Climate Forcings and Drivers

We begin with 'Earth's Energy Budget' and radiative equilibrium, before explaining 'radiative forcings' - the primary causes of climate change - as well as 'climate drivers' and 'climate feedbacks’. We also ask: how high will radiative forcing be in the future? and examine the representative concentration pathways” (RCPs) produced by the IPCC.
Climate Forcings - Solar energy reaches Earth's surface. Some melts the snow, the rest is reflected back into space.
Incoming solar energy has only a fraction of the radiative forcing effect of man-made greenhouse gases

In this article we deal with “climate forcings” ( also known as “radiative forcings“, or “climate drivers“) and the singular term “climate forcing“, which means something slightly different.

In climate science, the term “climate forcing” (singular) is a clunky way of describing the degree to which Earth’s climate is warming or cooling.

For example, suppose Earth receives 100 units of solar energy (in the form of sunlight) from the sun. And suppose that Earth radiates 90 units of this energy back into space. The difference between the two – 10 units – is called Earth’s radiative forcing, which in this case is a positive RF.

If the Earth received 90 units and radiated 100 units back into space, the planet would have a negative RF (minus 10).

The factors that have a direct effect on either incoming or outgoing solar energy are called “climate forcings” (plural) or “climate drivers”.

Climate change caused by global warming is now a scientific fact which is accepted by roughly 97 percent of all publishing climate scientists. 1 2 But what is causing this warming process? The answer is: climate forcings, or radiative forcings. To understand how these forcings work, let’s take a look at the science behind them.

What Is The Earth’s Energy Budget?

Diagram: Earth's Energy Budget. Climate forcings, Milankovitch cycles, albedo effect, greenhouse effect.
The Earth’s Energy Budget. Shows the various kinds of energy, light and heat, that enters and leaves the Earth system. Image: © NASA

The Earth receives a continuous amount of solar energy (sunlight) from the Sun. Some of this energy (about 29 percent) is reflected back to space; some (about 23 percent) is absorbed by the atmosphere; while the rest (about 48 percent) is absorbed at the surface of the Earth. 3

At the same time, in keeping with the laws of physics, the Earth and its atmosphere radiate energy back into space, which results in a rough balance between energy received and energy lost. This two-way flow of incoming and outgoing energy is known as Earth’s energy balance (or budget). For global temperature to remain stable over long periods of time, energy received and energy lost must be roughly equal. Scientists call this state of balance “radiative equilibrium”.

What Is Radiative Forcing?

“Radiative forcing” (also called climate forcing), is the difference between the amount of solar energy received by the Earth and the energy that is radiated back into space. 4 To put it another way, the difference between incoming and outgoing energy is known as a planet’s radiative forcing (RF). It can be positive or negative. Thus “positive radiative forcing” is when Earth receives more incoming energy than it radiates to space. The net gain of energy causes the planet to heat up. Conversely, “negative radiative forcing” is when Earth radiates more energy back into space than it receives from the sun. The net loss of energy causes the planet to cool.

Numerous studies of changes in Earth’s climate have reiterated the importance of global mean surface temperature as the primary index for climate change. Radiative forcing provides a way to measure and compare the contributions of different factors that influence surface temperature, although the integration of paleoclimate proxies with climate modeling is critical to improving the understanding of climate dynamics. 5

In any event, there is a nearly linear relationship between radiative forcing and global mean surface temperature in general research models. In practice, radiative forcing is relatively easy to calculate, uncomplicated to use in policy applications, detectable from space, and logically inferable from changes in ocean heat content. All in all, it is a highly useful metric for climate change study and research. It is commonly expressed in watts per square meter (W/m2) averaged over a set period of time.

Radiative Forcing in a Nutshell

Just in case the above explanation is too klunky for you, here’s a slightly simpler one.

Radiative Forcing (RF) is basically an energy imbalance imposed on the climate system either externally or by human activities. Think of it as an enforced heating up or cooling down. An external RF could be a change in the amount of solar energy from the sun. Quasi-external RF factors include major volcanic emissions. Human activities that can cause RF include: emissions of greenhouse gases, aerosols (and their precursors), deforestation or inappropriate land use change.

