How Climate Models Work and the Assumptions Underlying Them

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How Do Climate Models Work? by Roy W. Spencer, Ph. D....fears of manmade global warming — and potential legislation or regulations of carbon dioxide emissions — are based mostly upon the output of climate models, it is important for people to understand the basics of what climate models are, how they work, and what their limitations are....a climate model is a computer program mostly made up of mathematical equations. These equations quantitatively describe how atmospheric temperature, air pressure, winds, water vapor, clouds, and precipitation all respond to solar heating of the Earth’s surface and atmosphere. Also included are equations describing how the so-called “greenhouse” elements of the atmosphere (mostly water vapor, clouds, carbon dioxide, and methane) keep the lower atmosphere warm by providing a radiative ‘blanket’ that partly controls how fast the Earth cools by loss of infrared to outer space.The equation computations are made at individual gridpoints on a three-dimensional grid covering the Earth...In “coupled” climate models, there are also equations describing the three-dimensional oceanic circulation, how it transports absorbed solar energy around the Earth, and how it exchanges heat and moisture with the atmosphere. Modern coupled climate models also include a land model that describes how vegetation, soil, and snow or ice cover exchange energy and moisture with the atmosphere....The Importance of Energy Balance in Climate Models...used to study how the Earth’s climate might respond to small changes in either the intensity of sunlight being absorbed by the Earth, or in the case of anthropogenic global warming, the addition of manmade greenhouse gases that further reduce the atmosphere’s ability to cool to outer space....it is the balance between these two flows of radiant energy – solar energy in, and infrared energy out of the climate system — that is believed to control the average temperature of the climate system over the long run....If they are out of balance, the average temperature of the climate system can be expected to change....When averaged over the whole Earth, the rates of absorbed solar energy and infrared energy lost are estimated to be about 235 or 240 Watts per square meter. I say “estimated” because our satellite system for measuring the radiative energy budget of the Earth is still not quite good enough to measure it to this level of absolute accuracy.****That is, the uncertainty in the inputs and the output are each of the order of 5 watts/sq m. Roughly speaking, we don't know where the energy imbalance might be except that it's not clearly MEASURED to be closer to zero than the range of about +5 watts/sq m to -5 watts/sq m.We'll see below that the contribution of CO2 is believed to be of the order of 1.5 watts/sq m although it is in the direction of less radiated energy.Thus, the measured CO2 effect is not clearly dominant in the matter of energy imbalance.The question is whether it definitively tips the balance in the direction of warming.**** A variety of adjustable parameters in the model are tuned until the model approximates the average seasonal change in weather patterns around the world, and also absorbs sunlight and emits infrared energy to space at a global-average rate of about 235 or 240 Watts per sq. meter. The modelers tend to assume that if the model does a reasonably good job of mimicking these basic features of the climate system, then the model will be able to predict global warming. This might or might not be a good assumption – no one really knows. It is also important to understand that even if a climate model handled 95% of the processes in the climate system perfectly, this does not mean the model will be 95% accurate in its predictions. All it takes is one important process to be wrong for the models to be seriously in error. For instance, how the model alters cloud cover with warming can make the difference between anthropogenic global warming being catastrophic, or just lost in the noise of natural climate variability. Anthropogenic Global Warming in Climate Models Our addition of carbon dioxide to the atmosphere by burning fossil fuels is estimated to have caused an imbalance of about 1.5 Watts per sq. meter between the 235 to 240 Watts per sq. meter of average absorbed sunlight and emitted infrared radiation....This energy imbalance is too small to be measured from satellites; it must be computed based upon theory.
So, if the Earth was initially in a state of energy balance, and the rate of sunlight being absorbed by the Earth was exactly 240 Watts per sq. meter, then the rate of infrared loss to outer space would have been reduced from 240 Watts per sq. meter to 238.5 Watts per sq. meter (240 minus 1.5).
This energy imbalance causes warming in the climate model...the modeled climate system will warm up until energy balance is once again is restored...What Determines How Much the Model will Warm?
