Two timescales, not two unrelated ideas

Weather refers to the specific conditions being experienced at a particular time or over a short period, including temperature, humidity, air pressure and wind speed. It describes what the atmosphere is doing right now or during a short-lived event.

Climate is the average of these conditions over approximately 30 years. It shows the long-term pattern: averages, seasonal variation and the likelihood of extremes. One unusually cold day doesn’t disprove a warming climatic trend.

Organisms must cope with a place’s characteristic long-term temperature, water availability and seasonality, so climate helps determine where natural systems can develop. This starts to answer the topic’s first guiding question: climate filters which ecosystems can persist in each region.

Comparable ecosystems on a global scale

A biome is a group of comparable ecosystems that have developed in similar climatic conditions, wherever they occur. It’s a broad ecological grouping, not a single ecosystem.

Even when ecosystems are separated by oceans or continents, similar climates can give them parallel features. Comparable temperature and water availability favour plants with similar growth forms. In turn, this vegetation creates similar habitats and food-web structures. The species don’t have to be closely related; similar environmental pressures can lead to similar ecological roles and adaptations.

The major climatic controls on terrestrial biomes are:

• precipitation, which supplies biologically available water;
• temperature, which affects enzyme activity, photosynthesis, respiration and the length of the growing season;
• insolation, the incoming solar radiation that provides energy for photosynthesis and influences temperature.

These controls work together. High insolation won’t produce high plant growth when water is unavailable. Likewise, abundant rainfall cannot fully compensate for temperatures that keep water frozen and make the growing season very short.

Climate as an ecological filter

An abiotic factor is a non-living physical or chemical component of an environment that influences organisms. A particular combination of long-term temperature and rainfall will usually favour one natural ecosystem type over others. Warm, wet conditions, for instance, can support tall, multilayered forest. Warm but very dry conditions tend to favour sparse drought-resistant vegetation.

This remains a prediction rather than an absolute rule. Temperature and precipitation define the broad climatic envelope, but soils, topography, disturbance and human activity may stop the predicted biome from forming.

On a biome distribution graph, mean annual temperature appears on the horizontal axis and annual precipitation on the vertical axis. Biomes occupy zones rather than single points because ecosystems can tolerate a range of conditions, and climatic boundaries overlap.

Creating and interpreting a climate graph

Start with a consistent dataset and units. Plot each location using its mean annual temperature and total annual precipitation, then identify the biome region that contains the point. When several locations from the same biome are plotted, examine every point before outlining the climatic range—don’t impose a neat boundary unless the data support it.

Read both variables together when interpreting the graph. Moving upward shows wetter conditions, while moving right shows warmer conditions. A shift in either direction may take a location across a climatic boundary, making a different biome more likely.

Broad groups and subcategories

Biomes include freshwater, marine, forest, grassland, desert and tundra. Each group can be divided further. Forest biomes, for example, include tropical rainforest, temperate forest and boreal forest.

A limiting factor is an environmental variable that restricts the rate, abundance or distribution of an organism or ecological process. Productivity is a rate at which biomass or chemical energy is generated within an ecosystem. Biodiversity is the variety of life within an area, including diversity at genetic, species and ecosystem levels.

Climate affects productivity first. Productivity and environmental complexity then shape biodiversity. Greater plant production usually supplies more food and creates more habitats and ecological niches, though disturbance history and isolation matter too.

In freshwater biomes, productivity is commonly limited by nutrient availability, light penetration, temperature or flow. Marine productivity is often limited by light and nutrients, particularly where surface waters stay separated from deeper, nutrient-rich water. A large biome isn’t automatically productive—the open ocean is the classic reminder.

Unequal heating drives global circulation

The tricellular model is a model of atmospheric circulation that explains the behaviour of atmospheric systems and the distribution of precipitation and temperature at different latitudes. In each hemisphere, it separates atmospheric circulation into the Hadley, Ferrel and polar cells.

Latitude is the angular distance from the equator (north or south of it) as measured from the centre of the Earth (usually in degrees). It affects heating because solar energy is more concentrated at low latitudes. Toward the poles, the same energy spreads across a larger surface area.

Hadley cells: equatorial rain and subtropical dryness

Strong insolation heats the surface around the equator. The warm, moist air expands and rises near the intertropical convergence zone, producing low pressure. As the air rises, it cools. Water vapour then condenses, clouds develop and heavy convectional rainfall follows. This reliable warmth and rainfall supports highly productive tropical forests.

Higher in the atmosphere, air travels poleward before cooling and descending near $30^\circ$ north and south. Compression warms the sinking air, reducing its relative humidity and making cloud formation less likely. As a result, these subtropical high-pressure belts have dry climates, deserts and low productivity.

Surface trade winds carry air back toward the equator, completing each Hadley cell.

Ferrel and polar cells

Between approximately $30^\circ$ and $60^\circ$, the Ferrel cell carries relatively warm surface air toward the poles. Near $60^\circ$, this air meets colder polar air and is forced to rise. It cools as it rises, producing cloud and precipitation. Where seasonal temperatures permit sufficient growth, these conditions support temperate forests or grasslands.

