KEY POINTS:
Climate change pop quiz: Do you agree or disagree with the following two statements?
1). Global warming is happening and it's caused by us.
2). It's more cost-effective to do something about global warming now than to wait.
If you agree, you're in good company. The first is what the Intergovernmental Panel (IPCC) on Climate Change says in its just released summary for policymakers of its Fourth Assessment Report of the current science on global warming.
Actually, it didn't quite put it like that. The panel, which is made up of thousands of experts from more than 130 countries and which only makes statements when it has consensus, is more long-winded: "Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level."
The IPCC also says the understanding of anthropogenic (human-induced) warming and cooling influences on climate has improved since its Third Assessment Report in 2001, so much so that it now has: "very high confidence that the globally averaged net effect of human activities since 1750 has been one of warming". Very high confidence means a nine out of 10 chance of being correct.
The second statement about cost effectiveness is the gist of what economist Sir Nicholas Stern said in the Stern Review on the Economics of Climate Change, a 700-page report commissioned by the British Government and released in October. It discusses the effect of climate change and global warming on the world economy.
Stern and his team said: "The scientific evidence is now overwhelming: climate change presents very serious global risks, and it demands an urgent global response."
The Review presents what some have called "a dark and dramatic picture" of the possible consequences of global warming.
The summary of conclusions says: "Climate change will affect the basic elements of life for people around the world - access to water, food production, health, and the environment. Hundreds of millions of people could suffer hunger, water shortages and coastal flooding as the world warms."
Stern used formal economic models to show that if we don't act, the overall costs and risks of climate change will be equivalent to losing at least 5 per cent of global GDP each year, now and forever. If a wider range of risks and impacts is taken into account, the estimates of damage could rise to 20 per cent of GDP.
Contrast that, says Stern, with the costs of action to reduce greenhouse gas emissions to avoid the worst impacts of climate change and you'll find those costs can be limited to around 1 per cent of global GDP each year. From an economic perspective, it's a no-brainer.
To drive home his point Stern wrote this thundering paragraph: "The investment that takes place in the next 10-20 years will have a profound effect on the climate in the second half of this century and in the next. Our actions now and over the coming decades could create risks of major disruption to economic and social activity, on a scale similar to those associated with the great wars and the economic depression of the first half of the 20th century. And it will be difficult or impossible to reverse these changes."
None of this will come as any surprise to those who have seen Al Gore's movie An Inconvenient Truth. The former United States vice-president is a man on a mission - travelling from city to city around the world to give his sideshow about climate change. Which is what the movie is about - a slide show. If that sounds dull, it's testimony to the film-maker's art that it's not.
The movie, which weaves together aspects of Gore's life with the global warming picture, has been criticised as alarmist. But although some aspects are emotionally manipulative - such as the animation of a polar bear swimming in search of ice to rest on - mostly Gore comes across as calm and rational. And while he's deeply concerned, he's also optimistic there's plenty that can be done to avert the impending disaster.
Gore shows what that catastrophe might look like - with images of retreating glaciers, drought, tropical cyclones, disease and what could happen if the Arctic and Greenland ice sheet did melt. But he tempers the message by coming back to the science of how such events occur.
Not surprisingly, Gore's inconvenient truth is that greenhouse gases have increased over time, which is making the planet warmer, and human activity is to blame.
While Gore has undoubtedly helped in turning the debate about global warming, and the Stern Review has served as a rallying call to governments around the world to action, it's the work of the IPCC that has the most influence.
It has been reporting on climate change since 1990 and its latest report - which will be released in full in May - is its most comprehensive.
Significant in the summary of Climate Change 2007: The Physical Science Basis are the emissions scenarios which give projections for a range of global temperature increases dependent on six different futures.
"They show the emissions choices humans make over coming decades will make a substantial difference to the size of the climate changes experienced by the end of the 21st century," says Dr David Wratt, head of Niwa's National Climate Centre and a contributing author to the report.
"This moves the focus away from worrying about how 'terrible' the future might be, to thinking about approaches which might be taken to reduce future risk."
Its theme is likely to be echoed in subsequent IPCC reports: a summary of expected consequences of global warming in April; a summary of options for slowing down global warming in May; then the full reports covering science, consequences and mitigation; and in November a synthesis of all three. All will be in the convoluted language of consensus - as thousands of reviewers go through what can be said line by line until they reach agreement.
Not unexpectedly, the fourth report provides more certainty compared to the previous assessment in 2001 about rises in temperature and sea level. Some critics have interpreted this increased accuracy and refinement as the IPCC back-tracking.
Tied to the emissions scenarios, sea levels, for example, are now projected to rise between 18cm and 59cm, whereas in 2001 the projection was between 9cm and 88cm. The lowest end of the range in the 2007 report has increased to 18cm because that's how much sea level has increased globally each century since 1900. The upper end of the range has come down largely due to advances in the science - in particular the ability to now balance the "global sea-level budget" which better accounts for the various processes that contribute to sea-level rise.
But as Niwa scientist Rob Bell points out, the IPCC does include a caveat on these projections because of uncertainty about how much the land-ice flow from Greenland and Antarctica will contribute to sea-level rise. At present the contribution from the ice sheet melting is taken from rates measured in the decade 1993 to 2003.
