A Landsat satellite analysis of the Arctic tundra, 1985 to 2016, found roughly 37% of sampled sites growing visibly greener, against under 5% growing browner.
Tundra is defined partly by what it lacks: shrubs and trees, kept out by cold and a short growing season. If the tundra is greening, what does that suggest is actually happening to the biome itself?
The tundra is starting to convert into a different biome entirely. A biome's identity comes from its climate, not just its current vegetation. As Arctic warming lengthens the growing season and thaws permafrost, shrubs, and eventually trees, that used to be excluded by cold can survive further north than before. This is called shrubification: woody shrub cover in the western Canadian Arctic has expanded by roughly 2.2% per decade, and the Eurasian treeline has shifted about 50 km poleward.
What looks like "greening" from orbit is really a biome boundary moving, tundra retreating poleward as boreal-forest-like conditions expand into territory that used to be too cold for them. This subtopic is about exactly that link: why a given climate produces a given biome, and what happens at the edges when the climate itself changes.
- Landsat-based analysis (1985-2016) found approximately 37.3% of sampled Arctic tundra sites showing significant greening, versus about 4.7% showing browning.
- Greening was strongest at warmer sites, where summer air temperature, soil temperature and soil moisture had all increased.
- Shrub cover in the western Canadian Arctic expanded by an average of roughly 2.2% per decade in a Landsat-derived vegetation study.
- The Eurasian treeline, the boundary between tundra and boreal forest, has shifted approximately 50 km poleward.
Sources: NOAA Climate.gov, "Shrub Takeover One Sign of Arctic Change"; Landsat-derived tundra greening/browning analysis (1985-2016), cited in NOAA Climate.gov and related peer-reviewed literature; ScienceDirect, Arctic shrub expansion study, Western Canadian Arctic.
Climate versus weather
Weather describes the specific atmospheric conditions at a particular time or over a short period: temperature, humidity, air pressure and wind speed right now, or this week. Climate describes the average of these same conditions over a much longer period, roughly 30 years. A single unusually cold winter says nothing about climate; three decades of winters trending warmer does.
6.1 covers the atmosphere's structure and circulation in full, including the tricellular model this subtopic reuses below. Treat that page as the mechanism reference and this one as the biome-distribution application of it.
What is a biome
A biome is a group of comparable ecosystems that have developed under similar climatic conditions, wherever in the world they happen to occur. Ecosystems that formed under similar conditions in entirely different parts of the world often converge on many of the same features: a hot desert in the Sahara and a hot desert in the Atacama share far more in common with each other than either does with the nearest forest. Precipitation, temperature and insolation (the amount of solar radiation received) are the major influences shaping which terrestrial biome develops in a given location.
Abiotic factors determine biome distribution
For any given combination of temperature and rainfall, one particular natural ecosystem type is likely to develop. Plotting biomes on a graph with temperature on one axis and rainfall on the other produces distinct, largely predictable regions, hot and wet locations cluster into rainforest, hot and dry locations cluster into desert, and so on.
Diagram: Navarras, via Wikimedia Commons (CC0), based on original biome climate data.
Statement 2.4.3 explicitly asks you to create climate graphs (climographs) showing annual precipitation and average temperature for different biomes. Practice reading and matching climographs to biomes in the Skills tab.
The major biome groups
Biomes are commonly grouped into freshwater, marine, forest, grassland, desert and tundra, each with characteristic limiting factors, productivity and biodiversity. Each group can be further split into subcategories, temperate, tropical and boreal forests, for example, are all forests but differ substantially from one another.
Tropical rainforest
Hot desert
Tundra
Temperate grassland
Boreal forest (taiga)
Classify each description into the correct biome.
Warm and wet throughout the year, with the highest species diversity of any land biome.
Extremely low rainfall with high daytime temperatures, where water availability limits almost everything.
Bitterly cold with a growing season of only a few weeks, permafrost never far below the surface.
Moderate, seasonal rainfall, not quite enough to support forest, with periodic natural fires shaping the vegetation.
Long, cold winters and a short summer, dominated by cold-tolerant conifer trees.
