Antinuclear

In research published today in Nature, we reconstructed Indian Ocean Dipole variability over the last millennium. We found “extreme positive” Indian Ocean Dipole events like last year’s are historically very rare, but becoming more common due to human-caused climate change. This is big news for a planet already struggling to contain global warming.

So what does this new side-effect of climate change mean for the future?

The Indian Ocean brings drought and flooding rain

First, let’s explore what a “positive” and “negative” Indian Ocean Dipole means.

During a “positive” Indian Ocean Dipole event, waters in the eastern Indian Ocean become cooler than normal, while waters in the western Indian Ocean become warmer than normal.

Warmer water causes rising warm, moist air, bringing intense rainfall and flooding to east Africa. At the same time, atmospheric moisture is reduced over the cool waters of the eastern Indian Ocean. This turns off one of Australia’s important rainfall sources.

In fact, over the past century, positive Indian Ocean Dipoles have led to the worst droughts and bushfires in southeast Australia.

The Indian Ocean Dipole also has a negative phase, which is important to bring drought-breaking rain to Australia. But the positive phase is much stronger and has more intense climate impacts.

We’ve experienced extreme positive Indian Ocean Dipole events before. Reliable instrumental records of the phenomenon began in 1958, and since then a string of very strong positive Indian Ocean Dipoles have occurred in 1961, 1994, 1997 and now 2019.

But this instrumental record is very short, and it’s tainted by the external influence of climate change.

This means it’s impossible to tell from instrumental records alone how extreme Indian Ocean Dipoles can be, and whether human-caused climate change is influencing the phenomenon.

Diving into the past with corals

To uncover just how the Indian Ocean Dipole has changed, we looked back through the last millennium using natural records: “cores” taken from nine coral skeletons (one modern, eight fossilised)……….

positive Indian Ocean Dipole events have been occurring more often in recent decades, and becoming more intense…….

climate change is causing the western side of the Indian Ocean to warm faster than in the east, making it easier for positive Indian Ocean Dipole events to establish.

In other words, drought-causing positive Indian Ocean Dipole events will become more frequent as our climate continues to warm.   In fact, climate model projections indicate extreme positive Indian Ocean Dipole events will occur three times more often this century than last, if high greenhouse gas emissions continue.

This means events like last year will almost certainly unfold again soon, and we’re upping the odds of even worse events that, through the fossil coral data, we now know are possible.

Knowing we haven’t yet seen the worst of the Indian Ocean Dipole is important in planning for future climate risks. Future extremes from the Indian Ocean will act on top of long-term warming, giving a double-whammy effect to their impacts in Australia, like the record-breaking heat and drought of 2019.

But perhaps most importantly, rapidly cutting greenhouse gas emissions will limit how often positive Indian Ocean Dipole events occur in future.https://theconversation.com/a-rare-natural-phenomenon-brings-severe-drought-to-australia-climate-change-is-making-it-more-common-133058

Blazes across the country in the past few weeks have been so intense they have generated their own weather. They create rising air mixed with ash and smoke that results in thunderstorm clouds above the fires called pyrocumulonimbus (pyroCbs).

Some of these are strong enough and rise high enough to have channelled smoke into the stratosphere, a plume of which has crossed the Atlantic Ocean in an eastward direction. NASA says this plume has now made a full circuit around the Earth. There were at least 20 pyroCbs between 28 and 31 December, and more on 4 January, some of which injected smoke into the stratosphere.

The scale of the smoke in the stratosphere has now been calculated by David Peterson at the US Naval Research Laboratory, who is presenting his preliminary findings to the American Meteorological Society at a meeting in Boston later today.

“It’s very likely on a volcanic scale,” he says. “The big thing here is really the impact that this is having on the stratosphere.” Although not of the scale of the 1991 eruption of Mount Pinatubo, the largest in modern history, the effect is similar to a more moderate eruption, Peterson says.

In 2017, Peterson found that Canadian wildfires put as much smoke as a volcano into the stratosphere. He is now working to apply the same technique to the Australian fires and thunderstorms. “At this point I can tell you that this event is one of the largest, it’s very near the top. I can’t say for sure if it’s the biggest,” he says, in terms of the amount of smoke injected into the stratosphere.

While it is well known that a volcanic eruption can put enough aerosols into the atmosphere to have a cooling effect, the different chemistry of pyroCbs means the impacts of the fires on global temperatures aren’t yet entirely clear.

They may have a warming or cooling effect, and it isn’t known how long the smoke will persist at heights of between around 10 and 50 kilometres high, which is roughly where the stratosphere starts and finishes. Peterson says the biggest question is what role proyCbs are playing in the climate system. Some of the smoke plumes are also getting high enough to affect the ozone layer.

We may have answers to some of these unknowns soon though, thanks to NASA flying a plane earlier this year through the upper level of a pyroCb generated by US wildfires. “It wasn’t as massive as these Australia plumes but fortunately at an altitude the aircraft could get to it,” says Peterson. The resulting direct observations of the chemistry will, along with satellite measurements, help unlock the answers.

Alan Robock at Rutgers University in New Jersey says any potential cooling effect from the bushfire smoke is unlikely to be huge at a global level, but could cause cooling of several degrees Celsius at a local level. If the Australian pyroCbs produce twice as much smoke as those from Canada in 2017, “it still would not be a large or long-lasting impact on climate,” he says.

However, the smoke can persist in the stratosphere for half a year or longer, as at such heights it can be heated by the sun and lofted even further up, prolonging its lifetime.

“This is the same process we have modelled in our studies of the climatic consequences of nuclear war in which much more smoke from burning cities and industrial areas would be lofted into the stratosphere and last for years,” says Robock. As such, analysis of the smoke from the bushfires could help improve simulations of the impact of nuclear Armageddon.

Our knowledge of pyroCbs is at an early stage. These thunderstorms and the smoke they put into the stratosphere have only been detectable via satellite instruments since the early 2000s, and previously were thought to be the result of volcanic eruptions, until analysis traced them back to wildfires.