Guest essay by Dr. B Basil Beamish
In a recent WUWT post I looked at the number of hot days (Tmax > 35 °C) projected by climate models for Cairns as published in a joint CSIRO and Bureau of Meteorology report (McInnes et al, 2015). Several replies to that post queried the reference base of the number of hot days used from the report and I thank those of you who took the time to raise that issue with me. As a result I revisited the original technical report (CSIRO and Bureau of Meteorology, 2015) and discovered that my assumption these values were based on a 30-year moving average climatology was incorrect. In fact the values for the different locations in Table 7.1.2 of the report on page 98 for scenario 2030 RCP4.5 are a 20-year average for the period 2020-2039 centred on 2030.
I have therefore amended my graph for Cairns and Figure 1 shows the 20-year centred averages from 1980 through to the end of 2015. The average for the period from 1996-2015 is centred on 2006. The warming trend projections required to reach the modelled outcomes for Cairns shown in Table 1 are drawn on the graph.
Figure 1. Trend in the number of hot days for Cairns showing trends required to achieve the model projections contained in the CSIRO/BOM report.
The larger set of hot day projections for Australian regions contained in the more comprehensive technical report (CSIRO and Bureau of Meteorology, 2015) provides an opportunity to take a look at a wider snapshot of hot days in Northern Australia. Projected hot day averages for Darwin and Broome are shown in Table 1. The corresponding trends in hot days for each of these locations from 1980 through to 2015 are shown in Figures 2 and 3 respectively.
Figure 2. Trend in the number of hot days for Darwin showing trends required to achieve the model projections contained in the CSIRO/BOM report.
Figure 3. Trend in the number of hot days for Broome showing trends required to achieve the model projections contained in the CSIRO/BOM report.
From the trend lines shown in each of the graphs for Cairns, Darwin and Broome it is possible to calculate the number of hot days required in successive five year periods to remain on trend. These values are contained in Table 2. For each five year period that the number of hot days falls below the average number required to reach the model projection value, the number required in subsequent years will increase.
This comparison between observational data and model projections provides us with an opportunity to track the progress of any climate change taking place in these Northern Australian regions. A copy of the spreadsheet for Broome is made available here for you to verify the analysis provided and it can also be used to add in data from successive years to track the validity of the model projections as the graph is automatically updated.
Again I would emphasise:
- Any model projection needs to be validated against actual observations before any confidence can be placed in the model results.
- Accountability of model projections should be mandatory and comparisons similar to this one should be performed on a five yearly basis, particularly if the models are continually adjusted.
- As each five years of data becomes available it should become readily apparent whether the model projections are either validated or not.
CSIRO and Bureau of Meteorology, 2015. Climate Change in Australia Information for Australia’s Natural Resource Management Regions: Technical Report, CSIRO and Bureau of Meteorology, Australia, 216p.
McInnes, K et al, 2015. Wet Tropics Cluster Report, Climate Change in Australia Projections for Australia’s Natural Resource Management Regions: Cluster Reports, eds. Ekström, M et al, CSIRO and Bureau of Meteorology, Australia, 48p.
Data and Analysis Spreadsheet: broome-hot-days_wuwt (Excel .xlsx)