Last year, a remarkable April heat wave shattered all-time temperature records across Southeast Asia, prompting public health concerns, killing at least one elephant and making international headlines in the process. Now, scientists believe the event was driven by the combined influence of a strong El Niño event and human-caused climate change. And they say events like it will only become more common in the future.
A new study, published Tuesday in the journal Nature Communications, finds that the 2016 April extreme can be attributed about 49 percent to the influence of a severe El Niño event, which began in 2015 and lingered into the following year, with global warming accounting for another 29 percent, and the rest attributed to unknown factors. But the researchers note that the impact of global warming is catching up and may even become stronger than that of El Niño in the future.
"Basically, the global warming trend is going to overcome natural variability," said the study's lead author, Kaustubh Thirumalai, a postdoctoral scholar at the University of Texas at Austin's Institute for Geophysics.
April is typically the hottest month of the year in Southeast Asia, but over the past century its temperatures have been growing even more extreme. The 2016 event was the record-breaker, but extreme events like it have been occurring with increasing frequency for decades. In the new study, the researchers examined the influence of both El Niño and long-term climate change on nearly a century's worth of April extremes.
"We wanted to try to go into this business of 'attribution', in which people try to parse out the temperature differences that are caused because of natural variability or man-made anthropogenic variability," Thirumalai said. Scientists are growing increasingly interested in examining the extent to which human-caused global warming, vs. other climatic variables, is contributing to certain types of weather events, and how its influence is changing over time.
"I think these studies are very important in that they show how the relationships between these large-scale connected systems like El Niño and extreme events might change in a warming world," said Friederike Otto, deputy director of the University of Oxford's Environmental Change Institute and an expert on climate change and extreme weather events, who was not involved with the new study.
For the new study, the researchers used multiple historical data sets and modelled simulations to analyse April temperatures from the past century and their relationship to both El Niño years and the long-term progression of global warming. They found that just about all extreme Aprils occurred in El Niño years.
The reason likely has to do with El Niño's tendency to produce temporary drought-like conditions in Southeast Asia, the researchers suggest. Changes in the distribution of heat in the Pacific Ocean during El Niño years can alter atmospheric patterns throughout the region. The researchers note that post-Niño Aprils are typically associated with both reduced rainfall and cloud cover in mainland Southeast Asia.
The analysis also suggests that climate change has been making these April extremes more likely to occur over time. And, according to Thirumalai, it also tends to make these extremes even more intense than they would be otherwise.
"El Niño tends to prime this region for extremes, but global warming makes them worse," he said. 2016 is a prime example. It may still have qualified as an extreme event without the influence of climate change, but it's likely it would not have been as severe.
As it is, continued warming over time is only expected to increase the likelihood of these events in the future. According to Thirumalai, April extremes are occurring more and more frequently, and the analysis indicates that this is thanks to steady warming over the past few decades, an effect that will continue into the future.
Because of the strong connection between April temperatures and El Niño, the authors suggest that scientists may be able to better predict extreme events in advance through closer monitoring of factors, such as sea surface temperatures, which can help them see strong El Niño events coming. But Otto, the University of Oxford scientist, cautioned that this method might become less reliable as the planet continues to heat up and the influence of global warming on extreme Southeast Asian temperatures catches up to that of El Niño.
"In the past you needed El Niño to have extreme events like this," she said. "In the future this is not necessarily true."