Recent Vegetation Cover Dynamics and Climatic Parameters Evolution Study in the Great Green Wall of Senegal

The drought recorded in 1970s and 1980s, particularly in the Sahara and Sahel region has greatly affected the population as well as the economies and the eco-systems of this area. In 2007, the African Union launched a Pan-African program, the Great Green Wall for the Sahara, the Sahel Initiative (GGWSSI) to reverse land degradation and desertification by planting a wall of trees stretching from Dakar to Djibouti. The objective is to improve food security, and support local people to adapt to climate change. This paper aims to evaluate the impacts of the reforestation program in Senegal, fifteen years after it was launched. This study uses a time series of satellite-derived vegetation cover and climatic parameters data to analyze the sustainability of these interventions. Change detection approaches were applied to identify and characterize the drives of the eventual changes. A comparative analysis of reforestation on climatic parameters was explored through the temporal analysis of the vegetation index over the periods 2000-2008 and 2009-2020. An increase in vegetation activity was noted through the NDVI at the interannual (+2% to +8%) and seasonal (+1.5% to 7% for the wet season and 1% to 4% for the dry season) scale and a positive and significant evolution is noted on the trace of the GGW. Also, the period 2009-2020 recorded an increase in rainfall of 2% to 8% of the average value 2000-2020 and 4% to 8% of the rainy season. Soil moisture is the climatic parameter that has increased the most, with an increase of 25% to 54% of the 2000-2020 average, i.e. between 20 mm and 70 mm more. This study shows a significant improvement in the relationship between NDVI and climate parameters after the different reforestation actions of the GGW.

Estimation of the Carbon Sequestration Dynamics of Senegal’s Great Green Wall Based on Land Cover over the Past Three Decades

The severe drought observed in the Sahel during 1970s, 1980s and 1990s has deeply affected the population as well as the economies and the eco-systems of this climatic area. The GGW Initiative spearheaded by Africa Union in 2007 proposed to combat the land degradation and desertification by planting a wall of trees stretching from Dakar to Djibouti. A reforestation was then conducted in the Senegal’s GGW since 2006 as part as other areas in the Sahel. This paper aims to evaluate the carbon sequestration dynamics in the sites of the Senegal’s GGW over the last three decades. The method consists firstly of analyzing the evolution of land cover and land use dynamics based on ESA-CCI LC satellite data. There is an improvement of the surface areas of tree and shrub savanna of 11.40% (Tessekere), 8.25% (Syer) and 2.70% (Loughere-Thioly). The regreening of the different localities and a positive dynamic observed is explained by the return to normal rainfall and to reforestation actions, agroforestry practices, better management of natural resources undertaken. However, some non-reforested sites showed an opposite trend despite of the normal rainfall. Secondly, the results on land mapping are used as a proxy for the assessment of carbon stocks. The dynamic observed in vegetation cover since the beginning of the reforestation made it possible to sequester 5.8 million tons of carbon representing respectively 2.31% of African GGW. This gain in stored carbon is equivalent to 21.2 million tons of CO2 captured in the atmosphere. Through this study, it appears that carbon storage becomes significant 8 to 10 years after the start of reforestation. An urbanization without respect for the environmental factors could be a danger for the climate (case of Ballou).

Towards More Snow Days in Summer since 2001 at the Great Wall Station,Antarctic Peninsula:The Role of the Amundsen Sea Low

The variation in the precipitation phase in polar regions represents an important indicator of climate change and variability.We studied the precipitation phase at the Great Wall Station and Antarctic Peninsula(AP)region,based on daily precipitation,synoptic records and ERA-Interim data during the austral summers of 1985?2014.Overall,there was no trend in the total precipitation amount or days,but the phase of summer precipitation(rainfall days versus snowfall days)showed opposite trends before and after 2001 at the AP.The total summer rain days/snow days increased/decreased during 1985?2001 and significantly decreased at a rate of?14.13 d(10 yr)?1/increased at a rate of 14.31 d(10 yr)?1 during 2001?2014,agreeing well with corresponding variations in the surface air temperature.Further,we found that the longitudinal location of the Amundsen Sea low(ASL)should account for the change in the precipitation phase since 2001,as it has shown a westward drift after 2001[?41.1°(10 yr)?1],leading to stronger cold southerly winds,colder water vapor flux,and more snow over the AP region during summertime.This study points out a supplementary factor for the climate variation on the AP.

