Greenhouse Gases

How do Greenhouse Gases (GHGs) Work?

As solar energy radiates away from the sun, it is intercepted by different objects in space. One such object, located 150,000,000 kilometers away from the light source, is Earth. When the earth intercepts radiation from the sun, most of it is absorbed by the land and the oceans in the form of heat energy. This energy is then released by the Earth and re-emitted back into space (we know this because if it was not the case then the Earth would be continuously warming up from the sun's energy).

When this energy is released, a portion of it is absorbed by greenhouse gases in the atmosphere. This is important because the energy is then re-emitted from the gases back towards the surface. This phenomenon is known as the 'Greenhouse Effect', and without it, Earth's average surface temperature would consistently be almost 20 degrees Celsius below zero; 35 degrees colder than it is now, and completely unable to support life.

Historic GHG Concentrations

Scientific research has shown that over the course of our planet's history, Earth's surface and climate have gradually fluctuated between warm and cold temperatures. This explains how dinosaurs (large, cold-blooded reptiles) were able to survive from pole to pole during the Mesozoic era. Using ice cores to determine pre-historic concentrations of carbon dioxide in the atmosphere, scientists have determined that these temperature cycles are strongly correlated to atmospheric greenhouse gas levels. Essentially, as the atmospheric concentration of greenhouse gases increases, more energy radiating from the Earth is trapped, and the climate gets warmer. These patterns fluctuate from hot to cold climates over long timescales of hundreds of thousands of years.

Historic shifts between hot and cold climates are often used as an argument to justify the claim that climate change is “not caused by human activity”. This argument fails to comprehend the timescales at which global climate change occurs. Over the past 400,000 years, the concentration of atmospheric CO2 has cycled about five times between 180 p per million (ppm), and approximately 300 ppm. These cycles generally took between 75,000-100,000 years. However, in the past 250 years (since the Industrial Revolution), where we would expect to see the beginning of a decline in atmospheric CO2, we instead observe a dramatic increase. In fact, since 1750, atmospheric CO2 has increased 45% from 280 ppm to over 406 ppm. This is the first time in over 400 millennia that it has surpassed 400 p per million.

Climate Action

Information regarding the City's climate action initiatives and reporting can be found on the Climate Action page.

Water Vapour

It is interesting that while the greenhouse effect and climate change are largely attributed to atmospheric carbon dioxide concentrations, water vapour is actually a more abundant and important (in terms of warming effect) greenhouse gas. Water vapour does not garner much attention as a greenhouse gas due to its short atmospheric lifetime and because its presence in the atmosphere is largely independent of human activity.

When compared to CO2 or nitrous oxide, the water (hydrologic) cycle moves molecules through the atmospheric system quite rapidly. Depending on air temperature, the atmosphere can retain a specific amount of water vapour until it is saturated. Once this point is reached, the water is released in the form of rain or snow. This cycle occurs on a relatively short timescale; water molecules may only be present in the atmosphere for a couple hours or days.

Other, man-made greenhouse gases, like carbon dioxide, last much longer in the atmosphere. Though carbon dioxide is removed from the atmosphere by photosynthetic plants as well as ocean deposition, molecules are able to persist for up to 200 years, during which time they continue to warm the atmosphere and contribute to climate change. The same can be said of nitrous oxide, which slowly decomposes in the atmosphere over a period of about 120 years.

The other, very simple reason why climate scientists are more preoccupied with CO2 than water vapour is because the hydrologic cycle is largely independent of human activity. It is completely feasible to remove carbon from our energy and transportation systems and to reduce our GHG emissions from other activities; but it would be impossible to prevent water evaporating from the planet's surface into the atmosphere. The ineffectiveness of such a strategy is such that scientists have not even bothered exploring it. It makes more sense to address the emission of long-lived, human produced (anthropogenic) greenhouse gases.