- Biogeochemical cycles
- Book title
- Principles of environmental sciences
- Pages (from-to)
- Number of pages
- Dordrecht: Springer
- Document type
- Faculty of Science (FNWI)
- Institute for Biodiversity and Ecosystem Dynamics (IBED)
It is now often assumed that life first appeared on planet Earth about 3,500 million years ago. Since then ‘our’ Sun has changed considerably. While the flux of solar energy to the Earth has increased by about 30% over this period, though, this has not led to a corresponding increase in the Earth's temperature or the amount of ultraviolet radiation reaching the planet's surface.
The main reason for the absence of any major change in the Earth's temperature over this extended period is that the concentrations of so-called greenhouse gases — i.e. gases transparent to visible light but absorbing infrared radiation — such as carbon dioxide (CO2) and methane (CH4) have fallen dramatically. Ultraviolet irradiation of the Earth's surface has in all probability declined substantially since life's first origins, a crucial development because DNA and other vital cell components are easily damaged by ultraviolet radiation. The decrease in the UV radiation striking the Earth's surface is due to the presence of an ‘ozone layer’ in the stratosphere, the section of the atmosphere 15-50 km above the Earth's surface containing about 90% of atmospheric ozone. The ozone in this layer is a strong absorber of UV radiation. This long-term decline in atmospheric levels of greenhouse gases and the formation of the ozone layer are intimately linked to the development of life on Earth.
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