Concentration level of heavy metals in the Great Wall Bay， Antarctica in austral summer

Heavy metals in different marine environmental mediums from the Great Wall Bay, Antarctica were determined. Sea water samples from 14 stations, surface sediments from 2 stations and marine organisms from 1 station were analyzed for Cu, Pb, Zn, Cd, Cr, Hg concentrations, The results showed that the terrestrial input characteristics of Cu, Pb and Cr in sea water was evident. Zn concentration in the outlet water was high. The difference of each heavy metal concentration in two sediments was low. The enriching ability of seaweed to Zn was relatively high. Generally speaking, the concentration level of heavy metals in the Great Wall Bay was low.

Organic contamination in the Great Wall bay,Antarctica in austral summer

During 1992/1993 austral summer,oil, BHCs,DDTs,PCBs etc.were determined in the sea water, sediments and organisms samples of the Great Wall bay,Antarctica.In surface sea water,the mean concentrations of oil were 0. 26-0. 93 μg/L.DDTs were all lower than the DL of the method,that was<0.04ng/L. BHCs mean concentrations were<0.69-<1.94 ng/L. In other samples,oil,BHCs,DDTs,and PCBs were all relatively higher in patella than that in sediments and seaweed. PAHs could not be detected in all samples. By comparison,the concentrations level of organic contaminants in the Great Wall bay was low yet.

Seawater nutrient and chlorophyll α distributions near the Great Wall Station, Antarctica

We examined the influences upon nutrient, temperature, salinity and chlorophyll a distributions in Great Wall Cove（GWC） and Ardley Cove（AC）, near the Chinese Antarctic Great Wall Station, using measurements taken in January 2013 and other recent data. Nutrient concentrations were high, with phosphate concentrations of 1.94（GWC） and 1.96（AC） μmol·L-1, DIN（dissolved inorganic nitrogen） concentrations of 26.36（GWC） and 25.94（AC） μmol·L-1 and silicate concentrations of 78.6（GWC） and 79.3（AC） μmol·L-1. However, average concentrations of chlorophyll a were low（1.29 μg·L-1, GWC and 1.08 μg·L-1, AC）, indicating that this region is a high-nutrient and low-chlorophyll（HNLC） area. Nutrient concentrations of freshwater（stream and snowmelt） discharge into GWC and AC in the austral summer are low, meaning freshwater discharge dilutes the nutrient concentrations in the two coves. Strong intrusion of nutrient-rich water from the Bransfield Current in the south was the main source of nutrients in GWC and AC. Low water temperature and strong wind-induced turbulence and instability in the upper layers of the water column were the two main factors that caused the low phytoplankton biomass during the austral summer.

Seasonal variations in aerosol compositions at Great Wall Station in Antarctica

High volume aerosol samplers at Great Wall Station in Antarctica were used to collect 73 aerosol samples between January 2012 and November 2013. The main ions in these aerosol samples, Cl^-, NO3-, SO4^2-, Na＋, K＋, Ca2＋, Mg2＋, NH4＋, as well as methane sulfonic acid, were analyzed using ion chromatography. Trace metals in these samples, including Pb, Cu, Cd, V, Zn, Fe, and Al, were determined by inductively-coupled plasma mass spectrometry. Results showed that sea salt was the main component in aerosols at Great Wall Station. Most ions exhibited significant seasonal variations, with higher concentrations in summer and autumn than in winter and spring. Variations in ions and trace metals were related to several processes（or sources）, including sea salt emission, secondary aerosol formation, and anthropogenic pollution from both local and distant sources. The sources of ions and trace metals were identified using enrichment factor, correlation, and factor analyses. Clearly, Na＋, K＋, Ca2＋, and Mg2＋were from marine sources, while Cu, Pb, Zn, and Cd were from anthropogenic pollution, and Al and V were mainly from crustal sources.

