Smoke emissions from the 2019-20 Australian bushfires enhanced cloud cover over the southeastern Pacific Ocean, cooling sea surface temperatures and influencing the La Niña event of 2020-22 that led to widespread flooding, according to a new model-based study.
Journal/conference: Science Advances
Link to research (DOI): 10.1126/sciadv.adg1213
Organisation/s: National Center for Atmospheric Research, USA
Funder: The efforts of J.T.F. in this work were supported by NASA awards 80NSSC17K0565 and 80NSSC22K0046 and by the Regional and Global Model Analysis (RGMA) component of the Earth and Environmental System Modeling Program of the U.S. Department of Energy’s Office of Biological and Environmental Research (BER) via National Science Foundation IA 1947282. N.R. is also supported by the RGMA via National Science Foundation IA 1947282. Competing interests: J.T.F. is a member of the ATOC department at the University of Colorado. The authors declare that they have no competing interests.
Emissions from the 2019-20 Australian wildfires may have influenced multi-year cooling in the Pacific
A new model-based study suggests that smoke emissions from the 2019-20 Australian wildfires enhanced cloud cover over the southeastern Pacific Ocean, cooling sea surface temperatures and influencing the La Niña event of 2020-22. The findings highlight widespread multi-year climate impacts caused by an unprecedented wildfire season, including its potential influence on El Niño-Southern Oscillation (ENSO) climate variability, John Fasullo and Nan Rosenbloom say. The most recent Australian bushfires, which peaked around January 2020, emitted particulate aerosols into the atmosphere on par with major volcanic eruptions. Recent studies suggest that emissions from volcanic eruptions may produce a climate response similar to the La Niña cooling phase of ENSO – namely, surface cooling and a northward shift of intertropical convergence zone (ITCZ) wind patterns in the tropical Pacific. After the Australian wildfires, cooler surface temperatures were observed across the tropical Pacific and overlapped with the onset of the unexpected 2020-22 La Niña event – but few studies have explored the extent to which wildfire emissions may have contributed to this cooling trend. To investigate, Fasullo and Rosenbloom ran ensemble models in Community Earth System Model version 2 with and without emissions as observed by satellites during the 2019-20 Australian wildfires. Each of the two ensembles started from identical initial states and explicitly represented interactions between aerosols and cloud formation. The researchers observed cooling primarily caused by an increase in stratocumulus cloud cover over the southeastern tropical Pacific, brought on by an abundance of condensation nuclei from wildfire emissions. Enhanced cloud cover caused widespread surface cooling and a northward shift of the ITCZ, which potentially affected the onset of La Niña in late 2020. “This analysis […] illustrates an interaction between biomass aerosol forcing and ENSO that may become more prevalent under climate change as wildfires are projected to intensify and become more frequent,” the authors warn. For reporters interested in trends, a 2020 study published in Science Advances used an ensemble model to show that northward shifts of the ITCZ could be associated with a connection between ENSO and aerosols from volcanic.
These comments have been collated by the Science Media Centre to provide a variety of expert perspectives on this issue. Feel free to use these quotes in your stories. Views expressed are the personal opinions of the experts named. They do not represent the views of the SMC or any other organisation unless specifically stated.
Professor Pete Strutton is from the Institute for Marine and Antarctic Studies at the University of Tasmania and the ARC Centre of Excellence for Climate Extremes
“This work is a really interesting example of how regional processes, such as wildfires, can have global implications, steering the course of large-scale climate modes like La Niña.
It’s also an example of the kinds of processes we need to be able to capture in computer models that will predict our future climate. We can only achieve this through continued advances in atmospheric, terrestrial and ocean observations, through improved models and the computational systems that run them.”
Dr Tom Mortlock is a Senior Analyst at Aon and Adjunct Fellow at the UNSW Climate Change Research Centre
“This new study suggests that the 2019-20 Black Summer bushfires, fanned by strong westerly winds blowing smoke across the Pacific, increased cloud cover and cooled ocean temperatures to such an extent as to influence the onset and duration of the rare triple-dip La Niña that we have just come out of.
A long line of research has shown that volcanic eruptions can affect La Niña through a similar process, but this is the first time a bushfire event has been widespread enough to have an impact in climate models. In fact, if we look further back in the past, we can see a relationship between periods of volcanism and tropical cyclone activity in Australia, because of the same causal process.
It further highlights the interconnectivity of the climate system, and this doesn’t stop over the ocean. The prolonged La Niña, above average rainfall, and absence of bushfires on the east coast has led to high fuel growth and preconditions the landscape for burning when El Niño comes along.
We know from looking at historical insurance loss data, that bushfire losses are correlated to periods of El Niño. There is now a 60% chance that El Niño will begin to form this winter, peaking in spring and summer.”