Australian scientists claim to have provided a clearer answer to a 20-year climate puzzle – but the finding will not be welcome news for farmers, policymakers or the wider public.
The El Nino-Southern Oscillation, which operates over the Pacific and is viewed as an engine room for driving variability in the world’s atmosphere, has long been studied to understand how it will be affected by global warming from greenhouse gases.
New work by the Centre for Australian Weather and Climate Research, jointly run by the Bureau of Meteorology and CSIRO, shows that the impact of El Nino years – marked by a relative warming of waters in the eastern Pacific and shifting rainfall patterns – will be exacerbated by climate change.
”There’s an intensification of changes in rainfall that are driven by El Nino,” Scott Power, research leader and a senior climate scientist at the bureau, said.
Using the latest climate models, the team found western regions of the Pacific, such as east Australia, will have worse droughts during El Nino years, while the eastern Pacific will experience heavier rains. ”What we found was those two effects are intensified in the future, because global warming interferes with the impact El Nino has,” Dr Power said, citing peer-reviewed research to be published on Monday in the journal Nature.
Wenju Cai, a senior CSIRO research scientist, said the latest study showed a stronger agreement than earlier climate models.
The results may also show that changes in La Nina years – when western Pacific waters are relatively warm – could bring more extreme conditions to eastern Australia, he said. ”During an El Nino period, the drought in the western Pacific could be more intense, or during La Nina [ones], the floods could be more intense or maybe both,” Dr Cai said.
While separating the impact of greenhouse gas-induced global warming from natural variability can be difficult, human influence on El Nino patterns becomes clear from the latter part of the 21st century, Dr Power said.
”For these sorts of changes, the signal becomes larger as time goes on under the scenarios we’ve used,” he said.
Most of the hottest years globally and for Australia have been El Nino-dominant years, such as in 1997-98.
“When the world tends to warm up because of El Nino, so does Australia,” Dr Power said. “That’s because we tend to get less rainfall, so it dries out and clouds clear, we [then] get more radiation hitting the surface, less evaporation to moderate things, and temperatures go up.”
While more research is under way on El Nino periods – such as whether their frequency will change from the current three to eight years – climatologists have been surprised at the unusual heat recorded in Australia over the past year even though El Nino-Southern Oscillation conditions have remained neutral.
“It is sobering to see that we’re setting these records in non-El Nino years,” Dr Power said.
Record warm sea-surface temperatures and extended periods of heat over central Australia have put the country well on course to record its hottest calendar year.
A record hot summer and winter has also extended into September, which notched the biggest monthly departure ever from long-term averages for heat.
Dr Power, a co-ordinating lead author in the latest Intergovernmental Panel on Climate Change report, said global temperatures are likely to rise 0.3-0.7 degrees over 2016-2035, compared with the past 20-year period.
“The magnitude of the changes [on future El Nino periods] will critically depend on the amount of emissions that the world ultimately ends up producing over coming decades,” he said.
Read article at the Sydney Morning Herald