The climate on Earth from tropical to cold generally follows the characteristics of a latitudinal gradient from the equator to the poles. However, in the same latitude zone, there are still differences in the received solar radiation, even if the sun can shine for the same time. In addition, other factors, such as atmospheric circulation, land and sea location, topography, land cover, and human activities, may affect regional climate to varying degrees. Under the current global climate change background and human activity interference, it is of great significance to study the climate differences in the same latitude of the earth and whether the temperature and precipitation in typical areas will change significantly under a certain climate scenario mode.
Nanling is a rich mountain range, which embraces countless precious animals and plants, creating the largest "oasis" in the desert belt at the same latitude. Nanling is a magical mountain range that has undergone vicissitudes of life and formed many beautiful and peculiar geological landscapes (Zhou Ping, 2018). The Nanling Mountains have typical geology, geomorphological features and biodiversity, as well as the "Nanling Corridor" with human characteristics, which can be used as a base for comprehensive research with high potential value.
▲ Distribution map of the barrier zone in the Nanling Mountains
One of the natural geographical characteristics of Nanling is reflected in its function as a regional demarcation line and ecological barrier zone, which blocks the cold wave from the north and moves southward, and the warm and humid airflow from the south moves northward, which makes the difference in precipitation and temperature between the north and south sides of Nanling a large one, and it has become a better area for studying global climate change and mountain ecosystem response.
The typical areas from west to east in the same latitude zone of the Nanling Range include the Mexican Desert in the Americas, the Sahara Desert in Africa, the Arabian Peninsula Desert in West Asia, and the Thar Desert in northwestern South Asia. These regions at the same latitude have a tropical desert climate and a subtropical desert climate, except for the Nanling Ridge, which has a subtropical humid monsoon climate. Is there a special feature of the Nanling Mountains, where a humid climate occurs in the latitude zone dominated by deserts and deserts on the earth? How different is it from other typical areas with the same solar exposure time in the same subtropical high pressure in terms of precipitation, temperature, potential evapotranspiration, and sunshine percentage? What is the difference between dry and wet periods in the South Ridge and other regions at the same latitude? What is the reason for the different climates in this latitude zone? The exploration of these questions has important scientific value for the study of global change ecology and arid areas. Based on the observations of 91 meteorological stations and the atmospheric circulation model CCSM3, Zhou Ping and Liu Zhiyong (2018) analyzed the climate characteristic parameters of the typical region of the same latitude in Nanling, and predicted the temperature and precipitation in different regions in the future, further explored the sensitivity of net primary productivity to temperature and precipitation, and analyzed the possible reasons for the climate differences in the typical region of the same latitude zone of Nanling.
1. Differences in dry and wet characteristics in different regions
There are great differences in precipitation and seasonal distribution among the five typical regions in the same latitude zone.
The precipitation from more to least is the South Ridge> the Mexican Desert> the Thar Desert> the Arabian Peninsula Desert > the Sahara Desert, in which the precipitation in the South Ridge mostly occurs in spring and summer, the precipitation in the Mexican Desert mostly occurs in autumn, the precipitation in the Thar Desert mostly occurs in summer, the precipitation in the Arabian Peninsula Desert is concentrated in spring and winter, and the Sahara Desert has almost no precipitation throughout the year.
The order of potential evapotranspiration is the Sahara Desert> the Arabian Desert> the Thar Desert> the Mexican Desert > the South Ridge, which are in the same order of precipitation at the same latitude.
The meteorological dry period (P<0.5PET) and wet period (P>0.5PET is semi-humid, where P>PET is wetter) and the meteorological dry period (P0.5PET) is semi-humid, and PPET is wetter, is also quite different among regions at the same latitude. Among them, the South Ridge is in the wet and wet periods throughout the year, the Sahara Desert, the Arabian Peninsula Desert and the Thar Desert are in the dry period throughout the year, while the Mexican Desert has two dry periods and two wet periods a year, of which the winter drought lasts longer and the summer drought lasts shorter. The rainy season in the Mexican deserts is in the fall, and there is a period of spring drought during the season when the plants begin to grow. Unlike the Mexican deserts, the rainy seasons in the South Ridge Mountains are in spring and summer, with the same period of rain and heat during the season when most plants grow. Desert precipitation in Mexico varies greatly seasonally, with periods of continuous dryness, and the period of water demand for crop growth is also a period of relatively low precipitation (Molina et al., 2016).
