Climate Forcing

Climate forcing has to do with the amount of energy we receive from the sun, and the amount of energy we radiate back into space. Variances in climate forcing are determined by physical influences on the atmosphere such as orbital and axial changes as well as the amount of greenhouse gas in our atmosphere.

NASA/GISS Climate Forcing & Temperature

Climate forcings are the initial drivers of a climate shift. Solar irradiance is one example of a forcing. If the sun generates more light, the Earth will warm.Things that influence the total positive or negative forcing of the earth climate system include:

Surface reflectivity (albedo)
Greenhouse gases
Atmospheric aerosols (volcanic sulfates, industrial output)

Understanding climate forcing is actually easy. Force is generally understood as strength or energy, the cause of motion or change, or active power. This could be a person pushing a shopping cart or an engine that powers a car. For example, gravitational force is how the moons gravity causes the tides to change. Radiative forcing caused by greenhouse gases in our atmosphere keeps the earth from being a frozen ball in space, because the sun provides energy, and the greenhouse gases in our atmosphere trap the heat on earth... a positive forcing.

The National Academy of Sciences, National Research Council, Board on Atmospheric Science and Climate Present:

'Climate Change: Lines of Evidence - How Much Warming?'

That's it!

To understand the climate forcing, you just add all the components of forcing that are positive and negative and you end up with a view of the total forcing in the climate system of earth.

In our case, the sun provides our heat energy, the Milankovitch forcing is the general regulator of climate forcing over 100k year time scales. That combined with terrestrial components, such as greenhouse gases, aerosols, and changes in land use comprise the major factors that determine climate forcing.

Energy absorption (how much energy we get form the sun) and energy radiation (how much we radiate back out into space) part of the climate system. According the the Stefan Boltzmann Law, we receive on earth 240 W/m² and we radiate the same back to space.

Earth regulators include thermal inertia of the ocean, land use, and greenhouse gases & aerosols in the atmosphere. These things determine the speed of climate change in combination with the above factors, in general.

Some of the effects are faster and some are slower.

All these parts of the system, once measured and modeled reasonably give you an idea of the amount of forcing each imposes on the climate system. Some forces are positive and others are negative.

Changes in Climate "Forcings"

The following figure shows changes in climate "forcings" or factors that have contributed to climate change since 1750.

These agents can be categorized into three areas: greenhouse gases, other man-made (anthropogenic) forcings, and natural forcings. The greenhouse gases consist of carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂0) and chlorofluorocarbons (CFCs). Other anthropogenic forcings consist of black carbon (soot, formed by incomplete combustion), reflective aerosols (tiny airborne particles that reflect sunlight back to space), soil or dust, land cover changes, and forced cloud changes. Natural forcings include changes of the sun's energy. Figure is also available in PDF format. (Source: Figure 28 of Hansen et al. 2005. See references below.)

IPCC AR4 Forcing Components

$Radiative Forcing Components IPCC AR4 SPM – Figure SPM.2. Global average radiative forcing (RF) estimates and ranges in 2005 for anthropogenic carbon dioxide (CO2 ), methane (CH4 ), nitrous oxide (N2O) and other important agents and mechanisms, together with the typical geographical extent (spatial scale) of the forcing and the assessed level of scientific understanding (LOSU). The net anthropogenic radiative forcing and its range are also shown. These require summing asymmetric uncertainty estimates from the component terms, and cannot be obtained by simple addition. Additional forcing factors not included here are considered to have a very low LOSU. Volcanic aerosols contribute an additional natural forcing but are not included in this figure due to their episodic nature. The range for linear contrails does not include other possible effects of aviation on cloudiness. {2.9, Figure 2.20} – Source (page 4): http://ipcc-wg1.ucar.edu/wg1/Report/AR4WG1_Print_SPM.pdf$

Effective Forcings Employed in Current Climate Simulations

The following figures show effective global climate forcings employed in our current global climate simulations (e.g., Hansen et al. 2007a,b), relative to their values in 1880.

(a) The separate radiative forcing data are available as a text file. The figure is also available as a large GIF or PDF.

(b) The net forcing data are available as a text file. The figure is also available as a large GIF or PDF.

Further Details and Various Future Scenarios to 2100

References

Hansen, J., L. Nazarenko, et al. 2005a. Earth's energy imbalance: Confirmation and implications. Science 308, 1431-1435, doi:10.1126/science.1110252. (See also Imbalance webpages).
Hansen, J., M. Sato, et al. 2005b. Efficacy of climate forcings. J. Geophys. Res., 110, D18104, doi:10.1029/2005/JD005776. (See also Efficacy webpages).
Hansen, J., et al. 2007a. Dangerous human-made interference with climate: A GISS modelE study. Atmos. Chem. Phys., 7, 2287-2312.
Hansen, J., et al. 2007b. Climate Dynam., Climate simulations for 1880-2003 with GISS modelE. Climate Dynam., 29, 661-696, doi:10.1007/s00382-007-0255-8.