By Mandy Freund, Ben Henley, Kathryn Allen, and Patrick Baker
1 May 2018

(The Conversation) – Australia is a continent defined by extremes, and recent decades have seen some extraordinary climate events. But droughts, floods, heatwaves, and fires have battered Australia for millennia. Are recent extreme events really worse than those in the past?

In a recent paper, we reconstructed 800 years of seasonal rainfall patterns across the Australian continent. Our new records show that parts of Northern Australia are wetter than ever before, and that major droughts of the late 20th and early 21st centuries in southern Australia are likely without precedent over the past 400 years.

This new knowledge gives us a clearer understanding of how droughts and flooding rains may be changing in the context of a rapidly warming world.

Australia has been shaped by floods, droughts, and blistering heat. How big and how intense these events were is poorly understood due to the limited historical and observational records.

Historical records provide rough estimates of the extent and intensity of droughts in parts of Australia since the late 1700s. For example, captains’ logbooks from ships anchored off of Sydney describe the Settlement Drought (1790-1793), which threatened the tenuous foothold of early European settlers in Australia. And farmers’ records describe the Goyder Line Drought (1861–1866) that occurred in areas north of the known arable lands of South Australia.

Observational weather records provide more detailed descriptions of climatic variability. However, systematic recording of weather in Australia only began in the late 19th century. Since then many parts of the continent have experienced prolonged wet periods and droughts. The most well known of these are the Federation drought (1895-1903), the World War II drought (1939-45), and the recent Millennium drought (1997-2009).

All three droughts were devastating to agriculture and the broader economy, but each was distinct in its spatial footprint, duration, and intensity. Importantly, these droughts also differed in seasonality.

For example, the Millennium drought, which was most severe in southwestern and southeastern Australia, was caused by poor rainfall during the cool season. In contrast, the Federation drought, which affected almost the entire continent, was predominantly due to rainfall declines during the warm season.

Although the historical and observational records provide a wealth of information about the frequency of wet and dry extremes, they provide only part of the picture. [more]

Recent Australian droughts may be the worst in 800 years

Contextualising recent observed trends in regional Australian rainfall. Left panels show regional rainfall reconstructions since 1600 for the warm (red) and cool season (blue) with the 10-year low-pass Chebyshev filtered series shown as a black line. Grey bars along the x-axis denote non-verified periods for each reconstruction. Right panels show histograms of 30- and 50-year regional rainfall trends (mm yr−1). Grey shaded bars indicate the full range of the trends prior to 1970 (for 30-year periods) and 1950 for 50-year periods. Light red/blue colouring highlights the trends since 1970 (for 30-year periods) or 1950 for 50-year periods. The dark coloured bars indicate the trend in the most recent period. Bar heights are normalised by the maximum occurrence for each region. Graphic: M. Freund et al., 2018 / Climate of the Past

ABSTRACT: Australian seasonal rainfall is strongly affected by large-scale ocean–atmosphere climate influences. In this study, we exploit the links between these precipitation influences, regional rainfall variations, and palaeoclimate proxies in the region to reconstruct Australian regional rainfall between four and eight centuries into the past. We use an extensive network of palaeoclimate records from the Southern Hemisphere to reconstruct cool (April–September) and warm (October–March) season rainfall in eight natural resource management (NRM) regions spanning the Australian continent. Our bi-seasonal rainfall reconstruction aligns well with independent early documentary sources and existing reconstructions. Critically, this reconstruction allows us, for the first time, to place recent observations at a bi-seasonal temporal resolution into a pre-instrumental context, across the entire continent of Australia. We find that recent 30- and 50-year trends towards wetter conditions in tropical northern Australia are highly unusual in the multi-century context of our reconstruction. Recent cool-season drying trends in parts of southern Australia are very unusual, although not unprecedented, across the multi-century context. We also use our reconstruction to investigate the spatial and temporal extent of historical drought events. Our reconstruction reveals that the spatial extent and duration of the Millennium Drought (1997–2009) appears either very much below average or unprecedented in southern Australia over at least the last 400 years. Our reconstruction identifies a number of severe droughts over the past several centuries that vary widely in their spatial footprint, highlighting the high degree of diversity in historical droughts across the Australian continent. We document distinct characteristics of major droughts in terms of their spatial extent, duration, intensity, and seasonality. Compared to the three largest droughts in the instrumental period (Federation Drought, 1895–1903; World War II Drought, 1939–1945; and the Millennium Drought, 1997–2005), we find that the historically documented Settlement Drought (1790–1793), Sturt's Drought (1809–1830) and the Goyder Line Drought (1861–1866) actually had more regionalised patterns and reduced spatial extents. This seasonal rainfall reconstruction provides a new opportunity to understand Australian rainfall variability by contextualising severe droughts and recent trends in Australia.

Multi-century cool- and warm-season rainfall reconstructions for Australia's major climatic regions



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