One assumes scientific analysis is objective, so it may come as a surprise that this was challenged in a New Zealand High Court case, the results of which were released last week.
The New Zealand Climate Science Education Trust (NZCSET) contested the claim by the National Institute of Water and Atmospheric Research (Niwa) that New Zealand air temperatures had climbed by 0.9C over the past century. The trust maintains that objective analysis of the data shows a trend closer to 0.3C per 100 years.
Recent temperature trends were not in dispute. The court case centred on the fact that the temperature record before about 1965 had been adjusted downwards, a fact not denied by either party. The dispute was over the size of the downward adjustment.
There is little doubt the average annual temperature in New Zealand has been generally trending upwards in line with the expectations of many climate scientists. The question is whether all or part of the warming can be linked to artificial influences.
Long air temperature time series are more often than not beset with artificial, usually human-caused, discontinuities that contaminate what would otherwise be a near-natural record; for example, those sudden discontinuities caused by the relocation of weather stations, changes in instrumentation and observing practices, or gradual changes around the weather station that affect thermal conditions such as the growth or removal of vegetation and, in particular, urban growth and development leading to what is known as the urban heat island (UHI) effect.
Whether sudden or gradual, these changes can introduce inhomogeneities into the long-term temperature time series that distort or even hide the true climatic signal. Uneven spatial sampling because of irregular geographical distribution of weather stations also contaminates the temperature record.
The best-documented example of data contamination is the UHI effect in which data from urban stations are influenced by localised warming because of asphalt and concrete replacing grass and trees. There is a proven close correlation of city population size with urban heating influence on air temperature, which can account for an urban area being as much as 12C warmer than its rural surroundings. Many studies by climatologists have demonstrated that very small changes in population are enough to induce a statistically significant local warming.
The science of climate change depends entirely on reliable data, quality controlled and homogenised rigorously. Adjusting the data to achieve the reliability required is difficult and controversial. There are other problems.
Temperature trends detected are small, usually just a few tenths of one degree Celsius over 100 years, a rate that is exceeded by the data's standard error. Statistically this means the trend is indistinguishable from zero. Moreover, trends and temperature differences justified to one or two decimal places and significant figures are unreliable since the amounts are greater than the accuracy of the data allows, and multiple averaging of measurements does not make it more reliable.
Climate services of various countries provide clients with statistical information on climatic variables that is based on long-term observations at a collection of different weather stations. The importance of this statistical material stems from their widespread use as a major input for a large number of societal design and planning purposes, including setting greenhouse gas emissions policy and the economic consequences that follow. For these reasons it is important that climate services deliver the best estimates possible.
The NZCSET's lawyer summed it up when he told the court the trust was not asserting climate warming did not exist, "we're saying let's at least make sure that evidence of this for New Zealanders is accurate".
Despite the research work undertaken so far, there have been few attempts globally to reassess quantitatively the nature and reliability of homogeneity adjustments to complete national data sets. The High Court case highlights the situation in New Zealand where there have been no peer-reviewed science-based efforts to do this. A court ruling is no substitute.
Argument from authority has no place in science. This was the basis of NZCSET's case. Argument on the scientific facts and methods used in analyses must now take place. The question is: will it?
* Chris de Freitas is an associate professor in the School of Environment at the University of Auckland.
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