Nature is the key
Given the widely reported increase in the warming effects of constantly rising greenhouse gas concentrations, the fact that global temperature has not risen over the past 13 years continues to puzzle climate scientists.
The credibility of global warming alarm rests on the fact that global warming has to begin again. If it doesn't happen fairly soon, then the global warming alarmists' bubble will surely burst.
So it is not surprising that there have been many attempts to explain the temperature standstill.
The most recent is a paper by Robert Kaufmann of Boston University and colleagues in the Proceedings of the National Academy of Sciences, which claimed the temperature stasis has been caused by China emitting sunlight-reflecting aerosols from its rapidly increasing number of coal-fired power stations.
One does not have to look far to discover that aerosol data, obtained by satellites, show that global aerosol content - mainly sulphate particles - shows no trend between 2000 and 2006, years covered in the Kaufmann study. In 2007 Nasa reported that the global aerosol effect had reduced.
China has installed more modern desulphurisation units in its coal-fired plants than the rest of the world combined.
None of this appeared in the Kaufmann analysis.
Despite the temperature standstill, other scientists point to the existence of human fingerprints on human-caused dangerous climatic change as evidence of the role of humans.
The problem with this tack is that there are other mechanisms that can lead to the same fingerprints, therefore they are not exclusive evidence of human-caused change.
Moreover, none of the so-called fingerprints suggest that dangerous change is under way.
The confusion that exists might be explained like this.
Over the time since 1900 there is an empirical connection between carbon dioxide rise and average global temperature.
Calculations based on theory give a warming of about 1.6C for a doubling of carbon dioxide.
The warming from 1900 to 2010 is 0.8C, not far from that observed.
Few if any climate scientists would challenge the view that a further increase in concentration of carbon dioxide would cause further warming of the atmosphere that might be discerned.
However, empirical data from temperature measurements over time clearly show that carbon dioxide is not a main driver, as temperature does not rise monotonically with carbon dioxide.
For instance, there was almost 40 years of global cooling beginning in 1940 despite steadily rising carbon dioxide levels. Other factors also drive global climate change. It appears carbon dioxide is only a minor player.
The public are often told that recent floods, snow storms, droughts and other extreme weather phenomena are evidence of dangerous man-made global warming.
Sea levels are supposed to be rising at a rapid rate and will rise even faster in the future. So what is happening?
According to satellite observations, sea level is rising at around 2mm a year, which is entirely consistent with the rate of rise measured since the end of the Little Ice Age about 300 years ago.
There is no real world evidence of accelerated sea level rise. Nothing unusual is happening.
Most of the highly accurate sea level gauges installed around Australia and on Pacific atolls show that sea level rise is smaller than the global average.
Only computer models tell us that it will rise rapidly in the future.
Sea levels rose at about 3m a century about 15,000 years ago. Because coral atolls still exist, they obviously coped with this rapid rise. Recent floods, droughts, extreme cold, record snowfalls and the like are all consistent with what has happened over the past 150 years.
Although the cost of damage from hurricanes and tropical storms has increased, their number has decreased. Evidence from the more distant past indicates that current patterns of climatic extremes show no unusual trends.
Looking further ahead, there is evidence that the world might cool substantially as a result of a variety of solar effects. For example, history tells us that when there are few sunspots - as occurred in the Little Ice Age - the world cools.
Recent research led by Henrik Svensmark, a physicist at the Danish National Space Centre in Copenhagen, explains that, during sunspot-free periods, the stream of particles from the sun is much reduced.
As a direct result, more high energy cosmic rays reach lower altitudes and trigger the formation of more clouds. As clouds reflect sunlight, increased cloudiness causes cooling.
The same theory might also explain the recent period (1979-1988) of warming. The sun was active, the earth was shielded from cosmic rays, cloud cover reduced and the world warmed.
The main conclusion from the above is that the science is far from settled and the odds now favour the hypothesis that the main drivers of climate change are natural.
But the truth is nobody fully understands the climate.
That is why observed changes in world temperatures have made nonsense of computer model predictions.
Yet, even without computer models, we know there is no such thing as a constant climate.
A prudent government would make sure that we are prepared for a climate that is forever changing, be it warming or cooling.
* Chris de Freitas is an associate professor in the School of Environment at the University of Auckland.
The human component
Chris de Freitas does not accept that extreme weather events are linked to human-induced climate change.
This is in direct contradiction to the view expressed by Christchurch-born leading climate scientist Kevin Trenberth of the US National Centre for Atmospheric Research.
His summation of the facts is: "When natural variability is compounded by human influences on climate this is what we get. Records are not just broken, they are smashed."
The Intergovernmental Panel on Climate Change has concluded that in a warming world, there will be more high temperatures and heatwaves and fewer cold extremes.
There will also be more extremes in the water cycle, with droughts and floods becoming more common.
Rising air and sea temperatures mean there's more energy in the system, increasing the odds of extreme weather events.
Let's look at some facts: during the August 2003 European heatwave, temperatures in large areas of France were 7C above average, leading to 30,000 premature deaths and unprecedented decreases in crop yields.
Climate scientists have shown that such events are now twice as likely to happen compared with 50 years ago because of global warming.
Last year's Russian heatwave was the hottest summer on record with extensive wildfires and 50,000 deaths from smog and extreme heat.
Moscow average daily temperature at the peak was nearly 30C, and daily temperatures were around 7C above normal for more than a month.
We now understand that the heatwave was related to record warm temperatures in the Indian Ocean, which have a human component.
As average temperatures rise, the number of frost days decreases.
This has been particularly obvious in New Zealand.
Work published by Auckland climate scientists Jim Salinger and Georgina Griffiths shows that the number of frost days has decreased by between five and 15 days between 1950 and 2000, a reduction of around one-third.
Lisa Alexander at the University of New South Wales has examined the trend in heavy precipitation days worldwide.
There is a very clear increase in heavy precipitation days, even in locations where there is a decline in annual precipitation.
This is entirely consistent with our understanding that a warmer atmosphere holds more water vapour (moisture).
For every 1C of warming the air can hold an extra 6 to 8 per cent water vapour, meaning heavier rain when it rains.
The associated enhanced evaporation driving the increase in water vapour is consistent with drying soils and increasing risk of drought.
Thus climate scientists are concluding that the worldwide increase in heatwaves and flood-producing rainfalls, and the decreased cold nights and frost days, are all very likely a result of global warming.
Munich Re, one of the world's leading reinsurers, puts it like this: "The only plausible explanation for the rise in weather-related catastrophes is climate change. The view that weather extremes are more frequent and intense due to global warming coincides with the current state of scientific knowledge."
Finally, we may ponder the principle of academic freedom.
De Freitas is free to hold whatever opinions he chooses, and to voice them in public.
But when teaching a first-year university course, the onus on the lecturer is surely to present a comprehensive view of mainstream science, even if it goes against his or her personal views.
Academic freedom is a fundamental right, but it does carry responsibilities.
* Professor Keith Hunter is pro-vice-chancellor of sciences at the University of Otago and a former vice-president for physical sciences of the Royal Society of New Zealand.By Keith Hunter, Chris de Freitas