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.

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Climate change is slowing Atlantic currents that help keep Europe warm



File 20180412 584 n22937.png?ixlib=rb 1.1

Natalie Renier/Woods Hole Oceanographic Institution, Author provided

Peter T. Spooner, UCL

The ocean currents that help warm the Atlantic coasts of Europe and North America have significantly slowed since the 1800s and are at their weakest in 1600 years, according to new research my colleagues and I have conducted. As we’ve set out in a new study in Nature, the weakening of this ocean circulation system may have begun naturally but is probably being continued by climate change related to greenhouse gas emissions.

This circulation is a key player in the Earth’s climate system and a large or abrupt slowdown could have global repercussions. It could cause sea levels on the US east coast to rise, alter European weather patterns or rain patterns more globally, and hurt marine wildlife.

We know that at the end of the last major ice age, rapid fluctuations in the circulation led to extreme climate shifts on a global scale. An exaggerated (but terrifying) example of such a sudden event was portrayed in the 2004 blockbuster film The Day After Tomorrow.

The recent weakening we have found was likely driven by warming in the north Atlantic and the addition of freshwater from increased rainfall and melting ice. It has been predicted many times but, until now, just how much weakening has already occurred has largely remained a mystery. The extent of the changes we have discovered comes as a surprise to many, including myself, and points to significant changes in the future.

The circulation system in question is known as the “Atlantic Meridional Overturning Circulation” (AMOC). The AMOC is like a giant conveyor belt of water. It transports warm, salty water to the north Atlantic where it gets very cold and sinks. Once in the deep ocean the water flows back southwards and then all around the world’s oceans. This conveyor belt is one of the most important transporters of heat in the climate system and includes the Gulf Stream, known for keeping western Europe warm.

Climate models have consistently predicted that the AMOC will slow down due to greenhouse gas warming and associated changes in the water cycle. Because of these predictions – and the possibility of abrupt climate changes – scientists have monitored the AMOC since 2004 with instruments strung out across the Atlantic at key locations. But to really test the model predictions and work out how climate change is affecting the conveyor we have needed much longer records.

Looking for patterns

To create these records, our research group – led by University College London’s Dr David Thornalley – used the idea that a change in the AMOC has a unique pattern of impact on the ocean. When the AMOC gets weaker, the north-eastern Atlantic Ocean cools and parts of the western Atlantic get warmer by a specific amount. We can look for this pattern in past records of ocean temperature to trace what the circulation was like in the past.

Another study in the same issue of Nature, led by researchers at the University of Potsdam in Germany, used historical observations of temperature to check the fingerprint. They found that the AMOC had reduced in strength by around 15% since 1950, pointing to the role of human-made greenhouse gas emissions as the primary cause.

In our paper, which also forms part of the EU ATLAS project, we found the same fingerprint. But instead of using historical observations we used our expertise in past climate research to go back much further in time. We did this by combining known records of the remains of tiny marine creatures found in deep-sea mud. Temperature can be worked out by looking at the amounts of different species and the chemical compositions of their skeletons.

We were also able to directly measure the past deep ocean current speeds by looking at the mud itself. Larger grains of mud imply faster currents, while smaller grains mean the currents were weaker. Both techniques point to a weakening of the AMOC since about 1850, again by about 15% to 20%. Importantly, the modern weakening is very different to anything seen over the last 1,600 years, pointing to a combination of natural and human drivers.

The difference in timing of the start of the AMOC weakening in the two studies will require more scientific attention. Despite this difference, both of the new studies raise important questions regarding whether climate models simulate the historical changes in ocean circulation, and whether we need to revisit some of our future projections.

The ConversationHowever, each additional long record makes it easier to evaluate how well the models simulate this key element of the climate system. In fact, evaluating models against these long records may be a crucial step if we hope to accurately predict possible extreme AMOC events and their climate impacts.

Peter T. Spooner, Research Associate in Paleoceanography, UCL

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

Amazon: Deforestation Continuing to Fall


The link below is to an article that examines deforestation in the Amazon, with a trend now developing showing that deforestation is slowing, but more more needs to be done.

For more visit:
http://www.guardian.co.uk/environment/2012/aug/03/amazon-deforestation-falls-again