As the planet warms, subtropical regions of the Southern Hemisphere, including parts of southern Australia and southern Africa, are drying. These trends include major drought events such as Cape Town’s “Day Zero” in 2018.
Climate projections suggest this subtropical drying will continue throughout the 21st century. Further drying in these regions will place great stress on ecosystems, agriculture and urban water supplies.
Our new study, published today in Nature Climate Change, suggests the subtropical Southern Hemisphere drying trend may reverse, if global temperatures stabilise in a future world with zero net greenhouse gas emissions.
Dry places get drier, wet places get wetter
As global temperatures increase, some regions get wetter while others get drier. Climate models indicate that many parts of the tropics, where it is already very wet, will become wetter. The subtropics, which sit between the wet tropics and the wet mid-latitudes, are expected to get drier.
Over southern Australia, rainfall is expected to decline, particularly in the cool season (which is currently the rainy time of year). This has already happened in Perth and the surrounding southwest of Western Australia.
What will happen when warming slows or stops?
Climate models are typically used to explore future climate under transient or rising temperatures, at least until the end of the 21st century. International efforts to reduce greenhouse gas emissions are aimed at slowing and eventually stopping temperature rises so that the climate is stabilised. For example, the Paris Agreement aims to stabilise global warming within 1.5℃ or 2℃ above pre-industrial levels.
But if temperatures stop rising, how will rainfall patterns respond? To investigate, we used pre-existing climate model runs created by the international scientific community to project different conditions extending from the present to the year 2300.
The chart below shows two different scenarios: one in which greenhouse gases and temperatures level off around 2100 (this referred to as Extended Representative Concentration Pathway 4.5), and the one next to it (Extended Representative Concentration Pathway 8.5) in which greenhouse gases don’t level off until around 2250, creating a much warmer climate.
We found that rainfall in the Southern Hemisphere subtropics decreases while temperatures are rising rapidly, with most of the rainfall reduction occurring in the winter months. When temperatures begin to stabilise, subtropical rainfall starts to recover.
How rainfall reversal works
The subtropics are relatively dry right now because they are the region where dry air descends from the upper atmosphere to the surface, suppressing rainfall. Studies have shown that the subtropics may be expanding or shifting southward in the Southern Hemisphere as the global climate warms.
Our study found a link between the trend in Southern Hemisphere subtropical rainfall and the temperature gradient between the tropics and subtropical regions. This temperature gradient gets steeper during periods of rapid warming because the tropics warm faster. Once warming stops, the regions further from the Equator catch up and the temperature gradient gets weaker.
The pattern of temperature warming drives the shifts in rainfall: when the tropics are warming faster, the subtropics become drier as more moisture is exported to the tropics.
A wetter or drier future?
Our results suggest that stabilising global temperatures may lead to a reversal in the drying trend in the subtropics.
The path to stabilising global temperatures will be a long journey from the current trajectory of rising emissions, but this research is potentially good news for the future generations who will live in subtropical regions.
The authors would like to acknowledge Nathan P. Gillett, Katarzyna B. Tokarska, Katja Lorbacher, John Hellstrom, Russell N. Drysdale and Malte Meinshausen, who contributed to this study.
Kale Sniderman, Senior Research Fellow, School of Earth Sciences, University of Melbourne; Andrew King, ARC DECRA fellow, University of Melbourne; Jon Woodhead, Research Scientist, and Josephine Brown, Senior research scientist, Australian Bureau of Meteorology
Matthew Fraser, University of Western Australia; Ana Sequeira, University of Western Australia; Brendan Paul Burns, UNSW; Diana Walker, University of Western Australia; Jon C. Day, James Cook University, and Scott Heron, James Cook University
The devastating bleaching on the Great Barrier Reef in 2016 and 2017 rightly captured the world’s attention. But what’s less widely known is that another World Heritage-listed marine ecosystem in Australia, Shark Bay, was also recently devastated by extreme temperatures, when a brutal marine heatwave struck off Western Australia in 2011.
A 2018 workshop convened by the Shark Bay World Heritage Advisory Committee classified Shark Bay as being in the highest category of vulnerability to future climate change. And yet relatively little media attention and research funding has been paid to this World Heritage Site that is on the precipice.
