When I think about climate, greenhouse gases, carbon policy etc., I always worry about the certainty that people (typically politicians) want to attach to models (actually, that statement holds for macroeconomic models too, for the same reasons). The global climate is an incredibly complex system, comprising many individual agents and local systems that interact and lead to non-deterministic outcomes (thus the complexity, at least in part). In trying to understand such complex systems we construct models of their behavior. Even the best models are abstractions from some of the details of reality (as statistician George Box said, all models are wrong but some are useful).
Regarding climate, I’ve thought that the most with respect to clouds, and many different ways that clouds can affect climate. Capturing the effects of clouds in a model is difficult because there are so many variables — height, water vapor, etc. — and clouds have different effects depending on those variables and their interactions.
Thus I read with great interest at Ars Technica today about a new research study published in Nature Reports: Climate Change on the atmospheric effects of airplane contrails. Separate from any effects of the emissions from the combustion of jet fuel, the formation of contrails due to the production of water vapor as a by-product of burning jet fuel may itself contribute to greenhouse effects by increasing water vapor in the troposphere. According to the abstract:
An important but poorly understood component of this forcing is caused by ‘contrail cirrus’—a type of cloud that consist of young line-shaped contrails and the older irregularly shaped contrails that arise from them. Here we use a global climate model that captures the whole life cycle of these man-made clouds to simulate their global coverage, as well as the changes in natural cloudiness that they induce. We show that the radiative forcing associated with contrail cirrus as a whole is about nine times larger than that from line-shaped contrails alone. We also find that contrail cirrus cause a significant decrease in natural cloudiness, which partly offsets their warming effect. Nevertheless, net radiative forcing due to contrail cirrus remains the largest single radiative-forcing component associated with aviation.
While I remain cautious in drawing inferences from models of such complex systems, I think research like this at least gives us some insights into the dynamics of how a local system like cloud formation works; in particular, the “substitution” that occurs with the reduction in natural cirrus formation was something I always wondered about. Worth reading.