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State of the Climate: Thank goodness for ocean sinks currently holding more warming extremes at bay

Kate Doyle, Thursday December 20, 2018 - 08:05 EDT
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Trends in sea surface temperatures from 1950 to 2017. - ABC

The Bureau of Meteorology (BOM) and CSIRO's joint biennial and it is not the kind of report card you would want to take home to your parents just before Christmas.



An extra two years has firmed-up the data to demonstrate that climate change is happening now.

Dr Helen Cleugh, the director of the climate science centre at CSIRO, said the last time the planet saw levels of CO2 this high was at least 800,000 years ago.

She said atmospheric CO2 is up 46 per cent since before the industrial era began in the 1750s.

"We know from our analysis that the cause of the increases in CO2 concentration is human activities, through burning of fossil fuels and through land use change," Dr Cleugh said.



Ocean sinks

That CO2 is not just staying in the atmosphere.

"As a result of the increased greenhouse gases in the atmosphere we've actually got more energy in the Earth's climate system, and it turns out that over 90 per cent of that extra energy has actually been taken up by the ocean," Dr Cleugh said.

"Our oceans and land are performing an enormous ecosystem service at the moment because they're taking up a lot of the anthropocentric [human-generated] CO2 emissions."

The oceans take up the CO2 directly, removing it from the atmosphere, as well as absorbing heat from the atmosphere. The land also acts as a sink but to a lesser extent.

"That has two really important implications. The first is that it means that the oceans play a really important role in modulating the rate and pace of our changing climate. But the other is it leads to warming," Dr Cleugh said.

"A very live research question right now is will those oceans and land continue to take up CO2 into the future.

"At the moment we're not seeing any evidence of the weakening of that sink."



But Dr Cleugh said that models of our future climate suggest that the extra CO2 and heat would not be able to be taken up by the ocean forever.

A bit like sweeping dust under a rug, eventually only so much can fit.

"There are feedbacks that could lead to a weakening of those sinks, either on the land or in the ocean, and that would mean that warming in the atmosphere would proceed at a greater rate," she said.

Dr Cleugh said it is a very important scientific question to understand the way that the oceans are behaving.

"It turns out the Southern Hemisphere oceans are particularly important in taking out heat and CO2. So it's really important that we do that research in our own patch," she said.

Oceans already feeling the heat

Ocean temperatures, already up by around 1 degree Celsius since 1910, has contributed to more and longer marine heatwaves.

The back-to-back bleaching of the Great Barrier Reef in 2016 and 2017 have been well canvased, but the changing ocean is meddling with other ecosystems.

The report states that the Eastern Australian Current — of Finding Nemo fame — is extending further south, encouraging warming in the Tasman Sea and extending the habitat of other species south.

As the ocean warms it is expanding, which is coupling with ice melts to raise sea levels.



The increased CO2 in the water has also lead to a 30 per cent increase in ocean acidity since the late 1800s.

"This has significant implications for our marine ecosystem and the ability of corals to regrow, so it actually is linked back to the coral bleaching," Dr Cleugh said.

These changes are not happening evenly. Luckily for the Great Barrier Reef, so far it looks like the worst of the ocean warming acidification has happened to the south of Australia.

Fire danger and rain

Back up on the land and the warming atmosphere has changed rainfall patterns across Australia.

Dr Karl Braganza, manager of climate monitoring at the Bureau of Meteorology, said beyond background variability that naturally occurs in Australia the strongest rainfall change that has emerged is drying over the southern parts of the continent.

"Most significantly in the south-west of the continent. Since about 1970 we're seen a reduction in rainfall of around 20 per cent and as much as 26 per cent in some locations," Dr Braganza said.

He said the south-east of the continent has been drying too, but at 11 per cent since 1970.

"We believe that it's due to the same overall mechanism, which is a shift in circulation," Dr Braganza said.

"As the planet has warmed, we've pushed the cold fronts and cut off lows that bring this rainfall to the southern parts of the continent, further south."

According to Dr Braganza, the south-west has been drying at a greater rate because it is more dependent on those southerly systems coming through.

In the east there are more instances of factors like north-west cloud bands bring moisture from the tropics down to the south-east, as well as alternative sources of moisture like east coast lows.



All this heating and drying is leading to increased fire danger.

"What we can see is a clear shift towards a lengthened fire season, more fire weather during that season, and the severity of the fire weather has become more severe," Dr Braganza said.

He said it was significantly apparent over the fire-prone parts of the country in southern Queensland, New South Wales, Victoria, South Australia, south-west WA and Tasmania.

'Compound extreme events'

These trends have been consistent since the 1950s and it is not happening in isolation.

For the first time, the State of the Climate report draws attention to "compound extreme events", or when multiple variables culminate or coincide together.



"Fire weather is a good example of that. Often the worst fire weather occurs when you've had long term droughts, long term above average temperatures, maybe a short term heat wave, and then the meteorology that's consistent with severe fire weather and the ability for fire to spread," Dr Braganza said.

In some regions the trend towards increasing high fire danger days is combining with conditions that allow bushfires to generate thunderstorms which can lead to extremely dangerous fire conditions and can start more fires through dry lightning.

Examples include the Black Saturday fires of 2009 and the Canberra fires of 2003.

"It's those compound events that are going to be most challenging going forward in terms of adapting to climate change in Australia," Dr Braganza said.

But it is not drier everywhere. The report said rainfall during the northern wet season has been very much above average for the last twenty years.

"It's very likely that is, in part, driven by the increase in sea surface temperatures," Dr Braganza said.

Extreme rainfall events are also becoming more intense.

As the atmosphere warms, its capacity to hold water increases.

Rainfall is expected to increase by around 7 per cent per degree of warming and the statement said that observations in Australia generally show a larger than 7 per cent increase.

What about the future?

This Christmas carol is not going to have a redemption arc by morning.

"It takes some time to warm the oceans and that's where a lot of the energy and the enhanced greenhouse effect is going," Dr Braganza said.



"As the oceans continue to warm due to the emissions that we've historically put in the atmosphere, we can pretty much expect a commitment of further warming out to 2030 or 2040 by as much as another half a degree."

Dr Cleugh said we will see global ocean temperatures and ocean acidification continuing to rise because we have got a lot of the energy already locked in from past emissions.

"The temperature will increase and that will continue to contribute to the increasing number of hot days, heat extremes, and fewer cold days," she said.

"We expect tropical cyclones to decrease in number, but potentially increase in intensity.

"Extreme rainfall events are likely to become more intense.

"The harsher fire weather, particularly in southern and eastern Australia, will continue into the future and the trends in rainfall are likely to continue."


- ABC

© ABC 2018

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