A coral diary of Australian droughts before European settlement

In Britain, droughts were once defined as 15 consecutive days without rain. Imagine the shock when the English explorers encountered the climate in Australia. The journals of Captain James Cook and Joseph Banks are peppered with references to the scarcity of water during their four-month visit. "For the whole length of coast which we sailed along there was a sameness to be observed … Barren it may justly be called and in a very high degree … so barren [it] could not be supposed to yield much towards the support of man".

The crew was forced to replenish freshwater supplies from standing pools rather than from flowing streams. However, Banks also recognised that Australia was a land of extremes: on a number of occasions he noted erosion that could only have resulted from dramatic floods. "[The country] was most destitute of fresh water, probably that was the reason why so few inhabitants were seen: it seemed to be subject to a severe rainy season, so at least we judged by the deep gullies which we saw had been plainly washed down from the hills of a small height".

The accounts in Banks' diary fit with our experience of the Australian continent since European settlement, as expressed by Dorothea Mackellar: this is a land of droughts and flooding rains.

At the start of 2003 both extremes are yet again gripping Australia and reminding us of the vulnerability of our economy and society to climate variations. With a century or more of instrumental rainfall measurements the Bureau of Meteorology can put the current drought into perspective with earlier events, such as the "Federation Drought" (1895-1902) which inflicted a loss of 37 million sheep and 4.8 million cattle in Queensland alone.

The present drought ranks among the worst of the past century in terms of spatial extent and dryness for a nine-month period, although 1901-02, the final year of the eight-year Federation Drought, still holds the record at most rainfall stations across NSW and Queensland for the lowest rainfall levels measured.

Returning to the observations from The Endeavour", what fascinates me (as a PhD student studying past climates) is whether 1770 would rank as a drought year by present Bureau of Meteorology standards. Was it unusually dry, or were the reports from Banks and Cook coloured by a comparison with England?

This isn't just a matter of historical curiosity: written records can tell us about climate behaviour in times before instrumental records were collected. There are other sources too.

As The Endeavour sailed through the Great Barrier Reef, trying unsuccessfully to avoid the reefs, centuries-old corals were storing climate information in their skeleton. These coral witnesses are still around today and are proving to be an exciting archive of past climate information. The corals I am talking about aren't the decorative and colourful specimens, they are massive colonies of a species called 'Porites' that make up the foundation and framework of the Great Barrier Reef. These colonies can reach 8m across and 6m high, but only the surface 10 to 20mm layer is living, the rest of the structure is a skeleton laid down at about 1 to 2cm a year.

Over the past two decades researchers at the Australian Institute of Marine Science (AIMS) have been collecting core samples from these corals. The longest collected so far contains a continuous record back into the 13th century. Corals are also found in fossil reefs on land from times when sea levels were higher, providing windows into very different climates thousands of years ago.

A simple but visually striking example (52KB image - will open in a new window) of the records that we can extract from coral is due to a discovery by Dr Peter Isdale at AIMS. Under UV light, bright annual lines, reminiscent of tree-rings, appear in the coral. These are a visual diary of the timing, length and intensity of the wet-season rains and the number of river floods that reach the reef: the larger the flood, the brighter the line visible in the coral.

The strong wet seasons in 1981, 1979, 1974, 1968 and 1958 appear as intense banding in coral samples from the path of seasonal floodwaters from the Burdekin River, which drains into the Great Barrier Reef Lagoon. In some years two or more floods occurred during one wet season and this information is also captured by the coral (e.g. 1972 and 1958). The lack of any line in 1969 is evidence of a drought and was the third driest year in the Queensland rainfall record since 1891 and the lowest summer river flow for the Burdekin River over the period shown in these sections of coral. The consistently wet 1970s also stand out from drier decades in the 1980s and 1960s.

So what was it like when The Endeavour sailed through the Great Barrier Reef in 1770? A section of coral (100KB image - will open in a new window) growing from 1750 to 1825 shows a large variation in rainfall and river flows. Very intense lines indicate strong flooding (1801 and 1802, 1795), often following periods of drought (1799-1800 and 1793-94), and there are consistently wet decades with dependable wet seasons every year, such as the 1750s. There is also a striking period of below-average rainfall from the mid-1760s to the mid-1780s and four out of five years prior to Cook's visit were dry. It is hardly surprising that the Burdekin River mouth was not marked on The Endeavour's charts.

Another pattern of rainfall extremes is highlighted by this sample. Severe Australia-wide droughts and worldwide changes in rainfall patterns have been linked to the El Niño Southern Oscillation. During the Centennial drought of 1888-89 the meteorologist Charles Todd (better known for building the overland telegraph line from Adelaide to Darwin) was first to notice the connection between Indian monsoon failures and Australian droughts. This is a complex relationship which has waxed and waned over the past century but thanks to the historical record of Indian droughts, gathered by colonial scientists and in East India Company reports, we can see that Indian and Queensland droughts coincided throughout the 18th and early 19th Centuries. Matching reports from the early Governors of the NSW settlement confirm that the droughts were widespread events.

Corals trap many climate clues in their skeletons. These can be translated into records of rainfall, temperature, the saltiness of the surrounding sea, the sediment released from rivers during floods, and other environmental events in our coastal waters. At ANU state-of-the-art techniques have been developed to decipher the information stored in coral. In 1997 a collaborative research group, AUSCORE (Australian Coral Records), was set up to maximise Australia's contribution to international coral records research by sharing expertise, materials and measurement techniques. Our scientists are at the front of the field in gathering past climate information from coral archives.

Australians must live with recurring droughts and floods, and we need to be proactive in our management of future climate crises. Information on past climate is critical as it allows us to better understand what we will experience in the future.