Climate Sensitivity

John Tyndall FRS (1820-1893)

John Tyndall was the first to explain why the sky is blue. He was also one of the first people to adopt the term physicist rather than the term natural philosopher.

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John Tyndall FRS (1820-1893)
Wiki Commons

John Tyndall FRS (1820-1893)

John Tyndall was the first to explain why the sky is blue. He was also one of the first people to adopt the term physicist rather than the term natural philosopher.

Wiki Commons

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John Tyndall's discovery

John Tyndall from County Carlow made the breakthrough on understanding what was keeping the Earth at its comfortable temperature. He guessed that the atmosphere had an important role in this - but how?

To investigate this he devised very advanced instruments for analysis of how light and other radiation passed through the atmosphere. In doing this he discovered that while the wavelengths of light that we see in the rainbow can easily pass through the atmosphere, other invisible radiation that we cannot see directly is trapped. This radiation is sometimes called infra-red radiation and it is an important part of the heat energy being radiated by the Earth back to space. Eureka! He had found an unknown invisible mechanism to keep the Earth warm. This was a major breakthrough that would have won him a Nobel Prize if these existed at that time.

He then went further to isolate the parts of the atmosphere that were actually trapping the heat radiation and identified a number of gases, which although only making up a small portion of the atmosphere, carry out this vital role and make the Earth comfortable for us to live.    

These gases are now referred to as greenhouse gases as they behave somewhat like glass in a greenhouse, which lets sunlight energy in but stops the heat escaping (If you have ever sat into a car parked in sunlight you will be familiar with how powerful this effect can be).

John Tyndall had identified the natural greenhouse effect, which is vital to life on Earth. Some time later in 1896, Svante Arrhenius from Sweden made the first calculation of what would happen if the atmospheric concentration of one of these greenhouse gases, carbon dioxide CO_2, was doubled. He calculated that this would cause a global average temperature increase of 5 degrees Celsius. The most recent estimate for this figure produced by the Intergovernmental Panel  on Climate Change (IPCC) in 2015 is that temperatures will not exceed 3 degrees Celsius. The IPCC calculation is based on our most advanced analysis of the global climate system but the value is remarkably close to the value calculated over 100 years earlier.    

This calculation of the Earth's response to a doubling of CO₂ in the atmosphere is now known as “climate sensitivity”. It is a key factor for determining what actions are required at a global level to address climate change. The temperature of the Earth is determined by the balance between energy coming from the Sun in the form of visible radiation, sunlight, and energy constantly being emitted from the surface of the Earth to outer space in the form of invisible infrared radiation. The energy coming in from the Sun can pass through the clear atmosphere pretty much unchanged and heat the surface of the Earth. However, the infrared radiation emanating from the surface of the Earth is partly absorbed by some gases in the atmosphere, and therefore trapped. The effect of this is to warm the surface of the Earth and troposphere. The gases that do this work in the natural atmosphere are called greenhouse gases. These include gases such as carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O), which are the three most important greenhouse gases.

The natural greenhouse effect acts a bit like a blanket trapping some of the heat within the atmosphere. Without these gases the Earth would be approximately 30°C cooler and life would be significantly different. The thickening of this natural blanket through the emissions of additional greenhouse gases is the key driver of climate change.
 
The main impact of more greenhouse gases in the atmosphere is to trap additional energy or heat in the earths climate system. This is sometimes referred to as global warming. Its impacts are most clearly evident in the global temperature records. However this “warming” of the Earth has many knock-on effects on the global climate and those who depend on it.