Consumers may be happy, but climate change is making coffee farmers bitter. Diseases and pests are becoming more common and severe as temperatures rise. The fungal infection known as coffee leaf rust has devastated plantations in Central and South America. And while robusta crops tend to be more resistant, they need plenty of rain – a tall order as droughts proliferate.
The future for coffee farming looks difficult, if not bleak. But one of the more promising solutions involves developing new, more resilient coffee crops. Not only will these new coffees have to tolerate higher temperatures and less predictable rainfall, they'll also have to continue satisfying consumer expectations for taste and smell.
Finding this perfect combination of traits in a new species seemed remote. But in newly published research, my colleagues and I have revealed a little-known wild coffee species that could be the best candidate yet.
Coffee Farming in a Warming World
Coffea stenophylla was first described as a new species from Sierra Leone in 1834. It was farmed across the wetter parts of upper west Africa until the early 20th century, when it was replaced by the newly discovered and more productive robusta, and largely forgotten by the coffee industry. It continued to grow wild in the humid forests of Guinea, Sierra Leone and Ivory Coast, where it became threatened by deforestation.
At the end of 2018, we found stenophylla in Sierra Leone after searching for several years, but failed to find any trees in fruit until mid-2020, when a 10g sample was recovered for tasting.
Field botanists of the 19th century had long proclaimed the superior taste of stenophylla coffee, and also recorded its resistance to coffee leaf rust and drought. Those early tasters were often inexperienced though, and our expectations were low before the first tasting in the summer of 2020. That all changed once I'd sampled the first cup on a panel with five other coffee experts. Those first sips were revelatory: it was like expecting vinegar and getting champagne.
This initial tasting in London was followed by a thorough evaluation of the coffee's flavour in southern France, led by my research colleague Delpine Mieulet. Mieulet assembled 18 coffee connoisseurs for a blind taste test and they reported a complex profile for stenophylla coffee, with natural sweetness, medium-high acidity, fruitiness, and good body, as one would expect from high-quality arabica.
C. stenophylla growing in the wild, Ivory Coast. E. Couturon / IRD, Author provided
In fact, the coffee seemed very similar to arabica. At the London tasting, the Sierra Leone sample was compared to arabica from Rwanda. In the blind French tasting, most of the judges (81%) said stenophylla tasted like arabica, compared to 98% and 44% for the two arabica control samples, and 7% for a robusta sample.
The coffee tasting experts picked up on notes of peach, blackcurrant, mandarin, honey, light black tea, jasmine, chocolate, caramel and elderflower syrup. In essence, stenophylla coffee is delicious. And despite scoring highly for its similarity to arabica, the stenophylla coffee sample was identified as something entirely unique by 47% of the judges. That means there may be a new market niche for this rediscovered coffee to fill.
The taste testers approved of stenophylla's sweet and fruity flavour. CIRAD, Author provided
Breaking New Grounds
Until now, no other wild coffee species has come close to arabica for its superior taste. Scientifically, the results are compelling because we would simply not expect stenophylla to taste like arabica. These two species are not closely related, they originated on opposite sides of the African continent and the climates in which they grow are very different. They also look nothing alike: stenophylla has black fruit and more complex flowers while arabica cherries are red.
It was always assumed that high-quality coffee was the preserve of arabica – originally from the forests of Ethiopia and South Sudan – and particularly when grown at elevations above 1,500 metres, where the climate is cooler and the light is better.
Stenophylla coffee breaks these rules. Endemic to Guinea, Sierra Leone and Ivory Coast, stenophylla grows in hot conditions at low elevations. Specifically it grows at a mean annual temperature of 24.9°C – 1.9°C higher than robusta, and up to 6.8°C higher than arabica. Stenophylla also appears more tolerant of droughts, potentially capable of growing with less rainfall than arabica.
Robusta coffee can grow in similar conditions to stenophylla, but the price paid to farmers is roughly half that of arabica. Stenophylla coffee makes it possible to grow a superior tasting coffee in much warmer climates. And while stenophylla trees tend to produce less fruit than arabica, they still yield enough to be commercially viable.
The stenophylla harvest on Reunion Island. IRD / CIRAD, Author provided
To breed the coffee crop plants of the future, we need species with great flavour and high heat tolerance. Crossbreeding stenophylla with arabica or robusta could make both more resilient to climate change, and even improve their taste, particularly in the latter.
