''Why is the Australian public asked to swallow the 'carbon dioxide is a dangerous climate-changing pollution' crap when science shows no observed relationship between global climate and atmospheric carbon dioxide? There is no physical evidence showing a relationship between temperature and CO2, only computer models which give different answers according to whatever assumption data you put in. But there is a very close relationship between temperature and solar activity … Why, when thousands of respected scientists signed a petition saying they don't agree there is a problem, are we being forced to give up billions in tax dollars to waste on trying to stop carbon dioxide emissions?'' HARRY HOSTAN

DESPITE decades of climate science and temperature observations that show a substantial rise in the past 50 years, a sizeable minority of people believe the theory of global warming is, to quote Harry Hostan, ''crap''.

He and Stephen Harper have laid out several arguments that sceptics use to try to debunk mainstream climate science. This week the Climate Agenda will address the scientific questions they raise, and next week will answer the rest of Hostan's question about the economics of Australia's policy towards reducing carbon dioxide emissions.

1. Global temperatures have not risen since 1998 despite rising CO2 levels.

By any measure, 1998 was a standout in terms of the temperature on the earth's surface. A particularly strong El Nino that year made it pop out the top of the climate graphs, and the years since then have not reached its heights.

As always with complex science, there is a range of uncertainty with measurements, even of surface temperatures. For example, there are various different sets of data measuring global temperatures, partly because some take into account the Arctic Ocean, which is warming faster than anywhere else on the planet.

However, despite 1998 being a standout, some years since, particularly 2005, have come close and, globally, the hottest 12-month period ever recorded was between June 2009 and May 2010.

And all reputable climate measurements, including now being released recently released (which the sceptics hoped would disprove global warming), show significant and undeniable global warming in the second half of this century.

Professor Roger Jones of Victoria University said that temperature measurements at the surface of the earth only tell part of the story: heating can happen anywhere in the system, from the deep oceans to the atmosphere, and scientists still cannot really say where the extra energy being trapped in the atmosphere is being stored. It changes constantly, they say.

Jones's theory is that temperatures in 1998 represented a ''step change'' in global warming, rather than part of a smooth trend, and that we are now operating in a new, warmer paradigm. In south-east Australia, for example, maximum temperatures rose by almost 1 degree and have stayed close to those levels.

But whatever the fine measurements say, the effect of carbon dioxide as a trapper of electromagnetic radiation as heat (known since 1896) is well understood by science.

2.Alarmism is based on flawed models that do not reflect empirical measurements.

The use of computer modelling in climate science is both a blessing and a curse. Scientists build their models based on the best data they can find, tinker with them as new information comes to hand, and then use them to try to predict the future of the climate under various scenarios.

But the future is hard to discern. Particularly difficult to work out is how some of the effects of a warming planet - more clouds forming due to higher temperatures, more ice melting, oceans becoming less able to absorb carbon dioxide - will affect each other in what scientists call ''feedbacks''.

Professor Jones said the models were not, and may never be, finely enough calibrated to work out with any certainty what most people want to know: how climate change will affect their part of the world.

However, a report produced for the Copenhagen climate conference in 2009 shows that, at least for sea level rises, the predictions made by climate models have proved very accurate so far.

In 2006, climate scientist James Hansen compared his 1988 temperature predictions with reality, and found his own forecasts to have been strikingly accurate.

Sceptics argue, on the other hand, that the real-world measurements have been manipulated after the fact to make them conform with the models.

3. Two key theories of climate science - that water vapour is a positive feedback (amplifying the effect of climate change) and that a hot-spot in the troposphere at the equator will appear - have failed to show up in reality.

As temperatures on earth increase, the amount of water evaporated from the oceans will also increase, and so more water vapour will be in the atmosphere.

Satellites have observed an increase in atmospheric water vapour since 1988, and a 2007 paper which put together the observations of 22 different climate models showed that the increase was due to higher levels of carbon dioxide.

The mainstream science says that increasing water vapour will increase the effect of the atmospheric ''blanket'' over the earth, causing temperature rises to feed back on themselves, thus evaporating more water and perpetuating the cycle. These feedbacks are said to double or even triple the effect that carbon dioxide alone would have, accelerating and amplifying global warming.

The questioner suggests the opposite - that additional water vapour in the atmosphere will actually cool earth's temperature. This adopts the argument of one sceptical scientist in particular, Richard Lindzen of the Massachusetts Institute of Technology, who uses satellite measurements to argue that temperature rises are lower than surface temperatures suggest, and to say that clouds will reduce the impact of global warming, capping it at 1 degree.

