Neil Curtis

Emeritus Professor of Chemistry, Victoria University of Wellington

Although there is general acceptance by the overwhelming majority of atmospheric scientists that climate change driven by the effect of “greenhouse gases” on the world radiation balance is a real and serious problem, there remain many very vocal “deniers”.
 

Many of their objections are based on political or economic arguments, but they include a core of objections that have a scientific basis. An increase in mean world atmospheric temperatures over the last hundred years was the primary “evidence” which led to the climate change hypothesis, though this is now overwhelmingly supported (in the view of “acceptors”) by a variety of other evidence.
 
The “deniers” claim that there is no “scientific” or “statistically reliable” evidence for any significant world temperature increase. They consider that atmospheric temperature measurements are often faulty, and there are insufficient consistent and reliable atmospheric temperature measurements, over a sufficient time span, to conclude that there is any consistent global warming. They generally accept that a 0.4° C average temperature rise occurred during a 10-year period during the 1980s, but consider that there was no significant change before or after this period, and that this temperature rise is inconsequential in view of climate variability.
 
In the past, the mean daily temperature at the majority of meteorological stations was obtained by taking the average of the maximum and minimum temperature, measured on a special thermometer, in a specified enclosure. To obtain an average of a continually changing variable such as temperature, as many observations as possible should be taken, and statistical methods used to assess the mean and its confidence limits. The average of the largest and the smallest values can differ significantly from this. Reported measurements obtained by the average of the daily maximum and minimum temperature are therefore subject to an unknown bias, and there is no method of determining the true mean from such values, or their reliability. Many weather stations now have automatic equipment which averages temperatures recorded over short time intervals to provide a true mean, but it is not known how many of the world temperature records are so obtained, or how these relate to the records obtained by the older maximum-minimum averaged values. Changes in the environment of many weather stations over time are also considered by “deniers” to have made many time series of temperature records unreliable.
 
The measurements from weather stations around the world, many of unknown reliability, are then amalgamated by a complex procedure to provide an annual global temperature. These values are then compared with analogous values from the past, of generally decreasing reliability with time, to provide evidence for increasing temperatures. The “deniers” are unconvinced that there has been any significant increase in mean world temperature over the last century.
 
The calculated increases are then related to the supposed influence of increased atmospheric greenhouse gases on the world radiation balance and hence temperature. The “deniers” accept that the amount of “greenhouse” gases in the atmosphere has increased because of human activity, but claim there is no reliable evidence that this has had any significant effect on world temperatures.
A problem with all climate models is the poorly understood contribution of water. Water vapour is by far the most important greenhouse gas in the atmosphere, but though ubiquitous, its concentration is very variable. Water as clouds – of liquid water droplets or ice crystals – also have an effect on the radiation balance, by reflecting sunlight, thus lowering temperatures, absorbing sunlight, thus raising atmospheric temperatures, and by absorbing infrared radiation from the earth (acting as a blanket) and thus raising temperatures. Deniers can with some reason, claim that climate models incorporating such a large poorly characterised component should not be trusted. In their view all computational climate models are unverified and their resultant conclusions meaningless

These issues have all been considered and refuted, to their satisfaction, by the main-stream “acceptors” but the “deniers” remain unconvinced.

Time will tell, but perhaps by then it will be too late.