Very cold, dense air sinks at the poles, creating high pressure and dry conditions. At the surface, air moves away from the pole, then rises again near $60^\circ$ to complete the polar cell. Low temperatures, short growing seasons and low precipitation favour tundra and polar desert. Both have low productivity.

The model links the planet’s heat distribution to biome structure. Rising air usually creates wetter, more productive systems, whereas descending air produces dry ones. In reality, the boundaries are irregular because continents, mountains, oceans and seasonal shifts alter the idealized bands.

Oceans as heat stores and transport systems

Oceans absorb a large proportion of the incoming solar radiation. Because water has a high heat capacity, they can store substantial amounts of thermal energy, then release it slowly. Coastal temperatures are moderated as a result: summers tend to be cooler, while winters are milder than at inland locations at a similar latitude.

There is a net surplus of solar energy at low latitudes and a net deficit at high latitudes. Ocean currents and atmospheric circulation transfer heat towards the poles. Without this movement, the imbalance would cause unlimited warming in the tropics and cooling in polar regions.

Warm surface currents move heat away from the tropics. Cold currents carry cooler water back towards lower latitudes, sometimes suppressing temperatures along nearby coasts—especially when winds bring maritime air inland. Two places at the same latitude can therefore have different climates and potentially support different biomes.

Density-driven global circulation

Thermohaline circulation is a system of ocean-water movement driven by density differences caused by temperature and salinity. When surface water cools, sea-ice formation can leave it colder and saltier. Its density increases until the water sinks. Deep water then travels through ocean basins before eventually upwelling in other locations.

The great ocean conveyor belt is a connected global pattern of warm surface currents and cold deep currents that redistributes heat, dissolved gases and nutrients among ocean basins. This is a simplified model. In reality, several interacting pathways make up the circulation rather than one tidy belt. Even so, the model clearly links surface heat transport with deep-water formation and upwelling.

Climatic envelopes move

Global warming alters average temperatures, precipitation patterns, seasonality, snow cover, wildfire regimes and the frequency of climatic extremes. Since biomes are linked to climatic envelopes, their potential distributions also change. Biomes generally shift poleward and to higher altitude as organisms follow cooler conditions.

A whole biome doesn’t simply slide across a map. Mobile species may relocate quickly, while plants move slowly and soils barely move at all. Coastlines, mountains, cities and fragmented habitats can act as barriers. As a result, communities may be pulled apart, producing altered or novel ecosystems rather than exact copies of the original biome.

At high altitude, a biome can move upslope only as far as the summit, which reduces the area available to it. At high latitude, warming may allow shrubs or trees to spread into former tundra. This changes surface reflectivity, habitats and carbon storage. Warmer or drier conditions may also increase fire, converting forest into more open vegetation.

This addresses the second guiding question: changes in Earth systems affect both the location and composition of biomes. For a local engagement project, compare historical and recent records of flowering dates, tree lines, fire occurrence or species distributions. Then present the climatic causes and ecological consequences, without assuming that climate is the only cause of every observed change.

Why climates are classified

Climate classification groups places according to long-term patterns of temperature, precipitation and seasonality. Latitude and global atmospheric circulation largely produce the broad tropical, temperate and polar pattern. Distance from the ocean helps separate maritime climates from continental ones.

These associations are broad rather than precise borders. Ocean currents and relief can shift the observed biome away from the climatic prediction, as can soils and human land use.

Why the climatic prediction may fail

Temperature and rainfall indicate the most likely natural biome. Other factors can shift the outcome: secondary influences are environmental controls other than the main climatic variables that modify which ecosystem develops.

• Soil: low nutrient availability, poor drainage, shallow depth, salinity or unsuitable structure may restrict plants that the climate could otherwise support.
• Topography: elevation affects temperature, while slope aspect changes insolation. Mountains can also redirect winds, runoff and precipitation, sometimes producing a rain shadow.
• Natural disturbance: repeated fire, storms, flooding or herbivory may keep vegetation open in places where forest might otherwise form.
• Local hydrology: groundwater, rivers or seasonal waterlogging can support wetlands within a wider terrestrial biome.

Human intervention may override the natural pattern more completely. Logging removes forest structure. Agriculture replaces diverse communities with managed crops or pasture, while grazing and hunting alter food webs. Pollution changes soil or water chemistry. Urban development seals soil, fragments habitat and creates a warmer local climate.

Inferring the local natural biome

Start with regional temperature and rainfall records to identify the biome that would probably occur without urban or agricultural development. Next, compare less-disturbed nearby sites that have a similar altitude, slope, soil and drainage. Historical maps and pollen records can add evidence, as can surviving native species. Any conclusion should remain qualified because past disturbance and future climate change may prevent a full return to the inferred biome.

