Greenhouse gas increases and global climate change

Human activities are increasing the amounts of greenhouse gases in the atmosphere. For example, burning fossil fuels releases carbon dioxide, while some agricultural activities release methane. Higher concentrations of these gases change the balance between energy arriving at Earth and energy leaving it.

Earth receives energy from the Sun and releases energy back into space as infrared radiation. Greenhouse gases absorb some of this outgoing radiation and re-emit it in all directions, including towards Earth’s surface. When their concentrations increase, less energy initially escapes to space. The surface and the air near it warm as the Earth system retains more energy.

A rise in global average surface temperature can alter long-term climate patterns. For example, it can change where and when rainfall occurs and affect the frequency or intensity of some extreme weather events. These wider, long-term changes are known as global climate change. A rise in the global average does not mean that every place becomes warmer every year.

Using peer-reviewed evidence, many scientists conclude that human activities are increasing greenhouse gas concentrations and that this is likely to raise Earth’s average surface temperature. The causal chain is: human activity → more greenhouse gases → surface warming → changes in long-term climate patterns.

Evaluating climate change evidence

The climate system contains many interacting processes, so evidence about climate change needs careful interpretation. A scientific conclusion is more convincing when it is supported by good-quality evidence and when its limitations are made clear.

Climate models and uncertainty

Scientists use climate models to represent how parts of the climate system interact. A model simplifies reality: it cannot include every process in complete detail, so scientists must make assumptions about some of them. These simplifications and assumptions introduce uncertainty into the model’s results.

Uncertainty does not mean that a model or its results are useless. It means that their conclusions should be interpreted with an understanding of their limits, rather than treated as perfectly precise predictions.

Judging climate change claims

When evaluating a report, consider whether its evidence is reliable and complete enough to support its conclusions. Look at how the data were collected, whether relevant evidence has been considered, and whether the report recognises uncertainties and limitations. A conclusion goes beyond the evidence if it claims more certainty, or makes a broader claim, than the results justify.

Media reports may select only part of the available evidence or present a biased opinion. A striking claim should therefore be checked against the evidence and the scientific conclusions it is based on, rather than accepted uncritically.

Peer review and communication

In peer review, other scientists examine a study’s methods, data and conclusions. Their scrutiny can identify weaknesses and help researchers improve their work, making the findings more reliable. Peer review does not guarantee that a study is free from error.

Scientists also need to communicate results clearly to different audiences. Explaining both the evidence and its limitations helps readers understand what the findings support, as well as where uncertainty remains.

Human Sources of Atmospheric Carbon Dioxide

Human activities can increase the amount of carbon dioxide in the atmosphere by releasing stored carbon and by reducing its removal from the air.

Burning fossil fuels

Fossil fuels contain carbon. When they are burned, carbon in the fuel reacts with oxygen during combustion, producing carbon dioxide. This carbon dioxide enters the atmosphere, increasing its concentration.

Deforestation

Trees store carbon in their wood and other tissues. When trees are removed, carbon stored in them can be released as carbon dioxide if the wood is burned or decays.

Deforestation also reduces the number of trees available to take up carbon dioxide from the atmosphere during photosynthesis. It can therefore increase atmospheric carbon dioxide in two ways: by releasing stored carbon and by reducing carbon dioxide uptake.

Human Sources of Atmospheric Methane

Human activities increase the amount of methane, $$\ce{CH4}$$, in the atmosphere. Two sources are livestock farming and the disposal of organic waste in landfill. In both cases, microorganisms produce methane as they break down organic material.

Methane from livestock

Cattle are ruminants: animals with digestive systems in which microorganisms help break down their food. Some of these microorganisms produce methane during digestion. The cattle release the methane, which enters the atmosphere. Farming large numbers of cattle therefore increases atmospheric methane.

Methane from landfill

Organic waste, such as food waste, can be buried in landfill. Microorganisms break down this waste in conditions with little or no oxygen and produce methane. The methane can escape from the landfill into the atmosphere.