"If this contribution increases, as expected, with average temperature increases in the future, then the upper ranges of the sea-level projections in the Fourth Report would increase by 10 to 20 centimetres," says Bell.
Probably the most worrying scenario is that if there was sustained global warming of more than between 1.9 and 4.6C above pre-industrial temperatures, it could "lead to virtual complete elimination of the Greenland Ice Sheet and resulting contribution to sea level rise of about seven metres". Sustained means many hundreds of years.
But the report highlights paleoclimate information which shows that the warmth of the last half century is unusual in at least the previous 1300 years. "The last time the polar regions were significantly warmer than the present for an extended period (about 125,000 years ago), reductions in polar ice volume led to 4 to 6 metres of sea level rise."
Another aspect of global warming that's better understood in this report is the role aerosols play. Ironically these air pollutants produce a cooling effect in the atmosphere. While their role is now better understood because of improved satellite and ground-based measurements, and more comprehensive modelling, aerosols remain the dominant uncertainty in climate change mechanisms.
The report has more information too about the lag in the global climate system, which means that even if all sources of emissions were frozen today, average temperatures would continue to rise by 0.1C a decade.
New in this year's report is concern about how increasing atmospheric carbon dioxide concentrations leads to increasing acidification of the ocean. Projections based on the emissions scenarios give reductions in average global surface ocean pH (the lower the pH, the more acidic) of between 0.14 and 0.35 units over the 21st century, adding to the present decrease of 0.1 units since pre-industrial times.
Niwa scientist Brett Mullan explains the significance: "An increasingly acid oceanic environment is expected to adversely affect marine organisms that use calcium in building shells or backbones." By "adversely affect" Mullan means "dissolve". His main concern is for phytoplankton, microscopic organisms that form the basis of the food chain in oceans.
Such far-reaching effects bring home just how delicately balanced our climate system is - and how a few degrees' temperature increase on a global scale can upset that balance in many ways. It highlights too the silliness of asking whether there are advantages in a warmer world. While it may seem colder countries could benefit from such a change, the imbalance it brings to the global ecosystem - making oceans more acidic, creating extreme weather patterns and changing the spread of insect born diseases - has disastrous impacts.
It's hard to see how anyone could dispute the overwhelming evidence that has been amassed. While the IPCC analysis of the current science shows we don't have all the answers, it does show a huge majority of scientists deeply concerned about what is happening. This concern has led to something unprecedented in the scientific community - scientists from 130 countries working together to put a wrong situation right.
But a minority of scientists and economists remain unconvinced The same two statements at the beginning of this article were put to visiting economist Professor David Henderson after his lecture at Auckland University last week. Did the IPCC and Stern get it wrong?
"Whether they got it wrong, no one strictly knows," replies Henderson. "They're very over-confident in what they say ... I think I would say, and Chris de Freitas or others may want to correct my language, ... the extent to which any global warming that has taken place is anthropogenic [human induced] cannot be firmly established in the present state of knowledge."
Henderson, a former head of the economics and statistics department of the OECD, freely admits he's not a climate science expert, which is why he defers to people like de Freitas, an associate professor at Auckland University's School of Geography and Environmental Science, and New Zealand's leading global warming sceptic.
Henderson also puts forward the sceptics' opinion as though it was fact. What he says - that global warming cannot be attributed to human actions - is also patently wrong, as the IPCC has demonstrated. De Freitas and Henderson have strongly attacked Stern and the IPCC. But while de Freitas is ardent in his dispute with mainstream scientific views on climate change, Henderson is wishy-washy.
Asked whether he has any other major economic criticisms - apart from discount rates and estimates on the costs of mitigation - of the Stern Review, Henderson says, "It's not so much the economics as the stand which the Stern Review has taken with respect to climate science and biophysical impacts."
Which is really the only strategy sceptics have - to disagree with the fundamental evidence that climate scientists have amassed. What is disappointing is how little substance their arguments have - invariably ending up as attacking IPCC processes as being flawed, unprofessional and unscientific.
Henderson offers qualified support for global warming mitigation. "If I had the honour of being in Her Majesty's service and I was advising ministers I don't think I would wish to argue against some kind of action to mitigate emissions," he tells his Auckland audience. "The possible lines of action would include emissions trading, although my own preference is for a rather modest carbon tax with a possibility of increasing it."
To a sceptic's ears - and the Auckland congregation is largely from that camp - Henderson has uttered sacrilege. He defends his position by saying a broad-brush carbon tax would be much better than the ill-considered "extraordinary range of detailed regulatory intervention which governments are employing".
Henderson and most sceptics fear the world as we know it will change to such an extent that "alarmist politicians, bureaucrats and activists" will take an intrusive role in housing, heating, cooling, transportation, manufacturing, agricultural, business and consumer decisions.
He paints a dark and dramatic future, warning of food miles labels on goods in European supermarkets and the idea of personal ration books to govern energy consumption.
At least one in the audience warms to his hysteria: "Do you see squads of jack-booted carbon police walking around our business areas? How are people going to control this?"
The extreme alarmism is ironic because it employs the same tactics the sceptics accuse global warming advocates of using. The scare tactics are far from the truth.
While governments around the world are indeed talking about taking action to mitigate global warming, it is yet to translate into anything real. At present, the politicians' climate change rhetoric is just that - hot air.
