a drop in the ocean

Human-caused increases in atmospheric carbon dioxide have consequences besides climate change. For example, they are causing the ocean’s pH to drop with unwelcome consequences as corals struggle to accrete their calcium carbonate skeletons. What makes this process interesting from a scientific perspective is that a large time lag is built into this process as carbon dioxide gradually dissolves in the ocean. Alas this is not the only instance.

A review in TREE discusses the consequences of time lags in abstract terms. The authors argue that lags should be afforded greater attention for their role in regime shifts. Regime shifts occur when one ecological equilibrium gives way to another under the influence of some driving factor. One example of this is the degradation of Caribbean coral reefs under pressure from over-fishing, ocean acidification and other causes (I recently read this excellent book on the subject). A decline in coral cover has occurred over extended periods (punctuated by catastrophic events such as hurricanes). When changes occur gradually – over human generations – it can be difficult to perceive them as people in each generation base their expectations on personal experience or recent data – an effect which is called “the shifting baseline” (from a 1995 TREE article by Daniel Pauly – for an accessible account see this voice-overed slide show). Also, it can be difficult to assess the causes of gradual change.

Setting human understanding to one side, lags have real-world consequences because, by definition, they entail that a system is in a non-equilibrium state for a time. The focal article shows how this can impede ecosystem recovery more than equilibrium-only models predict. (Note also that they do not entail anything about the acceleration of a system just its velocity so sudden changes may occur – just later). More optimistically, they offer an opportunity to intervene to prevent regime shifts that are otherwise fated to occur. To use the phrase of the article, certain systems are likely “living on borrowed time”.

From an evolutionary perspective this offers one interesting possibility which is that adaptation can also occur during a lag period – leading to “evolutionary rescue”. This will be the topic of my next post (just as soon as I’ve read a recent article)! For now suffice it to say that adaptation is itself a non-equlibrium process and a brief survey of your environment, physical and social, suggests that, when lags are taken into account, equilibria might be the exception rather than the rule.

Correction: above I say that lags relate to velocity of change not its acceleration. This is probably inaccurate. In a complex system the rate of change and the change in that rate could both change. In a sense a lag means that both have changed relative to what we’d expect in the absence of a lag.

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