Earth’s Atmosphere & Climate Change
Earth’s Early Atmosphere
Scientists believe the Earth’s early atmosphere was very different from today. It was formed by gases released during volcanic activity. The early atmosphere is thought to have contained a large amount of carbon dioxide, water vapour, and small amounts of other gases such as methane and ammonia. There was little or no oxygen.
The Earth’s early atmosphere was produced by volcanic activity. It contained mainly carbon dioxide and water vapour, with little or no oxygen.
How the Atmosphere Changed
Over billions of years, the atmosphere changed dramatically. As the Earth cooled, water vapour condensed to form the oceans. Carbon dioxide dissolved in the newly formed oceans, reducing its concentration in the atmosphere. Carbon dioxide was also locked up in sedimentary rocks such as limestone and in fossil fuels over millions of years.
The most important change came with the evolution of primitive plants and algae. These organisms carried out photosynthesis, absorbing carbon dioxide from the atmosphere and releasing oxygen. Over time, the level of oxygen gradually increased.
Carbon dioxide was also removed from the atmosphere by other processes. It dissolved in the oceans when they formed, and marine organisms used it to build shells and skeletons made of calcium carbonate. When these organisms died, their remains formed sedimentary rocks such as limestone and chalk. Carbon was also locked away in fossil fuels — coal, oil and natural gas — formed from the remains of ancient organisms buried under layers of sediment over millions of years.
As oxygen levels rose, more complex organisms that depended on oxygen evolved. The build-up of oxygen also led to the formation of the ozone layer (\(\text{O}_3\)), which protects the Earth from harmful ultraviolet radiation. Today’s atmosphere contains approximately 78% nitrogen, 21% oxygen, and small amounts of other gases including about 0.04% carbon dioxide and varying amounts of water vapour.
Today’s Atmosphere
The composition of today’s atmosphere is approximately:
- Nitrogen — about 78%
- Oxygen — about 21%
- Argon — about 0.9%
- Carbon dioxide — about 0.04%
- Plus small amounts of water vapour and other gases.
Today’s atmosphere is approximately 78% nitrogen, 21% oxygen, 0.9% argon and 0.04% carbon dioxide.
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 radiates longer-wavelength infrared radiation back towards space. Greenhouse gases — including carbon dioxide, methane and water vapour — absorb some of this infrared radiation and re-emit it in all directions, warming the atmosphere.
Methane is produced by livestock farming, rice paddies and the decomposition of waste in landfill. Although present in much smaller quantities than carbon dioxide, methane is a more potent greenhouse gas — each molecule absorbs more infrared radiation than a molecule of carbon dioxide.
Climate Change
Climate change refers to long-term shifts in global temperatures and weather patterns. Human activities — particularly burning fossil fuels (coal, oil, natural gas) — have increased the concentration of carbon dioxide in the atmosphere. Livestock farming has increased methane levels. These increases enhance the greenhouse effect, leading to global warming.
Evidence for human-caused climate change includes the strong correlation between rising atmospheric carbon dioxide concentrations, increased fossil fuel consumption and rising global temperatures over the past 150 years. However, scientists acknowledge uncertainties: measurements taken at different locations can vary, and historical data may be less accurate than modern records.
Burning fossil fuels increases atmospheric CO\(_2\). Livestock farming increases methane. Both enhance the greenhouse effect and contribute to global warming and climate change.
The potential effects of increased global temperatures include: rising sea levels due to thermal expansion of water and melting ice caps; more frequent and severe extreme weather events such as floods, droughts and storms; changes in habitats and biodiversity, with some species unable to adapt quickly enough; and disruption to agriculture and food supplies. Mitigation measures include reducing fossil fuel use, increasing renewable energy, improving energy efficiency, and carbon capture and storage technologies.
Testing for Oxygen
The chemical test for oxygen is to place a glowing splint into the gas. If oxygen is present, the splint relights. This test was historically important in establishing that photosynthesis produces oxygen.
When evaluating evidence for climate change, always discuss both the correlation between CO\(_2\) levels, fossil fuel use and temperature, and the uncertainties in the data. The exam expects you to weigh the evidence rather than simply state that climate change is happening.