The Physics of Global Warming

When I talk about global warming, I say “Greenhouse gases are released, causing more heat to be trapped in the atmosphere and raising the global average temperature”. This is the basic version of the greenhouse effect. There’s a missing step: How do greenhouse gases physically cause more heat to be trapped? What is the physics behind it? My ruse has been uncovered, since my dissertation supervisor noticed that I didn’t actually go into the physics of climate change in my paper, so I’m going to need to look into it and tell you guys what I find out on the way!

What are Greenhouse Gases?

A greenhouse gas (GHG) is a gas that absorbs and emits radiant energy within the thermal infrared range, causing the greenhouse effect. The thermal infrared energy is what is known as heat, which are waves that humans can’t see but they can feel. GHG are Carbon Dioxide (from burning fossil fuels), Methane (from producing fossil fuels and agriculture), Nitrous Oxide (from agriculture and industrial activities), and Fluorinated gases like hydrofluorocarbons, perfluorocarbons, sulphur hexafluoride, etc (from industrial processes). Ozone (triple oxygen) and Water Vapour are also GHG, but we are less worried about these being emitted by humans. 

The Greenhouse Effect

The Greenhouse Effect is a naturally occurring phenomenon where heat is trapped close to the planet by GHG in our atmosphere (at a concentration of ab out 200 parts per million), which enables our planet to live in the Goldilocks Zone. This is where the conditions are just right for humans to thrive. About 30% of the sun’s energy is reflected back to space but 70% passes through the atmosphere. This heat is absorbed by earth and oceans which heats the planet. This heat is radiated back in the form of infrared radiation (heat). 10% passes through to space but 90% is reflected back. The planet is kept at an average of 15 °C. The problem Is that humans are interfering with this, releasing more GHG through burning fossil fuels, animal agriculture, and industry, to now have a concentration of about 400 parts per million. This causes more heat to be trapped and global temperatures to rise. But how do GHG do this?

GHG keep the Earth warm by absorbing energy and slowing the rate at which energy escapes to space, like an insulator. The main thing that differentiates GHG from other gases is a factor called global warming potential (GWP). This is a measure of the total energy that one ton of a gas absorbs over a period of time (usually about 100 years) compared to the emissions of one ton of carbon dioxide. This process is useful for scientists and policy makers to gather data on global warming. Carbon Dioxide has a GWP of 1, Methane has a GWP of 28-36 (GWPs are often presented as averaged due to multiple methods of calculating GWPs depending on the influence of future warming on the carbon cycle), Nitrous Oxide has a GWP of 256-298, and fluorinates gases tend to have a high GWP in the thousands or tens of thousands.  GWP is not an inherent property, but rather a metric of measuring the effect that a gas has on the warming of the atmosphere. Another similar method is Radiative Forcing (RF) which measures the difference between how much of the sun’s energy is absorbed the by earth and how much is released into space depending on one ‘Climate Driver’. A climate driver can be a GHG, a natural change in the sun’s energy output, volcanic eruptions, or changes in land use. 

You might be wondering why we are so concerned with carbon dioxide when fluorinates gases can be tens of thousands of times worse at trapping heat. This is because there are two other factors that affect how much a gas contributes to global warming. One is how much of it exists in out atmosphere, and another is its lifetime – how long it remains in the atmosphere. Water vapour is actually the most abundant GHG, but we only have a small effect on it through irrigation and deforestation. Carbon Dioxide makes up 0.04% of the atmosphere but about 80% of human GHG emissions. Its lifetime can’t be exactly quantified because it is part of the carbon cycle, but it can remain in the atmosphere for thousands of years, compared to 12.4 years for Methane, and 121 years for Nitrous Oxide. The highest lifetime is sulphur hexafluoride at 23,500 years (but it’s concentration in the atmosphere is 1000 times less than carbon dioxide). 

