A boundless lake appears in the desert, and the spring breeze blows through Yumen Pass?

Produced by: Science Popularization China

Author: Li Lingling (Jiangsu Second Normal University)

Producer: China Science Expo

Recently, CCTV News reported the rare spectacle of numerous lakes appearing in the Taklimakan Desert, which has once again sparked heated discussions on the topic of warming and humidification in the northwest. Some optimists are already imagining that the vast Gobi Desert in the northwest will return to the fertile Silk Road with the sound of bronze bells during the Han and Tang dynasties after being baptized by wind and sand for a long time.

Whether Northwest China really has such potential, we still need to clarify several issues:

(1) Is the climate in Northwest China really becoming warmer and wetter?

(2) Why is the Northwest region becoming warmer and wetter?

(3) What impacts will the warming and humidification of the northwest bring? Is it a good thing or a bad thing?

Before analyzing these issues in detail, let us first look at the millennial-scale changes in the climate of the Northwest region in historical periods.

(Photo source: CCTV News)

Millennial-scale climate change in Northwest China

When talking about the Northwest, many people may think of endless yellow sand and lonely smoke in the desert. Indeed, the Northwest is located in the heart of the Eurasian continent, far away from the ocean, and is one of the driest regions in the world at the same latitude.

However, the Northwest region has not always been dry in history. Over the past 2,000 years, the climate in the Northwest region has experienced two cold and dry periods and two warm and wet periods, which can be roughly divided into four stages, namely the cold and dry period from the Eastern Han Dynasty to the Northern and Southern Dynasties, the warm and wet period from the Sui, Tang and Song Dynasties, the cold and dry period from the Ming and Qing Dynasties, and the warm and wet period from 1800 AD to the present.

It can be seen that the configuration of temperature and precipitation is basically that warm periods correspond to wet periods, and cold periods correspond to dry periods. Does this mean that with global warming, the Northwest region is destined to develop in a warm and humid direction?

In fact, the relationship between temperature rise and precipitation caused by climate warming is far more complicated than we thought. Overall, over the past 100 years, the degree of drought in arid areas around the world has become increasingly severe, the area of ​​drought has continued to expand, and major drought events in arid areas have become more frequent.

The vast Sahara Desert

(Photo source: Veer Gallery)

For example, the area of ​​the Sahara Desert has expanded by more than 10% over the past century. Therefore, the matching relationship between temperature and precipitation requires specific analysis of specific regions. In addition, some scientists have proposed that temperature and precipitation have different response relationships on different time scales. In short time scales (interannual and interdecadal), it is a warm and dry match, while in longer time scales, it may be a warm and wet match.

So, what is the temperature and precipitation situation on a shorter time scale in Northwest China?

The Northwest region experienced a significant warm and dry period from the 1950s to the 1980s.

After the Little Ice Age in the Ming and Qing Dynasties, the climate in Northwest China became warmer and drier, especially from the 1950s to the mid-1980s. From 1920 to 1978, precipitation in northern Xinjiang was lower than the average level of the past 350 years. From 1950 to 1987, the runoff of 53 rivers in Northwest China showed a downward trend. The Tarim River dried up and the famous Lop Nur also dried up rapidly during this period, leaving behind a "sea of ​​death".

Not only that, drought also threatens the survival of the people in the Northwest. Between 1928 and 1930, a severe drought occurred in the Northwest, which was called "the worst drought in the history of the Northwest." It not only claimed the lives of millions of people, but was also one of the important reasons for the long-term poverty and backwardness of the Northwest society in the late modern period.

The desolate Lop Nur

(Photo source: Veer Gallery)

Since the mid-1980s, the Northwest region has experienced warming and humidification.

Since the 1980s, with the rapid warming, precipitation has increased in the western part of the Northwest, but the eastern part of the Northwest is still developing towards aridity. How are these east and west regions determined?

The main factor affecting the climate is the difference in wind belts. The water vapor in the air is mainly brought by wind from the ocean. The water vapor in the northwest and west is mainly brought by westerly winds from the Atlantic Ocean, the Black Sea, the Caspian Sea and the Mediterranean Sea, as well as the Norwegian Sea and the Barents Sea in the Arctic. The water vapor in the east comes from the southeast summer monsoon in the Pacific Ocean and the southwest summer monsoon in the Indian Ocean.

Therefore, the change in wind strength directly affects the amount of water vapor that can be input to the east and west of the northwest. Before the 21st century, precipitation showed a trend of increasing in the west and decreasing in the east, but since the 21st century, both the west and the east have entered a period of humidification, and after 1990, warming and humidification have become more intense, and the scope of humidification has also expanded eastward. Therefore, the western region has experienced a transition from a warm and dry climate to a warm and humid climate. This view has been widely recognized after more than a decade of research since it was proposed by Shi Yafeng and others in 2003.

Northwest region east-west division

(Image source: Reference 1)

Abnormal atmospheric circulation may be the main cause of humidification

As mentioned above, precipitation in the Northwest region is mainly affected by the westerly belt and the summer monsoon.

In the west, the westerly circulation has been strengthening since the mid-1980s, increasing the long-distance water vapor transport. In the east, the East Asian summer monsoon has also shown a significant strengthening trend, importing water vapor from the western Pacific Ocean into the northwest. At the same time, the South Asian summer monsoon has weakened, causing the Arabian Sea water vapor to be transported northward along the Qinghai-Tibet Plateau. This series of circulations work together to eventually lead to an increase in precipitation in the northwest.