An irregular Pacific fluctuation

The El Niño Southern Oscillation (ENSO) cycle is the fluctuation in wind and sea surface temperatures that characterizes conditions in the tropical Pacific Ocean. The two opposite and extreme states are El Niño and La Niña, with transitional and neutral states between the extremes. This ocean–atmosphere cycle works in both directions. Changes in the wind affect the ocean, while changes in the ocean feed back into pressure and rainfall.

During neutral conditions, easterly trade winds drive warm surface water westward. In the western tropical Pacific, warm water and rising moist air bring relatively low pressure and high rainfall. Farther east, colder deep water rises to replace the surface water that has been displaced.

The Walker circulation is an east–west atmospheric circulation in which surface trade winds move air westward across the tropical Pacific, air rises over warmer western waters, returns eastward at altitude and descends over the cooler eastern Pacific.

ENSO doesn’t follow a fixed clock. Both extreme states vary in timing, duration and intensity, and neutral or transitional conditions occur between events. Complex feedbacks between the ocean and atmosphere make events irregular in frequency and strength, so accurate long-range prediction is difficult.

Neutral conditions and upwelling

Upwelling is the upward movement of deeper water that replaces surface water moved away by winds or currents. Under neutral conditions, trade winds near north-western South America push warm surface water westward. Cold, nutrient-rich water can then rise to replace it. These nutrients support phytoplankton, the base of productive marine food webs.

El Niño

El Niño is an extreme ENSO state caused by weakening or reversal of the normal east–west Walker circulation. As the trade winds weaken, warm surface water spreads eastward. Density layering in the upper ocean becomes stronger: surface stratification increases, vertical mixing decreases, and the upwelling of cold, nutrient-rich water near north-western South America is suppressed.

Fewer nutrients reach the sunlit surface, so phytoplankton production falls. Zooplankton and fish populations may decline, along with seabird breeding success and fishery yields. Rainfall shifts eastward as well. El Niño commonly causes unusually wet conditions in the eastern tropical Pacific, while parts of the western Pacific face a greater risk of drought. Through teleconnections, it can bring wetter winters to southern California and drought to Papua New Guinea, although the exact pattern varies between events.

La Niña

La Niña is an extreme ENSO state caused by strengthening of the Walker circulation and reversal of other effects of El Niño. Stronger trade winds drive more warm surface water westward. This steepens the east–west differences in sea level and temperature, while enhancing upwelling near South America. In the eastern tropical Pacific, nutrient supply and marine productivity commonly rise.

More rainfall becomes concentrated over the western Pacific, raising flood risk in some western regions. Meanwhile, the eastern Pacific becomes cooler and drier. ENSO therefore acts directly on Pacific winds, currents, temperature and productivity. Atmospheric teleconnections also alter weather elsewhere in the tropics and subtropics.

One storm system, regional names

Tropical cyclones are rapidly circulating storm systems with a low-pressure centre that originate in the tropics and are characterized by strong winds. Air spirals in towards the centre before rising through deep thunderstorms. As water vapour condenses, it releases latent heat that helps power the storm.

A storm is classified as a hurricane or typhoon once its sustained wind speed exceeds $119\ \text{km h}^{-1}$; the name depends on where it originates. Hurricane is used in the North Atlantic and the eastern or central North Pacific, while typhoon is the regional name in the north-west Pacific. Elsewhere, including the Indian Ocean and South Pacific, the broader term tropical cyclone is widely used.

These storms need warm tropical water, abundant moisture and atmospheric instability to form. Vertical wind shear must be relatively low, and the system must be far enough from the equator for planetary rotation to organize the circulation. At the centre lies the eye, a relatively calm area of very low pressure. Around it, the eyewall contains the strongest winds and rainfall.

Severe storms can leave communities facing deaths, displacement and damaged infrastructure, as well as contaminated water and lost livelihoods. Awareness campaigns or fundraising may support recovery, but responsible action should reflect the priorities identified by affected communities instead of assuming that outside groups know what is needed.

Why warmer oceans can intensify storms

Global warming heats both the oceans and the air. Warm surface water transfers sensible heat and water vapour to a developing tropical cyclone. As the vapour condenses, it releases latent heat, which can strengthen rising air, reduce central pressure and increase wind speed. Warmer air can hold more water vapour too, creating greater potential for extreme rainfall.

The clearest physical expectation is greater intensity, heavier rainfall and a larger proportion of storms reaching the most severe categories. Warmer conditions may also make the most intense hurricanes and typhoons more frequent. Changes in the total number of tropical cyclones are less certain, however, because their formation also depends on wind shear, atmospheric stability and circulation patterns.

Evidence assessed by the IPCC shows that human influence has increased precipitation associated with tropical cyclones and has probably made especially intense storms more likely. Observations from recent decades also show that the global proportion of major tropical cyclones is increasing. However, incomplete older records and strong year-to-year variability make long-term trends in total storm frequency difficult to establish.

So the careful conclusion isn’t simply “more storms everywhere”. Ocean warming provides more energy and moisture when conditions already favour cyclone formation. The evidence for increasing rainfall and intensity is stronger than the evidence for a global rise in the total number of storms.