Olive trees in Lebanon – historically renowned for the high-quality olive oil they produce – are under threat from rising temperatures, new research finds.
Olives were first domesticated some 7,000 years ago in the Middle East. Olive oil has since become a staple of the Mediterranean diet and today drives a $3bn global industry. In Lebanon, olive trees are, on average, 150 years old, and occupy nearly one-quarter of the country’s agricultural surface.
New research, published in Nature Plants, presents 5,400 years of pollen data collected in the Lebanese city of Tyre. It finds that olive production has been closely linked to temperature for thousands of years, and reveals an optimal temperature for olive growth of 16.9C.
The researchers suggest that olives produced in Tyre were “sought after” in ancient times for their “high nutritional value and refined taste”, thanks to the dry climate of the city. However, they warn that rising temperatures will have “detrimental consequences” on olive tree growth by mid-century – especially in the country’s southern regions, which will become “too hot” for optimal flowering and fruiting.
Olive trees “form an important part of the Lebanese cultural heritage”, giving “a sense of unity and belonging in an otherwise politically segmented country”, a Lebanese scientist, who was not involved in the study, tells Carbon Brief.
He warns that the negative impacts of climate change on olive production will negatively impact the country’s culture and economy, at a time when “both are crucially needed”.
Cultivating the olive
Olives are among the oldest cultivated species in the world. The olive was first domesticated around 7,000 years ago in the Levant – an area generally defined as comprising today’s Lebanon, Syria, Iraq, Palestine, Israel and Jordan – and rapidly became a backbone of trade and commerce from the Mediterranean to western Iran.
Olive oil trading boomed during the Bronze Age, around 3300-1200BC, and olives soon became a symbol of peace and spirituality – holding cultural importance in ancient societies ranging from Egypt to Greece. Even today, meetings of the United Nations take place under a flag featuring two olive branches as a symbol of peace (pdf).
Lebanon is a small player in the global olive oil market, driving less than 1% of global production. Nevertheless, olive farming is a key sector for the Lebanese economy and is responsible for 7% of its agricultural GDP (pdf). The country’s olive trees are 150 years old on average, cover almost one-quarter of the country’s agricultural surface and are tended to by an estimated 170,000 Lebanese farmers (pdf).
Raed Hamed is a PhD student at Vrije Universiteit Amsterdam, who studies the impacts of climate variability on staple crop production and was not involved in the study. Hamed is a Lebanese national and tells Carbon Brief that olive trees “form an important part of the Lebanese cultural heritage”. He adds that the trees are found across the nation, giving “a sense of unity and belonging in an otherwise politically segmented country”.
To determine the historical activity of olive trees in the region, the study authors took a 390 centimetre (cm) sediment core from the Lebanese city of Tyre, located 83km south of Beirut.
Sediment cores are a key source of proxy data, which can give scientists a record of the world’s climate going back thousands of years, before dedicated measurements were collected. In this instance, the authors measured the density of pollen grains every 2cm throughout the sediment core, revealing the rate of pollen production and flowering in olive trees over a 5,400 year period.
Using a combination of statistical analyses and modelling, they used the pollen data to reconstruct historical temperature and rainfall levels in Tyre.
The plot below shows the results from the pollen analysis. It shows annual average temperature (red), total annual rainfall (blue) and olive pollen accumulation (green) from 8,000 years ago (bottom) to 2,000 years ago (top).
The right-hand column explains the changing relationship between humans and olive trees over time – including the domestication of olive trees and development of olive trade.
The soil core analysis reveals olive tree pollen in Tyre as early as 7,700 years ago – before the city was founded – indicating the presence of wild Lebanese olive trees. The authors add that when olive tree domestication increased during the Bronze Age,, so too did the pollen count in the soil core.
The results show that olive tree flowering has largely followed annual temperature trends. The paper says:
“The occurrence of olive trees in Lebanon seems to have been controlled by climate parameters since the Neolithic, even if human societies, since the Late Chalcolithic and Early Bronze Age, have domesticated the tree for economic reasons.”
Dr Luigi Ponti – a research scientist at Italy’s National Agency for New Technologies, Energy and Sustainable Economic Development, who was not involved in the study – calls the reconstruction using soil core data “an incredibly good idea”, noting that pollen emission data “has been shown to work as a predictor of olive yield” in previous studies.