Atmospheric circulation shapes biome distribution
The tricellular model of atmospheric circulation, the Hadley, Ferrel and polar cells covered in full on 6.1, explains far more than wind patterns: it explains why particular biomes sit at particular latitudes at all. Rising air near the equator cools and releases heavy rainfall, sustaining tropical rainforest. That same air sinks again around 30° latitude, arriving dry and warm, and hot deserts, the Sahara, the Arabian Desert, the Sonoran Desert, sit almost exactly along this band in both hemispheres. Latitude, the angular distance north or south of the equator, is therefore one of the strongest single predictors of which biome will develop.
6.1 builds the full tricellular model with an interactive diagram of all three cells and their surface wind bands. This subtopic assumes that mechanism and applies it specifically to explain biome location: rising air near 0° and 60° latitude tends to produce wetter biomes, sinking air near 30° and the poles tends to produce drier or colder ones.
Ocean currents redistribute heat
Oceans absorb enormous quantities of solar radiation, and ocean currents distribute the resulting heat around the world, shaping regional climate and, through it, biome distribution far from the coast itself. The detailed mechanism, thermohaline circulation and the global ocean conveyor belt, is HL-only content.
Credit: ESA, CC BY-SA 3.0 IGO.
4.1 covers thermohaline circulation and the North Atlantic conveyor belt in full detail, including how temperature and salinity differences drive the deep ocean currents that carry heat around the planet.
Global warming is shifting biomes
Global warming is changing regional climates, and biomes are shifting in response. The general trend is poleward and to higher altitude: warmth-dependent biomes expand into territory that used to be too cold for them, while cold-dependent biomes like tundra retreat toward the poles and mountain summits, exactly what the Arctic shrubification case in the hook demonstrates directly.
Quick check. A mountain's alpine tundra zone, above the treeline, is shrinking as trees establish themselves higher up the slope than before. What direction of biome shift does this represent?
A shifting mountain
A long-term research station on a temperate mountain has recorded, over 40 years, rising average temperatures, a longer frost-free growing season, and conifer trees now surviving 200 meters higher up the slope than historical records show.
Think it through, then check your reasoning against the model answer below. Explain what is happening to the mountain's biome boundaries, and identify which abiotic factor is most directly responsible.
- The treeline (the biome boundary between forest below and alpine tundra above) is moving upslope.
- Temperature is the most directly responsible abiotic factor: rising average temperatures and a longer frost-free growing season now allow trees to survive at elevations that used to be too cold for them.
- This is the same poleward-and-higher-altitude pattern described in 2.4.7, just observed directly through altitude rather than latitude.
- A strong answer explicitly names the biome boundary affected (treeline) and connects the mechanism to a longer growing season, not just "it got warmer."
- Arctic tundra shrubification and greening: NOAA Climate.gov, "Shrub Takeover One Sign of Arctic Change"; Landsat-derived greening/browning analysis (1985-2016); ScienceDirect, Western Canadian Arctic shrub expansion study.
- IB Environmental Systems and Societies Guide, first assessment 2026, Topic 2, Subtopic 2.4, statements 2.4.1-2.4.7.
Match the climograph to its biome
Application of skills, statement 2.4.3: create and interpret climate graphs (climographs) showing annual precipitation and average temperature for different biomes. Each climograph below plots real 30-year climate-normal data for one weather station: precipitation as green bars against the left axis, average temperature as a red line against the right axis. Read the shape of both together, then identify the biome.
Which biome does this climograph represent?
Which biome does this climograph represent?
Which biome does this climograph represent?
A real climograph plots two variables together: precipitation as vertical bars (left axis, usually in mm) and average temperature as an overlaid line (right axis, in °C), one point per month. When asked to construct one from data, always label both axes with units, and use a line, not bars, for temperature so the two variables stay visually distinct.
- Climate-normal data (1991-2020/2021 averages): Manaus, Brazil and Utqiagvik, Alaska from Climate-Data.org; Alice Springs, Australia from Climatestotravel.com (Copernicus C3S climate averages, 1991-2020). IB Environmental Systems and Societies Guide, first assessment 2026, Topic 2, Subtopic 2.4, statement 2.4.3.
Glossary
- Weather
- The specific atmospheric conditions at a particular time or over a short period.
