Frequently Asked Questions

Frequently Asked Question 2.1

How do Human Activities Contribute to Climate Change 

and How do They Compare with Natural Influences?

Human activities contribute to climate change by causing 

changes in Earth’s atmosphere in the amounts of greenhouse gases, aerosols (small particles), and cloudiness. The largest known 

contribution comes from the burning of fossil fuels, which releases 

carbon dioxide gas to the atmosphere. Greenhouse gases and aerosols affect climate by altering incoming solar radiation and outgoing infrared (thermal) radiation that are part of Earth’s energy 

balance. Changing the atmospheric abundance or properties of 

these gases and particles can lead to a warming or cooling of the 

climate system. Since the start of the industrial era (about 1750), 

the overall effect of human activities on climate has been a warming influence. The human impact on climate during this era greatly 

exceeds that due to known changes in natural processes, such as 

solar changes and volcanic eruptions.

Greenhouse Gases

Human activities result in emissions of four principal greenhouse gases: carbon dioxide (CO2), methane (CH4), nitrous oxide 

(N2O) and the halocarbons (a group of gases containing fluorine, 

chlorine and bromine). These gases accumulate in the atmosphere, 

causing concentrations to increase with time. Significant increases 

in all of these gases have occurred in the industrial era (see Figure 

1). All of these increases are attributable to human activities.

• Carbon dioxide has increased from fossil fuel use in transportation, building heating and cooling and the manufacture of 

cement and other goods. Deforestation releases CO2 and reduces its uptake by plants. Carbon dioxide is also released in 

natural processes such as the decay of plant matter.

• Methane has increased as a result of human activities related 

to agriculture, natural gas distribution and landfills. Methane 

is also released from natural processes that occur, for example, 

in wetlands. Methane concentrations are not currently increasing in the atmosphere because growth rates decreased over the 

last two decades.

• Nitrous oxide is also emitted by human activities such as fertilizer use and fossil fuel burning. Natural processes in soils and 

the oceans also release N2O. 

• Halocarbon gas concentrations have increased primarily due 

to human activities. Natural processes are also a small source. 

Principal halocarbons include the chlorofluorocarbons (e.g., 

CFC-11 and CFC-12), which were used extensively as refrigeration agents and in other industrial processes before their 

presence in the atmosphere was found to cause stratospheric 

ozone depletion. The abundance of chlorofluorocarbon gases is 

decreasing as a result of international regulations designed to 

protect the ozone layer.

• Ozone is a greenhouse gas that is continually produced and 

destroyed in the atmosphere by chemical reactions. In the troposphere, human activities have increased ozone through the 

release of gases such as carbon monoxide, hydrocarbons and 

nitrogen oxide, which chemically react to produce ozone. As 

mentioned above, halocarbons released by human activities 

destroy ozone in the stratosphere and have caused the ozone 

hole over Antarctica. 

• Water vapour is the most abundant and important greenhouse 

gas in the atmosphere. However, human activities have only 

a small direct influence on the amount of atmospheric water vapour. Indirectly, humans have the potential to affect 

water vapour substantially by changing climate. For example, 

a warmer atmosphere contains more water vapour. Human 

activities also influence water vapour through CH4 emissions, 

because CH4 undergoes chemical destruction in the stratosphere, producing a small amount of water vapour.

• Aerosols are small particles present in the atmosphere with 

widely varying size, concentration and chemical composition. 

Some aerosols are emitted directly into the atmosphere while 

others are formed from emitted compounds. Aerosols contain 

both naturally occurring compounds and those emitted as a result of human activities. Fossil fuel and biomass burning have 

increased aerosols containing sulphur compounds, organic 

compounds and black carbon (soot). Human activities such as 

FAQ 2.1, Figure 1. Atmospheric concentrations of important long-lived greenhouse gases over the last 2,000 years. Increases since about 1750 are attributed to 

human activities in the industrial era. Concentration units are parts per million (ppm) 

or parts per billion (ppb), indicating the number of molecules of the greenhouse gas 

per million or billion air molecules, respectively, in an atmospheric sample. (Data 

combined and simplified from Chapters 6 and 2 of this report.)

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Frequently Asked Questions

FAQ 2.1, Box 1: What is Radiative Forcing? 

What is radiative forcing? The influence of a factor that can cause climate change, such as a greenhouse gas, is often evaluated in 

terms of its radiative forcing. Radiative forcing is a measure of how the energy balance of the Earth-atmosphere system is influenced 

when factors that affect climate are altered. The word radiative arises because these factors change the balance between incoming solar 

radiation and outgoing infrared radiation within the Earth’s atmosphere. This radiative balance controls the Earth’s surface temperature. 

The term forcing is used to indicate that Earth’s radiative balance is being pushed away from its normal state. 

