Could you go back to the climate of the past with a reduction in CO2?
While the whole world is focused on achieving carbon neutrality (zero emissions of carbon dioxide, CO2), a New research shows that climate change in some regions is inevitable even if already high CO2 levels are reduced.
As CO2 decreases, the intertropical convergence zone (ZCIT, or ITCZ for its acronym in English) drifts south, which can trigger persistent El Niño conditions. El Niño refers to a phenomenon in which the temperature of the sea surface near the equator increases 1 to 3 ° C above its surroundings, causing droughts, storms and floods in some parts of the world.
ITCZ in a changing world of CO2: simulations
A research team of POSTECH led by Professor Jong-Seong Kug and Ji-Hoon Oh (Division of Environmental Sciences and Engineering) conducted a simulation in the Earth system model that can increase and decrease atmospheric CO2 concentration. The researchers observed that the ITCZ, which barely moved when the CO2 concentration increased, shifted sharply south when the CO2 level decreased. Even when the CO2 concentration returned to its original level, its center was still in the southern hemisphere.
The change in ITCZ, where 32% of global precipitation occurs, is an extremely important factor in determining the amount of precipitation in the tropics and subtropics. The change can change the Hadley circulation, the starting point of the global atmospheric circulation, to cause anomalies in the global climate.
Through this study, Professor Kug’s team has confirmed that as CO2 begins to decline, the ITCZ moves to the southern hemisphere, which remains hot, unlike the northern hemisphere which cools down with the reduction of CO2.
Reducing atmospheric CO2 could slowly return global mean temperature and precipitation to normal. However, the researchers claim that the climate can look completely different in some regions.
The change in precipitation due to the southward shift of the ITCZ is very similar to the pattern during an extreme El Niño. In other words, some regions are expected to experience abnormal weather conditions where an extreme El Niño persists.
Less CO2 would not improve things in some regions
Model simulations confirmed that even if the higher concentration of CO2 is reduced and returns to its original value, the Sahel area, including the Sahara desert and the southern Europe around the Mediterranean Sea, experienced a 20% decrease in average annual precipitation compared to current levels, leading to further desertification. In contrast, North and South America had an increase in rainfall of approximately 15%.
In fact, a more frequent flood risk was found in the western regions of North and South America, where the increase in rainfall was notable. In East Asia, including the Korean Peninsula, the possibility of more rainfall was found during the monsoon season due to increased rainfall in summer.
“It is impossible to adequately reflect the complex climate system if only 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 southward shift of the ITCZ, must be fully taken into account, Professor Kug added: “Greenhouse gases already emitted have lasting effects on the planet, so we must recognize its long-term impacts, as well as its immediate effects on climate change “.
Hysteresis of the intertropical convergence zone to CO2 forcing. Jong-Seong Kug, Ji-Hoon Oh, Soon-Il An, Sang-Wook Yeh, Seung-Ki Min, Seok-Woo Son, Jonghun Kam, Yoo-Geun Ham & Jongsoo Shin. Nature Climate Change (2021)