Why climate change will dull autumn leaf displays



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Autumnal displays may be dimmed in the future.
Shutterstock

Matthew Brookhouse, Australian National University

Every autumn we are treated to one of nature’s finest seasonal annual transitions: leaf colour change and fall.

Most of the autumn leaf-shedding trees in Australia are not native, and some are declared weeds. Nevertheless, Australia has a spectacular display of trees, from the buttery tresses of Ginkgo biloba to the translucent oaks, elms and maples.

Autumn colour changes are celebrated worldwide and, when the time is right, autumn leaves reconnect us to nature, driving “leaf-peeping” tourist economies worldwide.

However, recent temperature trends and extremes have changed the growing conditions experienced by trees and are placing autumn displays, such as Canberra’s, at risk.

Autumn leaf colour changes and fall are affected by summer temperatures.
Shutterstock

This year, Canberra, like the rest of Australia, endured its hottest summer on record. In NSW and the ACT, the mean temperature in January was 6°C warmer than the long-term average. So far, autumn is following suit.

These extremes can interrupt the ideal synchronisation of seasonal changes in temperature and day length, subduing leaf colours.

In addition, hotter summer temperatures scorch leaves and, when combined with this and the previous years’ low autumn rainfall, cause trees to shed leaves prematurely, dulling their autumn leaf displays.




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The subtlety of change

We learnt in childhood autumn colour change follows the arrival of cooler temperatures. Later we learnt the specifics: seasonal changes in day length and temperature drive the depletion of green chlorophyll in leaves. Temperature can also affect the rate at which it fades.

In the absence of chlorophyll, yellows and oranges generated by antioxidants in the leaf (carotenoids) as well as red through to purples pigments (anthocyanins), synthesised from stored sugars, emerge. Temperature plays a role here too – intensifying colours as overnight temperatures fall.

We’ve also come to understand the role of a leaf’s environment. Anthocyanin production is affected by light intensity, which explains why sunny autumns produce such rich colours and why the canopies of our favourite trees blush red at their edges while glowing golden in their interior.

However, early signs show this year’s autumn tones will be muted. After the record-breaking heat of summer and prolonged heat of March, many trees are shrouded in scorched, faded canopies. The ground is littered with blackened leaves.

Of course, we’ve seen it before.

During the Millennium Drought, urban trees sporadically shed their leaves often without a hint of colour change. Fortunately, that was reversed at the drought’s end.

But we’re kidding ourselves if we believe this last summer was normal or recent temperature trends are just natural variability. If this is a sign of seasons future, we need to prepare to lose some of autumn’s beauty.




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Lost synchronicity

Long-term and experimental data show that the sensitivity of autumn colour change to warmer temperatures varies widely between species. While large-scale meta-analyses point to a delay in the arrival of autumn colours of one day per degree of warming, individual genera may be far more sensitve. Colour change in Fagus is delayed by 6-8 days per degree.

Warming temperatures, then, mean the cohesive leaf-colour changes we’re accustomed to will break down at landscape scales.

In addition, as warm weather extends the growing season and deep-rooted trees deplete soil moisture reservoirs, individual trees are driven by stress rather than seasonal temperature change and cut their losses. They shed leaves at the peripheries of their canopies.

The remainder wait – bronzed by summer, but still mostly green – for the right environmental cue.

For years, careful species selection and selective breeding enhanced autumn colour displays. This rich tapestry is now unravelling as hotter summers, longer autumns and drought affect each species differently.




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Paradoxes and indirect effects

It seems logical warmer temperatures would mean shorter and less severe frost seasons. Paradoxically, observations suggest otherwise – the arrival of frost is unchanged or, worse, occurring earlier.

When not preceded by gradually cooling overnight temperatures, frosts can induce sudden, unceremonious leaf loss. If warm autumn temperatures fail to initiate colour change, autumn displays can be short-circuited entirely.

At the centre of many urban-tree plantings, our long association with elms faces a threat. Loved for the contrast their clear yellow seasonal display creates against pale autumn skies, elm canopies have been ravaged by leaf beetles this year. Stress has made trees susceptible to leaf-eating insects, and our current season delivered an expanse of stressed, and now skeletal, trees.

Autumn leaf displays drive tourism.
Norm Hanson/flickr, CC BY-NC-SA



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Change everywhere?