The largest source of uncertainty in climate modeling is this: will the climate system act to reduce, or enhance, the small amount of CO2 warming?...(it)has different ways in which an energy imbalance like that from adding CO2 to the atmosphere can be restored. The simplest response would be for the temperature alone to increase. ...the ~40% increase in atmospheric CO2 humans are believed to have caused in the last 150 years would only cause about 0.5 deg. C warming to restore energy imbalance. This theoretical response is called the “no feedback” case because nothing other than temperature changed.But a change in temperature can be expected to change other elements of the climate system, like clouds and water vapor. These other, indirect changes are called feedbacks, and they can either amplify the CO2-only warming, or reduce it. ...all 20+ climate models currently tracked by the United Nations’ Intergovernmental Panel on Climate Change (IPCC) now amplify the warming...mostly through an increase in water vapor — Earth’s main greenhouse gas — and through a decrease in low- and middle-altitude clouds, the primary effect of which is to let more sunlight into the system and cause further warming. These indirect changes in response to warming are called feedbacks. The models amplify the CO2 warming with positive water vapor feedback, and with positive cloud feedback. But is this the way that the real climate system operates? Uncertainties in Climate Model Cloud and Water Vapor Processes
...model equations are only approximations of the physical processes that occur in the atmosphere...some...are highly accurate, some of the most important ones...are unavoidably crude. This is because the real processes they represent are either (1) too complex to include in the model and still have the model run fast on a computer, or (2) because our understanding of those processes is still too poor to accurately model them with equations. This is especially true for cloud formation and dissipation, which in turn has a huge impact on how much sunlight is absorbed by the climate system. ... the manner in which clouds change with warming is of huge importance to global warming predictions....modelers are still struggling to get the models to produce cloud cover amounts and types like those seen in different regions, and during different seasons...It is probably safe to say that all climate modelers recognize that the modeling of cloud behavior accurately is very difficult, and is something which has not yet been achieved in global climate models. All of the IPCC climate models reduce low- and middle-altitude cloud cover with warming, a positive feedback. ...there is already some evidence showing up in the peer-reviewed scientific literature that this is (not correct).****Did warming cause decreased cloud cover or did decreased cloud cover cause warming?****...This is basically an issue of causation: one direction of causation has been ignored when trying to estimate causation in the opposite direction...The fundamental issue of causation in climate modeling isn’t restricted to just clouds. While warming will...cause an increase in low-level water vapor, precipitation systems control the water vapor content of most of the rest of the atmosphere...
precipitation is continuously reducing the greenhouse effect by converting that water vapor into clouds, then into precipitation.
...the physics of evaporation at the Earth’s surface is understood pretty well, the processes controlling the conversion of water vapor into precipitation in clouds are complex and remain rather mysterious. And it is the balance between these two processes — evaporation and precipitation — that determines atmospheric humidity.Even
in some highly complex ‘cloud resolving models’ – computer models that use much more complex computations to actually ‘grow’ clouds in the models – the point at which a cloud starts precipitating in the model is given an ad hoc constant value...a huge source of uncertainty, and one that is not appreciated even by most climate modelers (who) tune the models to approximate the average relative humidity of the atmosphere, but we still do not understand from ‘first principles’ why the average humidity has its observed value. We would have to thoroughly understand all of the precipitation processes, which we don’t. In the end, many of the approximations in climate models will probably end up being not very important for forecasting climate change…but it takes only one critical process to be wrong for model projections of warming to be greatly in error. The IPCC admits that their largest source of uncertainty is low cloud feedback, ...how precipitation efficiency might change with temperature is also a wild card in climate model predictions....climate modelers think global warming is the result of humans ‘upsetting’ the Earth’s radiative energy balance. And...adding more carbon dioxide to the atmosphere must have some effect…but how large is this change in comparison the energy imbalances the climate system imposes upon itself? It turns out that the modelers have made a critical assumption that ends up leading to the their conclusion that the climate system is very sensitive to our greenhouse gas emissions: that the climate system was in a state of energy balance in the first place....a pervasive, non-scientific belief in the Earth sciences that nature is in a fragile state of balance....a subjective concept, not a scientific one....Just because nature tends toward a balance does not mean that balance is in any way ‘fragile’. And what does ‘fragile’ even mean when nature itself is always upsetting that balance anyway?...if climate researchers ignore naturally-induced climate variability, and instead assume that most climate changes are due to the activity of humans, they will inevitably come to the conclusion