Analysis of annual CO₂ gas emissions in West Africa

The emission of greenhouse gases, particularly carbon dioxide (CO₂), is one of the main contributors to climate change. CO₂ emissions are mainly due to the use of fossil fuels and industrial production, and are constantly increasing at a global scale. West Africa is a developing region that is particularly vulnerable to the negative impacts of climate change [1], including drought, flooding and coastal erosion. In effect, during the 2000-2010 period, this region has lost 1.1% forest cover per year. In term of climate parameters, one observed an increasing of temperature from 0.2 to 0.8 C since the end of 1970s [1] and, a strong reduction of precipitation in the Sahel with a southward migration of 200 km of the isohyets. In additional, it estimated at 40 to 60% (15 to 30%) the decreasing of the annual averaging debit of the main rivers in West Africa [2]. Understanding CO₂ emissions in this region is therefore crucial for developing effective mitigation and adaptation policies. However, estimating annual CO₂ emissions in West Africa presents challenges due to variability and uncertainty in the available data [1] Several studies have been conducted to quantify CO₂ emissions in West Africa [1,3-5]. For example, a recent study conducted by [3] used a combination of satellite data and models based on forest fires and agricultural practices to estimate CO₂ emissions in the West African region. Results showed that forest fire emissions were highest in the arid Sahel regions, while agricultural emissions were more distributed across the region. Another study assessed CO₂ emissions in urban areas in the West African region, focusing on emissions from road traffic. The results showed that CO₂ emissions were concentrated in large cities such as Lagos, Abidjan and Accra [4]. Using long in situ time series of CO₂ concentration of Lamto station (Côte d’Ivoire) [1], observed a strong seasonal and interannual variability of CO₂ associated to the biomass fires regime and the large-scale circulation of air coming from North-east of the continent. These authors obtained an increasing rate of 7 ppm.yr^-1 of CO₂ concentration. This trend closes to the world trend, whilst this region is not classified as an important zone of CO₂ emission. These innovate and appreciated results provide by these studies open several perspectives fields such as the quantification of the different CO₂ emissions sources using time series at different timescales. This study aims to analyze CO₂ emissions in 16 West African countries over a 46-year period from 1970 to 2015 using data provided by the Emissions Database for Global Atmospheric Research (EDGAR) [6]. We use two trend analysis methods, the segmentation method and the empirical mode decomposition method, to assess the temporal trends and variability of CO₂ emissions in the region. In section 2, the study region, including its geography and the countries included in the analysis are described. Section 3 presents the data used in the study, including their source and temporal coverage. The trend analysis methods are presented in section 4. The results of analysis are presented in Section 5, followed by a discussion in Section 6. Finally, section 7 is devoted to conclusions, as well as avenues for future research.

The area of interest covers West Africa. It extends in latitude from the Equator (0°N) to the South of Algeria (25°N) and in longitude from Senegal (20°W) to Cameroon (15°E). The 16 countries in the study area are: Benin, Burkina Faso, Cabo Verde, Côte d'Ivoire, Gambia, Ghana, Guinea, Guinea Bissau, Liberia, Mali, Mauritania, Niger, Nigeria, Sierra Leone, Senegal and Togo (Figure 1).

 Figure 1: Studyarea. Scale: 1/2500000

Data

The data consist of time series of annual inventory emissions of carbon dioxide (CO₂) emissions from these 16 West African countries. They are provided by the Emissions Database for Global Atmospheric Research (EDGAR) [6] and span a period of forty-six years (from 1970 to 2015) at 0.1°x0.1° spatial and 1-year temporal resolutions [7]. These annual anthropogenic CO₂ emissions are due to fossil fuel use and industrial processes (cement production, limestone and dolomite carbonate use, non-energy fuel use and other combustions, chemical and metal processes, solvents, agricultural liming and urea, waste and fossil fuel fires).Short-cycle biomass burning (such as agricultural waste burning), large-scale biomass burning (such as forest fires), and carbon emissions/removals related to land use, land-use change, and forestry are excluded.