This dulled image of autumn is far from universal. Climates differ between locations. So too will the climate changes we’ve engineered and their impact on autumn displays.

Increased concentration of anthocyanins associated with warmer summers has, for example, created spectacular leaf displays in Britain’s cooler climates.

Of course, we’ll continue to experience radiant autumn displays too.

In years of plentiful rain, our trees will retain their canopies and then, in the clear skies of autumn, dazzle us with seasonal celebrations. However, that too may be tempered by the increased risk of colour-sapping pathogens, such as poplar rust, favoured by warm, moist conditions. And there are also negative consequences for autumn colour associated with elevated carbon dioxide concentrations.

Of course, we need to keep it in perspective – the dulling of autumn’s luminescence is far from the worst climate change impacts. Nevetheless, in weakening our link with nature, the human psyche is suffering another self-inflicted cut as collective action on climate change stalls.The Conversation

Matthew Brookhouse, Senior lecturer, Australian National University

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

2018-19 was Australia’s hottest summer on record, with a warm autumn likely too


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Dirty water from Queensland’s historic flooding, triggered by weeks of exceptional monsoon rains earlier in the year.
NASA Worldview/EPA

David Jones, Australian Bureau of Meteorology; Lynette Bettio, Australian Bureau of Meteorology, and Skie Tobin, Australian Bureau of Meteorology

Australian summers are getting hotter. Today marks the end of our warmest summer on record, setting new national temperature records. Worsening drought, locally significant flooding, damaging bushfires, and heatwaves capped a summer of extremes.

As we look to autumn, warmer temperatures overall and below average rainfall – especially in eastern parts of the country – are likely.

Australian summer mean temperature anomalies against the 1961–1990 average.
Bureau of Meteorology



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Very hot…

The starkest feature of this summer was the record warmth. The national average temperature is expected to be about 2.1℃ above average, and will easily beat the previous record high set in summer 2012-13 (which was 1.28℃ warmer than average).

Very low rainfall accompanied the record heat of summer. At the national scale, each month was notably dry, and total summer rainfall was around 30% below average; the lowest for summer since 1982–83. The monsoon onset was delayed in Darwin until the 23rd of January (the latest since 1972–73) and typical monsoonal weather was absent for most of summer.

Preliminary summer 2018–19 mean temperature deciles.
Bureau of Meteorology

In December 2018 Australia saw its highest mean, maximum and minimum temperatures on record (monthly averages, compared to all other Decembers). Notable heatwaves affected the north of Australia at the start of the month, spreading to the west and south during the second half of December. Temperatures peaked at 49.3℃ at Marble Bar in Western Australia on the 27th, with mid-to-high 40s extending over larger areas.

The heat continued into January, which set a national monthly mean temperature record at 2.91℃ above the 1961–1990 average. Heatwave conditions which had emerged in December persisted, with a prolonged warm spell and numerous records set. Eight of the ten hottest days for the nation occurred during the month, while a minimum temperature of 36.6℃ at Wanaaring (Borrona Downs) in western New South Wales on the 26th set a new national minimum temperature record.

Temperatures moderated a little in the east of the country for February, partly in response to flooding rainfall in tropical Queensland. Even so, the national mean temperature will come in around 1.4℃ above average, making this February likely to be the fourth or fifth warmest on record.

…and very dry

Australia has seen dry summers before and many of these have been notably hot. The summers of 1972–73 and 1982–83 – which featured mean temperatures 0.90℃ and 0.92℃ above average, respectively – both came during the latter stages of significant droughts, and were both records at the time.

As the State of the Climate 2018 report outlines, Australia has warmed by just over 1℃ since 1910, with most warming occurring since 1950. This warming means global and Australian climate variability sits on top of a higher average temperature, which explains why 2018-19 was warmer again.

A major rain event affected tropical Queensland during late January to early February, associated with a slow-moving monsoonal low. Some sites had a year’s worth of rain in a two-week period, including Townsville Airport which had 1,257mm in ten days. Many Queenslanders affected by this monsoonal low went from drought conditions to floods in a matter of days. Flooding was severe and continues to affect rivers near the Gulf of Carpentaria, as well as some inland rivers which flow towards Kati Thanda–Lake Eyre.

