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 greenhouse effect has developed over the course of the Earth’s history :

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.

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.

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.

The impacts of climate change

Warming Differentiated by Latitude

The expected increase in temperature varies according to latitude. The warming will be less pronounced in the tropics than at the poles. The increase in coastal temperatures will be smaller than that inland.

Assuming reasonable hypothetical conditions, which include continued levels of observed economic and demographic development and a balance between fossil and renewable energy sources, one can estimate the following increases in annual temperatures over a single century (period from 1999-2099):

• + 3,5°C in Southern Europe
  
• + 2,5°C in Southeast Asia
  
• + 4,9°C in the Arctic (North Pole)
  
• + 3,2°C in Central America
• + 2,6°C in Southern Australia
• + 3,3°C in West Africa
  

The global increase in average temperature would thus be +2.8°C.

Increase in Sea Level

Global warming affects sea level worldwide. Records indicate that sea levels have been continually rising since the 1870s.

The two principal factors affecting sea level rise are thermal expansion and the melting of terrestrial ice deposits (glaciers, polar ice caps, snow cover, permafrost). In the future, increases in sea level will most likely lead to population migration, as island and coastal populations are displaced by rising waters. 

Rising sea levels may also lead to the salinization of essential groundwater resources, hence making them unavailable for human consumption and agriculture.

Increased Frequency of Extreme Events

Extreme weather events include storms, hurricanes, floods and droughts, as well as abnormally mild winters and abnormally warm summers.

Because the factors driving extreme weather events are complex, it is difficult to attribute any one event to global warming. Focusing on measurable values such as temperatures and precipitation levels allows us to assess the frequency of “unusual” events with more accuracy.

Since the 1990s, scientists have measured both a significant decrease in the number of abnormally cold days and an increase in the number of abnormally warm ones. Precipitation anomalies have also been noted over the same time period. 

Impact on populations and ecosystems

Climate change will impact different ecosystems in different ways. More developed nations, with greater resources, will be better able to adapt to climate change than their less developed neighbours. Thus, there is a strong correlation between climate change impacts and economic inequality.

Any answer to this global issue requires international political and economic cooperation. The United Nations’ Framework Convention on Climate Change (UNFCCC) ratified by most nations of the world articulates this goal. The UNFCCC’s 1997 Kyoto Protocol set the first binding limits on greenhouse gas emissions from developed nations. 

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.