Earth's Atmosphere and Climate
The Current Atmosphere
The Earth’s atmosphere today is approximately 78% nitrogen, 21% oxygen, and about 1% other gases including argon (about 0.9%), carbon dioxide (about 0.04%), and small amounts of water vapour. This composition has been relatively stable for about 200 million years, though human activities are now causing measurable changes to the concentrations of certain gases, particularly carbon dioxide and methane. Understanding how and why the atmosphere has changed is essential for evaluating the evidence around climate change.
How the Atmosphere Evolved
The atmosphere has changed dramatically over Earth’s 4.6-billion-year history. Scientists believe the early atmosphere was formed by intense volcanic activity during the first billion years. It was very different from today: it consisted mainly of carbon dioxide with little or no oxygen, along with water vapour and small amounts of methane and ammonia — similar to the atmospheres of Mars and Venus today. As the Earth cooled, water vapour condensed to form the oceans, and some carbon dioxide dissolved in this water. The evidence for the early atmosphere comes from analysis of ancient rocks, volcanic emissions, and comparison with other planets in our solar system.
About 2.7 billion years ago, early organisms that could photosynthesise (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 gradually increased the oxygen level and decreased the carbon dioxide level. As oxygen accumulated, it formed the ozone layer (\(O_3\)) in the upper atmosphere, which blocked harmful ultraviolet radiation and allowed life to move onto land. Some carbon dioxide also dissolved in the oceans, was used by marine organisms to make calcium carbonate shells, and was eventually locked away in sedimentary rocks (limestone) and fossil fuels.
The early atmosphere was mainly \(CO_2\) from volcanoes. Photosynthesis by early organisms increased oxygen and decreased carbon dioxide over billions of years. Carbon dioxide was also dissolved in oceans and 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. Without it, the average surface temperature would be about −18°C instead of about 15°C. Short-wavelength radiation from the Sun passes through the atmosphere and warms the Earth’s surface. The warm surface emits longer-wavelength infrared radiation. Greenhouse gases (carbon dioxide, methane and water vapour) absorb some of this infrared radiation and re-emit it in all directions, including back towards Earth. This traps heat energy in the atmosphere and keeps the surface warmer than it would otherwise be. The greenhouse effect is essential for life on Earth, but problems arise when human activities increase the concentration of these gases beyond natural levels.
Climate Change
Human activities are increasing the concentration of greenhouse gases in the atmosphere, enhancing the natural greenhouse effect. Burning fossil fuels (coal, oil, natural gas) releases carbon dioxide that was locked away millions of years ago. Deforestation reduces the number of trees absorbing \(CO_2\) by photosynthesis. Agriculture (especially cattle farming and rice paddies) produces methane. Landfill sites also release methane as waste decomposes. The increase in greenhouse gases is enhancing the greenhouse effect, leading to global warming and climate change.
There is a strong correlation between increased fossil fuel use, rising atmospheric \(CO_2\) levels and rising global temperatures. Ice core data from Antarctica and Greenland shows that \(CO_2\) levels today are higher than at any point in the last 800,000 years. The rate of increase is also unprecedented. However, there are uncertainties in the evidence: climate models are complex and depend on many variables, natural climate variations (such as volcanic eruptions and solar cycles) also occur, and the exact extent and timing of future warming is debated. Despite these uncertainties, the scientific consensus — supported by organisations like the IPCC — is that human activities are the dominant cause of recent warming.
Potential Effects and Mitigation
Potential effects of climate change include: rising sea levels (from melting ice caps and thermal expansion of warmer ocean water), more frequent and intense extreme weather events (storms, floods, droughts, heatwaves), changes to rainfall patterns affecting agriculture, loss of biodiversity as habitats change faster than species can adapt, and disruption to food production, and displacement of populations in low-lying coastal areas. Mitigation strategies include: reducing fossil fuel use, increasing use of renewable energy (solar, wind, hydroelectric), carbon capture and storage technology, improving energy efficiency in buildings and transport, reforestation to absorb more \(CO_2\), and international agreements to limit emissions. Individual actions such as reducing energy use, using public transport, and reducing meat consumption also contribute.
Human activities (burning fossil fuels, deforestation, agriculture) are increasing greenhouse gas levels. This enhances the greenhouse effect, causing global warming and climate change. There is strong evidence for this but some uncertainties remain in predictions about the scale and timing of future impacts.