With stenophylla's rediscovery, the future of coffee just got a little brighter.
Aaron P Davis: Senior Research Leader, Plant Resources, Royal Botanic Gardens, Kew
Disclosure statement: Aaron P Davis receives funding from Darwin Initiative (UK).
Reposted with permission from The Conversation.
These longer periods of stratification could have "far-reaching implications" for lake ecosystems, the paper says, and can drive toxic algal blooms, fish die-offs and increased methane emissions.
The study, published in Nature Communications, finds that the average seasonal lake stratification period in the northern hemisphere could last almost two weeks longer by the end of the century, even under a low emission scenario. It finds that stratification could last over a month longer if emissions are extremely high.
If stratification periods continue to lengthen, "we can expect catastrophic changes to some lake ecosystems, which may have irreversible impacts on ecological communities," the lead author of the study tells Carbon Brief.
The study also finds that larger lakes will see more notable changes. For example, the North American Great Lakes, which house "irreplaceable biodiversity" and represent some of the world's largest freshwater ecosystems, are already experiencing "rapid changes" in their stratification periods, according to the study.
'Fatal Consequences'
As temperatures rise in the spring, many lakes begin the process of "stratification." Warm air heats the surface of the lake, heating the top layer of water, which separates out from the cooler layers of water beneath.
The stratified layers do not mix easily and the greater the temperature difference between the layers, the less mixing there is. Lakes generally stratify between spring and autumn, when hot weather maintains the temperature gradient between warm surface water and colder water deeper down.
Dr Richard Woolway from the European Space Agency is the lead author of the paper, which finds that climate change is driving stratification to begin earlier and end later. He tells Carbon Brief that the impacts of stratification are "widespread and extensive," and that longer periods of stratification could have "irreversible impacts" on ecosystems.
For example, Dr Dominic Vachon – a postdoctoral fellow from the Climate Impacts Research Centre at Umea University, who was not involved in the study – explains that stratification can create a "physical barrier" that makes it harder for dissolved gases and particles to move between the layers of water.
This can prevent the oxygen from the surface of the water from sinking deeper into the lake and can lead to "deoxygenation" in the depths of the water, where oxygen levels are lower and respiration becomes more difficult.
Oxygen depletion can have "fatal consequences for living organisms," according to Dr Bertram Boehrer, a researcher at the Helmholtz Centre for Environmental Research, who was not involved in the study.
Lead author Woolway tells Carbon Brief that the decrease in oxygen levels at deeper depths traps fish in the warmer surface waters:
"Fish often migrate to deeper waters during the summer to escape warmer conditions at the surface – for example during a lake heatwave. A decrease in oxygen at depth will mean that fish will have no thermal refuge, as they often can't survive when oxygen concentrations are too low."
This can be very harmful for lake life and can even increase "fish die-off events" the study notes.
However, the impacts of stratification are not limited to fish. The study notes that a shift to earlier stratification in spring can also encourage communities of phytoplankton – a type of algae – to grow sooner, and can put them out of sync with the species that rely on them for food. This is called a "trophic mismatch."
Prof Catherine O'Reilly, a professor of geography, geology and the environment at Illinois State University, who was not involved in the study, adds that longer stratified periods could also "increase the likelihood of harmful algae blooms."
The impact of climate change on lakes also extends beyond ecosystems. Low oxygen levels in lakes can enhance the production of methane, which is "produced in and emitted from lakes at globally significant rates," according to the study.
Woolway explains that higher levels of warming could therefore create a positive climate feedback in lakes, where rising temperatures mean larger planet-warming emissions:
"Low oxygen levels at depth also promotes methane production in lake sediments, which can then be released to the surface either via bubbles or by diffusion, resulting in a positive feedback to climate change."
Onset and Breakup
In the study, the authors determine historical changes in lake stratification periods using long-term observational data from some of the "best-monitored lakes in the world" and daily simulations from a collection of lake models.
They also run simulations of future changes in lake stratification period under three different emission scenarios, to determine how the process could change in the future. The study focuses on lakes in the northern hemisphere.
The figure below shows the average change in lake stratification days between 1900 and 2099, compared to the 1970-1999 average. The plot shows historical measurements (black), and the low emission RCP2.6 (blue), mid emissions RCP6.0 (yellow) and extremely high emissions RCP8.5 (red) scenarios.