- Climate
- The average of atmospheric conditions over a long period, roughly 30 years.
- Biome
- A group of comparable ecosystems that have developed under similar climatic conditions, wherever they occur.
- Insolation
- The amount of solar radiation received at a given location.
- Latitude
- The angular distance north or south of the equator, measured from Earth's center.
- Tricellular model
- The pattern of three linked atmospheric circulation cells (Hadley, Ferrel, polar) in each hemisphere that redistributes heat and shapes biome distribution.
- Thermohaline circulation
- A global ocean circulation system driven by differences in temperature and salinity, including the ocean conveyor belt; HL detail.
- Climograph
- A graph combining monthly precipitation (bars) and average temperature (line) for a given location or biome.
- Limiting factor
- An abiotic condition, such as water or temperature, that restricts the productivity or distribution of organisms in a biome.
- Climate type (HL)
- A broad classification of climate, tropical, temperate or polar, connected to characteristic biome types.
- El Niño Southern Oscillation, ENSO (HL)
- The irregular fluctuation in wind and sea surface temperature in the tropical Pacific, ranging between the El Niño and La Niña extremes.
- Walker circulation (HL)
- The normal east-west atmospheric circulation over the tropical Pacific Ocean, weakened or reversed during El Niño.
- El Niño (HL)
- The ENSO phase caused by a weakening or reversal of the Walker circulation, increasing surface stratification and reducing nutrient-rich upwelling off north-western South America.
- La Niña (HL)
- The ENSO phase caused by a strengthening of the Walker circulation, reversing the effects of El Niño.
- Tropical cyclone (HL)
- A rapidly circulating storm system with a low-pressure center, originating in the tropics and characterized by strong winds; called a hurricane or typhoon once sustained winds exceed 119 km/hr.
The questions and markschemes below attempt to mimic IB wording, phrasing and expectations, but are not IB-written questions.
Quick quiz
Score: 0 / 5
1. Climate is best described as:
2. Which two factors are the major abiotic determinants of terrestrial biome distribution?
3. Hot deserts commonly sit near 30° latitude because that is where the tricellular model produces:
4. Global warming is generally shifting biomes in which direction?
5. In a climograph, temperature and precipitation should be shown as:
Written practice questions
Total score: 0 / 18
Discuss whether Arctic greening could be considered an environmental benefit.
Check what your answer covers, up to 3 points per side:
Arguments that it could be considered beneficial
Arguments that it may not be beneficial
Award up to 3 marks for arguments that greening could be beneficial and up to 3 for arguments that it may not be, capped at 4 marks total to match this question. A full-mark response also reserves 1 mark for a supported judgement weighing both sides: do not credit a conclusion if only one side has been considered. This checklist cannot detect that judgement, so weigh it yourself.
Your score: 0 / 4
Given a location with an average annual temperature of about 28°C and annual precipitation of about 350 cm, identify the most likely biome.
Award the mark for the correct biome. Do not accept tropical seasonal forest/savanna because precipitation is too high.
Your score: 0 / 1
Given a location with an average annual temperature of about 27°C and annual precipitation of about 30 cm, identify the most likely biome.
Award the mark for the correct biome. Do not accept tropical rainforest/tropical seasonal forest/savanna.
Your score: 0 / 1
Compare tundra and hot desert biomes.
For full marks, the answer must include at least one similarity and one difference. Do not accept two separate descriptions without comparison.
Your score: 0 / 3
Explain why tropical rainforests commonly occur near the equator.
Do not accept only "it is hot near the equator" without linking to rainfall/rising air.
Your score: 0 / 3
Explain why many hot deserts occur around 30° north and south of the equator.
Do not accept only "deserts are hot" without linking to sinking dry air and low rainfall.
Your score: 0 / 4
Suggest why alpine tundra species may be especially vulnerable to warming.
Award 1 mark for a valid reason and 1 for development; a single developed suggestion may earn both marks. Do not accept only "they do not like heat" without linking to habitat/tolerance/competition/dispersal.
Your score: 0 / 2
To what extent is climate the main factor determining the global distribution of terrestrial biomes?
Essays like this are marked holistically against markbands, not ticked off point by point. Read the sections below, then rate your own answer against the bands.
