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Published: 2 December 2013

Indian Ocean linked to bushfires and drought in Australia

Wenju Cai

In a new study published in Nature Geoscience, we show that extreme weather events in Australia such as drought and bushfire are linked to temperature changes in the Indian Ocean. Much like El Niño in the Pacific Ocean, the Indian Ocean Dipole has far-reaching consequences, and these effects are likely to strengthen under climate change.

Dry paddocks in the Riverina region, 2007, during the Millennium Drought in southeastern Australia.

What is the Indian Ocean Dipole?

Like El Niño, the Indian Ocean Dipole (IOD) is an interaction between the ocean and atmosphere.

The IOD appears and develops in the Southern Hemisphere winter and matures in spring. In its positive phase, which is the one that interests us most, sea temperatures off the cost of Sumatra and Java are lower than normal. Meanwhile in the western equatorial Indian Ocean, off the coast of Kenya, sea temperatures are warmer.

(The IOD also has a negative phase, but this is much rarer, and its effects much more benign.)

These seemingly small changes in sea temperature have profound effects on the atmosphere. Convection — rising warm, moist air — and rainfall tend to follow the warmest sea temperatures. So changes in sea temperature dramatically alter atmospheric circulation and rainfall distribution.

The result, in the case of the IOD, is extreme weather in many parts of the world, including severe droughts in Indonesia and devastating floods in East African countries.

For Australia, our research confirmed links between this Indian Ocean phenomenon and extreme weather events in southeast Australia, for example, bushfires such as those that occurred on Black Saturday.

During a positive IOD event, south east Australia sees decreased rainfall and increased temperatures. This is because much of the moisture supplying rainfall over south east Australia in winter and spring come from the tropical eastern Indian Ocean. Less rain and clear skies lead to higher temperatures than normal.

Cool waters over the western Indian Ocean cause drought and extreme fire conditions in Australia.
Credit: CSIRO

More than a statistical fluke

In earlier studies, scientists showed that there are statistical links between the IOD and extreme weather in Australia.

In this new research we’re able to show that these linkages are not statistical flukes, and can in fact be predicted by climate models.

And because these events can be simulated by models, we can use these models to find out whether positive IOD events will become more common in a warming world.

We examined 54 climate models and experiments that participated in the Intergovernmental Panel on Climate Change’s Fifth Assessment Report. These model experiments include the historical period up to 2005, and a future period under a high emissions scenario. These experiments provide a large number of samples with thousands of years of virtual climate, which allows us to distil climate change signals.

How might the IOD change in the future?

Over the past 50 years, the IOD index (how we measure the difference in sea temperatures between the western and eastern Indian Ocean) has been trending upwards. Climate models suggest it will continue to do so over the next 100 years.

This predicts a drying trend over south east Australia, and more IOD events compared to the present climate.

In a warming world, the eastern Indian Ocean warms less than the west, and tropical rainfall and moisture move away from the eastern Indian Ocean, resembling a positive IOD event.

This slow warming pattern will lead to more frequent IOD events, and the associated dry conditions will be more intense, compared with the present-day climate. The bottom line is that the rain is moving away from Australia.

What does this mean for Australia?

Our major bushfires in summer have been linked with a positive IOD in winter and spring, and therefore the IOD offers a way of predicting summer bushfire conditions. This research enables us to better anticipate drought and increased bushfire risk.

This is because we have some four to six months of lead time before the fire season. An IOD in winter and spring is a warning sign of higher than normal fire risks in the upcoming summer.

In future climate, a decline in spring rainfall and a rise in temperature induced by an IOD event, exacerbated by a long-term drying trend in a warming climate, will greatly increase the risk of major bushfires.

The implications are of course far broader than Australia. The IOD has, to date, preconditioned wildfires in Indonesia, caused coral reef death across western Sumatra, and exacerbated malaria outbreaks in East Africa. We expect these extreme events to become more intense in the future.

Dr Wenju Cai is a CSIRO Principal Research Scientists, leading research into the use of climate change and variability predictions to maximise agricultural, urban and ecological water use opportunities. This article was originally published at The Conversation.







Published: 19 January 2015

Mapping East Asia’s disappearing tidal flats

Nick Murray

Who speaks for the tidal flat? There are many voices for the mangrove forest, the coral reef and the seagrass meadow, but the chorus for the mud, sand and silt flats that sit hidden under shallow water for most of the tidal cycle is often silent.

In China alone more than 1.2 million hectares of wetland reclamation has taken place in the last 50 years, perhaps accounting for more than 5 per cent of the worlds’ tidal wetlands.
In China alone more than 1.2 million hectares of wetland reclamation has taken place in the last 50 years, perhaps accounting for more than 5 per cent of the worlds’ tidal wetlands.
Credit: Nick Murray

Not only do hundreds of species of migratory bird depend on them for their existence, this coastal ecosystem also protects large chunks of humanity and provides ecosystem services to hundreds of millions of people around the world.

A zone under pressure

The problem for all coastal ecosystems is the shifting character of the coastal zone. The last 50 years has seen the global human population migrating rapidly to coastal regions. As a result, coastlines around the world have become a focus of expansion of urban, agricultural and industrial areas.

This development is having a major impact on coastal ecosystems, which has resulted in the widespread loss and degradation of ecosystems such as mangroves, seagrasses, coral reefs and tidal flats. And that has major consequences for humans and nature.