What Are Climate Forcings? 

“Climate forcings” (also called “radiative forcings”) are the factors that disturb Earth’s radiative equilibrium, and cause changes to the planet’s climate system, thus forcing temperatures to rise or fall. They are commonly divided into human-induced or natural climate forcings, as follows:

Natural Climate Forcings

Solar Radiation
The sun’s light and energy sustains all life on our planet. Variations in the amount or intensity of incoming solar energy have an immediate impact on Earth’s climate. These are usually caused by Milankovitch cycles – variations in Earth’s orbital eccentricity, axial tilt, and precession.

Volcanic Activity
Large volcanic eruptions, such as those of Mount Pinatubo (1991), Krakatau (1883) and Tambora (1815), inject large quantities of sulfur gases into the stratosphere, producing an aerosol haze which reflects incoming sunlight back into space. This leads to a significant cooling effect on climate.

Pinatubo, for instance, decreased global mean temperature by about 0.5°C. 6

During the eruption of the Philippine volcano Mount Pinatubo, on June 15, 1991, the ash cloud rose 40 kilometers (25 miles) into the atmosphere and satellites tracked it several times around the globe.

Volcanoes are one of the biggest natural contributors to the sulfur cycle and to atmospheric aerosols. Other sources of aerosols include: partially combusted carbon from forest fires, mineral dust whipped off the desert floor by strong winds and sea-salt blown upwards from ocean waves.

Human Induced Climate Forcings

Greenhouse Gases

Greenhouse gases – of which the main ones are carbon dioxide (CO2), Methane (CH4), Nitrous Oxide (N2O), plus the family of F-gases – are emitted by the burning of fossil fuels, or by other industrial processes.

Concentrations of these gases accumulate in the lower atmosphere where they trap heat rising from the Earth’s surface. This process, known as the greenhouse effect, is a naturally occurring phenomenon, which keeps Earth’s temperature at a cozy 15 degrees Celsius.

Unfortunately, the massive amounts of GHGs being emitted by humans has unbalanced the system, and has led to a rapid increase in temperatures around the world. See also: Global Temperature Projections For 2100.

Aerosols

Aerosols are tiny airborne particles of dust, smoke, and soot. Sulfate aerosols, which are discharged from burning coal, biomass, and other industrial processes, tend to cool the Earth. However, according to the UNFCC, they remain in the atmosphere for only a short time, compared to greenhouse gases. 7 Another potent but short-lived climate pollutant is black carbon – the sooty black particles given off by petrol and diesel engines and coal-fired power stations – which has a warming (and a cooling) effect.

A “forcing value” can be calculated for each of these factors for the period 1750-present. (See below.)

In its Fourth Assessment Report (2007), the Intergovernmental Panel on Climate Change (IPCC) states: “Radiative forcing is a measure of the influence a factor has in altering the balance of incoming and outgoing energy in the Earth-atmosphere system and is an index of the importance of the factor as a potential climate change mechanism. 8

Radiative Forcings Since 1750

Taking into account all positive and negative climate forcings as well as all interactions between climatic factors, the total net increase in surface energy due to human activity since 1750, is 2.29 watts per square metre. 9

Direct, Indirect & Non-Radiative Forcings

Radiative forcings can be further classified into direct, indirect and non-radiative types. Direct radiative forcings have a direct effect on Earth warming/cooling (example: an increase in sunlight). Indirect radiative forcings first affect climate system components which then cause warming/cooling.

An example is the emission of aerosols (caused by nature’s volcanoes or human factories) that speed up cloud formation leading to both greater sunlight reflection (cooling) and a greater greenhouse effect (cooling). Nonradiative forcings do not directly involve warming (example: evapotranspiration from man-made agricultural irrigation). 10 11 (See also: How Do Clouds Affect Climate?)

What Are Climate Drivers?

