By Roger Pielke Sr. Climate Science Weblog

Spencer R. Weart posts a remarkable weblog on Real Climate titled ”Simple Question, Simple Answer - Not”. He writes: “I often get emails from scientifically trained people who are looking for a straightforward calculation of the global warming that greenhouse gas emissions will bring.” and
“I’m not saying we don’t understand the greenhouse effect. We understand the basic physics just fine, and can explain it in a minute to a curious non-scientist. (Like this: greenhouse gases let sunlight through to the Earth’s surface, which gets warm; the surface sends infrared radiation back up, which is absorbed by the gases at various levels and warms up the air; the air radiates some of this energy back to the surface, keeping it warmer than it would be without the gases.) For a scientist, you can give a technical explanation in a few paragraphs. But if you want to get reliable numbers - if you want to know whether raising the level of greenhouse gases will bring a trivial warming or a catastrophe - you have to figure in humidity, convection, aerosol pollution, and a pile of other features of the climate system, all fitted together in lengthy computer runs.

Physics is rich in phenomena that are simple in appearance but cannot be calculated in simple terms. Global warming is like that. People may yearn for a short, clear way to predict how much warming we are likely to face. Alas, no such simple calculation exists. The actual temperature rise is an emergent property resulting from interactions among hundreds of factors. People who refuse to acknowledge that complexity should not be surprised when their demands for an easy calculation go unanswered.”

This is a clear admission of the complexity of the climate system which Climate Science has been emphasizing since it was initiated!

Indeed, since Spencer accepts that “The actual temperature rise is an emergent property resulting from interactions among hundreds of factors”, it also follows that we do not know even if the actual temperature will rise, since there are a variety of human cooling climate forcings, including from several of the aerosol effects (see), as well as natural cooling effects including atmospheric circulation changes, decreases in solar irradiance, and volcanic emissions. 

We recognized this complexity in several of our papers; for example, in Nonlinearities, feedbacks and critical thresholds within the Earth’s climate system. Climatic Change, 2004, where we write “The Earth’s climate system is highly nonlinear: inputs and outputs are not proportional, change is often episodic and abrupt, rather than slow and gradual. It is imperative that the Earth’s climate system research community embraces this nonlinear paradigm if we are to move forward in the assessment of the human influence on climate.”

In Non-linearities in the Earth system. Global Change Newsletter, 2003 we wrote: “The complex non-linear physical, chemical, and biological interactions among the components of the Earth System are becoming an increasingly important focus in global change research. These interactions between atmosphere, oceans, ice, and land are driven externally by the solar input of heat, and internally by geologic activity and the myriad processes that control the behaviour of each sub-system. Human activity is an integral component of these interactions.” and “The complexity of the Earth System’s behaviour makes it extremely difficult to accurately forecast the future of the Earth System, and presents a major challenge to the global change research community.”

Skillful multi-decadal global climate predictions (and, thus also multi-decadal regional climate predictions) are not yet possible. Spencer R. Weart’s weblog is a step forward in the wider recognition of the actual complexity of the climate system. See Roger’s post here.