Tropical thunderstorms are set to grow stronger as the world warms



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A supercell thunderstorm in the US state of Oklahoma.
Hamish Ramsay, Author provided

Martin Singh, Monash University

Thunderstorms are set to become more intense throughout the tropics and subtropics this century as a result of climate change, according to new research.

Thunderstorms are among nature’s most spectacular phenomena, producing lightning, heavy rainfall, and sometimes awe-inspiring cloud formations. But they also have a range of important impacts on humans and ecosystems.

For instance, lightning produced by thunderstorms is an important trigger for bushfires globally, while the hailstorm that hit Sydney in April 1999 remains Australia’s costliest ever natural disaster.


Read more: To understand how storms batter Australia, we need a fresh deluge of data


Given the damage caused by thunderstorms in Australia and around the world, it is important to ask whether they will grow in frequency and intensity as the planet warms.

Our main tools for answering such questions are global climate models – mathematical descriptions of the Earth system that attempt to account for the important physical processes governing the climate. But global climate models are not fine-scaled enough to simulate individual thunderstorms, which are typically only a few kilometres across.

But the models can tell us about the ingredients that increase or decrease the power of thunderstorms.

Brewing up a storm

Thunderstorms represent the dramatic release of energy stored in the atmosphere. One measure of this stored energy is called “convective available potential energy”, or CAPE. The higher the CAPE, the more energy is available to power updrafts in clouds. Fast updrafts move ice particles in the cold, upper regions of a thunderstorm rapidly upward and downward through the storm. This helps to separate negatively and positively charged particles in the cloud and eventually leads to lightning strikes.

To create thunderstorms that cause damaging wind or hail, often referred to as severe thunderstorms, a second factor is also required. This is called “vertical wind shear”, and it is a measure of the changes in wind speed and direction as you rise through the atmosphere. Vertical wind shear helps to organise thunderstorms so that their updrafts and downdrafts become physically separated. This prevents the downdraft from cutting off the energy source of the thunderstorm, allowing the storm to persist for longer.

By estimating the effect of climate change on these environmental properties, we can estimate the likely effects of climate change on severe thunderstorms.

Stormy forecast

My research, carried out with US colleagues and published today in Proceedings of the National Academy of Sciences, does just that. We examined changes in the energy available to thunderstorms across the tropics and subtropics in 12 global climate models under a “business as usual” scenario for greenhouse gas emissions.

In every model, days with high values of CAPE grew more frequent, and CAPE values rose in response to global warming. This was the case for almost every region of the tropics and subtropics.

These simulations predict that this century will bring a marked increase in the frequency of conditions that favour severe thunderstorms, unless greenhouse emissions can be significantly reduced.

Change in frequency (in days per year) of favourable conditions for severe thunderstorms for 2081-2100, compared with 1981-2000 averaged across 12 climate models under the RCP8.5 greenhouse-gas concentration scenario. Stippling indicates regions where 11 of the 12 models agree on the sign of the change.
CREDIT, Author provided

Previous studies have made similar predictions for severe thunderstorms in eastern Australia and the United States. But ours is the first to study the tropics and subtropics as a whole, a region that is characterised by some of the most powerful thunderstorms on Earth.

What drives the increased energy?

Different climate models, constructed by different research groups around the world, all agree that global warming will increase the energy available to thunderstorms – a prediction underlined by our new research. But we need to understand why this happens, so as to be sure that the effect is real and not a product of faulty model assumptions.

My colleagues and I previously proposed that high levels of CAPE can develop in the tropics as a result of the turbulent mixing that occurs when clouds draw in air from their surroundings. This mixing prevents the atmosphere from dissipating the available energy too quickly. Instead, the energy builds up for longer and is released in less frequent but more intense storms.

As the climate warms, the amount of water vapour required for cloud formation increases. This is the result of a well-known thermodynamic relationship called the Clausius-Clapeyron relation. In a warmer climate this means the difference in the humidity between the clouds and their surroundings becomes larger. As a result, the mixing mechanism becomes more efficient in building up the available energy. This, we argue, accounts for the increase in CAPE seen in our model simulations.