To further investigate how temperature and rainfall impact olive tree growth, the authors identify 325 current olive-growing areas around the Mediterranean Basin, and use climate databases to determine their modern climatic conditions.
Using these records, the paper identifies an optimal temperature for olive flowering at 16.9C, with a suitable range of 15.7C-18.3C. It also finds an optimal annual rainfall of 575 millimetres (mm), with lower and upper boundaries of 447mm and 672mm.
Dr David Kaniewski – a researcher in the department of biology and geosciences at the Paul Sabatier University and lead author of the study – tells Carbon Brief that this is the first time a study has found an optimum temperature for growing olives.
The authors then compare the temperature and rainfall preferences of modern-day olive trees with those of ancient olive trees at Tyre, as shown in the plots below.
Green dots show the activity of ancient olive trees in Tyre, at different temperatures (top) and rainfall levels (bottom), based on pollen measurements. The orange (top) and blue (bottom) shading indicate the suitable ranges of temperature and rainfall for olive tree flowering, based on data from modern-day olive growing regions.
The authors find that, historically, olive trees in Tyre produced the most pollen when temperature and rainfall matched the climatic conditions favoured by modern-day olive trees. This means that Mediterranean olive trees have not substantially changed their climatic preferences for the past 8,000 years, the authors conclude.
Olive trees are sensitive to seasonal changes in climate, so the authors use the same method to compare the climatic preferences of modern-day and ancient olive trees for specific months and seasons. Again, they find that these preferences are broadly similar.
The authors note that, when olives ripen and are harvested over October-November, they need at least 105mm of rainfall, with an optimum value of 135mm. In Tyre, they find that average rainfall over October-November averaged just 103mm for thousands of years. Interestingly though, rather than killing the olives, the authors suggest that this lack of water may have enhanced them.
The authors explain that when olive trees lack water, chemical compounds often build up in the olives, with the side effect of increasing their nutritional value and changing their flavour. They postulate that olive oil from Tyre, while not plentiful, may have been sought after in ancient times for its “high nutritional value and refined taste”.
Climate change and olive trees
To estimate how climate change may impact Lebanese olive trees over the coming century, the authors split the country into five regions. They calculate the temperature rise for each region by extrapolating the average rate of temperature rise over 1960-2020.
Based on this past warming, they project that the temperature will rise by 2.2-2.3C by the end of the century across the different regions of Lebanon. This is “consistent with” projections for the Mediterranean Basin under the low-emission SSP1-2.6 scenario from the Sixth Coupled Model Intercomparison Project, the authors say.
Kaniewski tells Carbon Brief that this method was chosen because it was “the only way to use the data directly recorded in Lebanon”, as model projections often have a large spatial resolution.
Hamed “commends” the paper, but tells Carbon Brief that its method of estimating future temperature rise is “crude”. He emphasises that the temperature projections in this study are only consistent with the “conservative” SSP1-2.6 projections, and the impacts of climate change could be more severe than those discussed in the study.
He adds that the combined impacts of heat and moisture can negatively impact crops, so “it would have been nice to explore in tandem future moisture trends as well as their potential interaction with temperature on olive flowering and production”.
The charts below show the temperature anomaly for five regions in Lebanon, compared to a 1961-90 baseline (green), and the optimal growing temperature for olive trees (red). The map shows the location of the five regions in Lebanon, coloured according to their average temperature in 2020.
The authors find that rising temperatures will have “detrimental consequences” on Lebanese olive tree growth and olive oil production by mid-century – especially in the country’s southern regions, which will become “too hot” for optimal flowering and fruiting.
Meanwhile, Western Lebanon will reach the upper threshold for suitable growing temperature over that same time period, the study finds. It adds that while eastern Lebanon will warm by more than 2C by mid-century, temperatures will still be below the 15.7C threshold – making it too cold for “optimal” olive production.
Hamed tells Carbon Brief that the negative impacts of climate change on olive production will negatively impact the country’s culture and economy, at a time when “both are crucially needed”.
And the paper warns of a wider impact on the Mediterranean region:
“At a Mediterranean scale, the effects of climate change on olive oil production and economy must be considered as a serious threat to current and future production.”
Kaniewski et al (2023), Climate change threatens olive oil production in the Levant, Nature Plants, doi:10.1038/s41477-022-01339-z
Climate change could make parts of Lebanon ‘too hot’ for producing olive oil
How 5,000 years of data reveals climate change ‘threat’ to Lebanon’s olive oil