The study is the longest tree-ring data series ever produced for the Greater Horn of Africa.
The availability of the data and information found in the TANA chronology can inform the development of long-term prevention and adaptive strategies to avert climate- and drought- related humanitarian crises.
For a long time, a majority of African countries have not been proactively mitigating the negative impact of drought and flood events. As a result, relief initiatives are often too late to stem the loss of lives and other social and economic impacts. To address this challenge, and assessing tree-based data from 1665 through 2014, scientists developed the TANA chronology, a historic dataset named after Lake Tana, the source of the Blue Nile River running through Ethiopia.
Scientists studying tree ring history to establish climate patterns – a practice known as dendrochronology – scrutinized 365 years of East Africa’s climate history to better anticipate and plan for the climactic challenges facing the region. Their research is compiled from 36 individual tree ring series of the Juniperus procera (African pencil cedar) species, which were then accurately cross-dated from four study sites in the upper Blue Nile River Catchment in Northern Ethiopia.
The study is the longest tree-ring data series ever produced for the Greater Horn of Africa and the availability of the data and information found in the TANA chronology can inform the development of long-term prevention and adaptive strategies to avert climate- and drought- related humanitarian crises. The data also contribute to achieving development priorities in the region, as outlined in multiple Sustainable Development Goals (SDGs). These include SDG 13, on building resilience and reducing the impacts of climate change by predicting periods where there is likelihood of dry and wet events; as well as “knock-on effects” to SDG 2, which seeks to end hunger, achieve food security and improved nutrition; as well as SDG 6, which aims to provide universal access to safe water and sanitation.
Collecting Reliable Data from the 19th to 21st Century
Climate data from the 19th century was reconstructed to provide information on precipitation variables. Looking at this period, average June to September and annual rainfall for the region was estimated at 909 mm and 1,109 mm for each century. Reconstructed data on rainfall patterns show below average rainfall in the region was at its peak in the period 1961 to 1990, while above average rainfall occurred in the period 1841 to 1870. Dry events, or periods where rainfall was below average, were likely to vary within a period of 2 to 3.8 years, and showed variations of 6 to 10 years when analyzed over a period of 10 consecutive years (decadal) and 12 to 24 years when analyzed over multiple decades (multidecadal), mainly during the second half of the 19th century and during 1960 to 1990. At the turn of 21st century (2001 to 2014), already eight below average rainfall years have occurred, showing increased frequency of dry events.
The Juniperus procera trees proved reliable sources of data for this study because their distinct annual growth ring boundaries are a result of the distinct rainfall seasons in the region. The accuracy of the collected data was confirmed by comparing existing data from a meteorology station near the study sites in northern Ethiopia with other existing data sources. Further, data from the study captured climate trends for the wider Greater Horn of Africa region and can be used as proxy, since it positively relates with historic events of droughts and flooding on the continent.
These and much more data explored in the study are not just numbers in history.
The study highlights the high potential of tree-ring research to reconstruct long-term climate variability in East Africa and to evaluate recent climatic trends and their adverse effect on human livelihoods. It underpins the importance of integrating event-focused (event-effect), and long-term trend based climate change studies to fully understand the weather and climate variations across the Greater Horn of Africa, rather than only focusing on long-term climate change investigations, which may underestimate inter-annual climate variability.
Knowing that below and above average rainfall can occur every 2 to 3.8 years is valuable information that governments can use to plan for and adapt to climate shocks over a period of at least two to five years. This can help mitigate the potential for climate and related humanitarian crises. The information can also support resilient agricultural practices including proper food collection, storage and preservation mechanisms; proper attention to infrastructure; and the introduction of drought resistant seeds. It can also be used to guide water management systems, especially since dry events are occurring with increasing intensity and frequency over long periods of time in East Africa.
Historic climate patterns are an important factor in understanding current climate fluctuations and are crucial for predicting future climate patterns.
Importance for Africa and the SDGs
One of the biggest challenges facing climate change mitigation in developing nations, particularly in Africa, is the lack of long-term climate data. According to the Intergovernmental Panel on Climate Change (IPCC), the available climate data on Africa cover a short period of time and are not continuous. Moreover, climate reconstructions are too limited and the lack of long-term records on elements such as rainfall changes do not allow for comprehensive investigations.
Dr. Solomon Dawit, who leads the East Africa Region CGIAR research program on Climate Change, Agriculture and Food Security (CCAFS) supports this conclusion. He points out that “insufficient knowledge of the mechanisms responsible for rainfall distribution over time coupled with a lack of knowledge and skills to predict rainfall anomalies contributes to inability to deal with extreme events such as drought.”
The TANA chronology, therefore, is a big step towards crossing this barrier. Historic climate patterns are an important factor in understanding current climate fluctuations and are crucial for predicting future climate patterns. Dr. Teferi Demissie, a former meteorologist at the National Meteorology Agency in Ethiopia, sees this study as a first attempt at understanding the past climate variations in Ethiopia using unique tree-ring proxy datasets. It can be a tool that provides information about potential future droughts and water scarcity.
Elizabeth Kahurani is a Communications Specialist with the World Agroforestry Centre (ICRAF). This article was written with contributions from Dr. Mulugeta Mokria, a joint PhD Researcher with The World Agroforestry Centre (ICRAF) and Erlangen University; Dr. Aster Gebrekirstos, a Scientist heading ICRAF’s Dendrochronology Lab; Dr. Abrham Abiyu Director of Forestry Research Directorate of Amhara Agricultural Research Institute; Prof Meine vanNoordwijk Chief Science at ICRAF and Professor at Wageningen University; Prof Achim Braeuning of the University of Erlangen.
For more information, please see the Journal Article: Mokria, M., Gebrekirstos, A., Abiyu, A., Noordwijk, M. V. and Bräuning, A. (2017), Multi-century tree-ring precipitation record reveals increasing frequency of extreme dry events in the upper Blue Nile River catchment. Glob Change Biol.. doi:10.1111/gcb.13809; http://rdcu.be/veVL