Causes of Climate Change

How causes change the Earth's temperature

Climate change is a long-term change in climate, including average temperatures and rainfall patterns. Global warming is specifically a rise in the Earth's average surface temperature. Natural processes have changed climate throughout Earth's history, while human activities now add an important warming influence.

The Sun supplies energy to the Earth. Some sunlight is reflected into space, while the land and oceans absorb much of the rest and warm up. The warm surface then gives out energy as long-wave radiation, also called thermal infrared radiation. Climate can change if the amount or distribution of incoming sunlight changes, or if the atmosphere changes how much outgoing energy escapes to space.

The natural and enhanced greenhouse effects

Greenhouse gases, including water vapour, carbon dioxide and methane, absorb some of the long-wave radiation emitted by the Earth's surface. They then emit energy in all directions, including back towards the surface. This warms the surface and lower atmosphere: the natural greenhouse effect. Without it, the Earth's average surface temperature would be about −18°C rather than about 15°C.

Human activities increase the concentration of greenhouse gases. More outgoing radiation is absorbed and more energy is emitted back towards the surface, producing additional warming. This is the enhanced greenhouse effect. It strengthens an existing natural process rather than creating a completely new one.

The distinction between incoming sunlight and outgoing long-wave radiation is important. Greenhouse gases warm the Earth mainly by changing the transfer of outgoing heat, not by making the Sun stronger.

Orbital changes: changing where and when sunlight arrives

The Earth's movement around the Sun changes gradually over tens of thousands of years. These changes, known as Milankovitch cycles, involve three features:

• Orbit shape: the orbit varies between more circular and more elliptical shapes, changing the pattern of the Earth's distance from the Sun during the year.
• Axial tilt: the angle of the Earth's axis changes. Greater tilt generally produces stronger seasonal contrasts, with hotter summers and colder winters.
• Axial wobble: the direction in which the Earth's axis points changes, altering when the seasons occur in relation to the Earth's position in its orbit.

Together, these affect where and when solar energy reaches the surface. Their climatic effect is not simply that an elliptical orbit always means a warmer Earth: the combination of changes matters. Orbital changes help explain long-term alternations between colder glacial periods and warmer interglacial periods, but operate too slowly to explain rapid warming over recent decades.

Volcanic activity: reflecting sunlight

Large volcanic eruptions can send ash and gases high into the atmosphere. Sulfur dioxide can form tiny particles that reflect sunlight back into space. With less solar energy reaching and warming the surface, global temperatures can temporarily fall.

For example, the 1991 Mount Pinatubo eruption released sulfur dioxide high into the atmosphere. Reflective particles formed, and global temperatures dipped for around two to three years. This illustrates why a large eruption can produce short-term cooling rather than warming.

Volcanoes also release carbon dioxide, a greenhouse gas. However, their carbon dioxide emissions are much smaller than those from human activities, so they do not explain recent global warming.

Solar output: changes in the Sun's energy

The Sun's energy output varies. Higher output means more incoming energy and can favour warming; lower output can favour cooling.

Solar activity follows an approximately 11-year cycle. Periods with more sunspots are associated with slightly higher overall solar output. Although sunspots themselves are darker areas, increased activity elsewhere on the Sun means its total energy output is slightly greater during these periods.

Solar variations influence climate, but they do not account for the sustained rapid warming of recent decades. A natural factor can affect temperature without being the main cause of the current warming trend.

Fossil fuels: adding carbon dioxide

Burning coal, oil and natural gas releases carbon dioxide into the atmosphere. These fuels are widely used for electricity generation, heating, transport and manufacturing.

For example, a coal-fired power station burns coal to generate electricity. This transfers carbon stored in the fuel into the atmosphere as carbon dioxide. Increasing carbon dioxide concentrations strengthen the greenhouse effect, leading to warming. Petrol and diesel vehicles contribute through the same basic process when their fuels burn.

Since the Industrial Revolution began around 1750, expanding energy use has increased the amount of greenhouse gases added by people.

Agriculture: several routes to greenhouse gas emissions

Agriculture contributes through different gases and activities:

• Cattle and manure release methane. Expanding beef and dairy production can therefore increase methane emissions.
• Rice cultivation releases methane. The decay of organic material in flooded rice fields produces this greenhouse gas.
• Artificial fertilisers can increase nitrous oxide emissions from soils. Nitrous oxide is another greenhouse gas.
• Farm machinery and the manufacture of fertilisers use fossil fuels, adding carbon dioxide emissions.

Agriculture can also involve clearing forests for crops or grazing. This connects agricultural expansion with deforestation: one activity can contribute to climate change through several linked processes.

Deforestation: releasing carbon and removing its uptake

Trees take in carbon dioxide during photosynthesis and store carbon as they grow. Removing forests leaves fewer trees to absorb carbon dioxide from the atmosphere.

If cleared trees are burned or decay, their stored carbon is also released as carbon dioxide. Deforestation therefore contributes in two ways: it can add carbon dioxide and reduce its removal.

Natural factors continue to influence climate, but their effects are too small or too slow to explain recent rapid warming. Human additions of greenhouse gases provide the main explanation for that warming.

Natural causes

• Orbital changes: orbit shape, axial tilt and axial wobble alter the distribution of sunlight over tens of thousands of years.
• Volcanic activity: large eruptions create reflective particles → less sunlight reaches the surface → temporary cooling. Pinatubo, 1991: cooling for around two to three years.
• Solar output: more solar energy can favour warming; less can favour cooling. Solar activity has an approximately 11-year cycle.

Human causes

• Fossil fuels: burning coal, oil and gas releases carbon dioxide.