We use the segmentation of [8] to detect the presence or absence of one or more break points in the inter annual behavior of the time series constituted by the annual anthropogenic carbon dioxide (CO₂) emissions released into the atmosphere in the different countries of the study area. The temporal evolution of annual CO₂ emissions can sometimes include breakpoints that express changes in the regime of this parameter. These changes clearly show an increase or decrease in emission intensity from one period to another. To highlight these likely changes in the year-to-date (YTD) CO₂ emissions, Hubert's statistical test or segmentation [8] was applied to the time series of each country. This technique allows the initial series to be broken down into two or more-time sub-series and to highlight the presence or absence of one or more breakpoints characteristic of probable changes in the regime of the parameter studied. The YTD time series of CO₂ emissions analyzed may also exhibit trends. To characterize these trends, the method based on empirical modal decomposition (EMD) [9] was applied to the time series of each country. This method decomposes the original signal into a finite number of intrinsic mode functions (IMF) with different time scales, and the trend function with at most one extremum corresponds to the last intrinsic function. This intrinsic trend corresponds here to the instantaneous rate of change of emissions in kton CO₂ per year (kton.yr^-1). The EMD method provides a better estimate of the trend than the conventional linear regression technique [9].

The temporal analysis of the annual CO₂ gas emissions exhibits several changes in interannual variability in some countries during the study period. These changes which are characterized by some breakpoint obtained by Hubert segmentation statistical test [8] (have unequal durations and intensities (Table 1). The number these breakpoints ranged between 2 and 7. The minimum and maximum numbers of breakpoint are observed in Nigeria (2), and Niger (7) and Senegal (7) annual CO₂ gas emissions time series evolution respectively. The first and the main breakpoints occur in 1972 in Nigeria and around the 1990s in all West Africa countries. The periodicity of the appearance of these breakpoints ranged between 1 and 25 years. Negative trends have been observed in Cabo Verde (-132.84%), Liberia (-145.06%), and Sierra Leone (-26.56%) before and after 1981, 1989 and1991, respectively. These values show a reduction of CO₂ emissions in these countries. Positive trends that signify an increase of CO₂ gas emissions have been observed in the other West African countries with varying intensity and duration from one country to another. The strongest trends of relatively short durations are generally observed in the 1970s. The maximum positive trend value has been recorded in Mauritania before and after the 1991s (Table 1).

Table 1: Summary of annual CO₂emissions breakpoint year obtained by segmentation method and associated rate of CO₂ gasemissions in West African countries.

Table 2 : Linear trend with slopes in kton CO₂ per year.

The maximum (~1.812kton. yr^-1) and minimum (~0kton. yr^-1) trends are observed in Nigeria and Cabo Verde, respectively.The upward linear trend shows the increasing CO₂ gas emissions in all West African countries over the 1970-2015 period.

Figure 2 : Intern annual variability of annual CO₂gasemissions in kton.yr^-1 in West African countries. The linear trend (in red) obtained by EMD was added on each figure.

