World Climate and Climate Change

The world has a bewildering variety of climates, from steaming rainforests to frozen ice caps. To make sense of them, geographers group similar climates together into a few great types. The most widely used scheme is Koeppen's. At the same time, the climate of the whole planet is no longer fixed. Human activity is warming the Earth. Understanding this change is one of the most important issues of our age and a constant feature of current-affairs questions.

It helps to separate two ideas. The elements of climate are the things we actually measure, chiefly temperature and precipitation (rainfall and snow), along with humidity, pressure and wind. The controls of climate are the factors that decide the values of those elements: latitude, altitude, the pressure and wind systems, distance from the sea, ocean currents and relief. The controls shape the elements. The elements together define the climate of a place.

The German scientist Wladimir Koeppen devised the most widely used system for classifying world climates. His key insight was that the natural vegetation of a region is the best single expression of its climate. So he based the boundaries of his climate types on the temperature and rainfall limits that plants need.

Koeppen used capital letters to name five major groups:

• A: Tropical climates: hot and wet all year.
• B: Dry climates: deserts and semi-deserts, where evaporation exceeds rainfall.
• C: Warm temperate (mild) climates: mild winters, found in the mid-latitudes.
• D: Cold (continental) climates: severe winters, found in the large northern landmasses.
• E: Polar climates: intensely cold, with no real summer.

Smaller letters are added to show finer detail, such as the season of rainfall or the degree of heat, but the five capital groups are the framework to remember.

Koeppen's is an empirical classification. It rests on observed values of temperature and precipitation, annual and monthly, rather than on theory. Four of the five major groups (A, C, D, E) are defined by temperature. Group B alone is defined by dryness. A sixth group, H: Highland, covers high-mountain climates governed by altitude.

The small letters make the system precise. A code such as Af or BWh captures a whole climate in two or three letters. The small letters that follow A, C and D describe the season of rainfall:

• f: no dry season.
• m: monsoon rainfall.
• w: dry winter.
• s: dry summer.

The dry (B) climates use a different pair of markers. A second capital gives the degree of aridity, and a small letter gives the temperature:

• S: steppe, the semi-arid climate.
• W: desert.
• h: hot, subtropical (low-latitude).
• k: cold, mid-latitude.

The main sub-types within each group are worth memorising:

• A (tropical): tropical wet (Af), tropical monsoon (Am) and tropical wet-and-dry or savanna (Aw).
• B (dry): steppe (BS) and desert (BW), each either hot (h) or cold (k).
• C (warm temperate): humid subtropical (Cwa, Cfa), Mediterranean (Cs) and marine west coast (Cfb).
• D (cold snow-forest): humid continental (Df) and dry-winter (Dw), found in the large continental interiors of the Northern Hemisphere.
• E (polar): tundra (ET) and ice cap (EF).

So Af reads as tropical with no dry season, the equatorial rainforest climate. Cs is the Mediterranean climate with its dry summer. EF is the ice cap. Learning the letters lets you decode any climate at a glance.

Working through the groups gives a picture of the world's climate regions. The tropical climates near the Equator are hot with heavy rain. They support rainforests. The dry climates of the subtropics lie under the descending air of the high-pressure belts. They give the great deserts such as the Sahara. The warm temperate climates lie in the mid-latitudes. They include the Mediterranean type, with its dry summers, and are pleasant and varied. The cold climates of the high northern latitudes have long, harsh winters and short summers. Coniferous forests grow there. The polar climates of the far north and south, and of high mountains, are frozen for most or all of the year.

Within a few degrees of the Equator the climate is hot and wet all year. The daily rhythm is remarkably regular. Mornings are clear and bright. The intense heat and humidity build up through the day until, most afternoons, convectional thunderstorms break out and drop brief, heavy rain. Convection here means warm moist air rising, cooling and condensing into towering storm clouds. Because this happens almost every day, rainfall is spread through the whole year, with no real dry season.

This climate supports the equatorial rainforest, the most luxuriant biome on Earth.

• Structure: tall trees whose crowns lock together into a continuous canopy, with woody climbers and epiphytes (plants that perch on other plants for support) threading through them.
• Diversity: an exceptional richness of species, more than any other biome.
• Productivity: very high primary productivity, meaning the living plants build up huge amounts of plant matter.
• Soils: poor and leached. Nutrients are washed downward by the heavy rain and are instead held in the living biomass, not the ground. The intense heat and moisture rot fallen leaf litter faster than in any other biome, so the soil surface is left almost bare.

