A process which is upsetting a natural equilibrium

The greenhouse effect is a natural phenomenon. Without the greenhouse effect, the average temperature on Earth would be much lower than the 15°C average observed. With a more pronounced greenhouse effect, whose main consequences are an increase in the average temperature of the Earth's surface and disruption of the water cycle, the planet's equilibrium is at risk.

Although many uncertainties remain, the 5th assessment report published by the Intergovernmental Panel on Climate Change (IPCC) in 2014 [IPCC, 2014], attributes the warming observed during the 20th century and into the 21st to human activity.


Greenhouse gases and radiative forcing

The Earth's radiation balance can be summarised as follows: Earth receives part of the sun's radiation in the form of visible rays. Incident solar flux is around 340 W.m-2. The Earth's surface absorbs about 50% of this incident energy, the remainder being absorbed by the atmosphere (20%) or reflected by clouds and reflective areas on the ground, such as deserts and glaciers (30%). The Earth's surface re-emits this incident energy in the form of infrared (IR) radiation.

The greenhouse effect is mainly produced by absorption of the long-wavelength infra-red (IR) rays radiated back into space by the Earth's surface, by clouds and by compounds naturally present in the atmosphere, such as: CO2, CH4, H2O, O3, N2O. These compounds therefore create a natural greenhouse effect, without which the average temperature on Earth would be around -18°C.


Figure 1: The Earth's radiation balance

Effet Serre Figure1

Source [IPCC 2014]


Although they are present as trace gases, the increase in concentrations of long-lived compounds (called Long-Lived Climate Forcers) such as CO2, CH4 and N2O therefore accentuate the greenhouse effect. Studies of situations in the past, through analyses of air bubbles trapped in deep ice layers, are producing a great deal of valuable information. Scientists have shown that cold episodes in the past usually coincided with low atmospheric concentrations of CO2 and CH4. In the case of CO2, fluctuations in concentrations are associated with changes in ocean currents and productivity: oceans are the planet's main long-term regulators of CO2. Changes in concentrations of CH4 affect terrestrial ecosystems and the permafrost regions in northern latitudes.

CO2 concentrations in the atmosphere are primarily due to CO2 emissions from the combustion of fossil fuels in the industry, residential and transport sectors. CH4 emissions are mainly due to agricultural practices such as rice growing or livestock farming. N2O emissions are mainly from the use of mineral fertilisers and animal manure in agriculture. New substances such as CFCs (chlorofluorocarbons), HCFCs (hydrochlorofluorocarbons), HFCs (hydrofluorocarbons), PFCs (perfluorocarbons), NF3, SF6, which are entirely of human origin, are powerful greenhouse gases.

Their effect is to disrupt the overall energy balance by additional trapping, expressed in W.m-2. The radiative forcing of greenhouse gases (i.e. their capacity for absorbing radiation) is positive. Radiative forcing by other compounds present in the atmosphere, such as certain aerosols, is negative, i.e., they cool the atmosphere.

Scientific understanding of the complexity of the phenomena involved is steadily advancing. The increase in the greenhouse effect is not only due to long-lived greenhouse gases. Many short-lived compounds also come into play, either directly (such as ozone and aerosols) or indirectly (such as CO, NOx, VOCs).

Among these short-lived compounds, black carbon (BC) has received a lot of media attention since it was shown by scientists that it plays a crucial role in the growing pace of climate change observed in certain regions (northern polar regions and the Himalayas in particular) [UNEP-WMO, 2011]. Black carbon is one of the components of the particulate matter released by the combustion of fossil fuels and biomass, especially when combustion is incomplete and poorly managed. Black carbon can be defined as a solid carbon compound that absorbs visible radiation at all wavelengths. It is present in soot, which is a complex mixture of compounds that also includes organic carbon (OC). Another compound in particulate matter also absorbs radiation. This is known as "Brown Carbon" (BrC), which is a class of organic carbon that absorbs visible and UV radiation. As black carbon is never released alone and can be emitted together with compounds that cool the atmosphere, the overall radiation balance of these various compounds is still uncertain.

The ozone (O3) in the atmosphere is also a greenhouse gas. Radiative forcing by ozone depends on its height above the Earth's surface. According to the 5th IPCC assessment report [IPCC, 2014], the ozone in the troposphere is the 3rd main greenhouse gas. However, stratospheric ozone has a cooling effect.

Figure 2 below shows radiative forcing in the main gases, aerosols and aerosol precursors and other causes from 1750 to 2011. It highlights the dominant role of the anthropogenic increase in the greenhouse gas concentrations in climate change.