Change in lake stratification duration compared to the 1970-1999 average, for historical measurements (black), the low emission RCP2.6 (blue) moderate emissions RCP6.0 (yellow) and extremely high emissions RCP8.5 (red). Credit: Woolway et al (2021).
The plot shows that the average lake stratification period has already lengthened. However, the study adds that some lakes are seeing more significant impacts than others.
For example, Blelham Tarn – the most well-monitored lake in the English Lake District – is now stratifying 24 days earlier and maintaining its stratification for an extra 18 days compared to its 1963-1972 averages, the study finds. Woolway tells Carbon Brief that as a result, the lake is already showing signs of oxygen depletion.
Climate change is increasing average stratification duration in lakes, the findings show, by moving the onset of stratification earlier and pushing the stratification "breakup" later. The table below shows projected changes in the onset, breakup and overall length of lake stratification under different emission scenarios, compared to a 1970-1999 baseline.
The table shows that even under the low emission scenario, the lake stratification period is expected to be 13 days longer by the end of the century. However, in the extremely high emissions scenario, it could be 33 days longer.
The table also shows that stratification onset has changed more significantly than stratification breakup. The reasons why are revealed by looking at the drivers of stratification more closely.
Warmer Weather and Weaker Winds
The timing of stratification onset and breakup in lakes is driven by two main factors – temperature and wind speed.
The impact of temperature on lake stratification is based on the fact that warm water is less dense than cool water, Woolway tells Carbon Brief:
"Warming of the water's surface by increasing air temperature causes the density of water to decrease and likewise results in distinct thermal layers within a lake to form – cooler, denser water settles to the bottom of the lake, while warmer, lighter water forms a layer on top."
This means that, as climate change causes temperatures to rise, lakes will begin to stratify earlier and remain stratified for longer. Lakes in higher altitudes are also likely to see greater changes in stratification, Woolway tells Carbon Brief, because "the prolonging of summer is very apparent in high latitude regions."
The figure below shows the expected increase in stratification duration from lakes in the northern hemisphere under the low (left), mid (center), and high (right) emission scenarios. Deeper colors indicate a larger increase in stratification period.
Expected increase in stratification duration in lakes in the northern hemisphere under the low (left), mid (centre) and high (right) emissions scenarios. Credit: Woolway et al (2021).
The figure shows that the expected impact of climate change on stratification duration becomes more pronounced at more northerly high latitudes.
The second factor is wind speed, Woolway explains:
"Wind speed also affects the timing of stratification onset and breakdown, with stronger winds acting to mix the water column, thus acting against the stratifying effect of increasing air temperature."
According to the study, wind speed is expected to decrease slightly as the planet warms. The authors note that the expected changes in near-surface wind speed are "relatively minor" compared to the likely temperature increase, but they add that it may still cause "substantial" changes in stratification.
The study finds that air temperature is the most important factor behind when a lake will begin to stratify. However, when looking at stratification breakup, it finds that wind speed is a more important driver.
Meanwhile, Vachon says that wind speeds also have implications for methane emissions from lakes. He notes that stratification prevents the methane produced on the bottom of the lake from rising and that, when the stratification period ends, methane is allowed to rise to the surface. However, according to Vachon, the speed of stratification breakup will affect how much methane is released into the atmosphere:
"My work has suggested that the amount of accumulated methane in bottom waters that will be finally emitted is related to how quickly the stratification break-up occurs. For example, a slow and progressive stratification break-up will most likely allow water oxygenation and allow the bacteria to oxidise methane into carbon dioxide. However, a stratification break-up that occurs rapidly – for example after storm events with high wind speed – will allow the accumulated methane to be emitted to the atmosphere more efficiently."
Finally, the study finds that large lakes take longer to stratify in spring and typically remain stratified for longer in the autumn – due to their higher volume of water. For example, the authors highlight the North American Great Lakes, which house "irreplaceable biodiversity" and represent some of the world's largest freshwater ecosystems.
These lakes have been stratifying 3.5 days earlier every decade since 1980, the authors find, and their stratification onset can vary by up to 48 days between some extreme years.
O'Reilly tells Carbon Brief that "it's clear that these changes will be moving lakes into uncharted territory" and adds that the paper "provides a framework for thinking about how much lakes will change under future climate scenarios."
Reposted with permission from Carbon Brief.