Earth's Atmosphere and Climate Change
The Current Atmosphere
The Earth’s atmosphere today is approximately 78% nitrogen, 21% oxygen, and around 1% other gases, primarily argon, with carbon dioxide at approximately 0.04% and small amounts of water vapour. This composition has been relatively stable for around 200 million years. However, human activities are now causing measurable increases in the concentrations of certain gases — particularly carbon dioxide and methane — with significant consequences for the global climate.
How the Atmosphere Evolved
Earth’s atmosphere has changed dramatically over its approximately 4.6-billion-year history. Scientists believe the early atmosphere was produced by intense volcanic activity during the first billion years. The early atmosphere consisted mainly of carbon dioxide, with little or no oxygen, along with water vapour and small amounts of methane and ammonia — resembling the atmospheres of Mars and Venus today. As the Earth cooled, water vapour condensed to form the oceans, and much of the carbon dioxide dissolved in this new ocean water.
Around 2.7 billion years ago, early photosynthetic organisms — including algae and cyanobacteria — evolved in the oceans. Photosynthesis absorbed carbon dioxide and released oxygen: \(6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2\). Over hundreds of millions of years, this process gradually increased oxygen levels and decreased carbon dioxide levels in the atmosphere. As oxygen accumulated, it formed the ozone layer (\(O_3\)) in the upper atmosphere, which blocked harmful ultraviolet radiation and allowed life to colonise land. Some carbon dioxide also dissolved in the oceans, was incorporated into the shells and skeletons of marine organisms, and was eventually locked away in sedimentary rocks (limestone) and, over geological time, in fossil fuels.
The early atmosphere was mainly \(CO_2\) from volcanic activity. Photosynthesis by early organisms gradually increased oxygen and decreased carbon dioxide over billions of years. Carbon dioxide was also removed by dissolving in the oceans and becoming locked in sedimentary rocks and fossil fuels.
The Greenhouse Effect
The greenhouse effect is a natural process that keeps the Earth warm enough to support life. Short-wavelength radiation from the Sun passes through the atmosphere and warms the Earth’s surface. The warm surface then emits longer-wavelength infrared radiation back towards space. Greenhouse gases — principally carbon dioxide, methane and water vapour — absorb some of this infrared radiation and re-emit it in all directions, including back towards the Earth’s surface. This traps heat energy in the lower atmosphere.
Without any greenhouse effect, the average surface temperature of Earth would be far below freezing. The problem arises when human activities increase the concentration of greenhouse gases beyond natural levels, intensifying the warming effect.
The three main greenhouse gases are carbon dioxide (\(CO_2\)), methane (\(CH_4\)) and water vapour. They absorb infrared radiation emitted by the Earth’s surface and re-emit it in all directions, including back towards Earth, warming the lower atmosphere.
Climate Change
Human activities are increasing the concentration of greenhouse gases in the atmosphere, enhancing the natural greenhouse effect. Burning fossil fuels releases carbon dioxide that was locked away millions of years ago. Deforestation reduces the number of trees absorbing \(CO_2\) through photosynthesis. Agriculture — particularly cattle farming and flooded rice paddies — produces methane. Landfill sites release methane as organic waste decomposes anaerobically. These processes together are enhancing the greenhouse effect, causing global warming and the broader pattern of climate change.
There is a strong correlation between the rise in fossil fuel use, increasing atmospheric \(CO_2\) concentrations, and rising global average temperatures. Ice core data shows that \(CO_2\) levels today are higher than at any point over hundreds of thousands of years of ice-core records, and the current rate of increase is faster than at any previous recorded time. However, climate models are complex and depend on many interacting variables. Natural climate variations also occur — volcanic eruptions can cause temporary cooling, and solar output varies over long cycles. Despite these uncertainties, the scientific consensus is that human activities are the dominant cause of the warming trend observed since the mid-20th century.
Atmospheric Pollutants
Burning fossil fuels produces not only carbon dioxide but also several directly harmful pollutants. You need to know the source, identity and effects of each.
Carbon monoxide (CO) — produced by incomplete combustion when oxygen supply is insufficient. It is a toxic, colourless, odourless gas that binds to haemoglobin far more strongly than oxygen does, so the blood cannot carry oxygen properly. High concentrations are fatal; lower concentrations cause headaches and dizziness.
Sulfur dioxide (\(SO_2\)) — formed when fuels containing sulfur impurities are burned. Reacts with water in the atmosphere to form sulfurous and sulfuric acid, falling as acid rain. Acid rain corrodes limestone buildings, damages tree foliage and acidifies lakes and rivers, harming aquatic life.
Oxides of nitrogen (\(NO_x\)) — formed when nitrogen and oxygen from the air react together at the high temperatures inside vehicle engines and industrial furnaces. Also contribute to acid rain and to the formation of smog in urban areas.
Particulates (fine soot particles and other solid fragments from incomplete combustion) — can penetrate deep into the lungs, causing respiratory and cardiovascular disease. They also reduce visibility and can influence global temperatures by scattering or absorbing sunlight.
Key pollutants from burning fossil fuels: CO (incomplete combustion, toxic to blood), \(SO_2\) (sulfur in fuels, causes acid rain), \(NO_x\) (high-temperature reactions in engines, acid rain + smog), particulates (incomplete combustion, respiratory disease).
Mitigating Pollution and Climate Change
Pollution from combustion can be reduced by using catalytic converters in vehicles (which convert CO and \(NO_x\) into less harmful \(CO_2\) and \(N_2\)), removing sulfur from fuels before burning, and using flue gas desulfurisation at power stations (reacting \(SO_2\) in waste gases with calcium oxide to produce calcium sulfate). Reducing greenhouse gas emissions requires switching from fossil fuels to renewable energy sources, improving energy efficiency, protecting and restoring forests, and — at the individual level — reducing energy consumption and modifying diets and travel choices.