World Climate and Climate Change

Written and reviewed by Subhra Gupta, Humanities faculty at UniDrill (M.A. Sociology, University of Delhi · 7 years teaching Humanities). Based on NCERT Geography Class XI (Fundamentals of Physical Geography), Chapter 11.

Can plants tell you the climate?

A traveller who goes from the Amazon forest to the Sahara, then to the Siberian taiga and on to Antarctica passes through every climate in this chapter. Koeppen's insight was that you do not need to feel the weather to name it. Vegetation is a living record of climate: tall evergreen forest means heat and rain all year, thorny scrub means drought, mosses and lichens mean cold. He picked temperature and rainfall values that matched the boundaries between plant types and used them to sort the world. The second half of the chapter asks a bigger question: if climates differ across the globe, have they also changed through time, and is human activity changing them now?

Three ways to classify climate

Climate can be studied by organising data into smaller units for easy description and analysis. There are three broad approaches.

Koeppen's scheme is the most widely used. It is empirical, based on mean annual and mean monthly temperature and precipitation. It was developed in 1918, has been modified over time, and is still in use. Koeppen introduced the use of capital and small letters to name groups and types.

Two views. For the exam: Koeppen's scheme is empirical and works from observed temperature and rainfall, with vegetation as the check. Another reading: climatologists who prefer genetic systems argue that grouping places by the air masses and circulation that cause their weather explains why a climate exists, not just what it looks like. Both are recognised approaches, and the choice depends on whether you want to describe or to explain.

Koeppen's five groups

Koeppen recognised five major climatic groups. Four are based on temperature and one (B) on precipitation. The capital letters A, C, D and E mark humid climates and B marks dry climates.

Groups are divided into types by small letters that describe the seasonality of precipitation and temperature.

Reading a code. The letters work like a short sentence. Af is a tropical climate (A) with no dry season (f). Cs is a warm temperate climate (C) with a dry summer (s). BWh is a dry climate (B), desert (W), found in low, hot latitudes (h). Group B types are named BSh, BWh, BSk and BWk, where the lowercase letter marks subtropical or mid latitude.

Pause and predict: a station has a coldest month averaging 24°C and gets afternoon thunder showers in every month of the year. Which Koeppen group and type is it?

Check your answer: Group A (coldest month above 18°C), type Af (no dry season). This is the tropical wet climate of places such as the Amazon Basin.

Beyond the textbook. For the exam: Group D has a coldest month of minus 3°C or below, and Group E has all months below 10°C. In reality: the two definitions overlap for the very coldest places, and standard Koeppen practice resolves this by placing a station in E when even its warmest month stays below 10°C, and in D when it has at least one month above 10°C. Why it matters: it shows why E is the polar group and D is the cold forest group, even when both have freezing winters.

Group A: tropical humid climates

These lie between the Tropic of Cancer and the Tropic of Capricorn. The sun is overhead throughout the year and the Inter Tropical Convergence Zone (ITCZ) is present, so the climate is hot and humid. The annual range of temperature is very low and annual rainfall is high.

Moving away from the equator, the wet season shrinks and the dry season grows, so Af, Am and Aw are three steps along one gradient. The dry-season forests drop their leaves because there is not enough water to keep them.

Group B: dry climates

Dry climates have very low rainfall, not adequate for plant growth. They cover a very large area, from 15° to 60° north and south of the equator. They arise in three settings, each with its own reason.

Dry climates are divided into steppe (BS, semi arid) and desert (BW, arid), and by latitude into subtropical and mid latitude types.

Steppe versus desert (subtropical). Both lie in the transition zone between humid and dry climates and share their general temperature and rainfall character. The steppe receives slightly more rain, enough for sparse grassland. Rainfall in both is highly variable, but variability hurts the steppe more because its people and plants depend on the rain, and bad years can cause famine. Rain in deserts falls in short, intense thundershowers that do not build soil moisture. Fog is common in coastal deserts that border cold currents. Summer maximum temperatures are very high, and annual and daily ranges are large.

Beyond the textbook. For the exam: the highest shade temperature recorded is 58°C, at Al Aziziyah in Libya, on 13 September 1922, and it belongs to the subtropical desert climate. In reality: the World Meteorological Organization reviewed this reading in 2012 and rejected it as invalid. The accepted record is 56.7°C at Furnace Creek in Death Valley, California, on 10 July 1913. Why it matters: a question may follow the textbook figure, so know both, but the climate type (hot subtropical desert) is the same either way.

Group C: warm temperate (mid latitude) climates

These extend from 30° to 50° of latitude, mainly on the eastern and western margins of continents. Summers are warm and winters mild. The textbook describes four types.