Radiative forcing is usually quantified as the ‘rate of energy change per unit area of the globe as measured at the top of the atmosphere’, and is expressed in units of ‘Watts per square metre’ (see Figure 2). When radiative forcing from a factor or group of factors 

is evaluated as positive, the energy of the Earth-atmosphere system will ultimately increase, leading to a warming of the system. In 

contrast, for a negative radiative forcing, the energy will ultimately decrease, leading to a cooling of the system. Important challenges 

for climate scientists are to identify all the factors that affect climate and the mechanisms by which they exert a forcing, to quantify the 

radiative forcing of each factor and to evaluate the total radiative forcing from the group of factors. 

FAQ 2.1, Figure 2. Summary of the principal components of the radiative forcing of climate change. All these 

radiative forcings result from one or more factors that affect climate and are associated with human activities or 

natural processes as discussed in the text. The values represent the forcings in 2005 relative to the start of the 

industrial era (about 1750). Human activities cause significant changes in long-lived gases, ozone, water vapour, 

surface albedo, aerosols and contrails. The only increase in natural forcing of any significance between 1750 and 

2005 occurred in solar irradiance. Positive forcings lead to warming of climate and negative forcings lead to a 

cooling. The thin black line attached to each coloured bar represents the range of uncertainty for the respective 

value. (Figure adapted from Figure 2.20 of this report.)

surface mining and industrial processes 

have increased dust in the atmosphere. 

Natural aerosols include mineral dust released from the surface, sea salt aerosols, 

biogenic emissions from the land and 

oceans and sulphate and dust aerosols 

produced by volcanic eruptions. 

Radiative Forcing of Factors Affected by 

Human Activities

The contributions to radiative forcing 

from some of the factors influenced by human activities are shown in Figure 2. The 

values reflect the total forcing relative to the 

start of the industrial era (about 1750). The 

forcings for all greenhouse gas increases, 

which are the best understood of those due 

to human activities, are positive because each 

gas absorbs outgoing infrared radiation in the 

atmosphere. Among the greenhouse gases, 

CO2 increases have caused the largest forcing 

over this period. Tropospheric ozone increases have also contributed to warming, while 

stratospheric ozone decreases have contributed to cooling. 

Aerosol particles influence radiative forcing directly through reflection and absorption 

of solar and infrared radiation in the atmosphere. Some aerosols cause a positive forcing 

while others cause a negative forcing. The direct radiative forcing summed over all aerosol 

types is negative. Aerosols also cause a negative radiative forcing indirectly through the 

changes they cause in cloud properties. 

Human activities since the industrial era 

have altered the nature of land cover over 

the globe, principally through changes in 

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Frequently Asked Questions

follow an 11-year cycle. Solar energy directly heats the climate 

system and can also affect the atmospheric abundance of some 

greenhouse gases, such as stratospheric ozone. Explosive volcanic 

eruptions can create a short-lived (2 to 3 years) negative forcing 

through the temporary increases that occur in sulphate aerosol 

in the stratosphere. The stratosphere is currently free of volcanic 

aerosol, since the last major eruption was in 1991 (Mt. Pinatubo). 

The differences in radiative forcing estimates between the 

present day and the start of the industrial era for solar irradiance 

changes and volcanoes are both very small compared to the differences in radiative forcing estimated to have resulted from human 

activities. As a result, in today’s atmosphere, the radiative forcing 

from human activities is much more important for current and 

future climate change than the estimated radiative forcing from 

changes in natural processes. 

croplands, pastures and forests. They have also modified the reflective properties of ice and snow. Overall, it is likely that more solar 

radiation is now being reflected from Earth’s surface as a result of 

human activities. This change results in a negative forcing. 

Aircraft produce persistent linear trails of condensation (‘contrails’) in regions that have suitably low temperatures and high 

humidity. Contrails are a form of cirrus cloud that reflect solar radiation and absorb infrared radiation. Linear contrails from global 

aircraft operations have increased Earth’s cloudiness and are estimated to cause a small positive radiative forcing. 

Radiative Forcing from Natural Changes

Natural forcings arise due to solar changes and explosive 

volcanic eruptions. Solar output has increased gradually in the 

industrial era, causing a small positive radiative forcing (see Figure 

2). This is in addition to the cyclic changes in solar radiation that 
From the report accepted by Working Group I 

of the Intergovernmental Panel on Climate Change 

but not approved in detail

Frequently Asked Questions

FAQ Citation:

These Frequently Asked Questions have been taken directly from the chapters of the underlying report and are collected here. 

When referencing specific FAQs, please reference the corresponding chapter in the report from whence the FAQ originated.

When referencing the group of FAQs, please cite as:

IPCC, 2007: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment 

Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, 

M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.