Clouds and Climate

I'm learning some atmospheric science so I understand what Server Sky will transmit data through, and what Power Sky will transmit energy through. Learning this has raised my awareness of arguments about anthropogenic global climate change, including some preposterous statements by some of the deniers. Here is a response to one of them:

Rather than argue personalities, let's talk about clouds and CO₂ and science. Terry is right that climate is complicated, and that is why his statement is ludicrous:

There is no way any statement of that form can be accurate, but the inaccuracy can be reduced by changing "0.1%" to a credible range more like "5% to 100%". That does not make any simplistic statement of that form true, just a tiny bit less false and misleading. It is surprising that this statement comes from a person who also insists that "warming" cannot be used to describe the Earth because the Earth does not have a temperature.

For all that follows, keep in mind that I am simplifying for clarity and brevity. If you want more accuracy, or support for my brief assertions, I can point you at papers and textbooks. There is no royal road to science - you gotta put in the time. I must make that time myself, because cloud opacity is critical to the behavior and availability of systems I am designing now.

A simple definition of cloud cover: "the percentage of the ground or ocean surface visible from space, looking down."

The earth is optically complicated. It absorbs or reflects short wavelength light from the sun, and emits longer wavelength infrared light. The power flows balance over the long term, with heat capacity and biomass energy storage complicating the short term. If you take a long term view - hundreds to millions of years - the storage terms become increasingly irrelevant to the behavior of the atmosphere.

When you make something hotter, it absorbs energy. The energy leaves as it cools. The three major heat storage systems are the atmosphere, the ocean, and the ball of rock itself. These all exchange heat with each other, but very slowly, proportional to the temperature difference between them. Heat diffusion times go up as the square of the distance. Think of how slowly a pot of water boils - minutes, for centimeter distances. Atmosphere, ocean, and lithosphere scales are many kilometers, so the times for significant heat flow between them are centuries.

The ocean/atmosphere interface speeds up heat flow a lot because of evaporation and condensation. The flow is mostly one way, out of the ocean, released from clouds. But vertical oceanic convection is pretty slow. We can treat the top few centimeters of the ocean as part of the atmosphere, poorly coupled to the deep ocean. Incoming sunlight to the planet is 175,000 terawatts, about 1370 watts per square meter face-on to the sun.

About 1000 W/m² reaches the surface, and much of that is reflected. 70,000 terawatts reaches the land surface, 20,000 terawatts is captured by plants, and about 200 terawatts is stored as biological energy, passing through bacteria, fungi, and animals to become infrared heat. Very little ends up stored in the ground - the development of termites and fungi put a stop to the Carboniferous era processes that deposited coal and shale. And a minuscule amount ends up as "information" or "organized complexity" - you can tote that up with the Shannon definition of bit energy, ½kT, and find that all the information in all structure in the world adds up to a few minutes of global sunlight.

Humans use 14 terawatts of artificial energy now, and this is expected to rise to between 50 and 100 terawatts by 2099 . This does NOT include the heavily modified energy flows caused by converting 40% of the land surface to agriculture, by far our largest interference with natural flows.

The detailed energy balance is far more complicated, but in the interest of brevity I will stop making statements like this - assume such a statement added every sentence.

After bouncing around through various systems, about 120,000 terawatts of thermal energy leaves the earth as infrared. Viewed from space, the earth appears partially reflective, and partially like an infrared black body with a temperature of -20C or so. Black body radiation is proportional to temperature to the 4th "power" (exponent 4). For smallchanges, an X% change in temperature results in a 4X% change in heat radiation. The "temperature" is relative to absolute zero, -273C, -460F. Kelvins, which is Celsius+273.15, are the easiest unit to work in.

To change the average black body temperature by 1C (or 1K), the absolute temperature changes by 1K/250K or about 0.4%. The outbound infrared heat flow changes by 1.6% (the derivative of the fourth power) . That is an enormous amount of heat flow, 2000 terawatts. So how does our measly 14 terawatts of artificial energy consumption have any affect on that?

