Glimpsing the Future Through Research into Past Climates

by Adin Richards '23
Edited by Maximilian Bonnici  - MS '20

Equatorial Africa is one of the world’s fastest growing regions, but it is also among the most vulnerable to climate change. Vital information for its farmers, consumers, and policy makers, and indeed for the global community, may lie in … lake sediments and computer models? The connection seems far from direct, but to a community researchers in the field of paleoclimatology, knowledge of how our world is responding to environmental change can be found in subtle clues to our past, chronicled in ancient deposits and mapped through new climate simulations.

Earth’s surface and biosphere are shaped by climate. The climate of the past, therefore, can be read from records of ancient environments preserved in a menagerie of media, from ice cores to tree rings. This is the guiding premise of paleoclimate research [1], a field which, though vital, may not seem an obvious first choice as a discipline of study. Indeed, for Brown University’s James Russell, it certainly wasn’t. Russell says that “there was no ‘aha moment’ when I knew what I wanted to study.” As with many of those going into the sciences, a curiosity about why the world looks the way it does was the impetus for Russell to begin what would turn into a career in research. We don’t, however, live in a world where pure inquisitiveness for its own sake is the only motivation for scientific pursuits. “Climate change was a growing concern during my time as an undergraduate,'' Russell notes. In graduate school, by studying paleoclimatology, he found “research that merged [his] interests in Earth History with the climate sciences.”

This merging comes from the ability of ancient climates to serve as indicators of how Earth responds to changing temperatures and atmospheric composition (phenomena which even passive news consumers are all too familiar with today) and as data for testing models of the complex systems which govern our planet. A key period of interest is the end of the Last Glacial Maximum (LGM) to the start of the Holocene, the epoch in which human civilization developed [2]. The interest in this approximately ten thousand year time span is due to its analogy to our present era, for the end of the LGM similarly marked a period of warming and an increase in atmospheric greenhouse gases (GHGs) [3]. Many questions about how our planet is responding to anthropogenic atmospheric changes still trouble researchers, and one crucial unknown is the exact effect which increased GHGs have on precipitation patterns, notably in tropical Africa [4] where fluctuations in rainfall could spell disaster for this climate-sensitive region, with ramifications for the economic and political stability of the region [5,6]. 

In addressing this gap in our knowledge, Russell and colleagues found a role for a paleoclimate investigation. It has been established through lake sediment analysis that around 14,700 years ago, about the middle of the aforementioned era of interest, the African continent experienced a strong episode of increased precipitation, with monsoons even reaching the Sahara, which was thereby turned into a fertile steppe [7]. This precise picture of past rainfall can be painted through various investigative strategies. One approach that is increasingly popular within the field, and that is used by Russell and associates at Brown, involves analysis of n-alkanes, or saturated hydrocarbons. Specifically, the hydrogen in these lipids will have a ratio of hydrogen to deuterium, a naturally heavier form of hydrogen. The exact ratio in which these two forms of hydrogen are found can indicate the environment of the plant that made the hydrocarbon. Knowing what specific ratios mean about the formative environment allows researchers to describe rainfall patterns going back thousands of years [8].

The bulk of the evidence indicates that there was a prolonged period of time, not too far back, when Africa was significantly wetter than it is today. We know this largely from lake sediments, but these do not answer the question of why there was an increase in precipitation and, importantly, what this episode has to tell us about our future. To tackle this question, Russell used several computer models of the past climate, one which tried to account for many changing factors that were likely influential in increasing rainfall and two which isolate specific variables. Of the many complex and not fully understood drivers of ancient Africa’s climate, the two that were isolated in Russell’s 2014 report in order to assess their specific importance were orbital forcing and GHG increase. Orbital forcing is the long term cyclical change in how heat from the sun is distributed across Earth’s surface. Russell and colleagues were able to conclude that the factor that most contributed to the increase in Africa’s rainfall at the time investigated was a natural increase in GHGs [4].

Plans to protect against possible increases in food insecurity in equatorial Africa may hinge on these and similar findings in the obscure field of paleoclimatology. In a 2018 report [6], the United Nations Economic Council for Africa asserted that “in many [Sub-Saharan] countries climate–related disasters were among the reasons for rising levels of hunger.” The same report discusses the relative confidence in warming trends but notes with concern that a similarly refined model of likely future precipitation has not been established [6]. A recent article in Nature discussing the implications of variable precipitation for water resource management in Sub-Saharan Africa further noted the clear need for “improvements in models of climate and hydrology.” [9] Understanding that GHGs were a primary contributor to increased rain in the past represents a vital step toward developing these models.

Russell sees his calling as a paleoclimate researcher as a venture in “understanding how past climates affected landscapes and ecosystems, and how those interactions produced the Earth we live in today.” Like us all, he notes that the Earth is a product of its history. It seems that the venerated axiom “Speak to the past and it shall teach thee” may well be a guiding light in our collective efforts to adapt to a changing world.

Works Cited:

​[1] NOAA. Paleoclimatology Data [Internet] [10/5/19]. Available from: https://www.ncdc.noaa.gov/data-access/paleoclimatology-data
[2] University of California, Berkeley. The Holocene Epoch [Internet] [10/5/19]. Available from: https://ucmp.berkeley.edu/quaternary/holocene.php
[3] Biello, D. What Thawed the Last Ice Age?. Scientific American [Internet]. 2012 [10/5/19] Available from: https://www.scientificamerican.com/article/what-thawed-the-last-ice-age/
[4] Otto-Bliesner, B.L., Russell, J.M., Clark, P.U., Liu, Z., Overpeck, J.T., Konecky, B., et. al. Coherent changes of southeastern equatorial and northern African rainfall during the last deglaciation. Science [Internet]. 2014 {10/5/19]; 346 (6214), 1223-1227. DOI: 10.1126/science.1259531
[5] Intergovernmental Panel on Climate Change. AR5 Climate Change 2014: Impacts, Adaptation, and Vulnerability [Internet] [10/5/19]. Available from: https://www.ipcc.ch/report/ar5/wg2/
[6] Food and Agriculture Organization of the United Nations. Africa Regional Overview of Food Security and Nutrition [Internet] [10/5/19]. Available from: https://www.uneca.org/sites/default/files/uploaded-documents/2018-Africa-Regional-Overview-of-Food-Security-and-Nutrition-Report/sofi_inbrief.pdf
[7] deMenocal, P. B. & Tierney, J. E. (2012) Green Sahara: African Humid Periods Paced by Earth's Orbital Changes. Nature Education Knowledge 3(10):12 Available from: https://www.ldeo.columbia.edu/~peter/site/Papers_files/deMenocal.Tierney.2012.pdf
[8] Sachse, D., Billaut, I., Bowen, G.J., Chikaraishi, Y., Dawson, T.E., Feakins, S.J., et. al. Molecular Paleohydrology: Interpreting the Hydrogen-Isotopic Composition of Lipid Biomarkers from Photosynthesizing Organisms. Annual Reviews [Internet]. 2012 {10/5/19]; 409, 221-249. Available from: https://doi.org/10.1146/annurev-earth-042711-105535
[9] Healy, R.W.. The future of groundwater in sub-Saharan Africa. Nature [Internet]. 2019 [10/5/19]. Available from: https://www.nature.com/articles/d41586-019-02337-6