Figure 2 : Radiative forcing  due to different causes from 1750 to 2011 [IPCC 2014]

Effet Serre Figure2

Source [IPCC 2014]


There is no doubt that the radiative forcing due to anthropogenic causes since the pre-industrial age (1750) is positive. It is estimated at 2.3 W.m-2 (1.1 to 3.3 W.m-2) [IPCC 2014].

The average contributions of the different gases to the radiative forcing during the industrial period are estimated as follows in the IPCC's 5th assessment report [IPCC 2014]:

CO2 +1.82 (±0.19) W.m-²

CH4 +0.48 (±0.05) W.m-²

N2O +0.17 (±0.03) W.m-²

CFCs/HCFCs +0.36 (±0.03) W.m-²

Ozone (tropospheric) +0.40 (0.20) W.m-²

Concentrations of long-lived greenhouse gases have increased since the beginning of the industrial age. The concentrations of a given compound in the atmosphere are the result of an equilibrium between emissions and disappearance of the compound. Long-lived compounds are chemically stable and can therefore persist in the atmosphere for decades and even hundreds of years. Shorter-lived compounds are chemically reactive and therefore disappear more quickly.

According to the annual GHG bulletin published by the World Meteorological Organisation [WMO 2016], GHG concentrations in the atmosphere in 2015 were as follows (observation data from the Global Greenhouse Gas Monitoring Network led by the WMO under its Global Atmosphere Watch programme).

    • CO2 : 400 parts per million (ppm), i.e. a 0.6% increase since 2014,
    • CH4 : 1 845 parts per billion (ppb), i.e. a 0.6% increase since 2014,
    • N2O : 328 ppb, i.e. a 0.3% increase since 2014.

CO2 concentrations reached the level of 400 ppm for the first time in 2015.

The table below shows changes in concentrations compared to the beginning of the industrial age [WMO 2016]:


Table 1: Changes in concentrations of certain GHGs since the beginning of the industrial age [WMO, 2016]

Atmospheric concentrations in 2016 400 ppm 1 845 ppb 328 ppb
Increase in concentrations since 1750a 44% 156% 21%
Increase in absolute values from 2014 to 2015 2.30 ppm 11 ppb 1.0 ppb
Relative increase 2014-2015 0.58% 0.60% 0.31%
Average annual increase since 10 years 2.08 ppm 6.0 ppb 0.89 ppb

a Assuming pre-industrial atmospheric levels of 278 ppm for CO2, 722 ppb for CH4 and 270 ppb for N2O

Before the industrial age (pre-1750), according to the 5th IPCC report [IPCC 2014], changes in concentrations were as follows:

    • CO2 concentrations in the 7 000 previous years show a slow increase of between 260 and 280 ppm. Going back to 800 000 years before 1750, CO2 concentrations varied from 180 ppm during the glacial periods to 300 ppm during the interglacial eras.
    • CH4 concentrations, at about 722 ppb in 1750, had changed only slightly during the 10 000 previous years.
    • N2O concentrations, at 270 ppb en 1750, had increased by only 10 ppb during the 11 000 years prior to the industrial age.

The WMO also emphasises [WMO 2016], that:

    • the atmospheric increase of CO2 from 2015 to 2014 is greater than between 2014 and 2013 and greater than, the average growth rate over the past 10 years,
    • the annual average growth rate of atmospheric CH4 has been increasing again since 2007 probably because of wetland emissions in tropical areas and anthropogenic sources in the mid-latitudes of the Northern hemisphere,
    • the atmospheric increase of N2O between 2015 and 2014 is the largest in the past 10 years, probably because of the increase in fertilizer use in agriculture and the increase in N2O releases from soils caused by an excess of atmospheric nitrogen deposits due to air pollution.


Global warming potential (GWP)

The GWP indicator was defined by the IPCC experts as a simple measure of the relative effects of emissions of different greenhouse gases. It is defined as the total amount of radiative forcing between today's situation and a given time in the future caused by a mass unit of gas emitted today. CO2 is used as the reference gas. The table below gives the GWP of several direct greenhouse gases. The IPCC experts cannot yet provide sufficiently reliable figures for indirect greenhouse gases.

The table below shows the updated GWP calculated for certain compounds [IPCC 2014]. 

Table 2: GWP of certain compounds according to the 5th IPCC assessment report [IPCC 2014]

Pollutants Lifetime
GWP up to different points in time

  years 20 years 100 years
CO2 variable 1 1
CH4 12 84 28
N2O 121 264 265
NF3 500 12 800 16 100
SF6 3 200 17 500 23 500
CF4 (perfluoromethane) 50 000 4 880 6 630
HCFC-22 12 5 280 1 760


Possible consequences of an accentuated greenhouse effect

The consequences for the world's climate of the accentuated greenhouse effect are not yet entirely certain although scientific understanding is steadily improving, as shown by the 5th IPCC assessment report [IPCC 2014]. There are still many uncertainties concerning the physical processes involved in CO2 absorption by oceans, for example.

