Slowing climate change could reverse drying in the subtropics


Kale Sniderman, University of Melbourne; Andrew King, University of Melbourne; Jon Woodhead, and Josephine Brown, Australian Bureau of Meteorology

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.




Read more:
‘Day Zero’: From Cape Town to São Paulo, large cities are facing water shortages


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.

Spatial plot of global rainfall projections for 2100 from IPCC AR5, showing percent change in annual rainfall for each °C of global warming, for the last two decades of the 21st century relative to 1986-2005. Subtropical regions, like the Mediterranean and southern Australia are projected to dry.
Modified from IPCC AR5 Ch. 12 Fig 12.10

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.

The drying trend in South-west Western Australia over the last century is significant.
BoM

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.

Smoothed global temperature and subtropical (25°S-35°S) winter (June through August) rainfall in Extended Representative Concentration Pathway (ECP) 4.5 and ECP8.5, from 2006 to 2300.
Author provided

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.




Read more:
The world’s tropical zone is expanding, and Australia should be worried


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.The Conversation

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

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Antarctica’s ‘moss forests’ are drying and dying



File 20180925 149958 iv4fh9.jpg?ixlib=rb 1.1
Lush moss beds in East Antarctica’s Windmill Islands.
Sharon Robinson, Author provided

Melinda Waterman, University of Wollongong; Johanna Turnbull, University of Wollongong, and Sharon Robinson, University of Wollongong

The lush moss beds that grow near East Antarctica’s coast are among the only plants that can withstand life on the frozen continent. But our new research shows that these slow-growing plants are changing at a far faster rate than anticipated.

We began monitoring plant ecosystems 18 years ago, near Australia’s Casey Station in the Windmill Islands, East Antarctica.

Casey Station is on East Antarctica’s coast. Click map to zoom.
Australian Antarctic Data Centre

As we report in Nature Climate Change today, within just 13 years we observed significant changes in the composition and health of these moss beds, due to the drying effects of weather changes prompted by damage to the ozone layer.

Living on the edge

Visitors to Antarctica expect to see a stark landscape of white and blue: ice, water, and sky. But in some places summer brings a surprisingly verdant green, as lush mosses emerge from under their winter snow blanket.

Because it contains the best moss beds on continental Antarctica, Casey Station is dubbed the Daintree of the Antarctic. Individual plants have been growing here for at least 100 years; fertilised by ancient penguin poo.




Read more:
Drones help scientists check the health of Antarctic mosses, revealing climate change clues


Antarctic mosses are extremophiles, the only plants that can survive the continent’s frigid winters. They live in a frozen desert where life-sustaining water is mostly locked up as ice, and they grow at a glacial pace – typically just 1 mm a year.

These mosses are home to tardigrades and other organisms, all of which survive harsh conditions by drying out and becoming dormant. When meltwater is available, mosses soak it up like a sponge and spring back to life.

The short summer growing season runs from December to March. Day temperatures finally rise above freezing, providing water from melting snow. Overnight temperatures drop below zero and mosses refreeze. Harsh, drying winds reach speeds of 200 km per hour. This is life on the edge.

Tough turf

When we first began monitoring the moss beds, they were dominated by Schistidium antarctici, a species found only in Antarctica. These areas were typically submerged through most of the summer, favouring the water-loving Schistidium. But as the area dries, two hardy, global species have encroached on Schistidium’s turf.

Like tree rings, mosses preserve a record of past climate in their shoots. From this we found nearly half of the mosses showed evidence of drying.

Healthy green moss has turned red or grey, indicating that plants are under stress and dying. This is due to the area drying because of colder summers and stronger winds. This increased desertification of East Antarctica is caused by both climate change and ozone depletion.

Moss beds, with moss in the foreground showing signs of stress.
Sharon Robinson, Author provided

Since the 1970s, man-made substances have thinned Earth’s protective sunscreen, the ozone layer, creating a hole that appears directly over Antarctica during the southern spring (September–November). This has dramatically affected the southern hemisphere’s climate. Westerly winds have moved closer to Antarctica and strengthened, shielding much of continental East Antarctica from global warming.

Our study shows that these effects are contributing to drying of East Antarctica, which is in turn altering plant communities and affecting the health of some native plant species. East Antarctica’s mosses can be viewed as sentinels for a rapidly drying coastal climate.

But there is good news. The ozone layer is slowly recovering as pollutants are phased out thanks to the 1987 Montreal Protocol. What is likely to happen to Antarctic coastal climates when ozone levels recover fully by the middle of this century?




Read more:
The ozone hole leaves a lasting impression on southern climate


Unlike other polar regions, East Antarctica has so far experienced little or no warming.

Antarctic ice-free areas are currently less than 1% of the continent but are predicted to expand over the coming century. Our research suggests that this may isolate moss beds from snow banks, which are their water reservoirs. Ironically, increased ice melt may be bad news for some Antarctic mosses.

East Antarctica is drying – first at the hands of ozone depletion, and then by climate change. How its native mosses fare in the future depends on how we control greenhouse gas emissions. But with decisive action and continued monitoring, we can hopefully preserve these fascinating ecosystems for the future.The Conversation

Melinda Waterman, Associate lecturer, University of Wollongong; Johanna Turnbull, Associate Lecturer in Biology, University of Wollongong, and Sharon Robinson, Professor, University of Wollongong

This article is republished from The Conversation under a Creative Commons license. Read the original article.