What is Global Warming: The Science of Global Warming

Friday, 19 December 2008 ·

In order to understand the mechanics of global warming, let’s see how the Earth is “powered” by the Sun (through solar radiation), how the energy (radiation) between these two entities is continuously recycled, and how this cycle maintains the equilibrium temperature of the Earth.

Here is how this process works: (14)

  • The Earth receives its original supply of shortwave radiation from the Sun
  • The Earth then reflects 30% of this solar radiation back into space in its original shortwave form
  • The Earth absorbs the remaining 70% of this solar energy and then re-radiates it back into space in the longwave form (infrared radiation)

So in order to maintain thermal equilibrium, the amount of shortwave and longwave radiation leaving the Earth must be equal to the amount of the original shortwave radiation received from the Sun.

Thus, for a certain amount of radiation being exchanged with the Sun the Earth will achieve a certain equilibrium temperature.

The Greenhouse Effect

Experts point out that for the Earth as a whole, the equilibrium “radiative” temperature (i.e. the temperature required to reradiate all the energy received from the Sun) at the outside of the atmosphere is −18°C. So if the Earth’s atmosphere was totally transparent to all wavelengths of radiation, then the average temperature at the surface of the planet would also be −18°C [making it most likely uninhabitable]. (15)

But thankfully, during the last step of the radiation cycle described above – when the Earth re-radiates solar energy back into space in the longwave form, the atmosphere absorbs (“traps”) some of this upward-going longwave energy.

The atmosphere then emits longwave radiation in all directions. Some of this radiation will go back down towards the Earth adding to the original shortwave solar radiation received by the planet.

This added amount of radiation raises the Earth’s equilibrium temperature [to around +15°C] and makes life as we know it possible on our planet. (16)

Therefore, the process of emission of longwave (infrared) radiation by the atmosphere which results in the warming of the Earth is called
the greenhouse effect.

Greenhouse Effect

The Earth’s atmosphere is a unique environment which consists of a number of naturally occurring gases.

The content of the atmosphere is roughly as follows: (17)

  • Nitrogen (around 78% of the total)
  • Oxygen (around 20% of the total)
  • Water vapor (substantial amount)
  • Carbon dioxide (small amount)
  • Trace gases (hydrogen, argon, helium and other gases)

It is exactly some of these gases in the atmosphere (e.g., water vapor and carbon dioxide) that trap the upward-going longwave radiation emitted by the Earth, re-emit it in all directions and thus contribute to the warming of the planet.

The gases that take part in this process are called radiatively active gases, or greenhouse gases.

Among the most important naturally occurring and man-induced greenhouse gases are:

  • Water vapor
  • Carbon dioxide
  • Methane
  • Nitrous oxide
  • Ozone

There are natural concentrations of greenhouse gases in the atmosphere which work to keep the planet warm.

These natural concentrations were pretty much constant during the Holocene (a period that started roughly 11,500 years ago) and before the industrial revolution. (18)

However, with the advent of the Industrial Age at the start of the 19th century, the concentrations of greenhouse gases have increased dramatically and have been attributed to human actions.

In simple terms, this means that the functioning of the modern industrial society produces additional amounts of greenhouse gases emitted into the atmosphere.

These additional amounts trap even more longwave radiation and re-radiate it back to the Earth.

This, in its turn, leads to the rise of the surface temperature and establishment of a new “equilibrium” temperature with a new given amount of radiation in the system.

Thus, human-induced increases in greenhouse gas concentrations are now so significant that they have been largely blamed for the recently observed accelerated rates of global warming.

Radiative Forcing

Heated Earth Global Warming
Heated Earth
Photo: Thiago Kunz

Following the discussion of the greenhouse effect above, it becomes clear that the rise in the average temperature of the Earth (i.e. global warming) can take place in 3 ways: (19)

  • An increase in the shortwave solar radiation that enters the atmosphere at the top
  • An increase in the percentage of the radiation that actually reaches the surface through the atmosphere (currently 70% of the total solar radiation), and
  • An increase in the concentration of the greenhouse gases in the atmosphere

Such increases (as well as decreases) in the quantity of radiation circulating through the Earth’s atmosphere are measured in terms of “radiative forcing”.

Therefore, radiative forcing is the net change in the amount of radiation that comes into, and goes out of, the atmosphere. It is measured in Watts per square meter (W m2).

This net change in radiation quantity may be compared to the total shortwave solar radiation of around 342 W m 2 per year that the Earth currently receives. (20)

Positive forcing (i.e. net increase in the amount of radiation) contributes to the warming of the Earth’s atmosphere. Greenhouse gases contribute to positive forcing.

Between the years 1765 and 2000 the Earth witnessed a net increase of 2.45 W m 2 in the total amount of radiation. Carbon dioxide, an extremely important agent of global warming, has contributed around 60% to this increase. (21)

This is an example of the positive forcing effect on the global temperature.

Negative forcing (i.e. net decrease in the amount of radiation) contributes to the cooling of the Earth’s atmosphere. Aerosols are an example of a climate agent that contribute to the negative forcing effect.

To summarize it all, the greenhouse effect and radiative forcing are among the most important concepts that lie at the heart of the science of global warming.