EXPERIMENT OF SHORT WAVE COMMUNICATION AND THE INVESTIGATION OF THE PROPAGATION BEARINGS BETWEEN THE GREAT WALL STATION AND XINXIANG

The distance from the Great Wall Station (62.2°S, 58.9°W) to Xinxiang (35.3°N, 113.8°E) is 16981 km. The path passes through the polar cap absorption region and the auroral absorption zone, and it is across the equator.In this paper firstly the effects of short wave communication and usable time blocks and frequency ranges between the Antarctic Great Wall Station and Xinxiang from December 1985 to March 1986 are introduced. The comparison between the usable frequency ranges with the estimated MUF is made. The upper limit of frequency ranges of communication along the short great circle path basically agrees with the MUF but there is difference between them along the long great circle path.Secondly, the result of the propagation bearings experiment in January to February 1986 is introduced in more detail, The propagation along the great circle path from the Great Wall Station to Xinxiang is the main propagation mode. But the propagation along non great circle paths occurs at times between Great Wall Station and Xinxiang. The non great circle path propagation varies with time because the ionospheric absorption and other conditions which support the non great circle path propagation are the function of the time. So the courses of the non great circle path propagation may be different in the different time. The mechanism of the constructing non great circle path propagation has been analysed. We preliminarily think the main cause of propagation along non great circle path is the ground scatter. The stronger radialization of the side lobes of the antenna and the less absorption of the ionosphere contribute to forming non great circle path propagation.

Sources and distribution of particulate organic carbon in Great Wall Cove and Ardley Cove, King George Island, West Antarctica

Concentrations of chlorophyll-a（Chl-a）, particulate organic carbon（POC） and its stable carbon isotope composition（δ13C） were analyzed to investigate the biogeochemical characteristics and sources of POC in Great Wall Cove（GWC） and Ardley Cove（AC） during the austral summer. POC concentrations ranged from 50.51 to 115.41 μg·L^-1（mean±1 standard deviation： 77.69±17.27 μg·L^-1） in GWC and from 63.42 to 101.79 μg·L^-1（82.67±11.83 μg·L^-1） in AC. The POC δ13C ranged from-30.83‰ to-26.12‰（-27.40‰±0.96‰） in GWC and from-28.21‰ to-26.65‰（-27.45‰±0.47‰） in AC. The temperature and salinity results showed distinct runoff signals in both GWC and AC, although the δ13C data and POC distribution indicate a negligible influence of land sources upon POC. The δ13C values suggest that POC is of predominantly marine origin. The POC/Chl-a ratio and the relationship between POC and Chl-a indicate that phytoplankton, organic detritus and heterotrophic organisms are significant contributors to POC in GWC and AC.

Features of visibility variation at Great Wall Station,Antarctica

The variation of visibility at Great Wall Station （GWS） was analyzed using manual observational data for the period of 1986 to 2012. Results show that the frequencies of occurrence of high （≥ 10 km） and low visibility （0-1 km） are 61.0% and 8.0%, respectively. Visibility at GWS shows an evident seasonal variation： The highest visibility between November and March, and the lowest visibility from June to October. Sea fog and precipitation are the main factors for low visibility during summer, whereas frequent adverse weather, such as falling snow, blowing snow, or blizzards, are responsible for low visibility in winter. The frequency of occurrence of low visibility has decreased significantly from 1986 to 2012. Conversely, the frequency of occurrence of high visibility has shown a significant increasing trend, especially during winter. The decreasing tendencies of fog, blowing snow, and snowfall have contributed to the increasing trend of high visibility during winter. Visibility at GWS exhibits significant synoptic-scale （2.1 to 8.3 d）, annual, and inter-annual periods （2 a, 4.1 a, and 6.9 a to 8.2 a）, among which the most significant period is 4.1 a. The visibility observed during 2012 indicates that instrumental observation can be applied in the continuous monitoring of visibility at GWS.