The modelling results produced for the 5th IPCC assessment report published show the following temperature changes according to several different scenarios for changes in average CO2 concentrations since the pre-industrial age [IPCC 2014].


Table 3: Changes in average temperatures according to different scenarios for changes in average CO2 concentrations since the pre-industrial age (mean 1850-1900) [IPCC 2014]

Scenario name Radiative forcing in 2100 (W.m²) CO2 concentration in 2100 (ppm) Estimated rise of temperature in 2100 (°C)
RCP2.6 (radiative forcing peak, then decline) 2.6 421 1.6
RCP4.5 (stabilisation, low level) 4.5 538 2.4
RCP6.0 (stabilisation, high level) 6.0 670 2.8
RCP8.5 (continuous increase) 8.5 936 4.3


Warming will differ depending on latitudes. It will be more pronounced in the higher latitudes (only slight changes in temperatures around the equator but a steep rise is predicted in the Arctic polar regions).

It will be more pronounced in winter than in summer and in continental regions.

Because of increasing evaporation, precipitation is also likely to increase. However, water resources will decline in some regions.

Average sea level is likely to rise as oceans dilate and glaciers and ice-caps melt, with dramatic consequences in highly populated delta regions (river estuaries).

Extreme climate events such as tornadoes, typhoons, floods, droughts, etc. will increase in frequency and severity.

Ocean waters absorb about a third of CO2 emissions, so that increasing concentrations are causing ocean acidification, which is a threat to marine ecosystems.

There is still much uncertainty as to the scale and consequences of an accentuated greenhouse effect. The roles of retroactive phenomena, both positive (release of CO2 and CH4 from northern permafrost regions, ocean warming, etc.) and negative (increase in biomass, cooling of the upper atmosphere implying slower ozone destruction, involvement of other physical and chemical mechanisms cooling the atmosphere (such as aerosols) are still not known with certainty.

Global warming could therefore have significant effects on the equilibrium of societies and economies across the planet.


International commitments to limit the increase in the greenhouse effect

The United Nations Framework Convention on Climate Change (UNFCCC), adopted on 9 May 1992 in New York, aims to stabilise GHG concentrations in the atmosphere at a level that would prevent dangerous disruption of the climate system due to human activities (Article 2). The UNFCCC was signed on 11 June 1992 by 166 Parties and has since been ratified by 195 Parties. It entered into force on 21 March 1994.

The developed nations and EU Member States with economies in transition, grouped together in Annex I to the Convention, have also undertaken, in pursuance of Article 4 § 2, to adopt policies and measures designed to stabilise their GHG emissions to 1990 levels by 2000. The developed nations and the European Union, as a whole, listed in Annex II to the Convention, have undertaken to finance the costs that the developing countries must bear in order to meet their commitments (Article 4 § 3).

The Parties listed in Annex I to the Climate Convention (before the new amendment decided on in Durban in late 2011) are as follows:

    • 26 industrialised countries: Australia, Austria, Belgium, Canada, Denmark, Finland, France, Germany, Greece, Ireland, Iceland, Italy, Japan, Liechtenstein, Luxembourg, Monaco, Norway, New Zealand, Netherlands, Portugal, Spain, Sweden, Switzerland, Turkey, United Kingdom and United States,
    • 14 European countries in transition towards a market economy: Belarus, Bulgaria, Croatia, Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Romania, Russian Federation, Slovakia, Slovenia and Ukraine,
    • the European Union as a regional economic integration organisation.

All the Parties to Annex I of the Climate Convention are also Parties to the Kyoto Protocol (see below), except the United States. Also to be noted is that although the Convention has been ratified by the EU, Malta and Cyprus as individual countries are not included in Annex I.

The Kyoto Protocol, adopted on 11 December 1997, aimed at reducing emissions, at international level, of six GHGs (CO2, CH4, N2O, HFCs, PFCs and SF6) by 5.2% compared to 1990 during the period from 2008 to 2012. It was signed by 84 Parties and has been then ratified by 193 Parties.

