Digging deep into the past to see the future of climate change

When did Australia’s climate become so dry? When did tropical reefs around Australia develop? And what will happen to Australia’s climate and reefs in the future?

The answer to these questions can be found by digging into the distant past. That means digging deep into the Earth’s crust, and you don’t always need to be on dry land to do that.

The US scientific ship JOIDES Resolution is capable of drilling deep under the ocean’s floor. It will depart Fremantle, in Western Australia, next month on a two month expedition to help shed light on some of the mysteries about Australia’s past. On board will be an international team of 30 scientists co-led by myself.

The JOIDES Resolution is named in honour of HMS Resolution that was commanded by Captain James Cook more than 200 years ago when he explored the Pacific Ocean. It is the flagship of the International Ocean Discovery Program (IODP), the world’s largest Earth science program whose country members include Australia and New Zealand.

The plan is to travel from Fremantle to Darwin and drill a series of cores of up to a kilometre deep into the seabed. This will give us a five million year history of climate and environmental change off the west coast of Australia.

Ocean drilling is the best method to directly sample the layers beneath the sea bed and it tells us how the Earth has worked in the past, how it is working now and how it might work in the future.

The origin of Australia’s western tropical reefs

The history of Australia’s climate is linked to oceanic conditions off its coastline.

Tropical reefs such as the Houtman-Abrolhos reefs off western Australia are controlled by the warm southward flowing offshore Leeuwin Current. This current is related to the global circulation system of ocean currents that travels through the Indonesian archipelago, called the Indonesian Throughflow.

Global ocean circulation controls the Earth’s climate. It transports water heat from the Pacific to the Indian Ocean and then to the poles.

Previous research has shown that ocean circulation in the Indian Ocean slowed down or nearly stopped near the Indonesian archipelago many times in the past. Every time this happened, the global climate changed, leading to cooling of the Indian Ocean and drier climates.

Our expedition hopes to study fossils and sediments in the layers from below the seabed to chart the history of these ocean features over millions of years. We seek to understand how global ocean circulation has changed and its climatic consequences.

The aim is to establish when tropical conditions suitable for reef growth first occurred and whether these conditions have changed over millions of years.

Looking into the past history of these reefs and ocean currents will improve our understanding of how modern reefs and currents off west Australia might behave with future climate change.

The history of the Australian monsoon

At present the climate of the northern half of Australia is highly influenced by seasonal rain variability. Monsoonal rains provide periodic relief to central Australia’s arid centre as the tropical monsoonal belt migrates southward from Indonesia during the summer months.

Even though there are large floods associated with these events, most of the Australian continent is still arid.

Previous work in the southern half of Australia suggests our present arid conditions are relatively recent in Australia’s geological history. These arid conditions were well established by around 1.5 million years ago.

But there are no similar histories of conditions in Australia’s northwest region. In addition, the long term history of the monsoon and aridity is not well known beyond a few hundred thousand years.

The reason for this is that no one has yet drilled deep enough beneath the sea bed to get million year scale records. In addition, long-term records of climate change in the arid zone of Australia do not preserve easily as conditions are so harsh.

Fortunately, sediment from monsoonal rivers flows into the sea in the region and is trapped in layers close to the continental margin, creating an excellent record of climate change.

We aim to drill into these layers to get a 5 million year record of the monsoon. Drilling these layers will allow us to understand how Australia’s climate and the Australian monsoon behaved during the last greenhouse period on Earth 3 million to 5 million years ago.

This was when the CO₂ levels in the atmosphere were more than 400 parts per million and the Antarctic ice sheet was smaller than today.

As this is the most recent period in Earth history when CO₂ levels were similar to today’s, our research should lead to a better understanding of the possible response of Australian monsoonal and arid areas to future climate change.

Our two month ocean expedition should hopefully lead to a greater understanding of Australia’s present tropical environment. How this environment behaved in the distant past, millions of years ago. How it came to be as it is today and how it might behave in future.

So we are in effect digging deep into our past to see our future.

Stephen Gallagher receives funding from the Australian Research Council and is a Chief Investigator on an ARC LIEF grant that funds Australia's membership to the International Ocean Discovery Program (IODP)