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Are Australia Bushfires Worsening from Human-Caused Climate Change?

Roy Spencer

The claim by many that human-caused climate change has made Australian bushfires worse is difficult to support, for a number of reasons.

Smoke plumes from bushfires in southeast Australia on January 4, 2020, as seen by the MODIS imager on NASA’s Aqua satellite.

Summary Points

1) Global wildfire activity has decreased in recent decades, making any localized increase (or decrease) in wildfire activity difficult to attribute to ‘global climate change’.

2) Like California, Australia is prone to bushfires every year during the dry season. Ample fuel and dry weather exists for devastating fires each year, even without excessive heat or drought, as illustrated by the record number of hectares burned (over 100 million) during 1974-75 when above-average precipitation and below-average temperatures existed.

3) Australian average temperatures in 2019 were well above what global warming theory can explain, illustrating the importance of natural year-to-year variability in weather patterns (e.g. drought and excessively high temperatures).

4) Australia precipitation was at a record low in 2019, but climate models predict no long-term trend in Australia precipitation, while the observed trend has been upward, not downward. This again highlights the importance of natural climate variability to fire weather conditions, as opposed to human-induced climate change.

5) While reductions in prescribed burning have probably contributed to the irregular increase in the number of years with large bush fires, a five-fold increase in population in the last 100 years has greatly increased potential ignition sources, both accidental and purposeful.

Historical Background

Australia has a long history of bush fires, with the Aborigines doing prescribed burns centuries (if not millennia) before European settlement. A good summary of the history of bushfires and their management was written by the CSIRO Division of Forestry twenty-five years ago, entitled Bushfires – An Integral Part of Australia’s Environment.

The current claim by many that human-caused climate change has made Australian bushfires worse is difficult to support, for a number of reasons. Bushfires (like wildfires elsewhere in the world) are a natural occurrence wherever there is strong seasonality in precipitation, with vegetation growing during the wet season and then becoming fuel for fire during the dry season.

All other factors being equal, wildfires (once ignited) will be made worse by higher temperatures, lower humidity, and stronger winds. But with the exception of dry lightning, the natural sources of fire ignition are pretty limited. High temperature and low humidity alone do not cause dead vegetation to spontaneously ignite.

As the human population increases, the potential ignition sources have increased rapidly. The population of Australia has increased five-fold in the last 100 years (from 5 million to 25 million). Discarded cigarettes and matches, vehicle catalytic converters, sparks from electrical equipment and transmission lines, campfires, prescribed burns going out of control, and arson are some of the more obvious source of human-caused ignition, and these can all be expected to increase with population.

Trends in Bushfire Activity

The following plot shows the major Australia bushfires over the same period of time (100 years) as the five-fold increase in the population of Australia. The data come from Wikipedia’s Bushfires in Australia.

Fig. 1. Yearly fire season (June through May) hectares burned by major bushfires in Australia since the 1919-20 season (2019-20 season total is as of January 7, 2020).

As can be seen, by far the largest area burned occurred during 1974-75, at over 100 million hectares (close to 15% of the total area of Australia). Curiously, though, according to Australia Bureau of Meteorology (BOM) data, the 1974-75 bushfires occurred during a year with above-average precipitation and below-average temperature. This is opposite to the narrative that major bushfires are a feature of just excessively hot and dry years.

Every dry season in Australia experiences excessive heat and low humidity.

Australia High Temperature Trends

The following plot (in red) shows the yearly average variations in daily high temperature for Australia, compared to the 40-year average during 1920-1959.

Fig. 2. Yearly average high temperatures in Australia as estimated from thermometer data (red) and as simulated by the average of 41 climate models (blue). (Source).

Also shown in Fig. 2 (in blue) is the average of 41 CMIP5 climate models daily high temperature for Australia (from the KNMI Climate Explorer website). There are a few important points to be made from this plot.

First, if we correlate the yearly temperatures in Fig. 2 with the bushfire land area burned in Fig. 1, there is essentially no correlation (-0.11), primarily because of the huge 1974-75 event. If that year is removed from the data, there is a weak positive correlation (+0.19, barely significant at the 2-sigma level). But having statistics depend so much on single events (in this case, their removal from the dataset) is precisely one of the reasons why we should not use the current (2019-2020) wildfire events as an indicator of long-term climate change.

Secondly, while it is well known that the CMIP5 models are producing too much warming in the tropics compared to observations, in Australia just the opposite is happening: the BOM temperatures are showing more rapid warming than the average of the climate models produces. This could be a spurious result of changes in Australian thermometer measurement technology and data processing as has been claimed by Jennifer Marohasy.

Or, maybe the discrepancy is from natural climate variability. Who knows?

Finally, note the huge amount of year-to-year temperature variability in Fig. 2. Clearly, 2019 was exceptionally warm, but a good part of that warmth was likely due to natural variations in the tropics and subtropics, due to persistent El Nino conditions and associated changes in where precipitation regions versus clear air regions tend to get established in the tropics and subtropics.

Australia Precipitation Trends

To drive home the point that any given year should not be used as evidence of a long-term trend, Australia precipitation provides an excellent example. The following plot is like the temperature plot above (Fig. 2), but now for precipitation as reported by the BOM (data here).

Fig. 3. As in Fig. 2, but for annual precipitation totals.

We can see that 2019 was definitely a dry year in Australia, right? Possibly a record-setter. But the long-term trend has been upward (not downward), again illustrating the fact that any given year might not have anything to do with the long-term trend, let alone human-induced climate change.

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