High temperature and melting snow + frequent precipitation → forming "desert lakes"

In addition to precipitation brought by external water vapor input, western rivers have another important source of water, which is glacier meltwater.

Against the backdrop of global warming, warming in the Northwest region has never stopped. From 1961 to 2018, the average annual warming rate in the Northwest region was 0.30℃/10 years, while the national average was 0.23℃/10 years and the global average was 0.12℃/10 years.

Perhaps everyone does not have a very direct feeling about this number.

We can look at it from another angle. Along with the warming, the glaciers are melting rapidly. It is estimated that the area of ​​glaciers in China has decreased by an average of 7% in the past 40 years. The annual runoff of glaciers is equivalent to that of the Yellow River . The high temperature and snowmelt combined with frequent precipitation have caused super-warning floods in various tributaries of the Tarim River. After the super-warning floods in the basin, the overflowing water from the river channel formed several small "water reservoirs", forming a "desert lake" landscape.

As glaciers retreat, don’t sit back and enjoy the fruits of your labor

The short-term impacts of glacier melting and retreat are more beneficial than harmful: ice and snow melt earlier, the glacier melting period within the year increases, and the time for the formation of ice and snow meltwater runoff is advanced. In the short term, this will help alleviate the spring drought and water shortage in the basin and is beneficial to economic and social development.

However, the long-term impacts outweigh the benefits: the glaciers continue to shrink, the glacier meltwater resources drop sharply, and the cold island effect in the high mountains weakens, which is not conducive to the formation of precipitation, further forming a vicious cycle (the "cold island effect" refers specifically to the oasis and lake areas in arid areas on the earth, where the summer temperature is lower than that of nearby deserts and Gobi, and the humidity is higher than that of the surrounding areas.) On the other hand, with global warming, the snow formed in winter decreases, causing the glaciers to retreat further.

The formation and melting of glaciers are also important for agricultural production. In winter, agricultural water demand decreases, and precipitation is frozen into ice and stored, and then melts when water is needed for agricultural irrigation in spring. The current warming will make this balance increasingly difficult to maintain.

Extreme precipitation increases, deserts also flood

In the past 30 years, the humidity in Northwest China has increased significantly, accompanied by an increase in extreme precipitation, especially in arid areas. Arid areas, especially deserts, have high alkalinity in the soil, which is not easy to absorb water and has poor water retention capacity. Extreme precipitation combined with glacial meltwater is very likely to cause floods.

In fact, according to historical data, the probability of flooding in the Tarim River and its tributaries is not small because of the simultaneous occurrence of rain and heat. For towns near the river, floods will bring disaster. Investigations in the Tarim Basin have found many remains of civilizations that are suspected to have been destroyed by floods, so more water in the desert is not necessarily better.

According to investigations, since the mid-1980s, precipitation in Xinjiang has increased from March to October, extreme precipitation events have increased, and the frequency of rainstorms and floods has increased by 3.8 to 5.6 times.

Ten-year changes in extreme precipitation intensity in Northwest China

(Based on the average value from 1961 to 1970)

(Image source: Reference 3)

The overall benefits of warming and humidification outweigh the disadvantages

Some studies have shown that the precipitation base in the arid northwest region is low and the evaporation capacity is strong. The increase in precipitation is not enough to offset the increase in water loss caused by rising temperature and increased evaporation. For a long time, the soil moisture in most parts of the region has been gradually decreasing.

Therefore, although the Northwest region has experienced significant humidification, the increase in precipitation is still far from changing the overall climate pattern, and cannot fundamentally change the basic pattern of drought and water shortage and desert landscape in the Northwest region, with Xinjiang as the main body.

However, it is undeniable that warming and humidification will bring many benefits to the recovery of ecological vegetation and agriculture. In the short term, the impact of warming and humidification in the northwest will generally have more benefits than disadvantages.

Conclusion

From this perspective, the wish of "turning the desert into Jiangnan" will be difficult to achieve in the short term.

In the face of the warming and humidification of the Northwest, how to improve the efficiency of water resource utilization, speed up the greening transformation of desert areas, and increase the protection of glaciers are difficult problems that need to be solved. In addition, how will the warming and humidification of the Northwest develop in the future and whether it will change the arid and semi-arid climate pattern of the Northwest? At present, the scientific community has not yet reached a conclusion. Therefore, to a certain extent, the mystery of the warming and humidification of the Northwest has not been completely solved.

Impact of warming and humidification in Northwest China

(Image source: Reference 2)

Editor: Sun Chenyu

References:

[1] Zhang Qiang, Zhu Biao, Yang Jinhu, et al. New characteristics of climate humidification trends in Northwest China[J]. Chinese Science Bulletin, 2021.

[2] Zhang Qiang, Yang Jinhu, Wang Pengling, et al. Research progress and prospects of warming and humidifying climate in Northwest China[J]. Science Bulletin, 2022.

[3] Zhu Biao. Changing characteristics of water vapor, potential evapotranspiration, extreme temperature and precipitation under the background of warming and humidifying climate in Northwest China, PhD dissertation of Lanzhou University, 2022.

[4] Shi Yafeng, Shen Yongping, Li Dongliang, et al. Characteristics and trends of the transition from warm and dry to warm and humid climate in Northwest China[J]. Quaternary Sciences, 2003.

[5] Chen Ying, Ma Yu. Spatial and temporal variation characteristics of different levels of rainstorm and flood disasters in Xinjiang[J]. Arid Land Geography, 2021.