This report gives a summary of the findings from a study carried out on the potential impacts of reforestation on mitigating climate change in Nigeria. The study is funded by the Ecological Fund Office (EFO) within the framework of Presidential Afforestation project. The aim of the project is two folds: (i) to study the impact different reforestation scenarios on climate change in Nigeria, and (ii) to quantify amount of carbon dioxide (CO2) that could be sequestered from the reforestation scenarios. It is envisaged that the outcome of the study would inform policy makers on the most suitable strategies for implementing any intended reforestation across the country.

In this study, the impacts of climate change on Nigeria is studied by using a regional dynamic model (RegCM3) to downscale and compare the present-day (1981 -2000) and future (2031-2050) climate simulations, obtained from a global climate model (GCM) called EH50M. The GCM used the IPCC A1B future scenario in the simulations. How reforestation could mitigate the impact of climate change was assessed using series of sensitivity experiments that simulate future climate with seven different reforestations scenarios as boundary condition. Four of these scenarios (i.e. R25, R50, R75 and R100) used different percentages (i.e. 25%, 50%, 75% and 100%, respectively) of random reforestations over the country, while the other three (RSZ, RMZ and RNZ) used zonal (South, Middle belt, and North) reforestation (Fig. 1). The amount of CO2 that can be sequestered in each reforestation scenario is quantified. The following is the step by step summary of the major findings, problems encountered and recommendations for future works.

Scientific Findings:

• The historical record between 1971 and 2000 shows a significant positive trend in temperature over the country. The trend, which is statistically significant (95% confidence level), is about 0.014oC per year for maximum temperature and 0.025oC per year for minimum temperature. This indicates that, over the period, the maximum temperature has increased by 0.4oC, the minimum temperature by 0.8oC. Also, the number of occurrence of heat waves (continuous hot days) has increased over the period.

 

• A significant positive trend (95% confidence level) of about 6 mm day-1 per year was found in rainfall over most part the country between 1971 and 2000. In addition, the onset of rainfall is becoming earlier, and the length of rainy season is increasing over the period.

• A comparison of present-day climate simulation with the observations shows that the combination RegCM3 and EH50M provide a realistic simulation of Nigerian climate. The models reliably reproduce all the important features in the spatial and temporal patterns in the climate (Fig. 2). For instance, the simulation captures the monthly variation of West African monsoon and the associated weather pattern very well over the country. The influence of topography (i.e. Nigerian coastlines, Jos Plateau and Cameroon Mountain) on the spatial distribution of rainfall and temperature are well simulated by the models. The maximum error in the simulated annual rainfall is about ±1 mm/day, with cold bias of about -3oC over the country.                                                                                                                                                                               

 

• The EH50M simulation captures the impact of increased in CO2 (IPCC A1B scenario) on global climate change, while RegCM3 captures the impacts of global climate change and local land-use modification on the regional climate over Nigeria.

• The future climate projection (2031-2050) shows an increase in temperature over the entire country, with the maximum increase (about 1.6oC) in north and minimum increase (1.20oC) to the south (Fig. 3a). The projection does not show any significant change in rainfall pattern over Nigeria, although it suggests a decrease in rainfall over the entire country, and an increase over the mountains like Jos Plateau and Cameroun mountain (Fig. 3b).

 

• An increase in evapotranspiration is suggested over the entire country in future (Fig. 3c). This is consistent with the increase in temperature, because with the increase in temperature, the capacity of the atmosphere to hold more water increases, hence the relative humidity of the atmosphere will reduce (Fig. 3c), and then evapotranspiration will increase. This will lead to a decrease in soil moisture, and may have negative impacts on agriculture.

• The 100% reforestation scenario lowers the projected increase temperature by more than 0.8oC at the northwest of the country, and by more than 0.6oC over the entire country (Fig 4e). In addition, it increases the rainfall by more than 10% in the north, but decreases it by more than 10% in the south (Fig. 5e). However, the deforestation increases both the evapotranspiration and relative humidity over the entire country. The impact of the deforestation is not limited to Nigeria; it extends to the neighboring countries. For instance, it reduces temperature (by up to 10%) and increases rainfall (by up to 10%) over Togo, but does the opposite over Chad (Figs. 4 and 5).

 

• The random reforestation scenarios (R25, R50, and R75) have a similar impact with that of R100, but with a varying pattern over the country (Figs. 4 and 5) They lower the projected temperature in the south and southwest, but increase it in north and northwest. As the percentage of the random deforestation increases, the zero line between the positive and negative temperature change shifts towards the northeast, and the magnitude of the temperature change increases as well. As with R100, the random reforestation increase rainfall in the north and decreases in the south of Nigeria. The pattern does not shift as the percentage of reforestation increases, but the magnitude of the rainfall change increases.

• With the zonal reforestation scenarios, the decrease in temperature occurs mainly over the reforestation zone, but with a region of increase in temperature north of the zone (Fig 4 and 5). The zonal reforestation does not produce and consistent pattern in the rainfall change distribution.