The IPCC agrees there is still uncertainty about the exact impact of water vapour and clouds but, Lindzen aside, most climate scientists agree that water vapour is the most efficient agent of global warming - more so than carbon dioxide itself. Some scientists believe vapour is responsible for between 36 per cent and 66 per cent of the greenhouse effect and say that, without it, the earth would be uninhabitably cold.

In the IPCC report, a particularly frightening graph appears that shows a ''hot spot'' about 10 kilometres above the surface of the earth in the lower atmosphere, or troposphere, above the tropics. The appearance of the expected hot spot would show warming at levels higher than the earth's surface, and is linked to the idea that water vapour will accelerate and amplify climate change.

Real world measurements show a hot spot appearing and disappearing along with climate variability, but it does not stay over the long term. The models, in this respect, do not reflect the reality.

Sceptics say this undermines the whole basis of believing in dangerous global warming. Defenders of the theory agree that there is some mystery here, but play it down and suggest it may be due to inaccuracies in measurements from weather balloons and satellites - reasons sceptics dismiss as a sham.

Professor Jones says that the tropics are not warming as quickly as middle latitudes and, on this issue, ''most of the models don't get the balance right''.

4. Thousands of respected scientists signed a petition saying they don't agree there is a problem.

In 1998, the Oregon Institute of Science and Medicine began circulating a petition, later posted online, asking scientists to sign up to a statement denying climate change.

''There is no convincing scientific evidence that human release of carbon dioxide, methane, or other greenhouse gasses is causing catastrophic heating of the earth's atmosphere and disruption of the earth's climate,'' the petition said, adding that carbon dioxide would instead ''produce many beneficial effects''.

To qualify for inclusion on the petition, the scientists need to have an undergraduate degree at least in earth science, computers and maths, physics, chemistry, biochemistry, medicine, engineering or general science.

Almost 32,000 people claiming to be American scientists (including 9020 with PhDs) have signed up, but as the institute does not release the full list of names, this cannot be verified.

The institute is located on a farm about 10 kilometres from the small town of Cave Junction, Oregon, but has been given credibility by conservative and sceptical commentators.

Those who believe in the mainstream science point out that, of the 10.6 million American scientists who fit the institute's qualifications, 32,000 is less than 0.3 per cent.

In a 2009 Pew survey of more than 2000 members of the American Association for the Advancement of Science, 84 per cent agreed that, ''warming is due to human activity'', compared with 10 per cent who felt that ''warming is due to natural causes''.

Stephen Harper: a contrarian's tale

1. What is your background?

I'm 51. I'm currently a builder. I also have a business degree and an applied science degree in wine science.

2. What sparked your interest in climate change?

The fact that a few years ago we were told the ''science is settled'' and anyone who asked any questions was an idiot and told to sit down and shut up.

My BS meter went off the radar and I started looking into it.

3. When did you become a sceptic?

Probably a couple of years ago; there was so much evidence, so many smoking guns. The whole point is we're meant to be sceptical. I'm just a product of the Enlightenment. If the people who want to put windmills all over the place were in charge 100 years ago, we wouldn't have half the technology or the lifestyle we've got today. They would have stopped everything.

4. What are your main sources of information?

I've read about 15 books. The first one was An Appeal to Reason: A Cool Look at Global Warming by former Thatcher government minister Nigel Lawson. Then there was Climate: The counter-consensus by Bob Carter. Websites? I look at wattsupwiththat.com and joannenova.com.au. Those two I find very interesting and fair.

5. What are your general political views?

I joined the Climate Sceptics Party two years ago, but I don't do anything. I probably would be libertarian. Each side of politics has got something to tell us … but the Greens are a complete disaster. I'd generally vote conservative. They're all corrupted, but it's the best of a bad bunch.

6. Do you ever have doubts?

It doesn't mean that some of these things aren't right to some extent, that humans are affecting the climate. I always want to keep an open mind and yes, we have caused some warming, but not very much; it's not catastrophic, it's not unprecedented, we just need to take a Bex and a good lie down and wait and see what happens.

7. Do you think there's a conspiracy to push global warming?

I do not think that there's a conspiracy to push global warming. There are many, many strands that keep the juggernaut rolling along. ''Noble cause corruption'' is a classic case in point. Morality has crept into science and some scientists have become convinced that the end justifies the means … This is never acceptable in science. There are elements of people in the United Nations who would love to have international government, but it's just one element of a much larger train that keeps rolling on.