▲ Annual distribution of dry and wet based on multi-year average daily precipitation and potential evapotranspiration
2. Differences in climatic characteristics of dry and wet seasons in different regions
The potential evapotranspiration, wind speed and sunshine percentages in the Sahara, Arabian and Thar deserts are higher than those in the Mexican desert and the South Ridge, but precipitation is the opposite. Although the sun's illumination time is almost equal in the same latitude region of the earth (Turton, 1987), the sunshine percentage varies greatly, with the highest being in the Sahara Desert (72%), and the other regions in descending order of Thar Desert (69.7%), Arabian Peninsula Desert (67.3%), Mexican Desert (35.9%), and Nanling (28.5%), and the difference mainly comes from the difference in sunshine hours in each region.
The Sahara Desert has less water vapour, less humidity, less cloud cover, less blocking of solar radiation, and more time than the irradiance threshold (120 W/m²) recorded for sunshine hours, resulting in higher sunshine hours and sunshine percentages. However, Nanling has relatively more water vapor, high humidity and more cloud cover, so the sunshine percentage is small.
The water vapor pressure in the dry season of the Mexican desert in the second stage was close to that of the Sahara Desert and the Arabian Desert, which was lower than that of the wet season and the Thar Desert and the South Ridge in the region, while the water vapor pressure in the wet season of the second stage was close to that of the South Ridge.
The average water vapor pressure in the Thar Desert is high (18.3 hPa) throughout the year, but it does not form precipitation in this region (precipitation is only 0.6 mm/d), and the southwest monsoon blows in summer and the northeast monsoon in winter, and the summer monsoon contributes 80% to precipitation (Singhvi et Al., 2010), which brings more water vapor to the Indian summer monsoon, but most of it forms precipitation in the area east of the Thar Desert, where there is also the highest precipitation center in the world, but it is difficult for water vapor to condense into rain over the Thar Desert. At the same time, probably due to the greenhouse effect of water vapor, the annual average daily temperature in this region is 26.3°C, which is higher than that of several other regions in the same latitude zone.
Table Differences in climatic parameters during the dry and wet seasons
3. Influencing factors of climate in different regions of the same latitude zone of Nanling
Although the sunshine hours of Nanling have the same in the same latitude zone, the sunshine hours and sunshine percentages are different in each region. The percentage of sunshine is the Sahara Desert, the Thar Desert, the Arabian Peninsula Desert, the Mexican Desert and the South Ridge, and a similar temperature pattern is formed.
Most of the Nanling zone at the same latitude is shrouded by the subtropical high all year round, and in the interior of this hot and dry air mass, the air flow is always sinking, and it is difficult for water vapor to condense into rain. The Sahara Desert is located in the tropical desert climate zone, under the control of the subtropical high pressure and trade winds all year round, the tropical continental air mass prevails, the air flow sinks, the temperature is high, the precipitation is very small, and the sunshine is strong, and the evaporation is vigorous, which aggravates the dryness of the climate. Nanling is located in the subtropical monsoon climate zone, which is an area controlled alternately by tropical ocean air masses and polar continental air masses, with high temperature and rain in summer and mild and little rain in winter. The northern part of the desert region of Mexico is located in the subtropical desert climate zone, affected by the subtropical high pressure and dry trade winds, the temperature in midsummer is similar to that of the tropical desert climate, there is tropical ocean air mass invasion in late summer, and polar continental air mass invasion in winter.
In addition to the above-mentioned solar radiation and atmospheric circulation, the climate of any part of the earth may also be affected by a combination of factors such as land and sea location, topography, typical vegetation, and human activities.
▋ Land and sea location
Although all regions are within the control range of the subtropical high, the ratio of ocean area to land and sea area in each buffer zone varies due to the distance of the center point from the ocean. The ratio of ocean area to land area in the buffer zone changes with the radius of the buffer zone, indicating that the Sahara Desert has the smallest proportion of ocean area in different buffer zones. followed by the Arabian Peninsula Desert; The Thar Desert, the Mexican Desert and the South Ridge are within the buffer zone with a radius of 300~800km, and the proportion of marine area has the upper hand. With a radius of more than 800 km, the proportion of marine area in the Mexican Desert and South Ridge is significantly higher than that of the Thar Desert. This pattern is close to the current pattern of dryness and wetness in various regions. It can be seen that due to the different locations of land and sea, the water vapor transport in the same latitude of the South Ridge is in the order of Sahara Desert, Arabian Peninsula Desert, Thar Desert, Mexican Desert and South Ridge. Of these areas, only Nanling, located on the world's largest continent, is close to the highest mountain on Earth and faces the world's largest ocean, so it has the greatest difference between land and sea heat, resulting in perennially humid climatic conditions.