Shark Bay, in WA’s Gascoyne region, is one of 49 marine World Heritage Sites globally, but one of only four of these sites that meets all four natural criteria for World Heritage listing. The marine ecosystem supports the local economy through tourism and fisheries benefits.
Around 100,000 tourists visit Shark Bay each year to interact with turtles, dugongs and dolphins, or to visit the world’s most extensive population of stromatolites – stump-shaped colonies of microbes that date back billions of years, almost to the dawn of life on Earth.
Commercial and recreational fishing is also extremely important for the local economy. The combined Shark Bay invertebrate fishery (crabs, prawns and scallops) is the second most valuable commercial fishery in Western Australia.
However, this iconic and valuable marine ecosystem is under serious threat. Shark Bay is especially vulnerable to future climate change, given that the temperate seagrass that underpins the entire ecosystem is already living at the upper edge of its tolerable temperature range. These seagrasses provide vital habitat for fish and marine mammals, and help the stromatolites survive by regulating the water salinity.
Shark Bay received the highest rating of vulnerability using the recently developed Climate Change Vulnerability Index, created to provide a method for assessing climate change impacts across all World Heritage Sites.
In particular, extreme marine heat events were classified as very likely and predicted to have catastrophic consequences in Shark Bay. By contrast, the capacity to adapt to marine heat events was rated very low, showing the challenges Shark Bay faces in the coming decades.
The region is also threatened by increasingly frequent and intense storms, and warming air temperatures.
To understand the potential impacts of climatic change on Shark Bay, we can look back to the effects of the most recent marine heatwave in the area. In 2011 Shark Bay was hit by a catastrophic marine heatwave that destroyed 900 square kilometres of seagrass – 36% of the total coverage.
This in turn harmed endangered species such as turtles, contributed to the temporary closure of the commercial crab and scallop fisheries, and released between 2 million and 9 million tonnes of carbon dioxide – equivalent to the annual emissions from 800,000 homes.
Some aspects of Shark Bay’s ecosystem have never been the same since. Many areas previously covered with large, temperate seagrasses are now bare, or have been colonised by small, tropical seagrasses, which do not provide the same habitat for animals. This mirrors the transition seen on bleached coral reefs, which are taken over by turf algae. We may be witnessing the beginning of Shark Bay’s transition from a sub-tropical to a tropical marine ecosystem.
This shift would jeopardise Shark Bay’s World Heritage values. Although stromatolites have survived for almost the entire history of life on Earth, they are still vulnerable to rapid environmental change. Monitoring changes in the microbial makeup of these communities could even serve as a canary in the coalmine for global ecosystem changes.
The neglected bay?
Despite Shark Bay’s significance, and the seriousness of the threats it faces, it has received less media and funding attention than many other high-profile Australian ecosystems. Since 2011, the Australian Research Council has funded 115 research projects on the Great Barrier Reef, and just nine for Shark Bay.
The World Heritage Committee has recognised that local efforts alone are no longer enough to save coral reefs, but this logic can be extended to other vulnerable marine ecosystems – including the World Heritage values of Shark Bay.
Safeguarding Shark Bay from climate change requires a coordinated research and management effort from government, local industry, academic institutions, not-for-profits and local Indigenous groups – before any irreversible ecosystem tipping points are reached. The need for such a strategic effort was obvious as long ago as the 2011 heatwave, but it hasn’t happened yet.
Due to the significant Aboriginal heritage in Shark Bay, including three language groups (Malgana, Nhanda and Yingkarta), it will be vital to incorporate Indigenous knowledge, so as to understand the potential social impacts.
And of course, any on-the-ground actions to protect Shark Bay need to be accompanied by dramatic reductions in greenhouse emissions. Without this, Shark Bay will be one of the many marine ecosystems to fundamentally change within our lifetimes.
Matthew Fraser, Postdoctoral Research Fellow, University of Western Australia; Ana Sequeira, ARC DECRA Fellow, University of Western Australia; Brendan Paul Burns, Senior Lecturer, UNSW; Diana Walker, Emeritus Professor, University of Western Australia; Jon C. Day, PSM, Post-career PhD candidate, ARC Centre of Excellence for Coral Reef Studies, James Cook University, and Scott Heron, Senior Lecturer, James Cook University
The Queensland Dragon Heath, or Dracophyllum sayeri, is a small, open-branched tree that grows up to 8 metres tall. It also looks decidedly as if someone has stuck pineapple plants or bromeliads on to the tips of its branches.
