This is basically a problem with epistemology: how we know what we know about the future. For future climate, we “know” about the future by using complex climate models. These have been evolving for several decades now and use some of the most complex and sophisticated computers on the planet. But they are really telling us what might happen, not what will happen. Looking at lots of climate models and lots of different scenarios for them, we can see areas where particular climate models are in agreement and where they differ.

Much analysis about future climate centers on looking for model consensus. I think of consensus as circles of certainty — basically, where they overlap. The inner circle of highest certainty is that we know that the climate is changing everywhere. All the models agree on this. But as we move outwards in time and space, the level of certainty falls dramatically. Nigel Arnell, for instance, is a leading climate scientist affiliated with the IPCC on freshwater issues. At Stockholm’s World Water Week last August, he declared, The spatial scale associated with reasonable levels of certainty about freshwater and climate change is roughly the size of the Mississippi river basin. That’s quite large  — about 1,151,000 sq miles or almost 300,000,000 hectares. The Mississippi river basin occupies about 15 or 20 percent of North America. That’s also pretty big.

This lack of certainty reflects how difficult it is to model the global climate system, which is extremely complex. I asked a climate modeler friend, When will have good predictions of precipitation? She said, Never. The uncertainties are simply too large. Some aspects of the climate system (air temperature, for instance) are pretty straightforward to predict. But others — sadly, most of the ones that are important for freshwater ecosystems, such as precipitation and evaporation — are not, mostly because they tend to be associated with a lot of stochastic (random) processes that simply cannot be predicted. This means we really don’t know that much about what will happen with freshwater ecosystems more than a decade or two away from now.

The role of models and climate change have come up recently from two recent sources. Freeman Dyson is a physicist at Princeton who was recently profiled as an important climate change skeptic by the New York Times. He is not, I think, dogmatic or doctrinaire on climate change (he voted for Obama, if that tells you anything). And he is clearly not stupid. But he appears to fundamentally believe, first, that humans probably don’t have the ability to change the climate, and even if we do, our ability to prepare for future changes is challenged by our dependence on climate models. The first assumption is patently wrong. We do actually have a lot of certainty about our ability to change the global climate system. But the second ... well, he makes a point I hear a lot. If we’re not sure what will happen where and when, then how do we make plans for building or modifying new water infrastructure like dams, which are supposed to last for decades?

This point should be well taken. As we look farther into the future, we really begin to need to know a lot more about how humans will continue to modify the climate, for instance — greenhouse gas emissions scenarios vary a lot, depending on economic and policy conditions. And those are not simple things to predict. For freshwater, we also need good hydrological models. These too are very difficult to construct, and they add uncertainty on top of climate uncertainty. Understanding ecological impacts adds another layer. And then there are potential synergies between climate, human behavior, and other natural systems that are extremely difficult to predict, such as shifts in precipitation that lead to changes in fire regime that alter sediment patterns, which then create quite different kinds of rivers and wetlands. Call that hyper-uncertainty.

Dyson also seems to be worried about climate change a potential distraction from other issues. If we focus too much on climate shifts, will we stop worrying about other, probably more pressing issues? This is also a fair point. But I think this is one we can deal with. In many ways, adapting ourselves to an emerging climate means doing much of what we have already been doing for some time, but trying to work more effectively and efficiently, and with mindfulness for the long-term interactions between our actions and the climate context.

Some of the answer comes from the second source of perspectives on models and climate change. Pilkey and Pilkey-Jarvis have written an interesting new book called Useless Arithmetic: Why Environmental Scientists Can't Predict the Future, which offers a severe critique of the use of models to deterministically set policy action. (And they are NOT climate skeptics.) They are scientists who do a lot modeling of complex systems, such as how sediment moves in near-shore coastal areas. This kind of modeling is very important if, for instance, you’re considering whether or not to replenish the sand on your community’s beach, which is a very expensive proposition for most towns and cities. In many cases, models that might be useful from a qualitative perspective (“sediment will probably move a great deal during large storms; removing a jetty could slow this process” ) are applied in a quantitative fashion (“add 43 metric tons of sediment, which will replenish this beach for 23 years” ). They are basically arguing for becoming a good and appropriate consumer of complex models of natural systems.

Something similar often happens with climate models, unfortunately. The challenge is to use models as a means of informing policy without strictly determining policy in a mechanistic fashion or — even worse — becoming crippled by uncertainty and not acting at all. This isn’t an easy situation. People that fund new dams or are planning on expanding city water services want to know how reliable their options are. Climate models appear to offer a seductive appeal: they provide numbers. But should have faith in these numbers?

The solution — if there is one — is in promoting flexibility (discussed briefly in a different context in another recent entry). Can you act in such a way that you do not regret your actions, given a particular range of climate scenarios?

But this topic will have to wait for more time ... and another entry.

Tags: modeling, synergies, policy