Putting current radiative forcing in context

Guest post by Jim

Abstract

Current radiative forcing from all greenhouse gases, omitting aerosols from nonmetal oxides and the like, is now just under 3.5 W/m², with an Annual Greenhouse Gas Index of about 1.52. This represents approximately 1.5% of average surface insolation.  Unlike solar energy, which rises and falls with the sun, this infrared energy captured from the earth itself accumulates second by second, day and night.

How significant is this? Consider this analogy. A typical adult on a 2000 kcal/day diet ingesting daily 1.5% calories in excess will gain 31 pounds per decade, with profound consequences. Now we have been burning fossil fuels at the same high rate as our over-eater for more than two decades. How can we possibly imagine that the consequences for the planet will be less severe?

Background

My most recent re-analysis of NOAA's greenhouse gas data stated that the current Annual Greenhouse Gas Index (AGGI) stood at 1.5177, on the scale where the 1990 AGGI is set to one.

NOAA's data from 1990 reported that total forcing was 2.301 W/m², so an AGGI of 1.5177 corresponds to radiative forcing of 3.492 W/m². This is the amount of energy emitted by the earth (which glows in the infrared) and captured by greenhouse gases in the atmosphere over each square meter of the earth's surface, each second. How does this compare to the energy from the sun?

The accompanying figure is from Total Solar Irradiance CDR, at the National Centers for Environmental Information, part of NOAA. It shows Total Solar Irradiance (TSI) at the top of the atmosphere, scaled to a uniform distance from the sun.

"The Total Solar Irradiance (TSI) Climate Data Record (CDR) measures the spectrally integrated energy input to the top of the Earth's atmosphere at a base mean distance from the Sun (i.e., one Astronomical Unit)."

There are a couple of things worth noting. First, TSI over the satellite era (1979-present) is almost constant, roughly 1362 W/m², clearly negating the denialisti claims that "It's the sun!". Second, historic reconstructions of TSI over the last four centuries show that variations in TSI average about 0.5 W/m², so an increase in radiative forcing of just 1W/m² is larger than most variations in the TSI signal. An increase of 3.5 W/m² is far larger than the usual variation.

Since the earth is nearly a sphere of radius R, its surface area, approximately 4πR² , is about four times its circular cross-section, about πR². So insolation - sunlight energy falling on each unit of area - over the entire globe is about one fourth that directly under the sun, (1362/4) W/m² . This implies that the average global insolation at the top of the atmosphere is about 340.5 W/m². At the surface, this is further reduced by the albedo (reflectivity) of the earth.

The American Meteorological Society sums up the current estimates of planetary albedo:

On the average, the earth reflects 31 units of solar radiation back to the space for every 100 units received (thus, the total earth albedo is 0.31). The cloud albedo accounts for 23 units of the 31. For individual clouds, local albedo may be in excess of 0.7.

So only 69% of that 340.5 W/m², about 235 W/m², hits the earth's surface. The radiative forcing from all greenhouse gases, except the short-lived species such as ozone, SOx, or NOx, is 3.492 W/m², about 1.486% of average surface insolation. Unlike solar energy, this  infrared energy captured from the earth itself accumulates second by second, day and night.

Comparison with diet

To see what impacts this energy has on the planet, let's look at the effects of over-eating on a typical adult who maintains a healthy weight with a 2000 kcal/day diet. That same percent, 1.486%, of their daily calorie intake is 29.7 kcal/day, almost the same as the energy content of 4 g of butter, a scant teaspoon. As we all come to know sooner or later, excess calories are stored in the body as fat, roughly 3500 kcal per pound of fat. This overeating will lead to weight gain, roughly one pound every 118 days, about 3.1 pounds each year, year after year. 

A decade of this overeating leads to a gain in weight of about 31 pounds. This will make that adult at least overweight, if not obese.  The CDC states

Obesity in children and adults increases the risk for the following health conditions:

• High blood pressure and high cholesterol which are risk factors for heart disease.
• Type 2 diabetes.
• Breathing problems, such as asthma and sleep apnea.
• Joint problems such as osteoarthritis and musculoskeletal discomfort.
• Gallstones and gallbladder disease.

Two decades will make them class III obese (formerly known as morbidly obese), if not worse. The Cleveland Clinic advises

"If left untreated, class III obesity may shorten life expectancy up to 14 years. In addition to contributing to potentially serious health problems, class III obesity is associated with reduced economic and social opportunities and reduced quality of life."

As stated in the abstract, we have been burning fossil fuels at the same high rate as our over-eater for more than two decades. How can we possibly imagine that the consequences for the planet will be less severe?