Human activity is like pulling on a blanket on a cold night. The energy to move the blanket is small, the effect on heat conductivity is enormous. CO₂ is opaque to infrared, very strongly opaque at the wavelengths produced by a 250K emitter. Practically, the effect of more CO₂ to insert extra "blankets" between the upper and lower portions of the atmosphere. 120,000 terawatts. 234 W/m² is still being converted into black body heat radiation, but there is more insulation in the way.

Think about blankets again - if the room you were in was at -450 degrees Fahrenheit, and one blanket made you comfortably warm, what would two blankets do? You would get hotter and hotter, until something cut your heat production in half. Perhaps death. Of course, you would just remove the extra blanket. But what if the blanket took 100 years to pull on, and even longer to pull off? What if heat made you too weak to move it? You would literally be toast.

Cutting the Earth's heat production is likely to come at the expense of energy flows going through living things, replacing plants with high-albedo, light-reflective desert. Remember, plants make 20,000 terawatts out of 120,000 terawatts of heat. Deserts make less heat, and instead return light which is not trapped by greenhouse gasses. So if temperatures rise, a new power flow balance will be found consisting of more reflective desert and less vegetation (supporting less animals, fungi, and bacteria). It is vitally important to remember that the plants are the only process that removes CO₂ from the atmosphere at the sub-century scale, so the plants are damaged, the blanket-removal process slows.

So, where do clouds enter into this ?

Clouds reflect light. But the details - those pesky details! - revolve around what a cloud really is, and does. Water vapor is lighter than air (atomic weight 18 versus atomic weight 28 for nitrogen). That means vapor rises. As it rises, it expands, and cools (why? look up "adiabatic expansion"). Hot air can hold more water vapor than cold air ( look up "vapor pressure") so at some point, the air saturates with water vapor, and some starts to condense as micron-scale droplets. As it condenses, the water sheds heat ("heat of vaporization") which slows down the cooling, until all the water condenses. The rising air keeps rising ("inertia"), and cooling, and at some point the droplets become tiny ice crystals. The air mass is no longer as buoyant, and may be slightly heavier than the surrounding air, so it is slowed by gravity, to bounce back down, compress, heat, melt the ice back into water and the water back into vapor. Rinse and repeat. These cycles drive thunderstorms.

Clouds are visible because the crystals and droplets scatter light. Scattered differently for each, and you can see the difference when you know what to look for. Knowing what to look for makes clouds awesome. It is the difference between looking at a football field, and a football game.

Within all this complication, the tiny droplets and ice crystals are slowly falling - very slowly, a lot of air drag on a tiny bit of weight. But a few merge, and fall faster because the surface to volume ratio drops. They run into more droplets or ice, fall faster, and so forth. Eventually, you have rain or snow. Much of which vaporizes again before it hits the ground.

While all this tumbling and state change is going on, energy is moving also. The movement of air and water also moves heat. Moisture rises because higher air is colder - until you get to the stratosphere, where the temperature starts increasing with altitude ("tropopause" "ultraviolet absorption"). The clouds stop rising, and spread with the winds at 10km altitude (very fast winds!). That is what makes the anvil shape of cumulonimbus thunderstorm clouds.

So, given all that enormous complexity, how can we ever hope to figure out what clouds do to climate?


Just as we can tell that an adult eating 500 calories a day is losing weight, and 4000 calories a day is gaining, we can count calories for the earth. We don't need to do micro-scale modeling in either case. Without good models, we can't tell exactly where the weight goes on (or the heat is stored), but we know it is going into the atmosphere or the body somewhere.

There are many atmospheric research satellites up there now, observing and probing the earth at many wavelengths and resolutions. We calibrate these satellites with data gained from radiosonde balloons and sounding rockets (as a fellow measurement engineer, Doug Strain would love the calibration techniques). The satellites capture and beam down torrents of data. We know vastly more about the atmosphere than we did a decade ago. And two of the easiest measurements to make are cloud cover and thermal/albedo optical emission.

If you surf to you can see a couple of plots made by Dr. Dennis Hartmann, an atmospheric scientist at University of Washington, and a helluva friendly and helpful guy. The top plot is made from ISCCP (International Satellite Cloud Climatology Project) data and the bottom plot is ERBE (Earth Radiation Budget Experiment) data. All the raw data are available for free

The top plot is stratus cloud cover, ranging from 0 to 60 percent on the plot. Stratus clouds are the low ( 1 km ) clouds that act like a "blanket" for infrared - not because they are clouds, but because they represent moisture. Plots of higher clouds (not shown) bring the totals higher. Totals range from 15% over northern Africa to 100% over much of the globe. By looking only at stratus, I exaggerate their effect.