Answers may demonstrate
- understanding of concepts and terminology such as weather, climate, biome, ecosystem, terrestrial biome, abiotic factor, temperature, precipitation, insolation, latitude, atmospheric circulation, Hadley cell, ocean currents, altitude, productivity, biodiversity, limiting factor, tundra, desert, grassland, tropical rainforest, boreal forest and biome shift;
- breadth in addressing and linking a range of climate-related factors, including average temperature, annual precipitation, insolation, length of growing season, soil moisture, evaporation, atmospheric circulation, rising and sinking air, latitude, ocean-current heat transfer and global warming;
- examples of climate determining biome distribution, such as tropical rainforests occurring in warm, wet equatorial regions, hot deserts occurring near 30° latitude where dry air sinks, tundra occurring in cold regions with short growing seasons, and tropical rainforests having high productivity because they are warm and wet year-round;
- examples of non-climatic or modifying factors, such as soil type, altitude, fire, ocean currents, mountain rain shadows, human land use, disturbance, grazing, deforestation or agriculture;
- balanced analysis of why climate is highly important, because temperature and precipitation strongly influence plant growth, productivity, biodiversity and limiting factors;
- balanced analysis of limitations, such as similar climates still supporting different ecosystems due to soils, disturbance history, species dispersal, human activity or local geography;
- evaluation of the idea that climate sets broad biome patterns, while other factors modify local ecosystem structure and exact boundaries;
- a conclusion that is consistent with, and supported by, the analysis and examples given, for example: climate is the main broad-scale factor determining terrestrial biome distribution because temperature and precipitation strongly control productivity and vegetation type, but it is not the only factor because altitude, soils, fire, ocean currents, disturbance and human activity can modify or override local patterns.
Indicative content
- Weather is short-term atmospheric conditions, while climate is the long-term average pattern.
- Biomes are groups of comparable ecosystems that develop under similar climatic conditions.
- Temperature and precipitation are major controls on terrestrial biome distribution.
- Insolation affects temperature and evaporation.
- High temperature and high rainfall support tropical rainforest.
- Low rainfall limits productivity in hot deserts.
- Low temperature and short growing season limit tundra productivity.
- Moderate seasonal rainfall and periodic fire help maintain temperate grassland.
- Boreal forests are shaped by long cold winters and short summers.
- Atmospheric circulation explains latitudinal patterns in rainfall.
- Rising moist air near the equator supports rainforest.
- Sinking dry air around 30° latitude supports deserts.
- Ocean currents redistribute heat and influence regional climate.
- Altitude can create colder conditions and shift biome boundaries upslope or downslope.
- Global warming is shifting biomes poleward and to higher altitude.
- Human activities may replace natural biomes with agriculture or urban ecosystems.
- Fire, grazing and soil conditions may influence whether forest or grassland develops.
Markbands
The response shows limited understanding of climate or biome distribution. The answer may list biomes without explaining why they occur where they do. Examples may be absent, vague or inaccurate. The response may be one-sided, with little or no consideration of factors beyond climate. There is little or no supported judgement on "to what extent."
The response shows sound understanding that climate influences biome distribution. There is some explanation of temperature and/or precipitation as controls on biome type. At least one relevant biome example is used. There is some consideration of other factors, such as altitude, soils, fire, ocean currents or human activity. The response includes some judgement, although it may be uneven or only partly supported.
The response gives a balanced and well-developed evaluation of climate as the main factor determining terrestrial biome distribution. It clearly explains how temperature, precipitation, insolation, atmospheric circulation and/or ocean currents shape biome patterns. Relevant examples are used effectively. It evaluates limitations and recognizes that local biome distribution is also influenced by altitude, soil, disturbance, fire, species dispersal and human activity. It recognizes that climate sets broad global patterns, while other factors modify local outcomes. There is a clear and supported conclusion that directly answers "to what extent."
Self-assessed band: not yet rated
- Arctic tundra shrubification data: covered in full in the Learn tab of this page.
- IB Environmental Systems and Societies Guide, first assessment 2026, Topic 2, Subtopic 2.4, statements 2.4.1-2.4.13.