Comparison of temperature extremes between Zhongshan Station and Great Wall Station in Antarctica

Although temperature extremes have led to more and more disasters, there are as yet few studies on the extremes and many disagreements on temperature changes in Antarctica. Based on daily minimum, maximum, and mean air temperatures(Tmin, Tmax, Tmean) at Great Wall Station(GW) and Zhongshan Station(ZS), we compared the temperature extremes and revealed a strong warming trend in Tmin, a slight warming trend in Tmean, cooling in Tmax, a decreasing trend in the daily temperature range, and the typical characteristic of coreless winter temperature. There are different seasonal variabilities, with the least in summer. The continentality index and seasonality show that the marine air mass has more effect on GW than ZS. Following the terminology of the Fifth Assessment Report of the Intergovernmental Panel on Climate Change(IPCC AR5), we defined nine indices of temperature extremes, based on the Antarctic geographical environment. Extreme-warm days have decreased, while extreme-warm nights have shown a nonsignificant trend. The number of melting days has increased at GW, while little change at ZS. More importantly, we have found inverse variations in temperature patterns between the two stations, which need further investigation into the dynamics of climate change in Antarctica.

THE SIGNIFICANCE OF THE GREAT WALL AS A GEOGRAPHIC BOUNDARY

The Great Wall is not only the symbol of the ancient Chinese culture but also an actual and significant geographic boundary. The arrangements of the wall in past dynasties are based on physiographic conditions, and are just consistent with the demarcation line between semi-humid and arid climates in the country. Therefore,the Great Wall becomes an indicator of the demarcation line between the two areas.The extension and retreat of the wall generally reflect the expansion and shrinkage of farming and animal husbandry areas. From the viewpoint of political geography, the Great Wall can be regarded as the balance belt of 'forces' where the political powers of different nationalities histoncally stood facing each other and had repeated trials of strength. On one hand, the man-made wall of 10,000 li had the function of separating two political units, to some extent, the extension and retreat of the wall reflect the changes of relative strength of the two sides. On the other hand,the conflicts of the advanced and backward forces objectively promoted the economic and cultural exchanges as well as the national assimilation. Today, the Great Wall has lost its function of military works for territorial expansion and defense, but it is still the significant indicator as a geographic boundary and a sensitive area for many geographic elements.

A Case study of a snowstorm at the Great Wall Station,Antarctica

A case of a snowstorm at the Great Wall Station was studied using data of NCEP（National Centers for Environmental Prediction） analysis,in situ observations and surface weather charts.The storm occurred on August 29th, 2006,and brought high winds and poor horizontal visibility to the region.It was found that the storm occurred under the synoptic situation of a high in the south and a low in the north.A low-level easterly jet from the Antarctic continent significantly decreased the air temperature and humidity.Warm air advection at high level brought sufficient vapor from lower latitudes for the snowstorm to develop. The dynamic factors relating to strong snowfall and even the development of a snowstorm were deep cyclonic vorticity at middle and low levels,the configuration of divergence at high level and convergence at low level,and strong vertical uplift.There was an inversion layer in the low-level atmosphere during the later phase of the storm.This vertical structure of cold air at low levels and warm air at high levels may have been important to the longevity of the snowstorm.

The climate background and synoptic situation of fog at Great Wall Station,Antarctica

Based on the observed and NCEP reanalysis data from 1985 to 2006, the climate background and synoptic situation of fog at Great Wall Station were analyzed.It is shown that the seasonal variation of fog is controlled by the change of general circulation and local pressure field.Three favorable typical synoptic situations for fog development arc found,the Front-of-A-Depression type,the Saddle-Shaped-Field type and the Passing-Weak-Cyclone type.The first one is the most important situation.Advection cooling fog is dominant at Great Wall Station,but there are other kinds of fog as well.As a result,some helpful principles for local fog forecasting are given.

A STUDY ON THE QUANTITY OF SHALLOW SEA BENTHOS IN GREAT WALL BAY, ANTARCTICA

Based on the investigation material acquired during the three cruises of the 4 Chinese Antarctic Research Expedition at 19 quantitative dredging stations and 4 trawling stations from the period December 1987 to March 1988, this paper studies the quantity of the benthos in Great Wall Bay,the distributive characteristics and variations, and points out that the stock number of the benthos in the bay is extremely large and that there are two large ranged high mass area and 2 - 3 low small-ranged mass area. The instability of the sediments is the cause for the low.mass areas. The warm season is the vigorous reproductive period of the various kinds of the benthos and the monthly variation of the quantity is remarkable.