In our new study, we tested this idea in a global climate model by artificially increasing the strength of the mixing between clouds and their surroundings. As expected, this change produced a large increase in the energy available to thunderstorms in our model.


Read more: Australia faces a stormier future thanks to climate change


Another prediction of our hypothesis is that days with both high values of CAPE and heavy precipitation tend to occur when the atmosphere is least humid in its middle levels (at altitudes of a few kilometres). Using real data from weather balloons, we confirmed that this is the case across the tropics and subtropics.

What this means for future thunderstorms

The models predict that the energy available for thunderstorms will increase as the Earth warms. But how much more intense will storms actually become as a result?

The answer to that question is currently uncertain, and answering it is the next job for me, and other researchers around the world.

The ConversationBut it is clear that through our continued greenhouse gas emissions, we are increasing the fuel available to the strongest thunderstorms. Exactly how much stronger our future thunderstorms will ultimately become remains to be seen.

Martin Singh, Lecturer, School of Earth, Atmosphere and Environment, Monash University

This article was originally published on The Conversation. Read the original article.

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Banded stilts fly hundreds of kilometres to lay eggs that are over 50% of their body mass



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Banded Stilts feed on a range of invertebrates (including brine shrimp and snails) at saline wetlands across southern Australia.
Ben Parkhurst, Author provided

Reece Pedler, Deakin University; Andy T.D. Bennett, Deakin University, and Raoul Ribot, Deakin University

The hot, dry Australian desert may not come to mind as an ideal location for waterbirds to breed, but some species wait years for the opportunity to do just that.

New research has shed light on one of Australia’s most enigmatic birds, the banded stilt. This pigeon-sized shorebird has long been a source of intrigue due to its bizarre and extreme breeding behaviour. They fly hundreds or thousands of kilometres from coastal wetlands to lay eggs that are 50-80% of their body mass in normally dry inland desert salt lakes, such as Lake Eyre, on the rare occasions they are inundated by flooding rain.

Such behaviour has been a mystery for decades; described for the first time in 1930, just 30 breeding events had been documented for the entire species in the following 80 years.

To investigate this behaviour, and to assess the stilts’ conservation status, we began a study in 2011, during which I was based in outback South Australia, ready to jump into a small plane after every large desert rainfall. We also satellite-tagged nearly 60 banded stilts, using miniature solar powered devices around half the size of a matchbox.

Sixty banded stilts were tagged with solar-powered satellite trackers.
Author provided

This focused survey effort – which required overcoming the logistical challenges of very remote sites, knee-deep mud, heat and flies – has revealed major new insights into how banded stilts breed and the incredible distances they travel: we recorded one bird that flew 2,200km in just two nights.

Fast movers

The research revealed that, on average, banded stilts respond within eight days to unpredictable distant flooding of outback salt lakes. They leave their more predictable coastal habitat to travel 1,000-2,000km in overnight flights to arrive at the newly flooded lakes and take advantage of freshly hatched brine shrimp.

Brine shrimp eggs lie dormant in the lakes’ dry salt crust for years or decades between floods, but upon wetting they hatch in their billions, creating a “brine shrimp soup” – a rich but short-lived banquet for the nesting stilts.

Banded Stilt nests, with clutches of eggs representing over 50-80% of female body weight, litter an island in recently flooded Lake Ballard, in the Western Australian Goldfields 2014.
Lynn Pedler, Author provided

During the six-year study, we detected this nomadic movement and nesting behaviour seven times more often than it had been recorded in the previous 80 years. Although the banded stilts were previously thought to require large once-in-a-decade rains to initiate inland breeding, we found that small numbers of banded stilts respond to almost any salt lake inundation, arriving, mating and laying eggs equivalent of 50-80% of their body weight, despite high chances of the salt lake water drying before the eggs could hatch or chicks fledge.

