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01 the band
Everything in your cup came from one stripe of the planet.
Not a preference. A constraint, and it has three parts.
Look at the map under the title. Coffee is farmed inside one band around the equator, which the review behind most of the numbers here puts at twenty to twenty-five degrees north down to about twenty-four degrees south.1 Nobody chose that line. It is what is left after three separate limits are laid on top of each other: the plant will not tolerate frost, it wants a temperature cooler than most tropical lowlands, and it wants a long wet season broken by a dry one.
The tropics do most of the first job. Between them the year has no real winter, only a wet season and a dry one, and that rules out most of the planet before you have said anything about the plant. The rest of this post takes the limits one at a time, then asks what the climate models say happens to the band.
For the enthusiast: why the tropics sit where they do, and how big the crop is
The tropics are not a human invention. The Earth spins on an axis tilted about 23.4 degrees relative to its orbit,21 and the Tropics of Cancer and Capricorn are simply the two latitudes where the Sun can get directly overhead.
The International Coffee Organization put world production at 168.2 million 60-kilogram bags in the 2022/23 crop year, and forecast 178.0 million for 2023/24.10 Of the 2022/23 total, 94.0 million bags were arabica and 74.2 million were robusta, so the split is roughly 56 to 44.10 Two species. One band. Everything else is detail.
The growing countries are not variations on one theme. The Ethiopian highlands and the slopes around Mount Kenya and the Aberdares are equatorial, very high, and sitting on rift volcanics. Colombia and Central America are volcanic mountain country a few degrees off the equator. Sumatra and Java are a young island arc. The robusta lowlands of Vietnam and the plateaux of Brazil are neither high nor equatorial, and they are in the band for different reasons again. Any one origin makes one part of the argument; the argument only closes if you look at several.
02 two plants
Arabica and robusta are not two grades. They are two different tolerances.
Arabica wants it cool, robusta warmer, and that gap is why there are two crops.
Two species of coffee are farmed at scale. Arabica does best at a mean annual temperature of 18 to 21 °C with 1,200 to 1,800 millimetres of rain a year. Robusta does best at 22 to 26 °C, and copes better than arabica with heavy rain above 2,000 millimetres.1 “Best” means where the plant yields well, not the only place it survives. Above 23 degrees arabica’s fruit ripens too fast, which usually costs quality, and continuous exposure to 30 degrees stalls its growth.1
Why does a few degrees matter so much to a tree? A leaf takes in carbon dioxide through tiny pores called stomata, and every time it opens them it loses water. Warm air pulls harder on that water, so a hot plant has to choose between starving and drying out. Arabica is native to cool Ethiopian highland forest, and robusta is a lowland plant.1 Every farm in the world is an attempt to rebuild one of those two places somewhere else.
For the enthusiast: the species, their native ranges, and the table
There are 124 known species in the genus Coffea.9 Coffea arabica is the one sold as arabica, and Coffea canephora is the one sold as robusta, which is strictly the name of its main cultivated group rather than the species.
Arabica’s native forest in Ethiopia lies at 1,600 to 2,800 metres, where the annual average is about 20 degrees and rainfall runs from 1,600 to more than 2,000 millimetres. Robusta can be grown from sea level up to about 800 metres.1 Push arabica’s leaf far enough into heat and it closes its pores, stops fixing carbon, and starts spending reserves it needed for next year’s crop.
You can see the gap on a map. Vietnam and the Brazilian state of Espírito Santo are warm and low, and they are robusta country. The Kenyan highlands, the Colombian Andes and the Ethiopian forests are cool and high, and they are arabica country. Indonesia grows both, arabica on the volcanic uplands of Sumatra and Java and robusta on the ground below: the two species answering different questions about the same island.
| Measure | Arabica | Robusta |
|---|---|---|
| Species | C. arabica | C. canephora |
| Optimum mean temperature | 18 to 21 °C | 22 to 26 °C |
| Rainfall | 1,200 to 1,800 mm/yr | the same, and copes above 2,000 mm/yr |
| Typical elevation | highland | sea level to about 800 m |
| Chromosomes | 2n = 4x = 44 | 2n = 2x = 22 |
| 2022/23 production | 94.0 million bags | 74.2 million bags |
03 the year of a cherry
The plant needs a dry season as much as it needs the rain.
The order the rain arrives in matters more than how much falls.
