Australian scientists have questioned the latest UN-backed global warming report, saying it underestimates the likelihood that major weather events caused by the Pacific will become more extreme as the planet warms.
A study published in the journal Nature Reviews a few days after the report of the Intergovernmental Panel on Climate Change was released this month showed that, according to the latest models, the frequency and severity of El Niño and La Niño events will increase this century.
The results also argue that events based on extreme sea surface temperatures have increased in frequency over the past 60 years compared to 1901-60, and large so-called the El Niño Southern Oscillation (ENSO) events since the mid-20th century have been larger than at any time in the past few centuries.
These findings, however, are not contradict the IPCC report, in which approvedthat there is “medium confidence” that “there is no model consensus on the systematic variation in the intensity of ENSO sea surface temperature fluctuations over the 21st century under any carbon emission scenario”.
IPCC verdict surprised the lead authors of the article, Wenzhu Tsai from CSIRO and Agus Santoso, climate researcher from the University of New South Wales. Although their work did not come out in January for the IPCC to review it for its report, it was based on model data, available to IPCC reviewers.
"Most models show sea surface temperature variability "as a factor in future extreme eventssaid Dr. Tsai. "I don't know how they missed it."
Dr. Santoso said: "This also surprises me a lot.", and "will begin to appear monstrous El Niños ".
Understanding how El Niño and La Niño will change in the face of warming will be vital. While these weather events occur in the Pacific, they have global impacts, ranging from droughts and bad bushfire seasons in Australia to changing rainfall patterns in California and the Brazilian Amazon.
The Nature Reviews article states that according to the latest models the frequency of El Niño events with extreme precipitation will double from about one event in 20 years in 1890-1990 to one in every decade in 1990-2090, even if the increase in global average temperature stabilizes at 1.5-2 degrees above pre-industrial level.
John Fyfe, lead author of the IPCC ENSO section and researcher at the Canadian Climate Modeling and Analysis Center, said the new work is "a very valuable contribution to understanding the projected changes in extreme sea-surface temperature and precipitation in the tropical Pacific."
However, Professor Fife noted that the magnitude and significance of projected changes may depend on the comparison period. The IPCC looked at 2081-2100 versus 1995-2014, while Nature compared the 21st century to the 20th. "For this reason, the two approaches have led to slightly different conclusions," he said.
Dr. Tsai, however, said that puzzling is the use of such a short period. "Should not use 20 years as a baseline"he said, noting that it can make a big difference which decade is used for comparison.
Dietmar Dommenget, Assistant Professor at the School of Earth's Atmosphere and the Environment, Monash University, said that the short comparison period of the IPCC is “problematic”. According to him, the period 1995-2014 was within two years on either side of the major El Niño or La Niño events, therefore a slight shift may lead to a different result.
However, Professor Dommenget noted that both the scientific article and the IPCC study found the effect of warming on precipitation patterns, even though there is great uncertainty about how sea surface temperature might affect the intensity of major weather events.
Professor Fife said that an increase in the amplitude of ENSO due to human activity has not yet been detected in the course of observations. Everything we're talking about here is about "event modeling," he said. "This is recognized in the IPCC report and in Dr. Kai's article."