Ch. 1 · Defining Radiative Forcing

Radiative forcing.

~3 min read · Ch. 1 of 6

The year 2019 marked a specific moment when human activity generated a radiative forcing of 2.72 watts per square meter relative to the baseline year 1750. This value represents the net change in downward minus upward radiative flux due to external climate drivers. Scientists measure this energy imbalance at the tropopause and top of the stratosphere to understand how Earth's temperature responds to changes. The concept distinguishes these external forces from internal feedbacks like water vapor or cloud formation that react to initial warming. A planet in equilibrium with its star exhibits zero net radiative forcing, maintaining a stable average global temperature. When this balance shifts, the resulting gain or loss of energy drives long-term climate trends rather than short-term weather fluctuations.

Historical Development Of Theory

Physicists developed comprehensive descriptions of radiative transfer during the first half of the 20th century before applying them to stellar atmospheres. Studies of radiative-convective equilibrium matured through the 1960s and 1970s as models began accounting for complex material flows within the energy balance. The term gained widespread traction in scientific literature by the 1980s after researchers distilled perturbation frameworks into usable metrics. Early work focused on homogeneous disturbances imposed onto the top-of-atmosphere energy balance to estimate state changes. Modern calculations now distinguish between instantaneous radiative forcing without stratospheric adjustments and effective radiative forcing which includes both tropospheric and stratospheric responses. These evolving definitions allow scientists to compare different drivers of climate change on a consistent scale.

Quantifying Atmospheric Gas Impacts

Carbon dioxide concentrations reached 415 parts per million by volume in 2019, representing a 50% increase since 1750. This specific rise corresponds to a cumulative radiative forcing change of plus 2.17 watts per square meter. Methane and chlorofluorocarbons contribute smaller roles over time despite their potent individual warming effects. Five major greenhouse gases account for about 96% of direct radiative forcing from long-lived increases since the industrial revolution. The remaining 4% comes from fifteen minor halogenated gases including hydrofluorocarbons and sulfur hexafluoride. A doubling of carbon dioxide concentrations would correspond to a further cumulative forcing change of plus 3.71 watts per square meter if emissions continue along current paths.

Solar Irradiance And Orbital Shifts

Total solar irradiance averages approximately 1361 watts per square meter at Earth's orbital distance of one astronomical unit. Variations occur between 1321 watts per square meter during aphelion in early July and 1412 watts per square meter at perihelion in early January. These fluctuations create seasonal weather patterns but contribute net-zero forcing over decades-long climate changes. Sunspot activity cycles every eleven years with average annual variations between 1360 and 1362 watts per square meter. Milankovitch cycles span periods of 40,000 to 100,000 years involving changes in orbital eccentricity and axial tilt. Such shifts cause total solar irradiance to fluctuate by about plus or minus 0.2% over these long durations.

Measuring Albedo And Aerosol Effects

Earth's bond albedo measures the fraction of incident solar radiation reflected back into space, averaging 0.30 globally. Clouds alone account for half of this reflectivity while atmospheric components contribute three-quarters of the total planetary albedo. Satellite instruments including MODIS and CERES have monitored these values continuously since 1998. Human activities like deforestation typically raise Earth's reflectivity whereas irrigation in arid lands may lower it. Major volcanic eruptions can significantly perturb planetary albedo for several years or longer through atmospheric particulates. Interannual variations of plus or minus 0.07 around the mean albedo occur throughout each year near solar equinoxes.

Attributing Climate Change Drivers

Human-caused radiative forcing reached 2.72 watts per square meters in 2019 relative to 1750 according to IPCC assessments. This warming is mainly due to increased greenhouse gas concentrations partly reduced by cooling from aerosols. NASA's Clouds and the Earth's Radiant Energy System instruments have measured top-of-atmosphere instantaneous radiative forcing since 1998. Data analysis shows radiative forcing rose by plus 0.53 watts per square meter between 2003 and 2018. About 20% of that increase associated with reductions in atmospheric aerosol burden while most remaining 80% attributed to rising greenhouse gases. Ground-based measurements at ARM sites in Oklahoma and Alaska found infrared heating experienced by surface dwellers rose by plus 0.2 watts per square meter during the decade ending 2010.

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Common questions

What was the human-caused radiative forcing value in 2019 relative to 1750?

Human activity generated a radiative forcing of 2.72 watts per square meter in 2019 relative to the baseline year 1750. This value represents the net change in downward minus upward radiative flux due to external climate drivers.

How did scientists develop the concept of radiative transfer during the 20th century?

Physicists developed comprehensive descriptions of radiative transfer during the first half of the 20th century before applying them to stellar atmospheres. The term gained widespread traction in scientific literature by the 1980s after researchers distilled perturbation frameworks into usable metrics.

What carbon dioxide concentration level existed in 2019 and how does it relate to radiative forcing?

Carbon dioxide concentrations reached 415 parts per million by volume in 2019, representing a 50% increase since 1750. This specific rise corresponds to a cumulative radiative forcing change of plus 2.17 watts per square meter.

How do Milankovitch cycles affect total solar irradiance over long durations?

Milankovitch cycles span periods of 40,000 to 100,000 years involving changes in orbital eccentricity and axial tilt. Such shifts cause total solar irradiance to fluctuate by about plus or minus 0.2% over these long durations.

When did satellite instruments begin monitoring Earth's bond albedo continuously?

Satellite instruments including MODIS and CERES have monitored these values continuously since 1998. Interannual variations of plus or minus 0.07 around the mean albedo occur throughout each year near solar equinoxes.

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