In Australia, we had a horrific start to 2020 – and that was well before COVID-19 took over our lives. A devastating drought across much of eastern Australia (which is also the most populous and agriculturally dense part of the continent) had tightened its grip across 2019, with many towns facing their own Day Zero. The news couldn’t really get any worse, and then it did – devastating bushfires commenced in late spring and continued into the southern hemisphere summer driven by extreme fire weather and catastrophically dry conditions.
This would all have been more than enough to bear, except that it wasn’t as though Australia hadn’t already just endured its most recent multi-year drought. The Millennium Drought spanned 1997-2009 and likely even started earlier in some regions. It was one of the ‘big three’ of the historical record, along with the Federation (1895-1902) and World War II (1937-45) droughts. The Millennium Drought changed water policy forever as drastic measures were introduced, and new levels of water restrictions were invented. Then, in classic Australian style but on a level not previously experienced, this drought ended in spectacular fashion with one of the wettest summers in the instrumental record in 2011, including major floods in the city of Brisbane and surrounding regions, and a follow-up wet summer and further flooding in 2012 and 2013.
If we put all this together the picture is alarming: for the last 25 years, a major portion of the continent experienced a devastating drought for at least 13 years, record-breaking wet for 2-3 years, a couple of normal years (whatever that looks like!) and then another drought of next level intensity spanning (so far) ~2016-2020. Climatologists know Australia has a highly variable climate, with multidecadal changes in rainfall, but this has been an abject lesson in how little we understand the magnitude of this variability.
How much of this is due to climate change?
Australia, like much of the subtropics, is projected to endure more extreme rainfall overlying a drying trend in the coming decades (arguably, it already is). So the experience of the last 25 years is highly likely to reflect underlying change due to rising emissions, particularly the influence of rising temperatures and changing rainfall on drought intensity. In southwest Western Australia, the picture is clearer - this region has undergone a significant drying trend since the mid-1970’s, probably driven by climate change. However disentangling the climate change signal from underlying variability is more difficult in eastern Australia.
One reason why it is difficult is short instrumental records. Rainfall stations covering the last 120-odd years may seem like enough data, until you throw in 3 or 4 multi-year droughts spanning anywhere from 5-14 years. At this point, your sample size of “How long do droughts last?” is woefully small. Furthermore, at least one of these big droughts is likely to already have some underlying climate change signal.
What is needed to develop a truer understanding of how bad droughts can be are palaeoclimate records that span much longer than the short historical record. These records need to be diverse. They should consist of those that are local to the region of interest as well as those that record remote drivers of climate. They need to give us an idea of drought intensity (how bad did it get?) and also duration (how long did it last?).
Finding this diversity in Australia is difficult, but there is now intensive effort by researchers spurred on by our back-to-back ‘record-breaking’ droughts to identify new palaeoclimate records that can fulfil these criteria (local, remote, intensity, duration). We don’t actually know what constitutes normal operating conditions for this continent over long timescales and without the influence of climate change. And certainly, recent palaeoclimate studies suggest drought duration in eastern Australia (unlike that of southwest Western Australia) can be much longer than the historical record shows. What we discover from these palaeoclimate records is likely to be discomfiting at best, but crucial to managing our industry, agriculture and day-to-day lives into the future.