and so presumably have some skill at predicting the future. My particular concern is actually that the models are too conservative - there was a report out last week which showed that the Arctic sea ice is disappearing much faster than even the most pessimistic models predicted: it's 30 years ahead of where it's supposed to be. Also, the most apocalyptic parts of the book are based not so much on models as on palaeoclimate research - so that is stuff that actually happened in the past, so far as we know, not that may or may not happen in the future .

Sceptics say that levels of carbon rise perhaps 800 years after global warming, and therefore can't be the cause of it.

Like so many aspects of the denialists' argument, this issue about the warming leading the CO, release in ice cores shows how they wilfully misunderstand the data to support preconceived erroneous conclusions. Yes, warming probably did lead initial CO., release, but over centuries this release then led to more warming, and therefore more CO2 in a positive feedback - which is precisely what is projected to happen in future, in the worst-case scenarios. So the fact that a positive feedback operates, which tends to exaggerate warming even under natural conditions, is hardly a cause for complacency.

They point to global cooling for the few decades after 1940. Do you agree that was due to "aerosols" – light reflecting particles in the air - before clean air acts came into place?

Of course - that's why most of the cooling was confined to the northern hemisphere, where most of the aerosols were released from factories and power stations.

Solar activity over the past few decades has changed little, and cannot explain the continued warming trend?

That's right. This is addressed in the latest IPCC report. In any case, remember Occam's razor: we don't need to invoke alternative causal mechanisms for current warming, because CO, explains it perfectly, just as physics predicts it should. Were the pyramids built by space aliens or by ancient Egyptians? Who knows? But I think the latter is far more likely than the former, and therefore far more worthy of scholarly study.

The troposphere should be heating first and fastest. There is some dispute it is not?

Nope, this last redoubt of scepticism was blown away last year when it was discovered that the apparent discrepancy between surface and tropospheric heating was an error in satellite data collection methods.

Don't hurricanes need areas of warm water and cold air to form, and if the globe is warming all over, won't that lessen their likelihood?

No, it will simply mean in theory that hurricanes have more "rocket fuel" to power them, and that they can extend over even greater areas of the global ocean. There is a theoretical possibility that due to polar amplification of global warming (it gets hotter relatively at the poles than at the equator) the temperature differential will be lessened, and that this will possibly lead to less mid-latitudinal cyclonic activity - but that's probably bad news if you live in the mid-latitudes (like the UK and NZ) and enjoy the currently plentiful rains. You won't be celebrating much if the North Island is left stranded in the subtropical dry zone. Critics cite periods such as the Medieval Warm Period, the Holocene Maximum and 17th century mini Ice Age as evidence that the climate is changing all the time. The first is believed to have enabled the flowering of western civilisation. We are probably as warm again now as in the Holocene Maximum, which was caused by a slightly different orbital configuration of how the earth revolves around the sun. The MWP and the Little Ice Age are now seen mainly as regional phenomena, despite their exalted mythical status among the denial lobby. To the extent that these fluctuations actually happened, their main forcing may have been solar.

Because methane is relatively rare in the atmosphere, it's more effective as a greenhouse gas than CO2 should we be working harder to contain methane release?

It's not the relative abundance of methane that governs its radiative forcing properties - that's simply a matter of the physics of the molecule. But yes, pound for pound methane is 20 times more effective in warming the planet than CO2, so stopping it is just as much of a priority . In addition, methane degrades into CO" so it has a double-whammy effect. Methane concentrations have actually stabilised in the atmosphere in recent years - the worrying thing is that no one seems to know why.

Greenhouse gases become less effective at heating the atmosphere as their concentrations increase. Will CO2 reach a level where its increase is less effective at raising temperatures?

Increases in CO₂, will always lead to increases in temperature, even if the relationship is not linear. It is clear from the geological past that eras of very high CO2 were eras of extreme greenhouse conditions - like some periods during the Cretaceous, for example. Of course the precise amount of warming for a precise amount of CO., is not known - that's because of the complexities of the water vapour feedback, clouds and so on. But we certainly know the direction of change beyond any reasonable doubt.

Is there a single humdinger scientific finding that sceptics haven't dare dispute?

No, because these people do not care about reality - they are driven by ideology. The whole sceptic debate is driven by a polarisation of world views between those who believe that humanity can do what it likes and damn the consequences, and those who want to take a more precautionary approach. Of course, environmental science overwhelmingly supports the second view, however you try to spin it.

Give any weight to the argument that funding comes easiest to those who "believe" global warming?

It's the most ridiculous thing I've ever heard. So the poor little ol' oil companies, automotive corporations , global commodities traders and monopolistic retailers face a conspiracy of money-grabbing scientists desperate to make a fast buck out of eco-alarmist governments. It's an inversion of reality at every stage.

What is the climatic forecast for New Zealand? One I read suggests we may be mostly spared the worst, and you suggest more migration of Pacific peoples.

Definitely - New Zealand has to be one of the most desirable refuges for a world that is going to become progressively less habitable. It is far enough south to probably be guaranteed sufficient rainfall into the foreseeable future, and so long as it can weather extreme floods and occasional droughts that will be part and parcel of a warmer future, it should still be able to feed itself too. There will also be plentiful fresh water on the South Island - at least until the last glaciers melt. My concern would be what to do with all the "boat people" who will be arriving from Australia and further afield - the process of drought-driven agricultural collapse is already underway in much of Australia, and there is nowhere for most of the population to go. How many more millions could NZ accommodate?

Though it seems to have some ability to balance itself, the Earth also seems to be fairly fragile and intelligent life is only possible because of some nice climatic coincidences. You are not confident that the Earth will balance itself?

The earth may well balance itself out at some point - I don't think we're very likely to turn it into Venus. But that new balance may not include most of humanity or large parts of the existing biosphere. Mass extinctions have happened in the past - often associated with climate changes - and we are doubtless heading into another one. I have huge moral qualms about the relativistic approach that some people adopt here - if billions of people and millions of species die is that not a problem? Sometimes I hear people saying that it's OK because the earth will right itself and new species will evolve. That's like saying the Nazi Holocaust didn't matter , because the high post-war birth rate soon replaced all those lost people.

A DEGREE - makes no difference on a day-to-day basis, but a big difference globally. During the last ice age 18,000 years ago, temperatures were about six degrees colder than today. Mark Lynas looked at past evidence to see what might happen if it keeps getting hotter.

ONE DEGREE - Fresh water lost from a third of world's surface this century.

TWO DEGREES - European heatwave of 2003, which cost over 20.000 lives, becomes norm.

THREE DEGREES - Amazon rainforest dies; widespread floods as 80% of Arctic sea ice melts.

FOUR DEGREES - China's, agriculture crashes: carbon cycle reverses - plants release CO₂

FIVE DEGREES - Refugees stream to cooler, wetter countries; potential invasions.

SIX DEGREES – You don't want to know.

Six Degrees: Our Future on a Hotter Planet, Mark Lynas. HarperCollins. $35. was released on May 1 2007.