Natural Cycle of GHG

Currently, GHG make up about 400 parts per million in the atmosphere. Naturally, they make up 200-280 parts per million, but they don’t stay in the atmosphere forever – this is what we mean when we talk about ‘lifetime’. 

I mentioned that Carbon dioxide has a difficult to measure lifetime because it is part of the carbon cycle. Carbon is a really important element for earth, for example, humans are carbon-based life forms. This is also found in the oceans, in sediment, in trees, and in the atmosphere. It has been used as a very powerful form of energy that powered the industrial revolutions of history. 

There are two main cycles of reactions: in the slow reaction (100-200 million years) carbon in the rain forms carbonic acid that dissolves rocks – chemical weathering. This releases calcium/magnesium/potassium/sodium ions, which travel to the ocean. The calcium ions combine with bicarbonate ions to corm calcium carbonate, which nature uses to make coral and plankton. When these organisations die, they sink to the seafloor and become sediment, eventually becoming limestone. Other carbon sources at this stage are coal and oil. When plates collide, the rock melts and turns into carbon dioxide, and volcanos return this carbon to the atmosphere. 

The fast carbon cycle moves 100-100,000 tons of carbon through each year. Phytoplankton and plants absorb carbon dioxide, which is returned to the atmosphere by (1) animals and humans eating plants, (2) plants and plankton dying, (3) fire consumes plants, (4) plants break down sugar to create energy. Some parts of these processes can be described as ‘carbon sinks’, which absorb more carbon than they release. Examples are forests or the ocean. It’s important to protect these resources to prevent too much carbon dioxide from being released into the atmosphere. 

Fluorinated gases travel to the upper atmosphere where they are eventually destroyed by sunlight. Methane has a half life of about 9.1 years, and nitrous oxide is eventually absorbed by bacteria or is destroyed by UV radiation. This process also depletes ozone which damages the natural state of the atmosphere.

How Do We Measure GHG?

As we’ve seen, it’s pretty complicated to measure the impact of a certain gas on global warming. You can’t just quantify the most dangerous gas, but the most abundant gas, its lifetime, factors that affect its lifetime like deforestation, and which gases humans are emitting the most. This data is really important to decide which actions we should take against climate change, such as writing legislation to limit methane emissions in animal agriculture or carbon capture and sequestration at coal plants. GHG can be measured in several ways in the laboratory. Carbon Dioxide can be measured by Manometry, which passes the gas through multiple dry ice traps to measure temperature in dry air form, and volume is measured by adding liquid nitrogen. You can also use an infrared gas analyser or titrimetry. Methane can be measured by differential absorption lidar (DIAL), where the DIAL takes vertical scans above methane sources, and then spatially separates the scans to measure the individual sources of methane emissions. 

Nitrous Oxide is measured differently, using an Atmospheric Chemistry Experiment – Fourier Transform Spectrometer (ACE-FTS). This is an instrument attached to the Canadian satellite SCISAT. The auxiliary Visible/Near-infrared Imager (VNI) monitors aerosols based on the extinction of solar radiation using two filtered detectors at 0.525 and 1.02 micrometres, which compares different way that nitrous oxides are released into the atmosphere. Carbon can also be measured by satellites using the Orbiting Carbon Observatory (OCO and OCO-2) using a spectrometer, and the Greenhouse Gases Observing Satellite (GOSat). This is a JAXA mission which also uses a Fourier Transform Spectrometer (TANSO-FTS) to measure the scene radiance of solar short wave infrared spectra (SWIR) reflected on the Earth’s Surface, as well as a Cloud and Aerosol Imager (TANSO-CAI) for cloud and aerosol observation. 

Final Thoughts on GHG

This was a bit more complicated than I expected. There are a lot of factors that go into ascertaining whether a certain gas is bad for the environment, and how bad. It's also more complicated than I thought to measure them. Carbon Dioxide is the GHG that we should be most worried about, and that a good way of limiting our emissions is by using renewable energy over fossil fuels, and by conserving forests and the ocean.