Australia has lowest number of tropical cyclones in 1500 years?

A new paper suggests there is an “unprecedentedly” low number of tropical cyclones around Australia at the moment. (How much should we spend to avoid this dreadful outcome I wonder?)

I am a little skeptical of how we can be so sure of the cyclone activity in, say, the year 900 AD. But nonetheless, the study is worth a look. Haig et al took stalagmites from two places in Australia (Chillagoe, Qld, and Cape Range, WA) and got very nice long year-by-year records of 18O and 16O data. They calibrated these against observational instrumental records — though I note these are but a tiny 20 years of data (1990 – 2010), and that during a period described by mainstream climate science (cough) as “unprecedented”.

Assuming that it is possible to pick apart normal rain and cyclonic rain, and that cyclone activity did not just shift to be more than 400 km away (where these stalagmites won’t record the cyclones) then it does appear that there are usually more cyclones in Australia than now. Note the top graphs are the WA site which go back to 500AD, and the lower pair are the QLD graphs “only” going back to 1300AD. Both graphs show an increase in storm activity during the Little Ice Age around 1700AD. If things do get cold in the next 20 years, will we get more cyclones and will fans of man-made global warming still be blaming CO2 if we do?

The paper has the usual obligatory vague claims pretending to connect this to anthropogenic causes, when there is nothing definitively unnatural shown in this long study, and given that the Earth has 1500 year and longer cycles, the long data is still not long enough anyway. We all know the debate is over and the science is settled, but note the 95% certainty is not quite evident in the conclusions (see my bolding).

The Australian region seems to be experiencing the most pronounced

phase of tropical cyclone inactivity for the past 550–1,500 years. The

dramatic reductions in activity since the industrial revolution suggest

that climate change cannot be ruled out as a causative factor. This

reduction is also in line with present projections for the late twenty first

century from global climate models, yet our results suggest that

this is occurring much sooner than expected. However, we cannot say

whether this downward trend in activity will be sustained.

These waffly phrasings of uncertainty will be studied one day as a comment on the politically distorted scientific culture of the early 21st century.

There are a couple of caveats, one is that normal rain and cyclonic rain are difficult to tell apart. The other is that these stalagmites can only record cyclones within 400km of the cave.

Tropical cyclones produce precipitation that is depleted in the heavier

oxygen isotope (18O) by >6%relative to average monsoonal precipitation,

owing to the recycling of water within the system, high

condensation efficiency and large size and longevity of such cyclones

as intense convective systems17. The resulting d18O content (expressed

as d18O5[(18O/16O )sample/(18O/16O )standard21)] x 1,000%) of tropical

cyclone precipitation at a site is influenced by a number of factors,

including the number of days since genesis (that is, rainout) and the

intensity of the storm, its source region18 and the distance of its centre

from the sampling path. Because tropical stalagmites are archives of

monsoonal d18O, signatures of past tropical cyclones are also recorded

within the d18Oof their carbonate layers, typically within 400km of the

storm centre19,20.

Figure 1 below shows a map of Australia with the two sites marked. The circles around the two centres are 400km radius, so all of the cyclones that track beyond those circles are not included. During the last 20 years, the two circles are very well located, but I wonder if weather patterns shift and cyclones simply move elsewhere. The black squiggly lines are the tracks of cyclones that wouldn’t register in these caves during the last 20 years.

ABSTRACT

The assessment of changes in tropical cyclone activity within the context of anthropogenically influenced climate change has been limited by the short temporal resolution of the instrumental tropical cyclone record1, 2 (less than 50 years). Furthermore, controversy exists regarding the robustness of the observational record, especially before 19903, 4, 5. Here we show, on the basis of a new tropical cyclone activity index (CAI), that the present low levels of storm activity on the mid west and northeast coasts of Australia are unprecedented over the past 550 to 1,500 years. The CAI allows for a direct comparison between the modern instrumental record and long-term palaeotempest (prehistoric tropical cyclone) records derived from the 18O/16O ratio of seasonally accreting carbonate layers of actively growing stalagmites. Our results reveal a repeated multicentennial cycle of tropical cyclone activity, the most recent of which commenced around ad 1700. The present cycle includes a sharp decrease in activity after 1960 in Western Australia. This is in contrast to the increasing frequency and destructiveness of Northern Hemisphere tropical cyclones since 1970 in the Atlantic Ocean6, 7, 8 and the western North Pacific Ocean6, 7. Other studies project a decrease in the frequency of tropical cyclones towards the end of the twenty-first century in the southwest Pacific7, 9, southern Indian9, 10 and Australian11 regions. Our results, although based on a limited record, suggest that this may be occurring much earlier than expected

REFERENCES

Haig, J., Nott, J. and Reichart, G. (2014) Australian tropical cyclone activity lower than at any time over the past 550–1,500 years, Nature 505, 667–671 doi:10.1038/nature12882 [Abstract]

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h/t Fred, and Robbie. And The HockeySchtick.

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