The heavy rains and floods in the north are actually related to the eruption of the Tonga volcano last year?

Last week, floods in North China, especially in Beijing, attracted the attention of the people across the country. Many people wanted to understand how Beijing, which was originally known for its dry weather, suddenly became a swamp.

Of course, people familiar with the climate of North China know that summer rainstorms are not uncommon here. But the intensity of this year's rainstorms can be said to be unprecedented. A weather station in Xingtai, Hebei Province recorded a total rainfall of 1,003 mm, equivalent to two years of rain in three days ; two weather stations in Beijing also recorded a total rainfall of more than 700 mm. Coupled with the high temperature that repeatedly broke 40℃ earlier, it can be said that extreme weather has been fully demonstrated in the past few months.

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Volcanic Eruptions and Extreme Weather

Why was this summer so extreme? The long-term root cause is of course climate change caused by human activities superimposed on natural changes, and this year's El Niño has exacerbated this trend. However, in addition to this, some researchers are also exploring the impact of a third factor, such as the Tonga submarine volcanic eruption in January 2022 .

Satellite image of Tonga volcanic eruption, source: Wikipedia

The first impression of a volcano is, of course, heat. But in fact, the heat directly emitted by ordinary volcanoes is very small and has no impact on the global climate. Climate scientists are concerned about another aspect of volcanoes: the gases it emits affect the atmosphere, thereby changing the level of heat absorbed by the Earth from the sun.

Normally, the biggest impact of volcanoes on the climate comes from sulfur. Volcanic eruptions throw a lot of material into the upper atmosphere, and materials like ash fall back to the ground within a few weeks, but sulfur dioxide and hydrogen sulfide are different. They react chemically in the stratosphere and become sulfuric acid in the form of aerosols. This sulfuric acid is too high up to fall directly to the ground as acid rain, but it will float in the stratosphere, surrounding the world, for years. This sulfuric acid has an extremely strong effect on reflecting sunlight, which greatly reduces the amount of heat the earth receives from the sun.

Therefore, under normal circumstances, the result of volcanic eruptions should be global cooling. There are some famous cases in history where volcanic eruptions have led to significant cooling, commonly known as " volcanic winter ". For example, the eruption of Mount Pinatubo in the Philippines in 1991 caused the maximum temperature in the Northern Hemisphere to drop by 0.5℃ in the next two years, while the eruption of an unknown volcano in 1808 and the eruption of Mount Tambora in Indonesia in 1815 caused the maximum temperature in the Northern Hemisphere to drop by 1.7℃ in 1816, resulting in the famous "year without summer". Climate models believe that the eruption of the Toba supervolcano 74,000 years ago may have caused a global cooling of 2.3℃~4.1℃, and may have greatly affected the survival of ancient humans at that time.

It is important to emphasize here that although volcanic eruptions can slow down the trend of global warming on paper, such violent disturbances can easily cause climate anomalies in local areas, which may intensify extreme weather , and their cooling effect is not long-lasting, so overall it may not be a good thing.

Next, let's talk about today's protagonist: On January 15, 2022, a strong volcanic eruption occurred on the seabed near Honga Tonga-Hunaha Apa'i Island in the Pacific island nation of Tonga. This may be the largest volcanic eruption so far in the 21st century. This volcano is a little different from normal: it spews very little sulfur, but an unusually large amount of water vapor.

Gas ejected from the Tonga volcano eruption. Image source: Wikimedia

Water vapor is also a gas that has a significant impact on the climate, but its effect is the opposite of sulfur, it does not cool down, but warms up. In fact, water vapor is the most abundant greenhouse gas in the atmosphere . However, most of the water vapor stays in the troposphere, and will turn into precipitation and fall back to the ground within a few days of rising, and it is difficult to climb higher - this is completely different from greenhouse gases such as carbon dioxide that can stay for decades or hundreds of years. Therefore, water vapor is usually not a direct driver of climate change. Only when other greenhouse gases change the characteristics of the atmosphere will water vapor increase the warming effect through feedback.

But this is only the usual case. The eruption of Tonga volcano directly threw a large amount of water vapor into the higher stratosphere, and the water vapor entering here is not so easy to fall back to the ground. It is estimated that this eruption emitted a total of 146 million tons of water into the stratosphere, directly increasing the water content of the global stratosphere by 10% to 15% . In contrast, it ejected only a pitiful 420,000 tons of sulfur oxides into the stratosphere, equivalent to 2% of Mount Pinatubo. This water will not directly lead to an increase in surface precipitation, but as a greenhouse gas, it can indirectly have various effects on the climate.

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When the Tonga volcano just erupted, the water vapor was concentrated near the eruption point. Now, more than a year has passed since the eruption, and the extra water vapor has spread to the world, especially in the mid- and high-latitude regions of the northern and southern hemispheres. Therefore, its impact was delayed for a year before it really manifested.

What impact do volcanic eruptions have on climate?

In early 2023, a paper in Nature Climate Change made a rough estimate of this. The researchers believe that the water and sulfur ejected from the Tonga volcano are dominated by water, causing the global average temperature to rise by 0.035°C. This number is not particularly large and can be considered within the normal fluctuation range of the climate system, but it is enough to increase the probability of an abnormally high average temperature of more than 1.5°C in the next five years by 7% .

However, this is only a rough global estimate, which does not take into account the specific spatial distribution of this water vapor, nor how it will affect the weather . If it is limited to a certain area, the impact it causes may be much greater than this number. Moreover, there are few precedents for such a sharp increase in stratospheric water content, and research is quite lacking. So we don't know how long the impact of this extra water vapor will last, perhaps years or even decades.

Of course, the research itself is about the relationship between Tonga volcanoes and global climate change itself. But if we take into account the changes that climate change has brought to us, including typhoons and floods, we may be able to better understand the huge risks that global climate change itself will bring to human society.

On a grand scale, the Tonga eruption was not a particularly large event. But as the baseline climate becomes increasingly extreme, fluctuations within the normal range will become more and more likely to have extreme consequences . Whatever the actual effects of this eruption, similar events in the future will almost certainly become more numerous and more severe.

References

[1] Tonga eruption increases chance of temporary surface temperature anomaly above 1.5 °C

[2] 10.1038/s41558-022-01568-2

[3] https://www.nature.com/articles/s41558-022-01568-2

Planning and production

Author: Fang Gang, a popular science author

Audit丨Xin Xin, Senior Engineer of China Meteorological Administration

Planning丨Xu Lai

Editor: Cui Yinghao

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