It has very specific habitat requirements, and is restricted to mountaintops where it receives high rainfall and misty conditions for at least 30 days of the year. The curved and pendulous leaves move in the slightest breeze, looking like they are dancing.
Walking towards Queensland Dragon Heath plants in the mist evokes a prehistoric feeling. I’m always subconsciously looking out and listening for approaching dinosaurs. One would think that the Dragon Heath plants, with their strap-shaped leaves, would be easy to spot in the vegetation. Not really: it is part of their camouflage on par with the stripes of tigers. When you are further than about 20 metres from a tiger in the bush, it melts into the vegetation.
For this reason, it is easier to spot the stems, with their flaky bark, than the bromelioid leaves of the Dragon Heath. It is of course another story searching for Dragon heaths in New Zealand. In the land of the long white cloud, Dragon Heath species vary from flat cushions, a mere centimetre high, to trees 18m tall, reminiscent of their Queensland counterpart.
It’s rather fun doing fieldwork there looking, at Dragon Heath in the hot sun at the foot of the mountains, then a few hours later trying to take notes with teeth chattering in blizzard conditions. Studying the New Zealand Dragon Heaths is definitely not for the faint-hearted.
The dragon family
The Queensland Dragon Heath belongs to the Ericaceae, a large family of 126 genera and about 4,260 species that grow everywhere from icy tundra to steamy tropical rainforests. Ericaceae includes heathers, rhododendrons, azaleas, and blueberries.
The Dragon Heath genus was first described by French biologist Jacques-Julien Houtou de Labillardière, based on a plant he collected in New Caledonia in April 1793. The leaves and stature of the plant reminded him of the dragon trees (Dracaena draco) of the Canary Islands; hence the name Dracophyllum. He described this plant in a book about his travels, aptly named Relation du voyage a la recherche de la Perouse, published in 1800.
Dragon Heaths grow across Australia, on the sub-Antarctic islands of New Zealand and New Caledonia. In Australia, they grow from Tasmania in the south to the tropical forests of Far North Queensland, as well as on Lord Howe Island.
Dragon Heaths vary widely, from tiny cushion plants 10mm tall (such as the cushion inka, Dracophyllum muscoides) to a much-branched tree 18m tall (the mountain neinei, D. traversii). The first DNA studies done on the genus Dracophyllum showed that they originated in Australia with two subsequent dispersals at least 16.5 million years ago, one to New Zealand and the other to New Caledonia.
The seeds of Dracophyllum are extremely light, similar to the dust-like seeds of orchids. They can travel very long distances by wind, making it easy to disperse to far-off places, especially during cyclones.
But the Queensland Dragon Heath is far more localised than its cousins. It is only known to grow on three mountaintops in Queensland (Mt Bellenden-Ker, Mt Bartle Frere, and the eastern slopes of Mt Spurgeon), all above 1,300m elevation. The species name D. sayeri is after the naturalist W.A. Sayer, who collected the type specimen in 1886 on Mt Bellenden-Ker, the second highest peak in Queensland.
It prefers to grow in fairly open low rainforest on mountain ridges where there is lots of air movement, and it can grow in thin layers of humus-rich granitic soils. Temperatures on the mountain peaks are normally low, with a maximum of 25℃ during the day and a minimum of 15℃ in the evenings.
A unique feature is the strap-shaped leaves that are parallel-veined, a character normally associated with monocots (lilies, grasses, sedges, and so on) rather than with dicots (plants with net-like veins).
The waxy light pink flowers are arranged in erect, loose-branching clusters. They produce ample nectar, which is popular with our feathered friends, the honeyeaters, which use their brushed-tip tongues to collect it.
Unfortunately the Queensland Dragon Heath is difficult to grow. It is really a plant for gardens in cooler climates and the chances of growing this plant is enhanced if the soil is inoculated with micorrhiza (fungal strands that exchange nutrients between their surroundings and their host plant) and the soil is kept moist. A thick layer of leaf mulch will keep the fine roots moist and cool.
I have succeeded in growing most of the New Zealand Dracophyllum species but had limited success with the alpine species. My next challenge is to grow the Queensland Dragon Heath successfully so that we can introduce this ancient-looking gem into horticulture.