Coffee does not flower on a calendar. A flower bud forms, then waits. A dry spell makes the bud ready, and the return of the rain opens it.5 Growers call it the flowering rain, and on a good farm a whole hillside opens white after the same shower. Rain spread evenly through the year cannot do that: the tree carries green, ripe and overripe fruit at once, and the crew has to pick the same rows several times.
After the flowers open the clock is long: 220 to 243 days on average to a ripe arabica cherry, and about 284 for robusta.5 Most of a year in which the farm is exposed to everything the weather does.
The cold end is the hard limit. Coffee is evergreen and never goes dormant, so it has no way to sit out a frost: its leaves die somewhere between minus 3 and minus 5 degrees.1 A frost does not reduce this year’s crop, it removes it.
For the enthusiast: the ripening range, and the 2021 frost
The clock starts at anthesis, the moment a flower opens. For the same arabica cultivar across different environments and altitudes, anthesis to ripe fruit ranges from 204 to 266 days.5 Section 05 comes back to that range. The altitude post cites longer times at cool sites; that is a different study of different farms, not a second reading of this one.
A frost that kills the wood takes next year’s crop too. Brazil is where this is not theoretical. Successive frosts hit the coffee regions in late June and July 2021. The ICO was still writing about it two crop years later, noting that the 2023/24 outlook would be shaped by “the impact of the July 2021 frost” as it continued to be resolved.10 A single week of cold in one country moved the world price of the whole crop. That is what having it concentrated inside a narrow band costs you.
04 a narrow genome
Arabica is a hybrid with almost no variation to breed from.
When the climate moves, most crops have variety to breed from. Arabica barely does.
This is the part I did not expect. The usual answer to a new climate is to look through the variety a crop already has, find the plants that cope, and breed from them. Arabica has strikingly little of that variety, because of how it was made: a single cross between two other coffee species, followed by several collapses in its population that threw variation away for good.34 The result is one of the lowest genetic diversities of any major crop.4 The obvious backup is the wild species, and at least 60 percent of them are threatened with extinction.9
The plant cannot move and cannot vary much. Everything that has to give will have to give on our side.the sentence this post has to earn
For the enthusiast: how arabica was made, the dates, and the bottlenecks
Arabica is allotetraploid. A normal diploid plant carries two sets of chromosomes, one from each parent. Arabica carries four, and they come from two different species: a whole genome from C. canephora and a whole genome from C. eugenioides, combined in a single hybridisation event and then kept together rather than sorted out.3 It has 44 chromosomes where its parents each have 22. Lashermes and colleagues established the parentage in 1999 using restriction fragment markers and genomic in situ hybridisation, and noted that the two genomes had barely diverged from their living relatives, which meant the event was recent.3
A chromosome-level assembly published in 2024 dates the founding event to between 350,000 and 610,000 years ago, and finds several population bottlenecks after it, including a split between wild accessions and the ancestors of cultivated varieties around thirty thousand years ago.4 A bottleneck is what happens when a population is reduced to very few individuals: whatever variation those few did not carry is gone, and no amount of later breeding brings it back. Arabica went through the founding one, then more, then domestication, then the handful of seeds that left Ethiopia and Yemen and became almost every tree grown in the Americas.
So when someone says the answer to a warming band is to breed a hotter-tolerant arabica, the honest reply is that the cupboard is nearly bare. The variation has to come from somewhere else: from C. canephora, which is where the disease resistance in most modern arabica varieties already came from, or from the wild species. Davis and colleagues assessed all 124 wild coffee species for the IUCN in 2019: 13 critically endangered, 40 endangered and 22 vulnerable.9 The bank you would need to draw on is being closed while you are deciding whether to use it.
05 altitude buys latitude
Air cools as it rises, at a rate you can measure and plan a farm around.
Near the equator the lowlands are too warm for arabica, so the crop climbs.
Rising air expands, and expanding cools it. The rate is called the lapse rate, and measured across the southern Ecuadorian Andes it is 6.9 °C for every kilometre climbed, on the annual mean.7 Put that slope together with the two optima from section 02 and the species sort themselves up one hillside.
Start at the height where the air averages 26 °C, the warm edge of robusta’s window, and climb. Robusta’s window is the first 580 metres. Then comes a one-degree gap, and arabica’s window runs from about 725 to 1,160 metres above the start. Where the foot of the slope is warmer, the whole ladder moves uphill; the spacing stays the same.