The term “climate drivers” is just another term for “climate forcings”. It describes those mechanisms or phenomena that influence global warming or cooling. Historically, the three critical influences have been: (a) Milankovitch cycles, involving small variations in the shape of Earth’s orbit and its axis of rotation, leading to changes in the solar radiation reaching the planet; (b) volcanic eruptions; and (c) meteorite collisions with Earth.

All three were (and are) naturally occurring events over which we had (and have) no control. However, none of these climate drivers are relevant to the present climate crisis, which is being driven by something entirely man-made. 12 13 14

What Is The Main Climate Driver Today?

The main driver of Earth’s recent warming is the increase in anthropogenic greenhouse gas emissions, caused by the burning of fossil fuels, and is entirely man-made. 15

It is the only factor that has changed significantly in the last 100 years.

For example, since pre-industrial times, carbon dioxide in the atmosphere has increased by more than 48 percent, from 280 ppm to 415 ppm. This is probably the highest level in 3 million years.

Likewise, methane concentrations have increased from 722 ppb to 1866 ppb – an increase of 250 percent, and the highest level for 800,000 years.

Nitrogen oxide levels have increased by 22 percent from 270 ppb to 330 ppb. 16

Graph Showing Relative Importance Of Radiative Forcings, or Drivers

Graph Showing Relative Importance of Radiative Forcings, or Drivers.
Separate global climate forcings relative to their 1880 values. Image: Goddard Institute for Space Studies (GISS).

The graph above clearly demonstrates, greenhouse gases have by far and away the greatest sustained effect on the climate system. Conversely, note the small influence of solar irradiance changes since 1880. In addition, aerosol emissions from volcanic eruptions also have a strong, cooling effect, although it is very short term.

Climate Feedbacks

In addition to the main climate drivers, there are a number of secondary influences known as climate feedbacks. These are internal climate processes that amplify or dampen the climate response to a specific forcing. Examples include: the increase in atmospheric water vapor (leading to a greater greenhouse effect) triggered by the warming effect of rising carbon dioxide (CO2) concentrations, in accordance with the Clausius-Clapeyron equation.

Two slightly more straightforward feedbacks are the thawing of permafrost and the burning of (say) Arctic wildfires. They are triggered as follows: global warming thaws out permafrost in the Arctic tundra, which results in the release of carbon dioxide and methane into the atmosphere, where it causes more warming. In the case of wildfires, rising temperatures create tinder dry conditions in forests which are then ignited by lightning. The burning trees release carbon dioxide into the atmosphere which causes more warming. In both cases, the initial warming is amplified by the feedback.

Sustained drought and higher than normal temperatures, caused by the effect of global warming on the Indian Ocean Dipole and the Southern Annular Mode, were responsible for the devastating Australian bushfires 2019-2020, which released an estimated 830 million tons of CO2 into the atmosphere, further stoking the greenhouse effect.

How High Will Radiative Forcing Be In The Future?

IPCC scientists have created four possible scenarios that can be employed in climate models in order to predict future climate patterns. Each scenario is based on specific greenhouse gas emissions and differing climate change mitigation strategies. The scenarios, called “Representative Concentration Pathways” (RCPs), are each named after the amount of radiative forcing (RF) they lead to in 2100, relative to 1750. 17

IPCC's AR5 Pathways Diagram
IPCC’s AR5 Representative Concentration Pathways used for projecting climate change to 2100. Image: Efbrazil (CC BY-SA 4.0)
  • RCP 8.5 shows what happens in the absence of any global plan of climate action to reduce emissions. It predicts very high CO2 concentration levels of 940 parts per million (ppm) by 2100. The scenario leads to global warming of between 2.6°C and 4.8°C (possibly as high as 5.5°C), with massive sea level rise and catastrophic effects on the planet’s biosphere. For more about the likelihood of this scenario, see: Our Climate Plan Can’t Cope.
  • RCP 6.0 demonstrates an intermediate scenario, in which emissions peak around 2080, then decline. Reductions in greenhouse gases are achieved via modest climate change mitigation strategies. Atmospheric CO2 levels rise less rapidly than in RCP8.5, but still reach 660 ppm by 2100 before stabilizing shortly afterwards. By 2100, RCP6.0 is projected to lead to global warming of up to 3.1°C.
  • RCP 4.5 represents another in-between scenario in which emissions peak around 2040, then decline. Curiously, concentrations of CO2 are a little higher than those of RCP6.0 until after mid-century, but emissions peak earlier (around 2040), and reach 540 ppm by 2100. RCP4.5 leads to warming of up to 2.6°C.
  • RCP 2.6 is the most rigorous of all the scenarios, and aims to limit global warming to below 2°C. It assumes that atmospheric carbon dioxide peaks at about 440 ppm around 2050, followed by a slow decline to around 400 ppm by the end of the century. The success of RCP2.6 is dependent upon global cooperation among all CO2 emitters, as well as the application of carbon capture and storage (CCS) technologies. It leads to warming of between 0.3°C and 1.7°C. 18

