The Greenhouse Effect (HL IB Physics)

• While only around 25% of the (primarily short wavelength) solar radiation is absorbed by the atmosphere on its way to Earth, around 80% of the (long wavelength) re-emitted radiation from Earth is absorbed on its way back into the atmosphere
  • For example, incoming UV radiation is absorbed by ozone
  • Re-emitted infrared radiation is absorbed by greenhouse gases

• This absorbed radiation keeps Earth at a habitable temperature
  • However, if there is an imbalance in the chemical composition of the atmosphere, this can lead to fluctuations in the Earth’s mean surface temperature

Resonance Model of Global Warming

• Incoming radiation from the Sun predominantly takes the form of ultraviolet and visible radiation
• Visible light is not absorbed by the atmosphere, instead, it is absorbed by the Earth’s surface
• At night, the Earth re-radiates this radiation as infrared
• Some of this radiation is absorbed by the Earth’s atmosphere and some of the radiation is reflected back into space
• The greenhouse gases present in the atmosphere absorb infrared radiation and reflect it back towards the Earth’s surface
  • The higher the concentration of greenhouse gases present, the more infrared radiation there is remaining in the Earth’s surface-atmosphere system
• Therefore, heat energy becomes trapped inside Earth’s atmosphere and accumulates
  • This leads to the greenhouse effect and an increase in average mean temperatures on Earth

Greenhouse gases absorb the long-wave radiation emitted by Earth, warming the atmosphere

Molecular Energy Level Model

• The greenhouse effect occurs due to the particular molecular structure of greenhouse gases
  • High-frequency UV light is energetic and able to break bonds within molecules
  • Infrared light, on the other hand, causes atoms to vibrate
• The greenhouse gases have a natural frequency that falls in the infrared region
  • This means when they absorb infrared light, they begin to resonate, causing the molecules to heat up
  • They absorb the infrared radiation and subsequently emit it back towards the Earth’s surface

Solar radiation is primarily short-wave, while the radiation that is re-emitted by earth is long-wave radiation

• Human activity is increasing the number of greenhouse gases in the atmosphere:
  • Carbon dioxide (CO₂) levels in the atmosphere have increased by more than 100 parts per million (ppm) to 420ppm in 2020
• Increased amounts of greenhouse gases have led to the enhanced greenhouse effect:
  • Less long-wave radiation (heat) can escape the atmosphere
  • Average global temperatures have increased over 1°C since pre-industrial times

The average global temperature has risen remarkably in the past 100 years

The greenhouse effect arises due to natural causes, but the enhanced greenhouse effect arises due to human activity

Human Sources of Greenhouse Gases