The Earth's Atmosphere and Greenhouse Effect
The Early Atmosphere
For the first billion years, the Earth’s atmosphere was very different from today. Scientists believe it was mainly carbon dioxide, with little or no oxygen — similar to the atmospheres of Venus and Mars today. There was also water vapour, nitrogen and small amounts of methane and ammonia. This early atmosphere was produced by volcanic activity.
The water vapour condensed as the Earth cooled, forming the oceans. Over billions of years, the atmosphere changed dramatically:
Step 1: The first simple organisms (algae and photosynthetic bacteria) evolved in the oceans. They used \(CO_2\) for photosynthesis and released \(O_2\), gradually increasing the oxygen level in the atmosphere.
Step 2: \(CO_2\) was also removed by dissolving in the oceans, where it formed carbonates in sedimentary rocks and the shells/skeletons of marine organisms.
Step 3: As \(O_2\) levels increased, more complex life evolved. The ozone layer (\(O_3\)) formed, shielding life from UV radiation.
Today’s atmosphere: approximately 78% nitrogen, 21% oxygen, 0.04% carbon dioxide, plus small amounts of other gases (argon, water vapour). This has been roughly stable for 200 million years.
The Greenhouse Effect
Greenhouse gases (carbon dioxide, methane, water vapour) absorb heat radiation (infrared) from the Earth’s surface and re-emit it in all directions, including back towards Earth. This natural greenhouse effect keeps the Earth warm enough to support life.
However, human activities are increasing the concentrations of greenhouse gases:
- Carbon dioxide — burning fossil fuels, deforestation.
- Methane — agriculture (cattle farming, rice paddies), landfill sites.
This enhanced greenhouse effect is causing global warming — an increase in global average temperatures. The consequences include: sea level rise (thermal expansion and melting ice), more extreme weather events, changes to ecosystems and species distribution, and impacts on agriculture.
Human activities increase \(CO_2\) (fossil fuels, deforestation) and \(CH_4\) (farming, landfill). Enhanced greenhouse effect → global warming → rising sea levels, extreme weather, habitat loss.
A carbon footprint is the total amount of greenhouse gases released over the life cycle of a product or activity. It can be reduced by: using renewable energy, improving energy efficiency, carbon capture and storage, reducing meat consumption, and reducing travel.