Questions and Answers About Earth’s Climate

For popular questions and answers on all aspects of our climate emergency, see: 50 FAQs About Global Warming and 50 Climate Change FAQs.

References

  1. Examining the Scientific Consensus on Climate Change.” Peter Doran, Maggie Zimmerman. Jan 20, 2009). Eos.90 (3): 22–23. []
  2. Consensus on consensus: a synthesis of consensus estimates on human-caused global warming.” John Cook; et al. (April 2016) Environmental Research Letters. 11 (4): 048002. []
  3. “Earth’s Energy Budget.” NASA Jan 14, 2009. (3) []
  4. “Radiative Forcing in the AR5.” Climate Change 2013. The Physical Science Basis. Working Group I. IPCC Fifth Assessment Report. Drew Shindell, Gunnar Myhre, Olivier Boucher, Piers Forster, Francois-Marie Breon, Jan Fuglestvedt. (4) []
  5. Paleoclimate Data–Model Comparison and the Role of Climate Forcings over the Past 1500 Years.” Steven J. Phipps, Helen V. McGregor, Joëlle Gergis, Ailie J. E. Gallant, Raphael Neukom, Samantha Stevenson, Duncan Ackerley, Josephine R. Brown, Matt J. Fischer, Tas D. van Ommen. Published Sept 9, 2013. (5) []
  6. Matthew Toohey, Kirstin Kruger, Hauke Schmidt, Claudia Timmreck, Michael Sigl, Markus Stoffel, Rob Wilson. “Disproportionately strong climate forcing from extratropical explosive volcanic eruptions.” Nature Geoscience, 2019 (6) []
  7. UNFCC, Climate Change Information Sheet 2. (PDF) (7) []
  8. IPCC. Fourth Assessment Report (AR4). []
  9. “Drivers of Climate Change.” IPCC AR5 Summary for Policymakers. Page 13. []
  10. “Climate Forcings and Global Warming.” NASA. Jan 14, 2009. (10) []
  11. Climate Forcing” – NOAA Climate.gov (11) []
  12. No evidence for globally coherent warm and cold periods over the preindustrial Common Era.” Raphael Neukom, Nathan Steiger, Juan Jose Gomez-Navarro, Jianghao Wang & Johannes P. Werner. Nature. July 2019. Nature 571, 550–554 (2019). (12) []
  13. Last phase of the Little Ice Age forced by volcanic eruptions.” Bronnimann, S., Franke, J., Nussbaumer, S.U. et al. Nat. Geosci. 12, 650–656 (2019). (13) []
  14. Consistent multidecadal variability in global temperature reconstructions and simulations over the Common Era.” Neukom, R., Barboza, L.A., Erb, M.P. et al. Nature Geoscience. 12, 643–649 (2019). (14) []
  15. “Climate Change 2007”, IPCC’s Fourth Assessment Report (AR4). (15) []
  16. “Climate Milestone: Earth’s CO2 Level Passes 400 ppm.” National Geographic. Robert Kunzig. 9 May 2013. (16) []
  17. IPCC WGI AR5 Box SPM.1 Summary for Policy Makers. (17) []
  18. IPCC Fifth Assessment Report. Summary for Policymakers. (18) []
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