QUANTITATIVE DISTRIBUTION OF NET PHYTOPLANKTON IN GREAT WALL BAY, ANTARCTICA

Samples of phytoplankton were obtained in February, 1985, in Great Wall Bay, Antarctica. Among the 31 species of phytoplankton identified, the cold - water species rank highest in proportion. The cell number of phytoplankton averaged 162. 5×104 cells/m3 in the bay. The most dominant species are Chaetoceros socialis Lauder, Rhizosolenia. alata f. inermis (Castr. ) Hustedt and Biddulphia striatas Karsten. The former species mainly appeared in the bay and the latter two species aggregated at the bay mounth.

ESTABLISHMENT OF SURVEYING SYSTEM OF THE GREAT WALL STATION

It is the main task of the antarctic surveying expedition to establish the surveying system. In three times of the antarctic surveying research expeditions, a complete and accurate surveying system has been established in the Chinese Great Wall Station area. The surveying system includes geodetic coordinate system, elevation system and gravity reference system. In this paper, the surveying methods and the mathematical models for establishing these systems are discussed, and the accuracy of results is analysed.

MASS MOVEMENT IN THE GREAT WALL STATION AREA, ANTARCTICA

This paper deals with the mass movement processes on the Fildes Peninsula, King George Island. Based on numerous field investigation, the authors consider that the slope mass movement and periglacial form on the peninsula have an internal relationship in genesis, reflecting in there main aspects. 1, All patterns of mass movement are developed on the slope surface; 2, different -order mass movement and periglacial type show their space-time distribution rules (their development stages); 3, The slope mass movement are developed from the periglacio - geomorphic processes. More detailed works have been done, such as fixed reapeated surveying whose results are concord and with the typical characteristics above mentimed very well.

^(210)Pb distribution characteristics in the lake sediment core at Great Wall Station,Antarctica

The results of 210 Pb measurements in the lake sediment core from Antarctica are reported: the relative specific activities of 210 Pb in the lake sediment were found to decrease exponentially with the depth in the study area; the sedimentation rate of the lake is about 0.072 cm/a. This paper shows that 210 Pb method is applicable to Antarctic lake sediment ,and during the 1960's the atmospheric nuclear tests have an effect on the rate of 210 Pb deposition in Antarctica.

MEDIUM-RANGE OSCILLATION OF METEOROLOGICAL ELEMENTS AT GREAT WALL STATION,ANTARCTICA

A method of multi-spectral analysis is used to study the spectral characteristics of surface and upper-level meteorological elements over the Great Wall Station (62°12'S, 58°57'W), Antarctica and their phasecorrelation, propagation of mean oscillation at 500hPa level in the Southern Hemisphere and their corresponding synoptic sense. the results are summed up as follows: 1. Over the sub-Antatctic zone, as in the Northern Hemisphere there generally exist quasi-weekly oscillation and quasi-biweekly oscillation. In different seasons the oscillations of meteorological elements are different: in winter season quasi-biweekly oscillation is dominant, while in summer season quasi-weekly oscillation is dominant. 2. From the Earth's surface to the lower stratosphere there is a distinct quasi-weekly oscillation at each isobaric surface, but the most intense oscillation appears at 200-300hPa, and the oscillations of height and temperature are propagated downward. 3. Both in winter and summer seasons the quasi-biweekly oscillation are propagated from west to east, and the mean velocity of its propagation is about 7-17 longtitude / day. 4. The quasi-biweekly oscillation and the quasi-weekly oscillation over the sub - Antarctic zone are closely related to the activity and intensity variation of polar vortex at 500hPa, while at 1000hPa they reflect an interaction between the circumpolar depression and the sub-tropical high. The quasi-biweekly oscillation may be a reflection of inherent oscillation of the polar vortex, where as the quasi-weekly oscillation is a result of forced oscillation by external disturbance.A large number of calculations and analysis made reveals the features of medium-range oscillation over the sub-Antarctic zone. The results are of significance for understanding the behaviour of synoptic dynamics and making the weather forecast.This work is supported by National Committee for Antarctic Research.