Day 2 Question 4: A critique of the comment that weather results from deterministic and stochastic factors

Question 4:

Criticize or defend the often heard comment that weather events (such as rainstorm, etc) result from a combination of deterministic and stochastic factors.

With the globalization of media, every major weather event becomes a widely discussed phenomena. The plethora of such weather events, particularly the onslaught of hurricanes we’ve witnessed in the past two years, begs the discussion of the cause of such events. People naturally wonder if we’ve altered climate by loading the atmosphere with greenhouse gasses to the point that dramatic weather events will become more common and more severe. Such claims by the public and the media then beg scientists to sort out the trend of global warming from the stochasticity, or randomness, of our climate.

One common question raised today is whether single events, like hurricane Katrina or hurricane Rita, were caused by global warming, or made more intense because of global warming. A second question is if events like those are going to become more common in a warming climate. The first of these questions gets to the heart of the topic at hand. The second question, however, is much simpler and should be handled first.

Global Climate Models (GCMs) used to predict future climates almost all agree that in an atmosphere artificially enriched in CO2, stronger hurricanes will become more common, while weaker storms will be less common (Knutson and Tuleya, 2004). Knutson and Tuleya’s study is based on the physical principles of tropical cyclones, and their results yield a deepening of the central pressure and more intense rainfall in a warmer climate.

These results do imply that more intense storms will become more likely, however, it is not appropriate to draw the conclusion that a single event, like hurricane Katrina, was made worse by global warming. In fact, it is impossible to definitively determine whether a single event was worsened by global warming because it is impossible to separate the effects of anthropogenic warming from natural cycles and the stochastic nature of weather on the scales which affect a single storm.

That said, on a larger scale, it is difficult, but not impossible to determine some of the influence that global warming is having on climate. The climate system, though, is extremely complex (Rind, 1999), and therefore behaves non-linearly and chaotically (Rial et al., 2004). This is not to say that weather events are totally random, rather that they may appear random because the factors affecting a single event are so complex and widespread.

Rahmstorf et al. (2005) suggest that every weather event is influenced by both stochastic chance and deterministic values. While this explanation is elegant, I disagree. I believe that weather is not random. I believe that everything that happens happens for a reason. Well, not just for one reason, but for a vast and complex set of reasons. This is, essentially, the premise of chaos.

If we take the stochastic nature of weather to be caused by the complexity of the climate system and the chaotic way in which it behaves, than the notion that weather events have a stochastic element is completely true. If we define stochasticity, however, as being simply random chance, than I no longer feel it is part of the equation. On the other hand, what exactly differentiates randomness and chaos?

Rahmstorf, S., et al. (2005), Hurricanes and global warming - is there a connection?, RealClimate, 181.

Rial, J. A., et al. (2004), Nonlinearities, Feedbacks and Critical Thresholds within the Earth's Climate System, Climatic Change, 65, 11-38.

Rind, D. (1999), Complexity and Climate, Science, 284, 105-107.