|Earth's changing distance from the Sun|
Motivated by the observation that for approximately 18 years, global warming has been essentially constant -- a behavior on Nature's part that not a single climate model predicted -- I decided to look more closely at the scientific basis for Catastrophic Anthropogenic Global Warming (CAGW), recently sanitized (to protect the children?) to the more innocuous "Climate Change". Catastrophic Warmists claim that human production of Carbon Dioxide (CO2) is warming the Earth via the greenhouse effect and that the magnitude of this warming will soon be so immense as to be termed catastrophic. (See below for a "spaghetti graph" of climate model predictions versus Nature's response).
CAGW skeptics (sometimes termed "climate deniers") believe that the Earth is indeed warming and that some of this warming is surely due to human-produced CO2, but skeptics suspect that the magnitude of this CO2-induced warming has yet to be correctly modeled by climate scientists and has probably been exaggerated. While atmospheric CO2 continues to increase -- CO2 concentration recently exceeded 400 ppm compared to a pre-industrial level of 280 ppm (parts per million) -- the average global temperature has remained essentially flat since the beginning of the 21th Century.
Climate scientists quantify the warming strength of a greenhouse gas as a "forcing" measured in Watts per square meter. If you aimed a 200 Watt heat lamp at a 1x1 meter Muslim prayer rug, you would be subjecting that prayer rug to a "forcing" of 200 Watts per square meter. (W/m^2).
The physics of the greenhouse effect predicts the forcing due to a doubling of CO2 to be 3.7 W/m^2 -- about the same effect on that prayer rug as a tiny Xmas tree light instead of a heat lamp. But these tiny Xmas tree lights are spread over the surface of the entire Earth so this seemingly small forcing represents an immense total input of heat into the Earth's atmosphere.
The Earth's temperature rise due to 3.7 W/m^2 of forcing is estimated to be about 1.2 degrees Centigrade.
But CO2 forcing is only a small part of the story. Climate scientists argue that "feedback effects" will amplify this modest temperature rise to truly catastrophic values. Estimates of CO2 - induced warming WITH FEEDBACK range from 2 to 6 Centigrade degrees for a doubling of CO2. Differing estimates of this crucial feedback factor account for the spaghetti-like quality of climate model predictions.
The main source of this conjectured feedback is water vapor which is a more powerful greenhouse gas than CO2. When increased CO2 heats the atmosphere, more water vapor can be stored in the heated air. This extra water vapor causes more global warming than the CO2 alone.
BARE CO2 plus FEEDBACK = Catastrophic Anthropogenic Global Warming.
The crux of the global warming controversy is whether current climate models are reliable enough to guide public policy. The correct estimation of feedback factors is a big part of this controversy.
The global warming debate is much too large a topic for a blog post. Here I want to discuss a discovery that is probably not original but which I have never seen discussed in the global warming literature. Does Earth have a "second season"?
Everyone knows that the Earth goes around the sun and that Earth's orbit is not circular but is shaped like an ellipse (a fact discovered by Johannes Kepler in the 17th Century). Traveling this oval path, the Earth is closest to the Sun in January (producing record high winter tides in Santa Cruz) and furtherest from the Sun in July.
When the Earth is close to the Sun (perihelion) it should be gathering more heat. And at aphelion, less heat will fall upon us. Just for fun I decided to calculate this dependable yearly change in solar radiation and compare this number with the 3.7 Watts/m^2 forcing due to CO2 doubling.
This is a simple physics problem. The change in Earth-Sun distance between perihelion and aphelion is 3.33%. Elementary calculus shows that the inverse-square law turns this 3.33% distance change into a 6.66% intensity change in the amount of solar radiation reaching Earth.
I looked up the intensity of sunlight at the top of the atmosphere. Bare solar radiation is 1350 Watts/m^2. But not all of that sunlight reaches the Earth's surface. The fraction of light reflected from a planet's surface is called its "albedo" (from the Latin term for "whiteness"). Mainly due to its bright white clouds, Earth's albedo is about 30% (compared to the dull Moon's 12% reflectivity). Subtracting reflected sunlight gives a nice round number for solar intensity at the Earth's surface -- 1000 Watts/m^2.
The 6.66% change in solar intensity amounts to a yearly change of 66.6 Watts/m^2 at the Earth's surface. That's a change in radiative forcing that's 18 times greater than the forcing (3.7 Watts/m^2) due to CO2 doubling!!! What a surprise!! Periodic solar forcing is immense!
Not so fast, Nick. Most of this solar radiation strikes the (spherical) Earth at a slant. And none of this radiation strikes the night side. To get a fair estimate of the "effective yearly change in total solar forcing" this number must be divided by 4. (Books on climate change explain why this correction factor for the spherical Earth is just 4 and not some complicated factor of pi.)
Dividing by four gives an effective yearly change in total solar forcing of 16.6 Watts/m^2 -- a factor of 4.5 greater than the calculated forcing due to a doubling of CO2.
Nearly 5 times as intense as the CO2 forcing -- and many times faster: Rate of solar forcing is measured in months compared to decades for CO2!
And that's just the BARE solar forcing before water-vapor FEEDBACK is factored into the equation.
Big BARE ANNUAL SOLAR FORCING FLUCTUATION plus big FEEDBACK = big ANNUAL TEMPERATURE SWING.
This simple calculation suggests that in addition to the familiar seasons due to the tilt of the Earth's axis -- an asymmetric effect that produces Winter in Melbourne while it's Summer in New York, Earth should enjoy a symmetric "second season" during which the whole Earth warms in January and the whole Earth cools in July.
Does Earth's second season actually exist? Is our much vaunted science capable of measuring this periodic pulsation in the Earth's heat budget. Does our planet possess instruments sensitive enough to register this regular thermal heartbeat superimposed on the ordinary seasons that we know so well?
Therefore I am suggesting a new goal for 21th-Century climate scientists comparable to the search for a new planet or a new elementary particle. I propose we mount a well-funded international search for Earth's Second Season.
|Spaghetti Graph: comparison of 90 climate models compared with satellite-measured average global temperature..(Click to enlarge.)|