In terms of the human cost, coastal ecosystems are a frontline defence that protects billions of dollars of infrastructure from storms and sea level rise, and maintaining their integrity is among the most cost-effective options for coastal protection.

Tidal flats are a widespread coastal ecosystem that is frequently overlooked in the planning and management of coastal resources. They are among the most widespread of any coastal ecosystem and, as well as providing ecosystem services to hundreds of millions of people worldwide, they sustain a suite of threatened and declining species.

For instance, tidal flats support the majority of the world’s migratory shorebird species, enabling their yearly migration from the arctic to areas as far south as Patagonia. Unfortunately, their proximity to centres of human population have also made these areas targets for cheap and rapid coastal development.

Drawing a mud map

So, what’s the magnitude of the problem?

Until now we have had no way of knowing just how much of this declining coastal ecosystem has been destroyed, or how much and where it remains. The principal reason for the lack of accurate maps of this ecosystem is due to the rapidly changing conditions they encounter: changing tides either expose or cover them, severely limiting the application of classical remote sensing methods.

To solve this problem, our small team of remote sensors and spatial ecologists have been developing methods to map tidal flats over very large areas.

Using the heavily developed tidal flats of mainland East Asia as a case study, we have developed a rapid mapping approach for identifying the distribution of tidal flats while assessing their changing status at continental scales.

The tidal flats in this region – which fringe the countries of North Korea, South Korea and China – are among the largest in the world, measuring up to 20 kilometres wide in some places. Our methods – utilising free data from the US Geological Survey’s Landsat archives and freely available regional tide models – allow fast implementation across thousands of kilometres.

Indeed, with more than 28,000 images to choose from, we determined the changing status of tidal flats across more than 14,000 kilometres of coastline.

Easily overlooked, and invisible for much of the tide cycle, mud flats are disappearing right before our very eyes. And their loss comes with an enormous cost.
Easily overlooked, and invisible for much of the tide cycle, mud flats are disappearing right before our very eyes. And their loss comes with an enormous cost.
Credit: Nick Murray

Impacts of reclamation

Our results demonstrate that tidal flats in East Asia are being destroyed at rates similar to other major at-risk ecosystems, such as tropical forests and mangroves. The principal cause of these losses related to coastal development. Changes to sedimentation regimes due to the damming of major rivers is also an issue as this results in offshore losses of tidal flats.

In East Asia, land scarcity is a severe issue and often the cheapest method of acquiring land for large coastal developments is through land creation, often termed reclamation. Tidal flats, which are generally characterised by low-sloping flats in areas protected from severe weather, have proven an ideal environment for cheap and rapid coastal development.

This radical transformation involves the construction of seawalls, infilling and finishing for land use. These areas are then developed into new parcels of land for aquaculture, agriculture, suburbs and industry.

Loss of coastal wetlands to land reclamation is a global problem that is severely affecting the world’s coastlines. In China alone more than 1.2 million hectares of wetland reclamation took place in the last 50 years, perhaps accounting for more than 5 per cent of the world’s tidal wetlands according to some estimates.

This is clearly a symptom of China’s rapid coastal urbanisation. This arc of growth will form one of the world’s largest urban areas by 2030 – a continuous coastal urban corridor over 1800 kilometres long.

The rapid pace of coastal population growth and sea-level rise – as well as increasing demand for aquaculture, coastal wind farms, and tide energy – will certainly apply further pressure to the world’s tidal flats in the future.

The loss of tidal flats along migratory pathways, especially staging sites (where birds must replenish their energy stores during migration for long, energetically expensive flights) can have extreme consequences for shorebird populations. For the millions of shorebirds that migrate through the East Asian-Australasian Flyway, the intertidal areas of Asia are a crucial migratory bottleneck.
The loss of tidal flats along migratory pathways, especially staging sites (where birds must replenish their energy stores during migration for long, energetically expensive flights) can have extreme consequences for shorebird populations. For the millions of shorebirds that migrate through the East Asian-Australasian Flyway, the intertidal areas of Asia are a crucial migratory bottleneck.
Credit: Nick Murray

Uncertain future

An effective conservation strategy must manage the complex economic and social trade-offs that drive coastal development.

Decision-making that simultaneously plans for coastal development and coastal conservation along the world’s most rapidly developing shores is clearly needed.

For example, places where natural values have effectively been lost due to sediment depletion and coastal subsidence could be prioritised for development. As part of a carefully integrated plan, this could ease pressure on a functioning network of coastal protected areas and ensure continued delivery of ecosystem services.

Not only might this avert catastrophic extinctions of coastal biodiversity, it will also help us ensure we have a coastline capable of adapting to an increasingly uncertain future.

Dr Nick Murray is a Research Associate at the Centre for Ecosystem Science, University of New South Wales. He carried out this research in association with the Environmental Decisions Group (EDG), while completing his PhD at the University of Queensland. The EDG is a network of conservation researchers developing the science of effective decision making to better conserve biodiversity, and includes a number of Australian and International research centres, including CSIRO. This article first appeared in Decision Point – a free monthly online publication from the EDG.

More information

‘Tracking the rapid loss of tidal wetlands in the Yellow Sea’, published in Frontiers in Ecology and the Environment

‘Continental scale mapping of tidal flats across East Asia using the Landsat archive’, published in Remote Sensing

‘IUCN situation analysis on East and Southeast Asian intertidal habitats, with particular reference to the Yellow Sea (including the Bohai Sea)’, IUCN occasional paper






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