Several aspects of Jim Salinger's op-ed "Climate hurtling towards a hothouse Earth" (Herald 24/5/13) are quite misleading.
It is true most climate scientists would agree that rising carbon dioxide in the atmosphere due to fossil fuel use could affect global climate. The basic physics is there to support this view. But there is no evidence that the putative change would be large or damaging. Output from computer models is not evidence unless model performance has been validated. So far, it has not.
The so-called evidence of minor human-caused climatic change can also be attributed to causes or processes other than those related to the increase of carbon dioxide in the atmosphere.
What is rarely mentioned by climate alarmists is the incontrovertible fact that adding more carbon dioxide to the atmosphere has an ever-decreasing effect on global temperature. To illustrate this, compare covering a glass window with very thin paint. The first coat of paint cuts out some light, the second some more; but each subsequent coat has an ever decreasing effect on light shining through.
It is true, the warming effect of increasing carbon dioxide concentrations never reaches zero (saturation); but, for significant global warming to occur, increased concentrations must set in motion positive (or destabilising) feedback processes.
Such processes would cause temperatures to rise by some other mechanism. One such mechanism is increased evaporation caused by higher temperatures leading to rising water vapour concentration, which is by far the most important greenhouse gas. This would increase retention of energy from the Sun and lead to further warming, and so on.
To date, scientific evidence suggests that negative (stabilising) feedback processes prevail; possibly due to the cooling effect of increased cloudiness from water vapour increase. If true, this means it is unlikely higher concentrations of carbon dioxide in the atmosphere will greatly influence global climate.
Negative feedback processes are played down by climate alarmists who assume climate is governed by positive feedback processes which they claim will lead to runaway global warming. Four billion years of global climate history shows that negative feedbacks prevail.
Climate warming does not confirm that carbon dioxide is causing it. The evidence would have to distinguish between human-caused warming and natural warming. This has not been done.
Climate is always warming or cooling. There are natural variability theories of warming. Much of the talk of "increasing evidence for global warming" is actually evidence of climate variability.
Whatever the cause of the current warm phase, its occurrence is not unprecedented. Global warming happened from 1850 to 1940, then cooling to 1979. During the Medieval Warm Period from 900 to 1200AD, the Vikings sailed in arctic waters that are now covered with sea ice, and farmed Greenland soil that is now too cold for agriculture.
From the results of research to date, it appears the influence of increasing carbon dioxide on global warming is almost indiscernible. Future warming could occur, but there is no evidence to suggest it will amount to much.
One could reasonably argue that lack of evidence, one way or the other, is no reason for complacency.
I will concede that.
Chris de Freitas is a teacher and researcher in the School of Environment at the University of Auckland.