Research: 2023 Global Warming Spike Was Not Caused by CO2 but El Niño-Southern Oscillation

Model calculations and observations do not show global warming caused by greenhouse gases.

According to a recently published study by the University of Miami and Princeton University, last year's high temperatures were caused by a weather phenomenon known as El Niño, which followed the long-lasting La Niña.

Between 2022 and 2023, the global-mean surface temperature rapidly increased (0.29±0.04K). Such a large interannual increase in global-mean surface temperature (GMST) above 0.25K is not unprecedented. Global-mean surface temperature has been rising since 1850 and more rapidly since the mid-20th century. Observational analyses showed that GMST reached its highest recorded value in 2023, making it the warmest year on record. The rapid increase in 2023 by 0.29 ± 0.04K compared to 2022 (a change that typically occurs over 1-2 decades) has raised societal and scientific concerns and it is considered to be due to anthropogenic reasons, i.e. an increase in greenhouse gases in the atmosphere.

But alongside this, there has been some debate about whether natural causes, such as increased solar activity, volcanic-induced stratospheric water vapor increases and other natural phenomena, such as the El Niño-Southern Oscillation (ENSO), could be behind the sharp rise in temperatures. However, the latter, i.e. internal variability within the climate system, has not been addressed or taken into account in most studies to date. Thus, the Miami and Princeton study focused on it, resulting in the understanding that ENSO is actually the main cause of global warming spikes.

El Niño (the positive phase) or La Niña (the negative phase) occur in the tropical Pacific Ocean every few (usually 3-7) years. They affect global temperature, winds, sea level, atmosphere, clouds, moisture, and radiation. El Niño brings anomalous warmth with a delay, which increases the global-mean surface temperature. A long La Niña event is considered to be a situation where the Niño Index exceeds -0.5K for at least 18 consecutive months. 2023 was an El Niño event, but it was preceded by a La Niña in 2020-2022. The same was the case in the 1970s when a long La Niña (1973-1976) was followed by an El Niño. In other words, the abrupt warming in 2023 is not unprecedented, and the observational record shows a link between warming and El Niño-La Niña. Admittedly, given the short record of 74 years of observational data and two events, it is difficult to draw conclusions based on these alone. Therefore, all available global climate model simulations spanning multiple centuries to millennia, constructed without human influence, were included.

For one point, the authors of the study found that global warming spikes can happen without any human influence and that the La Niña-El Niño transition is 6 times more likely to be followed by global warming. However, the study did not aim to refute the existing literature on the impact of anthropogenic activity as a long-term cause of global warming. Rather, it highlighted the fact that the temperature records across years may be influenced by natural events, with internal factors playing a central role in the process.