Three general climate types
Climate can be classified into three general types, each connected to characteristic biome types. Tropical climates (seasonal or equatorial) support rainforest, savanna and seasonal grassland biomes. Temperate climates (maritime or continental) support temperate forest and grassland biomes. Polar climates support tundra and polar desert biomes.
Tropical
- Seasonal (wet/dry season) or equatorial (wet year-round)
- Warm temperatures with little seasonal variation
- Biomes: rainforest, savanna, seasonal grassland
Temperate
- Maritime (coastal, moderated by ocean) or continental (interior, more extreme)
- Clear seasonal temperature variation
- Biomes: temperate forest, temperate grassland
Polar
- Cold year-round, minimal precipitation
- Very short or absent growing season
- Biomes: tundra, polar desert
When the predicted biome does not develop
The biome predicted by a given temperature and rainfall pattern may not actually develop in a location, because of secondary influences or human intervention. A site with rainforest-predicting climate cleared for farmland will support cropland, not rainforest, regardless of what the climate alone would produce; the underlying climate still constrains what could develop there without that interference.
El Nino Southern Oscillation
The El Nino Southern Oscillation (ENSO) cycle is the irregular fluctuation in wind and sea surface temperature that characterizes the tropical Pacific Ocean, ranging between two opposite extremes, El Nino and La Nina, with transitional and neutral states between them. Both the frequency and intensity of these events are irregular and hard to predict.
El Nino occurs when the normal east-west (Walker) atmospheric circulation weakens or reverses. This increases surface-water stratification and reduces the upwelling of cold, nutrient-rich water near the coast of north-western South America. La Nina occurs when the Walker circulation strengthens instead, reversing El Nino's effects. ENSO directly affects conditions in the tropical Pacific, and indirectly affects weather and marine productivity across the wider tropics and subtropics.
4.1 covers a concrete, well-documented consequence of this mechanism: severe El Nino events weaken the trade winds driving upwelling off Peru, deepening the thermocline and cutting off the nutrient supply that sustains the Peruvian anchoveta fishery, most notably during the severe 1997-98 El Nino. That page covers the ecological and fisheries impact; this page covers the atmospheric cause behind it.
Tropical cyclones
Tropical cyclones are rapidly circulating storm systems with a low-pressure center, originating in the tropics and characterized by strong winds. Once sustained wind speeds exceed 119 km/hr, the same storm is classified as a hurricane or a typhoon, the name depends purely on where in the world the storm originates, not on any difference in the storm itself.
Warming and cyclone intensity
Rising ocean temperatures caused by global warming are increasing both the intensity and frequency of hurricanes and typhoons, because warmer water and warmer air both hold more energy available to power the storm.
Quick check. Which best explains why global warming is linked to more intense hurricanes and typhoons, not just more frequent ones?
- Hurricane Harvey made landfall in Texas in late August 2017, stalling over the Houston area and producing catastrophic rainfall and flooding.
- Multiple independent attribution studies estimated that human-caused warming increased Harvey's extreme rainfall by roughly 15% to 37%, with one widely cited study (World Weather Attribution) putting the increase at about 15% and the event itself as roughly three times more likely because of warming.
- The physical mechanism is well established: under the Clausius-Clapeyron relationship, the atmosphere's maximum moisture content rises by roughly 6-8.5% for every 1°C of warming, so a warmer atmosphere can simply hold and release more rain.
- Harvey is one of the most thoroughly studied individual storms in the attribution science literature, making it a well-evidenced case rather than an inferred one.
Sources: World Weather Attribution, "Climate change fingerprints confirmed in Hurricane Harvey's rainfall, August 2017"; Risser and Wehner (2017), Geophysical Research Letters; van Oldenborgh et al. (2017), Environmental Research Letters.
- El Nino/La Nina mechanism and Peru anchoveta cross-link: covered in full on 4.1 Water Systems.
- Hurricane Harvey attribution figures: World Weather Attribution; Risser and Wehner (2017), Geophysical Research Letters; van Oldenborgh et al. (2017), Environmental Research Letters.
- IB Environmental Systems and Societies Guide, first assessment 2026, Topic 2, Subtopic 2.4, statements 2.4.8-2.4.13.