This soil paradox matters in the exam. A rainforest looks immensely fertile, yet once it is cleared the nutrients in the burnt vegetation are quickly used up or washed away, and the land soon fails for farming.

The tropical wet-and-dry, or savanna, climate has a marked dry season. Its vegetation is grassland dotted with scattered small trees. Forest does not take over here for three reasons working together: recurrent fire, grazing by large herbivores, and seasonal, unreliable rainfall. Each keeps tree growth in check and lets the grasses dominate.

The defining feature of a monsoon climate is the seasonal reversal of winds. In summer, winds blow onshore from sea to land and bring heavy rain. In winter, the flow reverses and dry winds blow offshore from land to sea. This complete switch in wind direction, not just a change in rainfall, is what marks a true monsoon.

The warm temperate (C) group holds several distinct mild climates that are easy to confuse.

• Mediterranean (Cs): mild temperatures, roughly 6–16°C through the year, with moderate annual rainfall of about 50 cm. Its signature is a winter rainfall maximum and a dry, warm summer.
• Marine west coast (Cfb): also called the West European type. Temperatures are mild with a low annual and daily range, the sea keeping extremes away. The westerlies (the prevailing west-to-east winds of the mid-latitudes) bring precipitation throughout the year, so annual rainfall is high, between 50 and 250 cm.
• Humid subtropical (Cfa, Cwa): the China type, warmer in summer than the marine climate, with rain in all seasons or a dry winter. Unlike the equatorial climate, it has a clear cool season.

The contrast is the point. The Mediterranean type is dry in summer, the marine type is wet all year with a small temperature range, and the humid subtropical type runs hotter than either.

Across the high northern latitudes of Canada, Russia and Scandinavia stretches the boreal coniferous forest, or taiga. It is the world's most extensive forest belt and covers the largest share of global forest area.

The brightness of these surfaces matters for warming. Albedo is the fraction of incoming sunlight that a surface reflects rather than absorbs. Bright surfaces have high albedo and stay cool; dark surfaces have low albedo and warm up. Ordered from most to least reflective, the major cold-and-tropical biomes run as follows:

• Snow-covered tundra: highest albedo, reflecting most sunlight.
• Taiga (boreal conifers): high, but lower than open snow.
• Tropical deciduous forest: lower still.
• Tropical evergreen rainforest: lowest, its dense dark canopy absorbing most of the light.

The greenhouse effect is a natural and necessary process. Certain gases in the atmosphere (carbon dioxide, methane, water vapour and others) let the Sun's short-wave energy pass down to the surface but trap part of the long-wave heat that the Earth radiates back. Like the glass of a greenhouse, they hold in warmth. Without this effect the Earth's average temperature would be well below freezing and life as we know it could not exist.

The problem arises when human activity adds too much of these gases. Burning coal, oil and gas, and clearing forests, have raised the level of carbon dioxide sharply, strengthening the greenhouse effect and trapping more heat than before.

The result is global warming, a steady rise in the average temperature of the Earth's surface over the past century. This warming drives wider climate change: the melting of glaciers and polar ice, a rise in sea level that threatens low-lying coasts and islands, more frequent and intense droughts, floods and storms, and shifts in the timing of the seasons and the monsoon.

Because the causes are global, the response must be global too. Nations have come together in agreements to cut emissions of greenhouse gases and to switch to cleaner sources of energy. Individuals can also help by saving energy, planting trees and reducing waste.

Some scientists have gone further and proposed geoengineering: deliberate, large-scale intervention in the climate system to counter warming. One branch, called solar radiation management, tries to reflect or release more of the Sun's energy back to space rather than cut emissions. Two suggested techniques stand out.

• Stratospheric sulphate aerosols: injecting fine sulphate particles high into the stratosphere so they reflect part of the incoming sunlight, mimicking the cooling that follows a large volcanic eruption.
• Cirrus cloud thinning: thinning the high, wispy cirrus clouds that trap outgoing heat, so that more long-wave radiation can escape to space.

These remain proposals under study, not settled policy, but they appear in current-affairs questions on climate solutions.