Rare ground data confirm significant warming and drying in western equatorial Africa

Abstract

Background The humid tropical forests of Central Africa influence weather worldwide and play a major role in the global carbon cycle. However, they are also an ecological anomaly, with evergreen forests dominating the western equatorial region despite less than 2,000 mm total annual rainfall. Meteorological data for Central Africa are notoriously sparse and incomplete and there are substantial issues with satellite-derived data because of persistent cloudiness and inability to ground-truth estimates. Long-term climate observations are urgently needed to verify regional climate and vegetation models, shed light on the mechanisms that drive climatic variability and assess the viability of evergreen forests under future climate scenarios. Methods We have the rare opportunity to analyse a 34 year dataset of rainfall and temperature (and shorter periods of absolute humidity, wind speed, solar radiation and aerosol optical depth) from Lopé National Park, a long-term ecological research site in Gabon, western equatorial Africa. We used (generalized) linear mixed models and spectral analyses to assess seasonal and inter-annual variation, long-term trends and oceanic influences on local weather patterns. Results Lopé’s weather is characterised by a cool, light-deficient, long dry season. Long-term climatic means have changed significantly over the last 34 years, with warming occurring at a rate of +0.25 °C per decade (minimum daily temperature) and drying at a rate of −75 mm per decade (total annual rainfall). Inter-annual climatic variability at Lopé is highly influenced by global weather patterns. Sea surface temperatures of the Pacific and Atlantic oceans have strong coherence with Lopé temperature and rainfall on multi-annual scales. Conclusions The Lopé long-term weather record has not previously been made public and is of high value in such a data poor region. Our results support regional analyses of climatic seasonality, long-term warming and the influences of the oceans on temperature and rainfall variability. However, warming has occurred more rapidly than the regional products suggest and while there remains much uncertainty in the wider region, rainfall has declined over the last three decades at Lopé. The association between rainfall and the Atlantic cold tongue at Lopé lends some support for the ‘dry’ models of climate change for the region. In the context of a rapidly warming and drying climate, urgent research is needed into the sensitivity of dry season clouds to ocean temperatures and the viability of humid evergreen forests in this dry region should the clouds disappear.

Data-driven estimates of evapotranspiration and its controls in the Congo Basin

“…The results in Table 1 agree well with those of Nicholson et al (2019), which found CHIRPS2 and PERSIANN-CDR to be more accurate than TRMM and GPCC in the Congo Basin after 1998. Prior work has also demonstrated that CHIRPS2 is among the best P products available for central Africa and outperforms TRMM and PERSIANN-CDR on a monthly basis (Dembélé and Zwart, 2016;Camberlin et al, 2019;Nicholson et al, 2019), consistent with the results in Table 1. Given the decreasing availability of Congolese rain gauge data in the GPCC database and the difficulty of measuring P with satellite remote sensing in central Africa (McCollum et al, 2000;Yin and Gruber, 2010;Awange et al, 2016;Nicholson et al, 2018), it is not surprising that the GPCC-based products generally displayed higher errors.…”

“…While PER-SIANN is also available without the GPCC gauge corrections that make PERSIANN-CDR so similar to GPCC v7 and TRMM 3B43 over the Congo Basin (Nguyen et al, 2018), it was not used here because it severely overestimates P across Africa (Beighley et al, 2011;Thiemig et al, 2012). Several recent studies have found that TRMM Version 7 3B43 and PERSIANN-CDR both perform reasonably well over central 4192 M. W. Burnett et al: Data-driven estimates of evapotranspiration and its controls in the Congo Basin Africa (Munzimi et al, 2015;Awange et al, 2016;Camberlin et al, 2019).…”

“…To that end, the Climate Hazards InfraRed Precipitation with Station data version 2.0 (CHIRPS2) product, which uses two thermal infrared datasets and interpolated gauge data, was also included as a more independent dataset (Funk et al, 2015a). The CHIRPS2 product was recently found to be among the most accurate rainfall datasets on monthly timescales within the Congo Basin (Camberlin et al, 2019).While CHIRPS2 does incorporate some gauge data that overlap with the GPCC product, it is not scaled to fit GPCC data to the same extent as TRMM 3B43 or PERSIANN-CDR are over central Africa (Nicholson et al, 2019) -in fact, the near-total lack of rain gauges within the basin in both CHIRPS2 and GPCC leads to a low correlation between the two datasets within the study area (Funk et al, 2015a), indicating the P datasets maintain a high degree of independence. Finally, a recent gauge-based dataset developed for the Congo Basin, NIC131-gridded, served as another independent precipitation data source with coverage through 2014 (Nicholson et al, 2018).…”

“…Likewise, microwave backscatter data from large areas of the Congo's evergreen forests imply canopy biomass and/or water content peak during JJA and, to a lesser extent, DJF (Guan et al, 2013;. But phenological observations from Gabon indicate that new leaf growth is sup-pressed during JJA and that the full mature-leaf tree canopies common in JJA contain an elevated fraction of senescing leaves, so dry-season backscatter peaks in Congolese evergreen forests may not be attributable to widespread leafflushing events (Bush, 2018). Overall, the phenological synchronicity of leaf-out events appears to be low in the evergreen forests of central Africa (Couralet et al, 2013;Bush, 2018), so the magnitude and hydrologic effects of evergreen leaf flushing are probably smaller and more temporally distributed in the evergreen forests than in the deciduous woodlands of the basin.…”

“…2016; Dezfuli, 2017). Such shifts are particularly concerning because Africa's tropical rainforests are already significantly drier than other humid tropical forests and exist at the climatic threshold of conversion from evergreen to deciduous trees (Guan et al, 2015;Philippon et al, 2019;Bush et al, 2020).…”