Temporal trends in annual anthropogenic CO₂ emissions were analysed during the 1970-2015 period over the 16 West African countries. The results for each country indicated that CO₂ emissions exhibited a high annual variability with an increasing trend. Nigeria, Côte d'Ivoire, Ghana and Senegal have the highest CO₂emission values (>1340 Kton) with rates of 12.10%, respectively. These high values are probably due to the increase in anthropogenic activities associated with population and economic growth in these countries. Similar trends are also observed in the study of [10] over the 1960-2010 period in 39 African countries. In fact, these four countries (i.e., Nigeria, Côte d'Ivoire, Ghana and Senegal) are the most industrialised and highly populated in West Africa with population growth rates of 2.63%, 3.26%, 2.62% and 2.65% per year respectively during the 45 years [11]. This is corresponding to a population increase of roughly 231%, 330%, 226% and 220% respectively over the 1970-2015 period, which is of the same order of magnitude as the increase in bio fuel emissions. Moreover [12,13], highlighted that bio fuel emissions have increased at a higher rate than other sources due to an increase in low-income population in sub-Saharan Africa, where biomass constitutes about 80% of the total energy consumption. In addition, results of this study corroborate those of [14] on carbon emissions between 1990 and 2019 in West Africa. These authors showed that the regional net warming potential between 1990 and 2019 was estimated to be 11.44 Pg and Nigeria had the highest potential at 18.7%, closely followed by Mali and Ghana at 15% and 13.2%, respectively. Similarly, using Kaya identity, [15] indicated that the largest CO₂ emitters in West Africa in 2017 were respectively Nigeria (86 MtCO₂), Ghana (14 MtCO₂), Cote d’Ivoire (10.2 MtCO₂) and Senegal (8.5 MtCO₂), in agreement to our results. The authors explained this result by many factors, including GDP. In fact, they noted that countries with high CO₂ emissions per capita also have high GDP per capita. Our results showed that anthropogenic CO₂ emissions increased significantly in 32% of the countries of the West African sub-region in agreement with GDP per capita in the same period [16] arrived at the same conclusion for other regions. These authors highlighted that in South Africa, between 1994 and 2019, CO₂gasemissions levels were generally correlated with economic growth. Furthermore, in Africa, many studies have concluded that pollutant emissions are increasing [1,12,16]. In the present study, trends of CO₂emissions are in the same order of magnitude compared to those obtained for the other pollutants (i.e., BC, OC, NOx, CO, SO₂ and NMVOCs) in different African regions. For example, [12] showed that all pollutant emissions are globally increasing in Africa during the period 1990-2015 with a growth rate of 95%, 86%, 113%, 112%, 97% and 130% for BC, OC, NOx, CO, SO₂ and NMVOCs, respectively. These authors also pointed out that West Africa is the highest emitting region for some pollutants such as CO, mainly due to domestic fires and traffic activities. CO is an atmospheric pollutant that is emitted mainly from anthropogenic sources [17]. It was reported that many processes that emit CO also emit CO₂, and therefore knowledge of CO emission rates can provide additional information on CO₂ emissions [18,19] The work of [1] using atmospheric concentrations of CO₂ and the tracer CO measured continuously at Lamto showed that both compounds had strong similarities between their seasonal cycles and similar emission sources as well as high and very significant correlation values depending on the time of year and concluded that the trend of CO partly explains that of CO₂. Therefore, in this study different trends of CO₂ by country are not surprising since the values of CO₂ obtained in some regions of West Africa from direct measurements are very high compared to those in other regions of the world.

Unlike the four countries mentioned above, Cabo Verde, Gambia, Guinea Bissau and Liberia showed lower levels of CO₂ emissions and a reduction in CO₂ emissions for particular years (Table 2), indicating a low level of industrialisation in these countries [20] . Furthermore, for many years, most tropical countries such as Ghana [21] and Côte d'Ivoire [5] have considered themselves as being net carbon sinks or, at worst, carbon neutral. This assertion is based on the relatively low level of industrialisation in these countries. But given the intensification of industrial processes and the use of fossil fuels, it is conceivable that their CO₂ emissions will increase significantly as indicated by the upward trends (in red) observed in long-term change in temporal evolution CO₂ gas emission in each West African country (Figure 2).

Anthropogenic CO₂ emissions in 16 West African countries over a 46-year period from 1970 to 2015 were analyzed on an annual scale using EDGAR data. The results show that CO₂ emissions exhibit high annual variability for each country. Furthermore, the results have shown that CO₂ emissions have increased significantly in 32% of West African sub-region countries, in agreement with GDP per capita over the same period in these countries. Nigeria, Côte d'Ivoire, Ghana and Senegal presented the highest CO₂ emission values (> 1340 kton) associated with strong upward trends. In contrast, CaboVerde, Gambia, Guinea-Bissau and Liberia showed the lowest emission levels with reductions in specific years. Statistical analysis using the HUBERT segmentation method highlighted the different breakpoints and their duration in the CO₂ emissions time series of each of these 16 countries. The application of this method enabled us to identify that the breakpoints in the various time series are in the interval [2,7]  indicating several regime changes in CO₂ emissions. However, the time series observed in Gambia (2 points) shows the lowest number of breakpoints, where Senegal and Nigeria (7 points) show the highest number of breakpoints. Furthermore, EDGAR data used here are based on inventories, statistical models and generated knowledge, presenting sometimes very large uncertainties throughout the African region due to the scarcity of quality data. This can underestimate and/or overestimate the sources and trends of CO₂ in particular and GHGs in general in the region. Thus, to reduce uncertainties, an extensive network of GHG measurements in Africa is a privileged observable that can both help diagnose the impact of human activities and help model the various components of the carbon cycle.

The authors thank the Emissions Database for Global Atmospheric Research (EDGAR) which provided us with high-quality data.

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