No returning to climate of the past even with CO2 reduction

While the entire world focuses on achieving carbon neutrality – zero carbon dioxide (CO₂) emissions – a new research shows climate change in some regions is inevitable even if the already increased CO₂ level is reduced. As CO₂ decreases, the intertropical convergence zone (ITCZ) shifts southwards, which can trigger persistent El Niño conditions. El Niño refers to a phenomenon in which the sea surface temperature near the equator rises by 1 to 3°C above its surroundings, causing droughts, storms, and floods around the world.

A POSTECH research team led by Professor Jong-Seong Kug and Ji-Hoon Oh (Division of Environmental Science and Engineering) conducted a simulation on the Earth system model that can ramp-up and ramp-down the concentration of atmospheric CO₂. The researchers observed that the ITCZ, which hardly moved when the CO₂ concentration increased, sharply shifted southwards when the CO₂ level decreased. Even when the CO₂ concentration was returned to its original level, its center still remained in the Southern Hemisphere.

The shift of the ITCZ, where 32% of the global precipitation occurs, is an extremely important factor in determining the amount of precipitation in the tropics and subtropics. The shift can change the Hadley circulation – the starting point of the global atmospheric circulation – to cause abnormalities in the global climate. Through this study, Professor Kug’s team has confirmed that as CO₂ begins to decrease, the ITCZ moves to the Southern Hemisphere which remains warm, unlike the Northern Hemisphere that cools down with CO₂ reduction.

Atmospheric CO₂ reduction could slowly return the average global temperature and precipitation to normal. However, the researchers claim that the climate may appear completely different in some regions. The change in precipitation due to the southwards shift of the ITCZ is very similar to the pattern during an extreme El Niño. In other words, it is expected that some regions will experience an abnormal climate condition where an extreme El Niño persists.

The model simulations confirmed that even if the increased CO₂ concentration is reduced and returned to its original value, the Sahel zone including the Sahara Desert and southern Europe around the Mediterranean Sea experienced a 20% decrease in average annual precipitation compared to the current levels, leading to further desertification. In contrast, North and South America had an increase in precipitation by about 15%. In fact, a risk of more frequent flooding was found in the western regions of North and South America where the increase in precipitation was noticeable. In East Asia, including the Korean Peninsula, a possibility of more rain during the monsoon season was found due to the increased precipitation in summer.

“It is impossible to properly reflect the complex climate system if only the average global temperature and precipitation levels are considered when creating mitigation policies to prevent climate change, such as carbon neutrality or carbon reduction,” explained Professor Jong-Seong Kug. Emphasizing that regional changes such as the southwards shift of the ITCZ should be fully taken into account, Professor Kug added, “The already emitted greenhouse gases have lasting effects on the planet so we need to recognize their long-term impacts as well as their immediate effect on climate change.”

Recently published in Nature Climate Change, this study was conducted with the support from the Irreversible Climate Change Research Center funded by the National Research Foundation of Korea.