▲ The ratio of ocean area to land and sea area in the buffer zone varies with the radius of the buffer zone
▋ Topography
From the perspective of terrain roughness, the average elevation of the Thar Desert is the lowest, and the roughness of the terrain is also the lowest, and the area is a low-lying and flat basin. The average elevation of the Mexican desert is the highest, and the terrain roughness is in the middle. The average elevation of the Nanling Mountains is in the middle, and the terrain roughness is the highest compared with the other four regions at the same latitude, and the terrain is relatively complex and undulating, and the precipitation is also obviously affected by the topography in addition to the influence of monsoon circulation. Due to the topographic retardation, the spring frontal trough detours for a longer time and forms more precipitation (Huang Qizhang, 1990).
Table Topographic parameters and vegetation types of land cover by region
▋ Typical vegetation
Due to long-term climate change and the disturbance of human activities, the other four typical areas of Nanling and its same latitude zone are mainly covered by tropical desert and subtropical desert vegetation, except for Nanling, which is still mainly covered by subtropical evergreen broad-leaved forests originating from the early Cenozoic Tertiary and even the Cretaceous of the Mesozoic Era.
However, in terms of the formative period of these deserts and desert vegetation, there was a process of soil desertification and vegetation degradation during the Holocene, and even before the end of the Holocene wet period. The Sahara region was savannah woodland at different times during the Quaternary (Nicoll, 2018); The dominant tree species in the Chihuahua Desert in Mexico were Quercus (44.4%~66.5%), and the associated tree species were Elm, Oak and Willow. The Arabian Peninsula was once covered by savannahs during the humid period of 8600~8000 BC. However, at the end of the wet period in the Sahara region from 7600~6700 BC, most grasslands and woodlands were destroyed. The climate of the Mexican "Chihuahua Desert" became dry and cold after 7700 BC, with desert plants, including amaranth, dominating (64.7%) (Bruce, 2015). The savannahs of the Arabian Peninsula were gradually replaced by drought-tolerant, low, shrublands after 8000 BC (Dinies et al., 2015).
▋ Human activities
With the exception of the South Ridge, the other four regions experienced population decline and desertification of vegetation before the onset of the Holocene dry period (Saini and Mujtaba, 2012, Manning and Timpson, 2014, Bruce, 2015, Dinies). Al., 2015), which also suggests that the population of these regions may have peaked during the warm period, and then exceeded the carrying capacity of their natural ecosystems, and as the climate becomes more arid, the contradiction between population size and environmental pressures becomes more prominent, leading to a decrease in regional population density. These areas are relatively flat and have relatively long distances between land and sea, making them more vulnerable to climate change and anthropogenic disturbances.
In addition to the advantages of high terrain roughness and a relatively large proportion of land and sea area, the people living in Nanling have always had a simple environmental ethics (Liang An, 2011), and they have recognized the dependence of people on natural resources, so they have adopted a "nomadic farming" method to allow the cultivated land to recover naturally. In the existing primary forest, there are still many relict plants of the Tertiary and pre-Tertiary periods, and the species of these plant families are more than one-third of all the family species in Nanling (Pang Xiongfei, 1993). Although there is also a distribution of Quercus wugang belonging to the sclerophyll evergreen broad-leaved forest in Nanling (Xie Chunping, 2011), this tree species exhibits xerophyte characteristics, indicating that Nanling has also experienced dry climate characteristics, but Nanling has not experienced the same process of frequent human activities and vegetation desertification as the other four regions.
4. Prediction of typical regional climate characteristics in the same latitude zone of Nanling
Based on the measured daily data values of the meteorological station and the simulation of climate models, the climatic characteristic parameters of Nanling and typical areas at the same latitude were analyzed and predicted. The drought period of the Sahara Desert, the Arabian Peninsula Desert and the Thar Desert in the same latitude zone of the Nanling Mountains lasted for a long time, while the Mexican Desert only had a seasonal dry period. There were wet periods in both the South Ridge and the Mexican Desert, but the South Ridge had the same period of rain and heat during the growing season, while the Mexican Desert had different periods of rain and heat during the plant growing season. Under the future B1 climate change scenario, the temperature of all regions in the same latitude zone of Nanling will increase significantly, and the precipitation will generally show an increasing trend, but there will be significant differences in each region, and there will be local areas with significant decrease in precipitation, and there will be more areas with significant increase in overall precipitation than in areas with significant decrease.