that the climate system is fragile: that is, that feedbacks are positive....If the warming observed during the 20th Century was due to human greenhouse gas emissions, then the climate system must be pretty sensitive (positive feedbacks). But if the warming was mostly due to a natural change in cloud cover, then the climate system is more likely to be insensitive (negative feedbacks). And there is no way to know whether natural cloud changes occurred during that time simply because our global cloud observations over the last century are nowhere near accurate enough....climate modelers simply assume that there are no natural long-term changes in clouds, water vapor, etc. But they do not realize that in the process they will necessarily come to the conclusion that the climate system is very sensitive (feedbacks are positive). As a result, they program climate models so that they are sensitive enough to produce the warming in the last 50 years with increasing carbon dioxide concentrations....but this is simply reasoning in a circle. Climate modelers have simply assumed that the Earth’s climate system was in a state of energy balance before humans started using fossil fuels. A plot in the paper (from Loehe, 2007)**** shows 2000-years-of-global-temperatures including the Medieval Warm Period (600-1150 C.E.(Vikings in Greenland) and the Little Ice Age (1450-1850 C.E.(from which we may be still emerging.) Not only do the models not "post-dict" these, they ignore them and assume constant temperature.****...changes in solar activity are one possible explanation for these events, ...also possible that there are long-term, internally-generated fluctuations in global energy balance brought about by natural cloud or water vapor fluctuations. For instance, a change in cloud cover will change the amount of sunlight being absorbed by the Earth, thus changing global temperatures. Or, a change in precipitation processes might alter how much of our main greenhouse gas — water vapor — resides in the atmosphere. Changes in either of these will cause global warming or global cooling....Ultimately, the climate researcher (and so the politician) must take as a matter of faith that today’s computerized climate models contain all of the important processes necessary to predict global warming...
validating the predictions of any theory is so important to the progress of science. The best test of a theory is to see whether the predictions of that theory end up being correct....we have no good way to rigorously test climate models in the context of the theory that global warming is manmade....because warming due to more carbon dioxide is, for all practical purposes, indistinguishable from warming due to, say, a natural increase in atmospheric water vapor....“But what could cause such a natural change in water vapor?” Well, how about just a small change in atmospheric circulation patterns causing a decrease in low cloud cover over the ocean?...how about the circulation change causing a change in wind shear across precipitation systems? ...just because we don’t understand all of the ways in which nature operates doesn’t mean that we humans are responsible for the changes we see in nature....natural changes in climate I am talking about can be thought of as ‘chaos’. Even though all meteorologists and climate researchers agree that chaos occurs in weather, climate modelers seem to not entertain the possibility that climate can be chaotic as well...If they did believe that was possible, they would then have to seriously consider the possibility that most of the warming we saw in the 20th Century was natural, not manmade. But the IPCC remains strangely silent on this issue. The modelers will claim that their models can explain the major changes in global average temperatures over the 20th Century. ...(1) not likely that theirs is a unique explanation, and (2) this is not an actual prediction since the answer (the actual temperature measurements) were known beforehand....years of considerable trial-and-error work has gone into getting the climate models to reproduce the 20th Century temperature history, which was already known to the modelers....an exercise in statistical ‘curve-fitting’****hindcasting of temperature fluctuations in the past are not done or are off by embarrassing numerical discrepancies.****...any modeler who claims they have found the only possible cause of global warming is being either disingenuous, or they have let their faith overpower their ability to reason. Even the IPCC (2007) admits there is a 10% chance that they are wrong about humans being responsible for most of the warming observed in the last 50 years. That, by itself, shows that anyone who says “the science is settled” doesn’t know what they are talking about.****How they can restrict to 10% the chance that they don't know what they're talking about is unclear.****

Conclusion

There is no question that great progress has been made in climate modeling. I consider computer modeling to be an absolutely essential part of climate research. After all, without running numbers through physical equations in a theoretically-based model, you really can not claim that you understand very much about how climate works.But given all of the remaining uncertainties, I do not believe we can determine — with any objective level of confidence — whether any of the current model projections of future warming can be believed. Any scientist who claims otherwise either has political or other non-scientific motivations, or they are simply being sloppy ****or fanatical believers in Gorism, whose tenets were simplistically founded decades before the more sophisticated models tried to account for effects other than the simplistic.**** Ref.Loehle, 2007. A 2,000 year global temperature reconstruction on non-tree-ring proxy data. Energy & Environment, 18, 1049-1058.