Clarification of NZ Climate Science Coalition claims re NZ temperature trends
The New Zealand Climate Science Coalition yesterday issued a media release claiming that a UK-based research group has
calculated a much higher temperature trend for New Zealand over the period 1979 to 2005 than shown by NIWA data. The
coalition claims that the Intergovernmental Panel on Climate Change (IPCC) "believes" this higher figure, and that this
alleged disagreement amongst experts shows that the science of climate change is "far from settled".
The coalition is not comparing apples with apples.
DETAILS OF THE UK DATASETS
The UK research group referred to by the coalition is the Climatic Research Unit of the University of East Anglia,
Norwich, which is led by Dr Phil Jones (the "Jones et al" data).
Jones et al provide several large, complex datasets. These data are averages over squares of 5 degrees latitude by 5
degrees longitude over the globe (so there are only a handful of squares which relate to New Zealand).
There are two key datasets: 1. Land-based data only. The NIWA temperature trends are from land-based climate station
measurements, so this land-based dataset is the appropriate one for comparison purposes. The trend from the Jones et al
land-based data and from NIWA's land temperature data are in close agreement. The 1979–2005 trend from the NIWA data is
0.25 degrees Celsius. The trend which NIWA scientists have calculated from the Jones et al land-based data is 0.21
degrees Celsius.
2. Synthesised land-based and sea surface temperature data. This appears to be the dataset which the coalition has
used, and it includes more ocean area than land area for the New Zealand grid squares. For the period the coalition has
chosen (1979–2005), the ocean data shows greater warming than the land data. Using this dataset, we calculate the trend
as about 0.6 degrees. This is how the coalition gets a much higher temperature trend.
The ocean temperature is affected by a combination of local conditions and transport of heat from other places. This
means that we do not expect the land and ocean temperatures to show the same trends over a period of only a couple of
decades.
ACCESS TO NIWA DATA
In their media release, the coalition asks the Minister responsible for climate change issues to "direct NIWA to
release its (climate) station data". Warwick Hughes of the coalition already has NIWA's corrected data for each
individual station in the New Zealand temperature time series, which we provided to him at no cost. For raw climate
data, NIWA charges the marginal cost of supply. We also charge for a value-added service of provision of climate data
via the internet. Mr Hughes has been offered access to these services subject to our standard rates, terms, and
conditions.
ENDS