Climatic and environmental change in Africa during the last two centuries

Abstract

Climatic and environmental changes in Africa during the last 2 centuries have been examined, using both systematic rainfall records and proxy information concerning lakes and rivers and the occurrence of famine and drought. The rainfall records provide excellent detail for the 20th century. The proxy data have been used to produce a semi-quantitative data set spanning most of the continent and having an annual time resolution. These provide an overview of conditions during the 19th century. Various issues related to the causes of these 2 centuries of variability are also considered: atmospheric and oceanic processes, desertification, surface albedo, mineral dust and hydrological feedbacks. The most significant climatic change that has occurred has been a long-term reduction in rainfall in the semi-arid regions of West Africa. This was on the order of 20 to 40% in parts of the Sahel. There have been 3 decades of protracted aridity. Nearly all of Africa has been affected by increased aridity, particularly since the 1980s. Few changes in temperature have been demonstrated. These have occurred on a much smaller scale and are of considerably lower magnitude than those over the continents. The rainfall conditions over Africa during the last 2 to 3 decades are not unprecedented. A similar dry episode prevailed during most of the first half of the 19th century. By mid-century, conditions more typical of the 'normal' for the current century again prevailed. Thus, the 3 decades of dry conditions evidenced in the Sahel are not in themselves evidence of irreversible global change. On the other hand, the processes controlling rainfall over most of the continent are now reasonably well understood. One of the most important factors, particularly in the Sahel, is sea-surface temperatures. It has been hypothesized that anthropogenic changes in the land surface, particularly land use change and desertification, have contributed significantly to the decline in rainfall. Current evidence suggests that if changes in the land surface (e.g., vegetation cover, surface albedo, soil moisture) signficantly impact climate, they are much more strongly controlled by natural climate variations, such as the recent decline in rainfall, than by human-induced land-use change or degradation. Unfortunately, we still do not have any accurate large-scale assessments of the extent, nature and degree of such changes. The dreaded 'desertification' process appears to be confined to relatively small scales. However, there is still cause for concern because the net effect of the various feedback processes involved in land degradation appears to be destabilization of ecosystems. Thus, a priority must be large-scale monitoring of the land surface and estimates of the degree of change. KEY WORDS: Africa · Climate · Rainfall · EnvironmentResale or republication not permitted without written consent of the publisher

The response of African land surface phenology to large scale climate oscillations

“…7 shows the regions with significant correlations (p b 0.1) between SOS and MEI and cumulative NDVI and PDO. The figure reveals a familiar and wellknown north-south division of ecoclimatic regions (Nicholson, 2001). Table 2 reveals that 5.1% of the pixels correlated with MEI in the June-August period have a q-value of less than 50%, meaning that for only 5.1% of the pixels the expected proportion of false positives incurred when calling that pixel significant is less than 50%.…”

The global footprint of persistent extra‐tropical drought in the instrumental era

“…The 1870s and early 1890s in the Nile records are more characteristic of a La Niña-like response, with both periods recording a consistent lack of El Niño-related lowstands (Quinn, 1993). Further south, historical accounts and lake level reconstructions from equatorial east Africa indicate lower water levels in the earlyto mid-nineteenth century, followed by, in most cases, steadily rising levels in the following one to two decades (Sieger, 1887(Sieger, , 1888 (as referenced by Walsh and Musa, 1994); Owen and Crossley, 1989;Nicholson, 1998Nicholson, , 1999Nicholson, , 2001Nicholson and Yin, 2001;Verschuren, 2001). However, the resolution and the complexity of interpreting these historical lake level records, hinders their interpretation on the multi-year timescales required here.…”

“…However, the resolution and the complexity of interpreting these historical lake level records, hinders their interpretation on the multi-year timescales required here. In southern Africa, a region characteristically wet during most of the identified extra-tropical drought events, missionary documents from the Kalahari region Nash and Endfield, 2002), climate chronologies from the former Cape Province of South Africa (Lindesay and Vogel, 1990), and semi-quantitative southern African rainfall timeseries from Nicholson (2001) each indicate a dry late 1850s to early 1860s. Meanwhile, the 1870s and 1890s are both wet (Lindesay and Vogel, 1990;Nicholson, 2001), equivalent to a La Niña rainfall signal in this region.…”

“…In southern Africa, a region characteristically wet during most of the identified extra-tropical drought events, missionary documents from the Kalahari region Nash and Endfield, 2002), climate chronologies from the former Cape Province of South Africa (Lindesay and Vogel, 1990), and semi-quantitative southern African rainfall timeseries from Nicholson (2001) each indicate a dry late 1850s to early 1860s. Meanwhile, the 1870s and 1890s are both wet (Lindesay and Vogel, 1990;Nicholson, 2001), equivalent to a La Niña rainfall signal in this region. In summary, the historical and proxy evidence from northern and southern Africa display a mixed signal during the 1860s, whilst the 1870s and 1890s periods share the African hydroclimatic footprint of subsequent instrumental global drought regimes more convincingly.…”

Building Resilience to Drought in the Sahel by Early Risk Identification and Advices

“…In the late 1970s, as a consequence of recurrent droughts that occurred in East and West Africa (Nicholson 2001), famine struck millions of people, creating a need for national authorities and the international community to coordinate efforts to limit the dramatic impact on the affected population. Since then, an integrated famine early warning system (EWS) has been established in West Africa by the Permanent Inter-State Committee for Drought Control in the Sahel (CILSS), which is still one of the most effective and reliable mechanisms operating in sub-Saharan Africa in order to prevent and manage food crises (Genesio et al 2011).…”