Pause and predict: the Mediterranean climate has a dry summer and a wet winter. What shifts across the seasons to cause that?

Check your answer: the wind and pressure belts. In summer these regions come under the subtropical high, whose sinking air blocks rain. In winter the westerlies move over them and bring rain.

Group D: cold snow forest climates

These occupy the large continental areas of the northern hemisphere between 40° and 70° N in Europe, Asia and North America. Winter becomes more severe with increasing latitude.

Group E: polar climates

These lie poleward beyond 70° latitude and have two types.

How to place any station. Check dryness first, because B is defined by moisture and not by temperature. Then use the coldest month: 18°C or more means A; above minus 3°C but below 18°C means C; minus 3°C or lower means D; and if every month is below 10°C, it is E.

Climate change: the Earth has always changed

The climate we know has probably prevailed for about 10,000 years, with small and sometimes wide fluctuations. Evidence shows that climate change is a natural and continuous process.

Causes of climate change

Causes are grouped into astronomical and terrestrial ones.

Beyond the textbook. For the exam: more sunspots mean cooler, wetter weather, and fewer sunspots mean warmer, drier weather, although the findings are not statistically significant. In reality: satellite measurements show the sun is, if anything, very slightly brighter when sunspots are many, and changes in the sun's output since the middle of the twentieth century cannot explain the recent rise in global temperature. Why it matters: sunspots are an exam fact, but not the main explanation of current warming.

The greenhouse effect

The atmosphere behaves like a greenhouse. It lets incoming short wave solar radiation pass, but absorbs most of the long wave radiation that the Earth's surface sends back up. The gases that absorb long wave radiation are greenhouse gases (GHGs), and the warming they cause is the greenhouse effect. The name comes from glasshouses used in cold areas to keep plants warm: the glass is transparent to incoming short wave radiation but opaque to outgoing long wave radiation. A closed car in summer, or a closed vehicle in winter, shows the same effect.

The effect in one line: sunlight (short wave) → warms the surface → surface sends out long wave radiation → GHGs absorb it → the lower atmosphere warms.

Nitric oxide (NO) and carbon monoxide (CO) are not primary GHGs, but they react with GHGs and affect their concentration.

A GHG's effectiveness depends on how much its concentration has increased, how long it lasts in the atmosphere, and which wavelength of radiation it absorbs. The longer a molecule stays in the air, the longer the atmosphere takes to recover from the change it brings. CO₂ takes 20 to 50 years to adjust to changes in sources and sinks, and is rising at about 0.5 per cent a year. Climate models use a doubling of CO₂ over the pre-industrial level as an index for estimating climate change.

Pause and predict: CFCs appear twice in this chapter, once as a greenhouse gas and once as the cause of the ozone hole. Is that the same problem?

Check your answer: no. As a GHG, a CFC molecule absorbs outgoing long wave radiation, which causes warming. As an ozone destroyer, it drifts into the stratosphere and breaks down ozone, which lets ultraviolet rays through. One gas, two separate effects.

The ozone hole

Ozone forms in the stratosphere, where ultraviolet rays convert oxygen into ozone, so ultraviolet rays do not reach the surface. CFCs drift into the stratosphere and destroy ozone. The large depletion over Antarctica is called the ozone hole, and it lets ultraviolet rays pass through to the troposphere.

Kyoto Protocol and global warming

Beyond the textbook. For the exam: the Kyoto Protocol bound 35 industrialised countries to cut GHG emissions by 2012. In reality: the next major agreement, the Paris Agreement of 2015, took a different design. Almost every country sets its own national target rather than a fixed legal cut, and the shared aim is to hold the rise in global average temperature well below 2°C above pre-industrial levels. Why it matters: it shows that international climate action has moved from binding targets for a few countries to national pledges from nearly all.

Once warming sets in, it will be difficult to reverse. Its effects may not be uniform everywhere, but a rise in sea level from melting glaciers and ice caps and from thermal expansion of the sea may inundate large coastal areas and islands.

Beyond the textbook. For the exam: 1998 is the warmest year in the chapter's record. In reality: many later years have been warmer, and the last decade is the warmest on record. Why it matters: the textbook records stop at 2000, so treat 1998 as the answer only to a question about that dataset.

Exam traps

60-second revision

Where this chapter connects

• Previous: Water in the Atmosphere explains the rainfall types and distribution that define the dry and wet seasons used here.
• Next: Water (Oceans) moves from the atmosphere to the ocean, where heat and currents shape climate.
• Related: Atmospheric Circulation and Weather Systems explains the ITCZ, subtropical highs and westerlies behind each climate. Solar Radiation, Heat Balance and Temperature covers the heat budget that the greenhouse effect disturbs. Natural Vegetation shows the plant cover that Koeppen used.