That is also the honest answer to “why not grow it further from the equator instead”. Going up lowers the temperature and leaves the seasons alone. Going away from the equator adds a winter, and a winter means frost, which section 03 covered. There is a quieter gain to height too: cooler air stretches the ripening, which is where the 204 to 266 days of section 03 come from.5
For the enthusiast: the three lapse rates, the arithmetic, and real farms
Córdova and colleagues measured the lapse rate over a station network running from 2,600 to 4,200 metres, which is above coffee country rather than in it and is the honest caveat on borrowing the number. They found 6.9 °C per kilometre for the mean, 8.8 for the daily maximum, and 5.5 for the daily minimum.7 Days cool faster than nights as you climb, so the daily swing narrows by roughly 3.3 degrees for every kilometre. A highland farm is cooler in the afternoon by more than it is cooler at dawn, which is the shape a plant wants if what hurts it is the hottest few hours of the day.
The arithmetic, so it can be checked. It needs only the slope and the optima, not a guess at how warm the bottom of the hill is:
# lapse rate: Cordova et al. 2016, tropical Andes, annual mean [7] # optima: DaMatta & Ramalho 2006 [1] LAPSE = 6.9 # degrees C lost per 1,000 m climbed START = 26 # start where the mean is robusta's warm edge for name, lo, hi in [("robusta", 22, 26), ("arabica", 18, 21)]: bottom = (START - hi) / LAPSE * 1000 top = (START - lo) / LAPSE * 1000 print(f"{name:8s} {bottom:5.0f} m to {top:5.0f} m up") robusta 0 m to 580 m up arabica 725 m to 1159 m up
Real farms land where that arithmetic says they should. Arabica in Kenya, Colombia and Ethiopia is grown high on mountain slopes, because those countries are close to the equator and the lowland is too warm. Brazil manages arabica lower because much of its growing country is elevated plateau rather than mountain and sits well south of the equator, so latitude and altitude do the job together. Vietnam’s robusta sits in the lowland range, and so does most of Indonesia’s, with the arabica on the same islands moved up onto the volcanoes.
Moving poleward does cool the mean, and a few places genuinely gain from warming for that reason.8 But latitude also changes daylength, which the flowering cycle is tuned to. And a longer fill means a denser seed: the trade has long sold that as “high grown”, and for once the marketing and the physiology point the same way.
06 what the volcano left
Volcanic soil is not simply fertile. It is deep, light, and very good at hiding phosphorus.
The soil’s gift to coffee is its structure. Its catch is chemical.
The best-known coffee origins sit on volcanoes with a regularity that stops looking like coincidence: Central America, the northern Andes, Sumatra and Java, the Ethiopian highlands, the flanks of Kilimanjaro. Soils formed in volcanic ash are called andisols, and they are rare: roughly 1 percent of the world’s ice-free land.11
What makes them good for coffee is that they are light and porous, so roots push deep and water drains away instead of drowning them. The catch is that the same tiny minerals that hold water also grab phosphate, a nutrient no plant can do without, and keep most of it from the root.12
I want to be careful about how far this goes. Coffee does not need volcanic soil; it needs deep, well-drained ground with organic matter in it, and volcanic highlands in the tropics happen to make a great deal of exactly that. I have not found a study that separates the soil from the altitude, shade, variety and processing that come with it, so “volcanic soil makes it taste better” is not something this page can claim.
For the enthusiast: the three measurements, a Costa Rican study, and where volcanoes are
What makes a soil andic is not vibes, it is three measurements. Under the USDA classification a soil material must contain less than 25 percent organic carbon and then meet one or both of two sets of requirements. The first is all of the following, in its fine-earth fraction: aluminium plus half the iron, extracted with ammonium oxalate, totalling 2.0 percent or more; a bulk density, measured at 33 kilopascals of water retention, of 0.90 grams per cubic centimetre or less; and a phosphate retention of 85 percent or more. The second route asks for far less retention, 25 percent, and makes up the difference with a measured fraction of volcanic glass.12 It is the first route that a weathered tropical andisol takes.
Bulk density of 0.90 or less. Bulk density is the mass of a volume of soil including the air in it; a litre of andisol weighs less than 900 grams. Light means porous, and coffee is intolerant of waterlogging and roots deep when it is allowed to. A measured Costa Rican coffee andisol came in at 0.74 grams per cubic centimetre with organic matter above 5 percent.6
Aluminium plus half the iron above 2.0 percent. This is a proxy for minerals with no proper crystal structure: allophane, imogolite and ferrihydrite.12 When volcanic glass weathers fast, in a warm wet highland, it does not have time to organise into ordinary clay. It forms tiny hollow particles with an enormous surface for their weight, which is why these soils manage to be free-draining and moisture-retentive at once.