Preliminary summer 2018–19 rainfall deciles.
Bureau of Meteorology

The outlook for autumn

Spring 2018 saw a positive Indian Ocean Dipole which faded in early summer. At the start of summer sea surface temperature anomalies in the central Pacific exceeded 0.8℃, which is the typical threshold for El Niño affecting the oceans, but these declined as summer progressed. Combined with a lack of coupling between the atmosphere and ocean, the El Niño–Southern Oscillation remained neutral, though normal rainfall patterns shifted to oceans to the north and east, leaving Australia drier as a result.

As we move into autumn, the El Niño–Southern Oscillation and Indian Ocean Dipole tend to have less influence at this time of year. The onset of new Indian Ocean Dipole or El Niño/La Niña events typically happens in late autumn or winter/spring.

Over recent years, autumn rainfall has also become less reliable, with declines in cool season rainfall in the southeast and southwest. Temperatures are also rising, in a local expression of the global warming trend.




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The Bureau’s outlook for autumn shows high probabilities that day and night-time temperatures will remain above average for most of the country. We expect to see continued below-average rainfall in much of the east, where drought is currently widespread.

Looking to the winter, the Bureau’s ENSO Wrap-Up suggests the Pacific Ocean is likely to remain warmer than average. The potential for an El Niño remains, with approximately a 50% chance of El Niño developing during the southern hemisphere autumn or winter, twice the normal likelihood.

Rainfall outlook for autumn 2019.
Bureau of Meteorology


For more information watch BOM’s March–May 2019 Climate and Water Outlook video and subscribe to receive Climate Information emails.The Conversation

David Jones, Climate Scientist, Australian Bureau of Meteorology; Lynette Bettio, Senior Climatologist, Australian Bureau of Meteorology, and Skie Tobin, Climatologist, Australian Bureau of Meteorology

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

Australia simmers through hottest autumn on record


Andrew King, University of Melbourne

It’s the same old story: with 2016 on track to become the hottest year on record globally, and record-breaking heat already evident around the world, Australia has just experienced its hottest autumn on record.

Figures from the Bureau of Meteorology indicate Australia has experienced its hottest autumn on record.
Bureau of Meteorology

The Bureau of Meteorology has reported that for average temperatures across Australia, this has been the hottest March-May period ever recorded – beating the previous record, set in 2005, by more than 0.2℃.

Within this period, March was also the hottest on record, while April and May were each the second-warmest in a series extending back to 1910.

Temperatures were well above average across much of the country, especially in the east.
Bureau of Meteorology

Why so hot?

El Niño events tend to cause warmer weather across the east and north of Australia and the major El Niño of 2015-16 undoubtedly contributed to the extreme temperatures experienced across these areas.

However, climate change also played a significant role in our warmest autumn. Previous work, led by ANU climatologist Sophie Lewis, indicates that the human influence on the climate has made a record-breakingly hot autumn roughly 20 times more likely.

In other words, without climate change we would be much less likely to experience autumns as warm as this one has been in Australia.

How we’ll remember autumn 2016

In the past few months, Australia has seen many extreme hot weather events. Melbourne experienced its warmest March night on record, while Sydney had a run of 39 days with daytime highs above 26℃, as the summer heat continued long into March.

But it’s the coral bleaching event on the Great Barrier Reef that will likely linger in our memories the longest. Some 93% of the reef was found to be affected by bleaching and recent surveys have revealed that more than one-third of coral in the northern and central parts of the reef have died.

Without climate change, a bleaching event like this would be virtually impossible.

The extreme heat over Australia this autumn and the associated damage to the reef are also having an effect on the election campaign. As public concern over the future of the reef grows, the parties are being asked to defend their climate change policies.

Both major parties have made election commitments to the reef, with the Coalition announcing an extra A$6 million to tackle crown-of-thorns starfish (adding to a further A$171 million committed under the 2016 budget), and Labor an extra A$377 million over five years (A$500 million in total). While both Labor and the Coalition aim to improve water quality in the reef through their policies, the coral bleaching and death this year is linked with warm seas.

Whether we’ll be able to save parts of the reef largely depends on whether we reduce our greenhouse gas emissions and manage to prevent the rising trend in temperatures from continuing.

The Conversation

Andrew King, Climate Extremes Research Fellow, University of Melbourne

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