 

• With 100% reforestation in Nigeria about 11,800 million tonnes of CO2 would be sequestered per year (Table 2). This value reduces to about 5,000 million tonnes with north zone reforestation, 4,000 million tonnes with middle zone reforestation, and 2,600 million tonnes with south zone reforestation (Table 2).

 

Problem encountered:

• Due to the insufficiency of meteorological stations together with the problem of missing data in Nigeria CRUD gridded data was used to validate the result of the model.
• Insufficiency infrastructure to carry out the study for more years and at a higher resolution to be able to obtain more accurate result.
• Time constrains were also a major source of challenge faced during the period of this research.

Recommendation:

• To test the robustness of the result presented in this report, future studies should use more regional climate models (WRF and PRECIS) in downscaling other IPCC climate change scenarios (A2 and B1) from at least 5 GCMs simulations. And the simulation period should be extended to 40 years.
• Before embarking on any reforestation activities, the results presented in this study should be adapted to various socio-economical and heath sectors to study the impact of the reforestation on agriculture, health, and water resource management in the country. This will help to provide baseline information on reliable adaptation and mitigation strategies to address climate related problems.
• All the simulations in this study were performed outside Nigeria, because of lack of infrastructure and human capacity on climate modeling in the country. Nigerian government should provide more infrastructure for climate modeling in Nigerian university, and encourage the younger scientists to embrace climate modeling studies. The Climate Research Group in South Africa, where all the simulation for this were performed, is willing to assist in building human capacity on climate modeling in Nigeria.
• We recommend more meteorological data with better station network over Nigeria. This would be useful for model validation and for a better understanding of the Nigerian climate.

Figure 1: Landuse patterns used in the study: Present-day landuse pattern with (a) no reforestation (NRF), (b) north zone reforestation (NZR), (c) middle zone reforestation (MZR), (d) south zone reforestation (SZR), (e) 100% random reforestation (R100), (f) 75% random reforestation (R75), (g) 50% random reforestation (R50), and (h) 25% random reforestation (R25) in Nigeria.


Figure 2: Observed (a, c) and Simulated (b, d) annual temperature (a, b; oC) and rainfall (c, d; mm/day) over Nigeria for present-day climate (1981-2000).


Figure 3: Projected changes in future (2031-2050) climate (June-September) over Nigeria: (a) Temperature change (oC), (b) Rainfall change (%), (c) Evapotranspiration change (%), and Relative humidity change (%).


Figure 4: Impact of reforestation on future temperature (June-September). (a) Future temperature (mm/day) with no reforestation (NRF); changes in the future temperature (oC) due (b) north zone reforestation (NZR), (c) middle zone reforestation (MZR), (d) south zone reforestation (MZR), (e) 100% random reforestation (R100), (f) 75% random reforestation (R75), (g) 50% random reforestation (R50), and (h) 25% random reforestation (R25) in Nigeria.


Figure 5: Impact of reforestation on future rainfall (June-September; mm/day). (a) Future temperature with no reforestation (NRF); changes in the future rainfall (%) due (b) north zone reforestation (NZR), (c) middle zone reforestation (MZR), (d) south zone reforestation (MZR), (e) 100% random reforestation (R100), (f) 75% random reforestation (R75), (g) 50% random reforestation (R50), and (h) 25% random reforestation (R25) in Nigeria.

Table 1: Reforested land (x 10⁶ hectares) in Nigeria under the reforestation scenarios

Reforestation Scenarios	Mangrove	Fresh Water Swamp	Rain Forest	Tall Grass Savanna	Short Grass Savanna	Marginal Savanna	Total (Nigeria)
R25	0.32	0.48	2.68	11.72	7.52	0.76	23.84
R50	1.08	0.92	4.80	23.08	15.64	1.48	47.64
R75	1.52	1.68	6.40	35.56	23.32	2.12	71.48
R100	2.00	2.08	9.16	47.84	30.28	2.84	95.28
RNZ	0.00	0.00	0.00	6.48	24.08	2.84	33.60
RMZ	0.00	0.00	0.96	30.64	6.20	0.00	38.00
RSZ	2.00	2.08	8.20	10.72	0.00	0.00	23.68

Table 2: Sequestered CO2 (x 10⁶ tonnes/year) in Nigeria under the reforestation scenarios

Reforestation Scenarios	Mangrove	Fresh Water Swamp	Rain forest	Tall Grass Savanna	Short Grass Savanna	Marginal Savanna	Total (Nigeria)
R25	35.27	52.91	295.38	1,291.75	828.84	428.88	2,972.73
R50	119.04	101.40	529.05	2,543.84	1,723.85	8,35.189	5,922.93
R75	167.53	185.17	705.40	3,919.39	2,570.29	1,196.35	8,841.19
R100	220.44	229.25	1,009.60	5,272.90	3,337.43	1,602.66	11,791.25
RNZ	0.00	0.00	0.000	714.21	2,654.06	1,602.66	4,992.98
RMZ	0.00	0.00	105.81	3,377.11	683.35	0.00	4,188.33
RSZ	220.44	229.25	903.79	1181.54	0.00	0.00	2,609.97