Phosphate retention of 85 percent or more. The same surfaces grab phosphate ions and will not let go, so most of any phosphorus fertiliser ends up where the root cannot take it. Phosphorus is in every molecule of the plant’s energy currency and in the backbone of its DNA. In the Costa Rican study, yield tracked soil texture and phosphorus rather than the nitrogen strategy being tested, and the sector with 50 percent more clay and 47 percent more silt doubled the yield of its sandier neighbour.6
Volcanoes cluster where one tectonic plate dives beneath another and where crust is pulling apart, which is why the Smithsonian’s Holocene catalogue puts more than half of the world’s recently active volcanoes around the rim of the Pacific alone.13 Several of those arcs run through the tropics: Central America, the northern Andes, the Sunda arc through Sumatra and Java, the Philippines and New Guinea. The East African Rift adds a second source down Ethiopia, Kenya and Tanzania, and Kenya is the cleanest single example of all three factors lining up: equatorial, so no winter; high, on the flanks of Mount Kenya and the Aberdares, so the temperature is in the arabica window; and volcanic, so the soil is deep and free-draining. Those are the arcs on the map in the hero.
07 2050
Every serious model says the band shrinks. They disagree by a lot about how much.
The direction is settled. The size is not.
These studies model climatic suitability: they learn which combinations of temperature and rainfall coffee is grown in today, then ask how much land will still offer them in a warmer climate. Suitable area is not production, and losing it is not the same as coffee disappearing. It is a statement about where the crop can be grown without doing something new.
The losses run from about half of today’s suitable land worldwide2 to 73 to 88 percent in Latin America.14 The one bar that goes the other way is Ethiopia with farms moved uphill alongside forest conservation, which gives at least four times the suitable area of doing nothing.16 Read the chart as a spread and not as competing answers; section 08 is about why it is so wide.
For the enthusiast: the five studies, one at a time
Globally. Bunn and colleagues trained machine learning models on geo-referenced production locations and projected that climate change will reduce the global area suitable for coffee by about 50 percent, a figure that holds across emission scenarios.2 Impacts are highest at low latitudes and low altitudes, and still negative but less pronounced higher and further from the equator.2 That is the lapse-rate figure from section 05 running in reverse. The band does not vanish, it climbs, and mountains get narrower as you go up.
Globally, but grading the land. Grüter and colleagues ran 14 global circulation models under three emission pathways and separated land by suitability class. The highest class falls by more than 50 percent globally under all three pathways, with declines in the main producing countries from around half to almost all of it depending on country and scenario.8 Coffee came out the most vulnerable of the three crops they studied.8
Latin America. Imbach and colleagues modelled the largest growing region at higher resolution: suitable area down 73 to 88 percent by 2050, which they describe as 46 to 76 percent greater loss than the global assessments had estimated.14 They also modelled the bees, since coffee yields improve with insect pollination even though arabica can self-pollinate, and found mean bee richness falling 8 to 18 percent inside the future suitable areas, though every one of those areas kept at least five bee species.14
Brazil. Koh and colleagues looked backwards as well as forwards. Temperatures in the Brazilian coffee regions rose about 0.25 degrees per decade between 1974 and 2017, rainfall fell during blooming and ripening, and yields in southeast Brazil dropped by more than 20 percent.15 That is not a projection; it already happened. Looking ahead, they cite work putting the loss of low-risk area in São Paulo state at around 20 percent under modest warming and as much as 75 percent under severe scenarios.15
Ethiopia, and the one hopeful line. Moat and colleagues found that by the end of this century 39 to 59 percent of Ethiopia’s current growing area could become unsuitable without significant intervention, leaving 41 to 61 percent of it.16 Relocating coffee areas uphill, combined with forest conservation and re-establishment, could deliver at least a fourfold increase in suitable farming area against that no-action case.16 That is the sun bar running off the chart, and the only row that goes the other way.
08 the evidence is thin
What those percentages do not know.
Some reasons say the losses are overstated; others say the models measure different things.
