Carbon economics from A to Z

Climate change

                                  
1- What is the greenhouse effect?
A natural atmospheric phenomenon
The greenhouse effect is caused by gases that naturally occur in the atmosphere (the so-called “greenhouse gases”, or GHGs).The figures below show how the greenhouse effect has developed over the course of the Earth’s history.  The data presented shows the energy flows, expressed in W/m2.

Fig. 1. Hypothetical Initial State: the atmosphere contains no GHGs. The sun supplies energy through its rays. The rays are absorbed by the planet and heat the surface. The Earth, in return, radiates an equal quantity of energy in the form of infrared radiation (IR), assuring an energy balance. In the absence of all GHGs, the terrestrial temperature would be -19°C.
Fig. 2. Transition Phase: as GHGs accumulate in the atmosphere, they reflect a portion of the infrared radiation back towards the surface. The Earth’s temperature increases until the energy radiated is equal to that absorbed.
Fig. 3. Current State: the presence of GHGs leads to an increase in surface temperature, reaching +14°C. 

Over the course of the industrial era, this naturally occurring greenhouse effect has been amplified, as human activities have released more and more greenhouse gases into the atmosphere.  This build-up has caused the average surface temperature of the Earth to rise. 

What are the major greenhouse gases?

As mentioned above, the unique property of greenhouse gases is that they trap and reflect some of the infrared energy radiating from the Earth’s surface.  Different gases trap more or less energy, depending on what is called their Global Warming Potential (GWP).  The GWP of a gas is the ratio between the energy that 1 kg of the gas would reflect toward the surface over a 100 year period and the energy that would be reflected by 1 kg of CO2 over the same period. The GWP depends on the concentration and lifespan of each gas. For example, 1 kg of CH4 and 25 kg of CO2, emitted at the same time, would heat the atmosphere an equally over one century.

Among the greenhouse gases, six are produced at least in part by human activities :
- carbon dioxide (CO2)
- methane (CH4)
- nitrous oxide(N2O)
- fluorinated gases (HFC, PFC et SF6)
The buildup of these gases in the atmosphere is responsible for the enhanced greenhouse effect that leads to global warming.

2- Evolution of atmospheric GHG Concentrations and temperature increases
Atmospheric GHG concentrations in the past
Human activities disrupt the natural GHG cycle. For example carbon that has been stored underground for thousands of years in the form of oil and coal is released massively and rapidly into the atmosphere when these fossil fuels are burnt to produce energy.
As industrial activities have developed the atmospheric concentration of greenhouse gases has increased.

The fluorinated gases (HFC, PFC and SF6) are entirely of human origin and were not present in any concentration before the industrial era.

What impact on the global average temperature?

While today’s atmospheric CO2 concentrations are similar to those reached at other points in the Earth’s history, it is important to note that in the past, these CO2 “peaks” were achieved after tens of thousands of years of accumulation, while today’s elevated CO2 levels are due to a rapid rise in concentrations (+35%) over the last two centuries.

Average global temperature has increased by approximately + 1 °C over the last century.
Much of this warming occurred over the past 25 years, during which temperatures increased at an accelerated level.

|Next|