The Earth's Atmosphere: Weather is not climate

The argument often made by deniers and repeated by naive readers is "They (scientists), cannot even predict the weather for more that three days, so how can they know what would happen in 50 or 100 years? On the face of it, this sounds like a sensible question.

The physics of planetary temperature is a very different problem, however, than weather prediction. It is actually much simpler than understanding the dynamics of air masses, which are what govern the weather. Determining planetary temperature is a more straightforward problem: calculating the "energy budget" of the planet.

Pool balls and light bulbs

c2sx As an illustration, imagine two objects, a white pool ball and a source of heat such as a 100W bulb. If you move the ball closer and closer to the source of heat, its temperature will increase. You may have to wait a bit after each move, to overcome thermal inertia, but eventually the ball heats up to a temperature at which the energy absorbed by the ball is radiated back into space. The ball has then reached dynamic equilibrium and its energy budget is balanced.

When you look at the temperatures of the planets you see a progression from cold to hot as we approach the sun. There are some small deviations from the temperature expected solely on the basis of distance. To understand those with our illustration, substitute a ball painted black. It will absorb more heat and its temperature will be higher.

This higher temperature means an increase in the radiation of the extra absorbed energy. That is the greenhouse effect. This is a simple description of a "balanced energy budget." More complexly stated, the composition of the planetary atmosphere, represented by the color of the ball in our example, changes its "albedo," the ratio of energy reflected to energy absorbed.

Obviously, the calculation of the energy budget for a real planet is a bit more complex. Our planet both rotates and orbits the sun . There are seasons. The intensity of sun's output itself is changing, as is the amount of sunlight reaching particular parts of the earth at any given time. Bodies of water, which act as heat reservoirs, are being filled and emptied, the effect of thermal inertia.

Introducing the ants

Now imagine thousands of ants, living on the surface of a ball which has gray splotches. They will run around, often in response to the local temperature, in an unpredictable, chaotic manner. Their chaotic motion will not affect the energy budget, the average temperatures, or climate of the ball. These ants represent the weather, the motion of air masses, precipitation, and the evaporation of water.

The ants also can represent the behavior of life, which has no effect on the energy budget, unless and until the ants start painting the surface of the planet a different shade of gray. That has happened in the history of our planet several times. It happened when life started, when animals appeared, and it has been happening again, since industrial revolution.

Let's abandon the pool balls and ants for a look at the Earth: c2s
The configuration of the sun, Earth, and dust clouds in space keep changing when observed on geological time scales. These changes produce semi-periodic changes of climate , ice ages and interglacial periods. Beyond that, the activity of the sun itself has several irregular cycles.
It is important to understand that climate models are not attempting to predict either weather or climate. Both are chaotic systems, in the mathematical sense, unpredictable in the long run. Models of climate are calculating the energy budget of the planet. They are describing what happens to the energy which comes from the sun. While some models of the processes within the sun and other stars do exist, they only predict the long term fate of stars. They do not predict the irregular fluctuations and cycles of their output.

From an excess of O2 to an excess of CO2

c2e
During the early years of our planet, the energy content of the Earth was increasing, the converse of our situation now. Nor were the ratios of gasses in the atmosphere stable. Rather, the concentration of oxygen in the atmosphere was growing dramatically. This process had been going on since the Earth cooled enough for water vapor to condense and form oceans, followed by life appearing there in the form of microbes. The concentration of oxygen peaked at 30%. How and why did this happen?
As we all learned at school, plants use solar energy, water from the soil, and carbon dioxide in a process called photosynthesis, which releases oxygen into the atmosphere. In that splendid process, plants also reduce the energy-poor compounds (oxides such as water and CO2) into energy-rich compounds (wood, sacharides such as sugars and cellulose.) and are eventually transformed into fossil fuels.
This process flourished uninhibited until the Cambrian “explosion” a half billion years ago, when animals and multi-cellular organisms which used oxygen appeared. After many milenia and events, the rise of new species and the extinction of others, the atmosphere stabilized at ratios of 20% oxygen and 256 ppm of CO2.

It remained stable even when, some 10 million years ago, a new animal started walking on two legs and invented fire. However, about 350 years ago these bipeds invented the steam engine and started converting the accumulated fossil fuels back into oxides, to CO2 (Carbon dioxide) and H2O (water), at an ever increasing rate. c4v

Writings of that period, even as late as the early 20th century, speak exuberantly of "progress" and see the supply of such energy as inexhaustible. No one apparently could envision the rapid growth of the world's population coupled with the explosion of devices engineered to specifically to use those energy supplies stored so long ago. In fact, many of us are still reluctant to face the fact that the end of these sources is in sight.

The thin blue ribbon

c4v means "click for video". The thin blue ribbon you see at the edge in this photo is the Earth's atmosphere. In this five-minute video, American astronaut Dr.Sally Ride describes her first impression of that blue ribbon in 1983. Illustrated lavishly with photos from space taken then and in the 25 years since, we see clear changes in the Earth's surface.
The temperature, as shown in the previous post, increased by a half degree centigrade in that time. A half degree is a small change, when compared to swings of a season, the weather, or latitude, but over vast areas and time periods it leads to significant changes, as shown in the video. Do look at it and leave a comment

Dr. Ride is optimistic. Are you? In our next post, we will examine what humans can do about these problems, what was tried in the past and failed, what new solutions are being proposed.

References

Qualitative theory behind a simple model Unless you are totally allergic to even simple equations (the Stefan-Boltzman law), look at this elegant, "order of magnitude" estimate of GH effect.