I found the range uncomfortable, so I went looking for why it is so wide. Six reasons, one line each. The models learn where coffee grows now; they do not simulate the plant.17 Most leave out the extra carbon dioxide, which raised yields in a field trial.18 The real limit may be how dry the air is, not how hot.19 Shade helps, but heavy shade costs cup quality.20 The regional and global answers do not agree.14 And the studies are not measuring the same thing.
There are five levers people can pull: move uphill, grow under trees, breed for heat, time the flowering with irrigation, and the carbon dioxide nobody pulls. None of them moves frost.
For the enthusiast: the six reasons in full
For the enthusiast: the five levers, and the thing none of them move
- Move uphill. The biggest single effect in the literature, and the only projection that turns positive. It needs land that is currently forest.16
- Grow it under trees. Buffers the hottest hours. Depends on tree age, and heavy shade costs cup quality.20
- Breed for heat. Limited by one of the lowest genetic diversities of any major crop, and the wild reservoir is itself at risk.49
- Irrigate, and time it. A controlled water deficit then irrigation is already used to synchronise flowering. It does nothing about heat.5
- The CO2 offset. Not a lever anyone pulls, but it is in the ground truth and out of most of the models.18
- Frost. No lever. An evergreen with no dormancy dies at minus 3 to minus 5 degrees, and that sets the poleward edge for good.1
09 use your agent
Use your agent with this page.
Any part of this page can become a question for your AI.
Every section and figure has an ask agent button. Pick how deep to go (Explain it simply, Go deeper or How do we know?) and copy that prompt. Selecting a passage offers one too. This one covers the whole post:
For the enthusiast: the tools
Each ask agent button opens a menu: pick how deep to go (explain it simply, go deeper, or how do we know), and it copies that prompt. Section 00 offers two more for the whole post. A browser agent that speaks WebMCP calls the page’s read-only tools directly. One without a browser reads the plain-text version, where-it-grows.txt, where every section and figure is numbered.
- start_guided_reading a teaching plan: learn step by step, a quiz, or just the answer.
- get_outline the post’s claim, each section’s claim, and the figures under their sections.
- explain_section any section for a
child, astudentor anexpert. - explain_figure any figure at the same three levels: what it asks, how to read it, what it shows.
- get_evidence the study behind a claim and what it actually measured.
- get_glossary any technical word on the page, in one line.
Coffee is the most universal subject on this site, and one piece of prose cannot be pitched at every reader. The three levels are written by hand, not generated at runtime. The tools read this page and nothing else: no network, no key, no backend. Their answers load only when an agent first asks.
Checking whether this browser exposes the tool API.
10 what I take from it
Three things I did not know a week ago.
I started this because I kept reading an altitude on the side of a bag and could not actually defend what it meant.
The soil story is backwards from how it is sold. What volcanic ground gives coffee is depth and drainage, a structural gift rather than a chemical one, and I now think the altitude that comes free with a volcano does more of the work than the ash.
The genetics are the quiet emergency. The crop facing a warmer band came from a single cross and has been squeezed through bottlenecks since,4 and most of the wild species you would breed from are threatened.9
The disagreement is the finding. Five studies point the same direction, the one hopeful result needs land that is forest today, and the range between 502 and 88 percent14 measures how much we do not know rather than offering a menu.
So here is what an altitude on a bag actually says, whichever country it comes from. The farm sits inside an 18 to 21 degree window that the lowland below it has not had for a very long time. Its afternoons are cooler by more than its mornings are. And the window is moving up the hill faster than the hill is getting taller.
For the enthusiast: the longer version
I had absorbed “volcanic soil, therefore fertile” without ever asking what the mechanism was. The mechanism includes a property that is actively unhelpful: a phosphate retention of 85 percent or more is part of the definition of the soil order.12
Everyone writes about the climate projections. Very few people write about the fact that the crop facing them was assembled from a single hybridisation between 350,000 and 610,000 years ago,4 and that 60 percent of the wild species you would breed from are threatened with extinction.9 Those two facts belong in the same paragraph more often than they appear in one.
My first instinct with the projections was to average them, which is exactly the wrong move: they have different scopes and different definitions of the thing being lost. What I will be watching is not the headline percentage. It is whether the wild species survive long enough to be useful, and whether the countries that can move their crop uphill are allowed to do it without felling the forest that keeps the hillside cool in the first place. Moat’s fourfold figure and the deforestation it would take are the same sentence.16
Every number on this page traces to one of them. The list is also in where-it-grows.txt.