The bottom plot is a plot of net radiation forcing, the amount of heat/optical energy leaving the earth versus the amount arriving from the sun. Both plots are averaged over a year, though day and month scale data can be downloaded from NASA for your own analysis.

Take a look at that image. It might help if you downloaded it and brought it up in an image editor, displayed beside this email. These plots are easier to "browse" with the "select by color" tool found in many image editors, such as the free "GIMP" graphics editor.

Obviously, the earth radiates a lot of heat at the poles. Relative to inflow, there isn't much incoming with the sun low on the horizon, or during the 6 month night. In the equatorial band and the temperate zones, there is more incoming light, and the effect of clouds versus thermal emission is easier to see.

We can pick a couple of points, lets say Africa around Lake Chad (the squiggle above the notch of the Gulf of Guinea), and central China ( an orange spot in the top picture, a red spot in the bottom ). You can pick lots of pairs of spots, and get somewhat different results, but the average trend will be the same.

Stratus Cloud

Radiative Forcing



-60 W/m²



0 W/m²



-60 W/m²

So, the net effect of 35% increase in stratus cloud cover is 60 W/m² of heat outflow. About 2 W/m² per percent of cloud. About 0.7% of the 234 W/m² thermal radiation. Divide that by 4, to get the percentage change in black body absolute temperature, about 0.18%, and multiply by 250K, to get 0.45 degrees (K or C) per percent of cloud.

The IPCC claims the most likely (not worst case) warming will be 4C over the next century. The much larger CO₂ effect is partly masked by the most likely cloud and smog effects. But if we generously assume CO₂ is entirely responsible for only the 4C rise, without compensation, we still need 8.8% cloud increase to compensate for it. Not 0.1% as Terry claims (shifting responsibility to Doug Strain, who can no longer defend himself).

Keep in mind that all of the above is way oversimplified hand waving. Some of it is stated poorly, and there are likely mistakes. With more complete cloud models, and a more realistic CO₂ contribution, the discrepancy is larger than a factor of 88. However, to understand that, you must learn some atmospheric science. It's good to understand where your next breath is coming from.

I find this stuff fascinating as hell - and understand how a scientist can spend a lifetime studying the atmosphere. The richness and complexity of the behavior is astounding. Scientists are learning to model these systems with increasing precision. They keep many of the world's largest supercomputers busy. They are quickly getting better at refining ranges. But in 2011, they cannot tell you precisely what will happen in 50 or 100 years.

It's climatological Russian Roulette; there is a bullet in there, the only question is how many more times we can pull the trigger.

Your investment adviser can't tell you to the penny what your stocks will be doing in 20 years, but they can tell you with some certainty that your $100K will not turn into $100 or $100M. The better they understand trends and your spending habits, the more accurate their predictions become. Never perfect.

So too, our investment in our planet. Right now, we are maxing out our credit cards, not investing at all. Whether we go environmentally bankrupt in 5 years or 5000 years, the cost of bankruptcy is the squandering of a 3.7 billion year investment that we did not make, and does not belong to us. We are stewards, not owners.

I see a solar system flooded with with 380 trillion terawatts of solar energy, blowing out into intergalactic space, forever. I claim that with hard work we can not only stop squandering nature's investment, but we may be able to compound the sum enormously, using all that extra energy. Whether we are capable of seizing this unused wealth and adding to nature's capital is unknown - anyone who tells you surely yes or surely no is a hubristic fool. But the prospects are enticing, and the available tools (including atmospheric science) are surprisingly capable, even in 2011. Prudent investment will reap rewards that beggar the imagination.

Argue from inertia and ignore change all you want. I am working hard to make mother Gaia, who has been so generous to me, far more wealthy. Perhaps that will partly justify the obscene executive bonuses we humans are now giving ourselves.

CloudsClimate (last edited 2011-07-16 21:33:18 by KeithLofstrom)