Who was Wallace “Wally” Broecker?
One of the most influential and outspoken climate scientists died last month aged 87. Wallace Broecker, who spent his entire career at Columbia University’s Lamont-Doherty Earth Observatory, gained recognition through his research in the fields of oceanography and paleoclimatology. He is also accredited as having popularised the term “global warming” in his 1975 publication “Climatic Change: Are We on the Brink of a Pronounced Global Warming”. In this paper, he argues that the climate is about to drastically warm due to the increase in CO2, a notion widely perceived as ‘fake news’ due to the cooling trend during the mid-20th century1. His discoveries in his over 60-year scientific career had great impacts on our understanding of the Earth system and climate. He was an early advocate of action against climate change and he was actively broadening his network to high ranking politicians such as Al Gore and people in business to explain the science behind climate change and convey the importance for climate action. He did so by publishing articles in popular press and speaking on television and radio2. Arguably his biggest contribution to our current understanding of global climate change is the discovery of the thermohaline circulation of the oceans, which his brother-in-law captured with the song “Uncle Wally’s Tale”.
“He has singlehandedly pushed more understanding than probably anybody in our field” – Richard Alley, Climatologist at Pennsylvania State University
The Thermohaline Circulation
Most people might be familiar with the concept of the thermohaline circulation through the Gulf Stream that brings warmth to western Europe and giving it its mild winters. To illustrate, New York is frequently faced with strong blizzards during winter whereas Rome, Barcelona and Lisbon are cities known for their warm and mild climate; however, all four cities are roughly on the same latitude band!
The great ocean conveyor belt, as Broecker referred to the thermohaline circulation, is a collective of currents that transports warm water close to the surface towards the poles and cold water at the bottom back south. Through this circulation all oceans are connected. Broecker contributed greatly to the understanding of this circulation through the discovery that a complete circulation of a water parcel takes between 1000 and 2000 years. On such timescales, ocean-atmosphere interactions become more relevant because of the faster response time of the ocean to an atmospheric forcing. It was prior understanding that such a circulation operates on the timescales of 10,000 years2.
Water that reaches the poles cools until it is cold enough to sink to the bottom. This process is mainly driven by the density of water. As water cools and approaches 4 degrees Celsius, its density increases. Additionally, water high in salinity also increases in density. Thus, in order to sink, the water needs to be cold and highly saline. In the North Atlantic, this process is referred to as the North Atlantic Deep Water (NADW) formation. The NADW formation was the centre of attention of scientific research in the 80s and 90s and Broecker was on the forefront of this research. His investigations revolved around abrupt climate changes during the last glacial period (the last ice age). In Greenland ice cores and sediment cores from the North Atlantic, patterns of abrupt climate changes have been observed. In his paper “An Unstable Superconveyor”, Broecker3 identified a shutdown in the thermohaline circulation as the driving factor.
The “Achilles Heel” of the Climate System
A shutdown of the thermohaline circulation can be caused by a great influx of freshwater that prevents NADW formation because the diluted seawater is less dense and thus cannot sink. Abrupt climate changes, such as the ‘Heinrich events’ that occurred during the last ice age, occurred at times of a shutdown of the conveyor belt. It is thought that, in Broecker’s words, an ‘armada of icebergs’ originating in the Hudson Straight in Eastern Canada flushed the Atlantic with a large amount of freshwater. This influx of freshwater was thought to be large enough to cause the shutdown4–6. While Broecker initially believed that a shutdown of the ocean conveyor only occurred during ice ages, modelling studies soon showed that a shutdown is also possible during an interglacial period, such as the one we are experiencing today7. The warming caused by climate change will enhance the hydrological cycle, which manifests itself through increased precipitation and a stronger freshwater flux into the North Atlantic due to the melting of glaciers and ice sheets. This gradual increase in freshwater would lead to a decrease in strength of the thermohaline circulation until it eventually stops. However, in contrast to the abrupt climate changes of the past, this process is thought to take at least a century8. The consequence of a shutdown would be significant cooling of approximately 5-8 degrees C over Europe. It was the earlier belief that a shutdown would cause immediate climate change leading us into the next ice age (e.g. The Day After Tomorrow by Roland Emmerich). However, this process is projected to proceed slowly and to merely offset the warming due to climate change, abrupt not on human but geologic timescales. Due to this variability in the ocean conveyor and the danger of a shutdown, Broecker called it the ‘Achilles heel’ of the climate system9.
Uncle Wally’s Tale
Uncle Wally’s Tale is a story about the great ocean conveyor that distributes heat across the planet and plays an integral part in the climate system. It is Wallace Broecker’s research into this topic that greatly affected our understanding of how our planet works and how climate change may alter the climate system. The ocean is now an integral part in climate science and the conveyor belt an important variable to consider in climate models trying to predict the climate response to global warming. His impact, however, reached further than the scientific community because of his work in publicising climate science, advocating action against climate change, and educating the public, politicians, and business leaders. It was his conviction that the climate system is an “angry beast” and we are “poking it” by adding greenhouse gases into the atmosphere requiring urgent action.
- Broecker, W. Climatic Change : Are We on the Brink of a Pronounced Global Warming? Science (80-. ). 189, 460–463 (1975).
- Kimball, R. Profiles in Science for Science Librarians: Wallace Broecker. Sci. Technol. Libr. 30, 212–228 (2011).
- Broecker, W. An unstable superconveyor. Nature 367, 414–415 (1994).
- Hemming, S. R. Heinrich events: Massive late Pleistocene detritus layers of the North Atlantic and their global climate imprint. Rev. Geophys. 42, (2004).
- Broecker, W., Bond, G., Klas, M., Clark, E. & McManus, J. Origin of the northern Atlantic’s Heinrich events. Clim. Dyn. 6, 265–273 (1992).
- Bond, G. et al. Evidence for massive discharges of icebergs into the North Atlantic ocean during the last glacial period. Nature 360, 245–249 (1992).
- Broecker, W. Converging Paths Leading to the Role of the Oceans in Climate Change. Annu. Rev. Energy Environ. 25, 19 (2000).
- Broecker, W. Global warming: Take action or wait? Chinese Sci. Bull. 51, 1018–1029 (2006).
- Broecker, W. Thermohaline Circulation, the Achilles Heel of Our Climate System: Will Man-Made CO2 Upset the Current Balance? Science (80-. ). 278, 1582–1588 (1997).
Photograph retrieved from: https://blogs.ei.columbia.edu/2019/02/19/wallace-broecker-early-prophet-of-climate-change/
Figures: Great Ocean Conveyor Belt & Angry Beast retrieved from ref 3 (Broecker, 1994)