Causes of Climate Change

The greenhouse effect is not the problem. Without it, Earth would swing between extremes like Mars does and nothing would live here. The problem is that we have thickened the blanket — and worse, warming has started triggering processes that release yet more greenhouse gas. This page is about both halves of that: what we added, and what our warming has now set off on its own.

The greenhouse effect

Radiation from the sun passes through the atmosphere and hits Earth. The surface absorbs that energy and re-emits it at longer wavelengths — as infrared radiation, which we experience as heat. A greenhouse gas is one that absorbs this re-radiated radiation and traps it in the atmosphere, so it is not lost to space.

The name comes from the comparison with glass in a greenhouse, which lets light in and keeps heat from escaping. And the effect is genuinely necessary: without the insulating effect of greenhouse gases, Earth would experience dramatic temperature fluctuations like its neighbours. Temperatures on Mars range between about 20 °C and −153 °C.

Human activities and carbon dioxide

Atmospheric carbon dioxide has fluctuated throughout Earth’s history for natural reasons — volcanic eruptions and the weathering of limestone rocks, for instance. What is different now is the scale and speed. Since the industrial revolution, which began in the late 1700s when burning fossil fuels to power factories, transport and homes became commonplace, carbon dioxide levels have risen to their highest in Earth’s history.

Fossil fuel combustion is not the only route. Carbon dioxide is also released when natural stores of carbon — carbon sinks — are damaged or destroyed. Carbon sinks include trees, soils, peat bogs and the oceans, and deforestation, soil degradation, peat harvesting and ocean warming all add carbon dioxide back to the atmosphere.

Human activities and methane

Methane (CH₄) is a simple hydrocarbon, present as a gas in the atmosphere and underground, and the main component of the fossil fuel natural gas. Some is produced by naturally occurring processes in certain bacteria, but levels have risen significantly over the last 150 years because of human activity.

• Guts of ruminant mammals such as farmed cattle. Intensive farming of these animals has greatly increased their contribution.
• Landfill sites, where organic matter such as food waste decomposes.
• Extraction of fossil fuels from underground.
• Rice paddy fields, where anaerobic bacteria in the waterlogged soil release methane.
• Permafrost — ground that stays frozen all year round. Warming of the poles releases methane from this natural store, which is a consequence of global warming rather than a direct human activity.

NOS: correlation and causation

Correlation analysis means measuring two variables and assessing the relationship between them to look for an association.

• Positive correlation — as one variable increases, the other also increases.
• Negative correlation — as one variable increases, the other decreases.

In climate change research, evidence from Antarctic ice cores shows a positive correlation between global temperatures and atmospheric carbon dioxide over hundreds of thousands of years. That is powerful, but on its own it is still only an association.

A correlation shows evidence of association, but in itself it does not provide evidence of causation. Two problems remain:

• It is not possible to say which variable influenced which.
• An unknown third variable might be responsible for both.

In this particular case, though, there is additional evidence supporting a causal link — we understand the physical mechanism by which greenhouse gases trap re-emitted radiation. The correlation plus that mechanism plus laboratory measurements together build a strong case, which no single piece of evidence could do alone.

Positive feedback: warming that causes more warming

Positive feedback is any mechanism that leads to additional and increased change away from equilibrium. The output of a process feeds back into the system in a way that moves it further from its average state. Positive feedback is therefore destabilising: it amplifies deviation and drives systems towards a tipping point where the state suddenly shifts to a new equilibrium.

Global warming has a positive feedback effect on the Earth and its atmosphere. In plain terms: global warming causes more global warming. Four mechanisms matter.

1. Loss of reflective snow and ice

The extent to which a surface reflects light is its albedo. Light-coloured surfaces such as snow and ice have a high albedo; dark surfaces such as rock, soil and open ocean have a low albedo. As polar ice caps melt, Earth’s overall albedo decreases, more of the sun’s energy is absorbed rather than reflected, and warming increases — which melts more ice.

2. Accelerating decomposition

Decomposition is carried out by living organisms such as bacteria and fungi, which break down dead matter and waste in a series of enzyme-controlled reactions. Those reactions run faster at higher temperatures, so as warming increases, rates of decomposition increase — and the respiration of decay microorganisms releases carbon dioxide.

Two stores matter most here:

• Peat bogs function as carbon sinks when they are stable. Increased decomposition releases huge volumes of carbon dioxide from them.
• Permafrost is a huge carbon sink precisely because it contains organic material that cannot decompose at low temperatures — decay organisms are inactive when it is cold. Warm it and that material starts decomposing.

3. Release of methane

Melting permafrost also releases methane, a potent greenhouse gas, because of the activity of methanogenic microorganisms in the frozen soil, which increases as the permafrost melts. These are species of archaea that produce methane as part of their metabolism.

4. Increasing drought and forest fires

Warming increases the frequency of extreme weather events, so droughts occur more often. Dry vegetation catches fire easily, so wildfires become more likely. Combustion of plant material releases carbon dioxide, and the resulting reduction in the number of photosynthesising plants means less carbon dioxide is removed from the atmosphere afterwards. That is a double blow: more released, less taken back.

Worked examples