Many times the eggs were abandoned as salt lake water dried. On other occasions some chicks survived long enough to learn to fly – although late-hatching chicks ran out of food or water and starved.

Once we found out that stilts needed much less rain to breed than previously thought, we used satellite imagery to reconstruct the past 30 years of flooding for ten salt lakes in South and Western Australia.

These models showed that conditions have been suitable for breeding more than twice as often as breeding events have actually been recorded. It seems that stilts’ nesting behaviour is so remote and hard to predict that scientists have been missing half the times it has happened.

Threats to banded stilt survival

Salt lakes in northwestern Australia are vital for banded stilts’ breeding. Our satellite tracking showed that birds from across the continent can reach these lakes after rain. Satellite images also suggested these lakes fill with water much more frequently than southern breeding sites.

These lakes are also largely free of native silver gulls (the common seagulls seen around our cities), which are predators of stilt chicks.

Silver Gulls fighting over a banded stilt chick on Lake Eyre. These gulls found in Australian cities also fly inland after rain and can decimate some Banded Stilt breeding attempts – eating thousands of eggs and chicks.
Reece Pedler, Author provided

But other southern Australian breeding lakes are dramatically affected by gull predation. In one instance, a colony of 9,500 pairs (around 30,000 eggs) had less than 5% of its chicks survive, despite abundant water and brine shrimp on offer. Observations made near the colony suggested that a chick was being eaten by gulls every two minutes. Nearly 900 chicks and 350 eggs were eaten in the 30 hours we watched the colony.

Unfortunately, even the lakes that are relatively gull-free are now under threat from human development, despite being in one of the most remote parts of the world. Lakes Disappointment, Mackay, Dora, Auld and others surrounding them in the Little Sandy and Great Sandy Deserts are the subject of plans for potash mining.

The most advanced plans relate to Lake Disappointment, where Reward Minerals plans to construct a series of drainage trenches and 4,000 hectares of evaporation ponds on the lake bed to harvest potash for use in fertilisers.

This action will create permanent brine pools in some parts of the lake, and prevent other areas from receiving any water. As surface water drains into evaporation ponds, it’s likely the first rains after a long dry spell will no longer prompt mass brine shrimp hatching. Without this brine shrimp “soup”, banded stilts cannot breed at the site.

A tiny island on Lake Torrens SA, covered by 70,000 Banded Stilt nests in 2010.
Paul Wainwright, Author provided

Meanwhile, the coastal habitat that supports banded stilt for the rest of the year is also changing. Sites that are home to thousands of birds, such as parts of the Dry Creek Saltfields and Bird Lake in South Australia, have been drained in the past two years.

If both the stilts’ inland breeding and coastal refuges are under threat, how can they survive?

Lessons for managing mobile species

This research offers insight into the conservation of highly mobile species, which may travel hundreds or thousands of kilometres in a year. Banded stilts are listed as vulnerable in South Australia, but have no conservation rating in the four other states in which they are found.

Individual banded stilts appear to operate over vast spatial scales, crossing between state jurisdictions in single overnight flights. Their episodic breeding events are hard to find and even more difficult to manage. Between breeding events, long-lived adults depend on refuges around the country which are being impacted by human activity, including potentially longer, harsher dry periods from climate change into the future.

These birds epitomise adaptation to unpredictable changes in their environment, but habitat loss and a warming climate may threaten them as much as any other species.


The ConversationThe authors would like to acknowledge L. Pedler, M. Christie, B. Parkhurst, R. West, C. Minton, I. Stewart, M. Weston, D. Paton, B. Buttemer and the South Australian Department for Environment, Water and Natural Resources, and Western Australian Department for Parks and Wildlife._

Reece Pedler, PhD student, Deakin University; Andy T.D. Bennett, Professor, Deakin University, and Raoul Ribot, Lecturer in Ecology, Deakin University

This article was originally published on The Conversation. Read the original article.