Analysis | Reconstructing the past 5,000 years of climate history in the Southern Hemisphere

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Antonio Lara, Ricardo Villalba, Rocío Urrutia-Jalabert, Álvaro González-Reyes, Juan Carlos Aravena, Brian Luckman, Emilio Cuq, Carmen Rodríguez, and Alexia Wolodarsky-Franke

The use of natural indicators to determine what the climate looked like in the past is vital for understanding the present-day climate and its variations over time.  These indicators, also called proxies (long-term climate archives)[1], can include coral reefs, ice cores, lakebed and seabed sediment, and tree rings.

In fact, it was tree rings that were used to determine temperature changes over the past 5,682 years in southern South America, making this the most extensive climate record for the Southern Hemisphere, providing information with annual resolution, and extending previous records by more than 2,060 years.  The researchers who participated in this  study published in the journal Quaternary Science Reviews worked with the rings of the Alerce tree (Fitzroya cupressoides), the second longest-lived species on the planet, which can live up to 3,620 years. Alerce grows exclusively in remote parts of Chile’s Los Lagos and Los Ríos regions and in Argentina near the Chilean border. This temperature reconstruction was achieved by dating and measuring the rings on 480 samples taken from both living and fallen trees – to a precision of 0.001 mm – establishing the relationship between ring width and temperature by using regional weather station records dating back to 1959.

The records show a warming trend in southern South America over recent decades (1960-2009), which is not exceptional in the context of the last 5,000 years (Figure 1). For example, other major warm periods occurred between 3140 and 2800 BC, and from 70 BC to 150 AD, coinciding with the absence of glacial advances.

Figure: Reconstruction of maximum summer temperatures over the past 5,680 years. The red line shows the trend of the reconstructed temperature.

One novel finding of this study is the long-term coincidence between temperature and the solar irradiation record at the global scale, in cycles of 293, 434, 512, and even 746 years. This points to the persistent influence of solar activity on climate at the multi-centennial scale for southern South America. For shorter timescales – years or decades – the study shows that temperature in this zone is primarily related to the internal climate variability of the Pacific Ocean, including the El Niño phenomenon. As other studies have also shown, this relationship indicates that the mid-latitudes of the Southern Hemisphere is one of the slowest-warming areas on the planet.

The study demonstrates the importance of the growth rings of ancient Alerce trees as valuable historic archives that contain substantial accumulated information about the climate.  This is a powerful reason to protect these forests, which are part of our globally important heritage and are threatened by illegal logging even today, as well as forest fires, which are increasing as a result of the rising temperatures and reduced precipitation associated with climate change in southern Chile.

The study also made it possible to characterize regional-scale natural variables and their drivers, which interact with current anthropogenic climate change to modulate the local climate and affect communities and ecosystems[2].

The database compiled for this study, with more than 388,000 tree-ring records, can be found at: the Center for Climate and Resilience Research.

Notes

[1] Edited. The term proxy was replaced by proxies in italics and «(long term climate archives)» was added.

[2] Edited. The third to last paragraph was changed and the second to last paragraph was inserted for more clarity.  27 May 2020.