2.4 Climate and biomes

Weather is the atmosphere now or soon; climate is the pattern of weather measured over many years.

Classify a claim by its time window and statistics. A single observation describes weather; a long-term average, trend or frequency describes climate.

Today’s 35°C is weather; a 30-year rise in average summer temperature is climate evidence.

A single unusual day is one weather observation; a climate claim requires a long record of averages, trends or event frequencies.

Climate is not ‘the weather somewhere else’; it is a long-term distribution of weather.

A biome is a broad ecological pattern shaped mainly by similar temperature and precipitation, even across different continents.

Climate limits water, energy and growing season; insolation helps set heating. Use climate first, then expect local soils, disturbance and species history to modify the community.

Two distant regions with warm temperatures and high year-round rainfall can both support rainforest, despite different species.

A biome groups many local ecosystems sharing broad climate constraints.

Biome boundaries are not exact lines; they are broad patterns, not one uniform community.

Use temperature level and seasonality first, then precipitation amount and timing, to infer a likely biome.

Warm and wet all year favours rainforest; persistent dryness favours desert; very cold temperatures and a short growing season favour tundra. Climate constrains producers, which then shape food webs.

A graph with 25°C throughout and 180 mm rain every month is more consistent with rainforest than savanna.

Seasonal water limitation; rainfall timing matters, not just annual total.

Do not name a biome from temperature alone; water availability and seasonality are essential.

Biomes are broad groups of comparable ecosystems. Major groups include freshwater, marine, forest, grassland, desert and tundra; each can be divided into more specific types such as tropical rainforest, temperate forest or boreal forest.

Use the limiting factor to explain the pattern: year-round heat and water support rapid plant growth in rainforest, lack of water suppresses growth in hot desert, and cold plus a short growing season limits tundra.

Compare biomes with the same chain: abiotic limit → primary productivity → habitat and resource availability → biodiversity.

These are broad tendencies, not fixed scores: local nutrients, disturbance, depth, flow and seasonality can change productivity and diversity within one biome group.

Rising air tends to bring rain; sinking air tends to suppress clouds, so circulation cells help explain biome belts by latitude.

Unequal solar heating drives three circulation cells in each hemisphere: Hadley cells from the equator to about 30°, Ferrel cells through the mid-latitudes, and polar cells at high latitudes. Warm moist air rises near the equator, bringing rain; descending air near 30° creates dry subtropical belts, while further rising and sinking zones help create mid-latitude and polar climate patterns.

A region near 30° latitude may be desert-prone because descending air warms and dries, limiting cloud formation.

The equatorial rising branch of the Hadley cells is generally wet; the descending branches near 30° are generally dry.

Latitude is a clue, not a guarantee; mountains, currents and seasonal circulation modify the pattern.

Ocean currents redistribute stored solar heat, so a coast’s climate can differ from what latitude alone predicts.

Wind and Earth’s rotation move surface water; warm currents generally moderate nearby coasts, while cold currents cool air and may reduce evaporation and rainfall.

A warm current beside a high-latitude coast can keep winters milder than an inland site at the same latitude.

Check current direction and sea-surface temperature; currents transport heat and moisture.

Ocean currents do not create energy; they redistribute heat already absorbed by the climate system.

Warming tends to move suitable climate conditions poleward or upslope, but real biomes may lag, fragment or disappear.

Species must disperse, establish and find soil and interactions in the new area. Mountains, land use, fragmentation and slow reproduction can block the climate envelope.

A cool-adapted plant may find suitable temperatures higher up a mountain, but if no connected habitat exists it cannot follow the climate quickly.

Species movement or habitat may be slower or blocked, so realized occupancy lags the climate shift.

A climate envelope is not a guaranteed species migration map; biological and human barriers matter.

First identify heat level, seasonal range and rainfall timing; only then infer the likely biome.

Equatorial climates stay hot and wet; seasonal tropical climates alternate wet and dry; maritime temperate climates have smaller temperature ranges; continental climates have stronger seasons; polar climates are cold with short growing seasons.

Two temperate sites may differ: the maritime site has mild winters, while the continental site has hotter summers and colder winters.

A smaller annual temperature range because nearby ocean stores heat.

‘Temperate’ is not one climate pattern; seasonality and maritime/continental position refine the prediction.

Climate gives a first biome prediction; soils, terrain, disturbance and human land use can change what actually grows.

Check soil depth and fertility, drainage, altitude, slope, fire and storms, then ask whether farming, cities, logging or altered fire regimes replaced the potential natural biome.

A warm, wet site may climatically support forest, but intensive cropland can occupy it after clearing.

Compare the climate-predicted potential biome with land-use history and current cover.

Current vegetation is not always the natural biome; distinguish potential natural vegetation from land cover.

ENSO links tropical Pacific sea-surface temperature with atmospheric circulation; El Niño and La Niña are opposite irregular extremes.

El Niño warms the central/eastern Pacific and weakens trade winds; La Niña cools it and strengthens them. Neutral conditions lie between, and events do not follow a fixed clock.

If eastern-Pacific water warms while trade winds weaken, the evidence points toward El Niño, not simply ‘a warm year’.

Coupled feedbacks vary, so timing and strength are irregular rather than exactly periodic.

ENSO is not just ocean temperature; the atmosphere and ocean state must be considered together.

El Niño weakens eastern-Pacific upwelling and can reduce fisheries; La Niña often strengthens upwelling and marine productivity.

Follow the chain: trade winds alter warm-water position, stratification changes nutrient delivery, phytoplankton changes, then fish and people respond. Remote rainfall effects are probabilistic, not guaranteed.

During El Niño, weaker upwelling can reduce nutrients near Peru, lowering phytoplankton and the fish that depend on them.

Warm, stratified surface water plus reduced nutrient-rich deep-water supply—not fish decline alone.

An ENSO association is not a deterministic forecast for every region; state the pathway and uncertainty.

A tropical cyclone is one warm-ocean, rotating low-pressure storm; the name changes with region.

Evaporation supplies moist air, condensation releases latent heat, rising air lowers pressure and draws in more air. Above sustained winds of 119 km h⁻¹, regional names include hurricane, typhoon and cyclone.

The same storm type is a typhoon in the western Pacific but a hurricane in the Atlantic.

Moist convection releases latent heat, lowering pressure and strengthening inflow.

These are regional names, not different physical categories; location determines the label.

In the syllabus model, rising ocean temperatures from global warming increase the intensity and frequency of hurricanes and typhoons because warmer water and air provide more energy to developing storms.

Warmer surface water increases evaporation. Moist air rises and condenses, releasing latent heat; this can lower central pressure, strengthen inflow and winds, and supply more moisture for heavy rainfall. A larger area or longer season of suitably warm water can also create more opportunities for storms to develop.

Evidence should compare consistent records across a stated region and time period, separating storm counts from measures of intensity such as sustained wind speed or rainfall.

The causal chain is warmer ocean and air → greater evaporation and energy transfer → stronger storm development, with evidence assessed separately for frequency and intensity.

Intensity and frequency are different measures. One severe storm or one active season is not enough to establish a long-term trend in either measure.