The net primary productivity of Nanling is not sensitive to temperature, but to precipitation. The sensitivity of the Sahara Desert and the Arabian Peninsula Desert to precipitation is significantly higher than that of the other three regions, and precipitation is the limiting factor affecting the growth of net primary productivity. In addition to the differences in sunshine percentage and atmospheric circulation, the sea and land positions and terrain roughness in the same latitude area of Nanling are also different, and the intensity of land cover and human disturbance is also different, which may be the reasons for the formation of climate differences in different regions of the same latitude zone of Nanling.
Climate change and human disturbances affect the Earth's ecosystems by affecting the soil, hydrology, and vegetation of the Earth's underlying surface. In arid regions, climate change alters the material cycling of ecosystems by altering precipitation and its spatial and temporal distribution (Chang et al., 2017). Desertification, on the other hand, affects regional microclimates by altering the biogeophysical processes of surface energy and water exchange. The increase of precipitation in the Thar Desert in the future may play a good role in promoting the growth of vegetation. Different deserts may manifest themselves differently, the same is warming, with tropical and subtropical deserts warming in summer and temperate deserts warming in winter (Mamtimin et al., 2011). It has long been widely recognized that climate change is the main factor in the formation of desert ecosystems. Pollen analysis and carbon isotope dating have revealed much evidence of climate change and desert formation. Before the desert was formed, before 8000 BC, both the Sahara, the Arabian Peninsula, and northern Mexico were in a wet period (Manning and Timpson, 2014, Bruce, 2015, Dinies et al., 2015).
In the past, it was generally believed that desertification was mainly caused by natural factors, and little attention was paid to the contribution of human activities. The Sahara region was relatively rich in water during the wet period, and archaeological finds of ceramics with harpoon, fishbone hooks, and comb motifs from 11,000 BC indicate that these civilizations developed in an environment where lakes, rivers, and inland deltas were widely distributed (Yellen, 1998, Drake et al., 2011). In the driest core of the Sahara Desert, Marinova et al. (2014) also found that there were traces of a more humid climate and human activity in 9400~8100 BC by studying widely distributed carbonate calcite and swimmer rock art images.
In terms of human indications, the population of the Sahara region increased rapidly after 11,000 BC, and the homogeneity of fishing gear in different regions indicates a rapid process of population expansion (Yellen, 1998, Drake et al., 2011). In 7600~6700 BC, that is, at the end of the wet period, the population showed a rapid downward trend (Manning and Timpson, 2014). The development of desertification has also led to the demise of ancient human civilizations that once developed in these places, and the Sahara Desert is now the driest region on Earth with 10mm of precipitation per year, and the least densely populated. These deserts or desert areas in the same latitude zone of the Nanling Mountains had good vegetation cover at the beginning of the humid period of the Holocene, and all experienced a process of population growth and then a sharp decline.
Wet-season vegetation has a higher net primary productivity of vegetation and can provide natural resources for a larger population, but human demand for nature may exceed the critical value of its ecosystem carrying capacity, resulting in more fragile soils, vegetation and water resources, and less resilience to climate change. Water is an important factor for the survival of life, and because the available water resources are reduced, it cannot support the survival of more people, so the population data is passively reduced. Changes in climate characteristics in this process may be slow, even for thousands of years, but once the tipping point is breached, it will be difficult to reverse. The dual disturbance effects of the natural environment and human activities lead to ecological imbalance and accelerate desertification. It can be seen that the impact of human activities on the formation of desertification cannot be ignored. Under the influence of human activities, the Sahara Desert region, which receives less water vapor transport due to the different locations of the sea and land, and the Thar Desert region, which has relatively less terrain roughness, have shown less resilience to climate change.
The Sahara Desert in this latitude zone of the South Ridge is home to the lowest net primary productivity on Earth and one of the lowest population densities in the world; The Thar Desert is currently the most densely populated place in the desert zone, while Nanling is one of the areas with the richest forest resources in China and one of the highest population densities in the same latitude zone. A better understanding of climate differences in different regions, as well as ecosystem resilience and carrying capacity, can help humans better cope with climate change and protect ecosystems from desertification as much as possible. Although Nanling has more suitable natural climatic conditions for animal and plant habitat and human life than the same latitude zone, due to the disturbance of human activities, the Nanling area is also facing the problem of serious rocky desertification and vegetation degradation, and special attention should be paid to reducing the negative disturbance of human activities on the ecosystem in the Nanling area under the background of future climate change.
This article is excerpted from "Research on Geographic Environment and Biodiversity in Nanling" (edited by Li Dingqiang and Zhou Ping. Beijing: Science Press, 2024.3), the title of the book "Chapter 1 Introduction", was added by the editor.