The first commitment period under the Protocol covered about 30% of worldwide GHG emissions [TCG, 2011]. Its aim is to identify the means for reducing emissions that incur the "least cost" in terms of progress and economic growth; it has introduced three "flexible mechanisms" for this purpose that are additional to national policies and measures for GHG emission reductions. These mechanisms apply to three different systems to achieve emission reductions:

    • Joint Implementation (JI): emission credits (certified emission reductions or CERs) obtainable as from 2008 by investing in emission reduction projects in another country. This mechanism applies to agreements between two countries that both have emission reduction commitments.
    • Clean Development Mechanism (CDM): this mechanism applies to projects involving industrial countries with developing countries. The industrialised countries may obtain emission reduction units (ERUs) by investing in projects to reduce or prevent emissions in developing countries.
    • Emissions Trading Mechanism: if a company's emissions exceed its assigned (emission) amount units (AAUs), it has to buy additional AAUs through a market or from other industrial companies. Emissions trading began in the European Union with the adoption of Directive 2003/87/EC establishing the EU GHG Emissions Trading Scheme (EU-ETS). The EU-ETS was introduced retroactively so that the first emissions trading period covered the years 2005 to 2007. This directive was amended by Directive 2009/29/EC in order to extend the EU-ETS beyond 2012 (currently 3rd period: 2013-2020).

Every year, the United Nations Climate Conference gathers the Convention Parties to pursue the discussions on global commitments to fight climate change.

The Parties to the Conference in Copenhagen (7 to 19 December 2009) [Tuddenham, 2009] recognised the scientific finding that the average rise in global temperatures must be contained below 2°C [compared to pre-industrial levels, although this is not specified in the text].

At the United Nations Climate Conference in Doha (Qatar) in 2012, the objective of keeping the average global temperature rise below +2°C by 2100 is confirmed, and a process has been launched to review by 2015 the adequacy of this goal in the light of the progress made towards its realization [Tuddenham, 2013]. However, the "Doha Gateway" does not, however, includeany new major commitment of emission reduction, although the formal adoption of the Kyoto Protocol second commitment period for 2013-2020 is a main outcome of the conference. 

Finally in 2014, during the United Nations Climate Conference in Lima from 1 to 12 December, the decision said " The Lima call" defines the roadmap towards the agreement to be concluded in 2015, by specifying particularly the contents and the style of the national contributions (INDC) of the Parties, which will have to go beyond the already signed commitments. However, this conference postpones to 2015 the concrete decisions on key issues [Tuddenham, 2015].

The Paris Agreement, adopted on 12 December 2015, is the first multilateral legal instrument binding both industrialized and developing countries, with the aim of reducing greenhouse gas emissions of all countries of the globe. To date, a total of 194 Parties have signed the Agreement and 122 Parties have ratified. It entered into force on 4 November 2016 [Tuddenham, 2016].

The key features of the Agreement include the five-year revision mechanism designed to strengthen the national contributions of the Parties (NDCs), and ultimately the overall ambition of the Agreement. Another important feature is the diffentiated enhanced transparency framework for monitoring, assessing and checking the performance of Parties (GHG emissions, climate policies and measures implemented; etc.).

The Agreement's weaknesses include the fact that there is no timeframe set for the GHG emissions peak and the long-term mitigation target is vague and does not come with a specific deadline for it to be met. In addition, due to a lack of political will and to rally the largest number of Parties, no sanctions mechanism is foreseen in the event of the Parties commitments not being met.


Are we on the right track to reduce GHG emissions to limit global temperature rise to +2°C?

Several recently published reports show that existing actions are a long way from meeting the stated aims and recommend speedy implementation of more ambitious reduction measures:

    • According to a recent report from the United Nations Environment Programme (UNEP 2014), projected emissions by 2020, based on effective implementation of the reduction commitments made by the Parties to date, will be 8 to 10 Gt CO2e higher than the maximum global GHG emissions level necessary by 2020 to meet the +2°C target (44 Gt CO2e),
    • The International Energy Agency's findings in its "World Energy Outlook 2014"  [IEA 2014] are very clear: "by 2040, the increase in energy-related carbon dioxide emissions (CO2), which increase by a fifth, implying an average global temperature rise of 3.6°C in long term".
    • In its 2013 update, the international Climate Action Tracker (CAT) [CAT, 2013], set up to monitor and assess reduction commitments, calculates that these would put the planet on a pathway that implies an average global temperature rise of +3.1°C by 2100, but underlines that most of the countries are not on track to meet their commitments, and thus the current trend is expected to lead to a temperature increase in the order of +3,7°C.
    • The chapter on climate change in the "Environmental Outlook to 2050" published by the OECD (Organisation for Economic Cooperation and Development) [OECD 2012] is equally clear: "the Baseline Scenario envisages that, without more ambitious [climate] policies than those in force today, GHG emissions will increase by another 50% by 2050, primarily driven by a projected 70% growth in CO2 emissions from energy use. This is primarily due to a projected 80% increase in global energy demand [by 2050]". An average global temperature rise of 3 to 6°C by 2100 is therefore possible.