Still not getting warmer:

Still not getting warmer:

In the previous blog I talked about a figure that has been floating around in blogworld making the case that IPCC predictions are wrong because they predict that the last four years should be getting warmer in each successive year and that has not been observed. This blog is about an interesting paper that also addresses that issue. The paper is by Judith Lean and David Rind. You might recall that I relied a lot on their work in the series of blogs on the Sun.

But first … Models are used to both isolate and quantify the processes that are important in the transfers of energy within the Earth’s climate system. The more recognized use of models is to make predictions. Here is a link to some material I have written that introduces the basics of modeling to, let’s say, science-interested people.

The recent paper by Lean and Rind investigates expected warming in the next 20 years or so. The paper relies upon a statistical model that is based on recent behavior that has been observed. Statistical models are one of a suite of model types that are used in science. Statistical models, physical models, and more intuitive, heuristic, models are used in concert with each other. One could view this hierarchy of models as a way to explore and, sometimes, manage and reconcile the uncertainties that are an intrinsic part of the scientific process. For those of you who are weather forecasters, this is same process that is used in that application. There is guidance from global and regional predictions. That guidance is made a more accurate prediction by local forecasters who have intuition on how local weather is related to global and regional models. This local intuition is based on understanding the statistics of past behavior. It’s using all available sources of information.

Here is Figure 1 from Lean and Rind:




Figure 1 from Lean and Rind (2009), Geophysical Research Letters. Figure taken from tinypic.com.

If you compare this figure with the one in my previous blog, there is an honest presentation. You see a longer record of both the observations and the IPCC projections. This presentation makes the historical variability clear, as well as the agreement and disagreement with the model representation of the last, approximately, 30 years. Also evident is the change in the character of the model representation of temperature as the models move from representing the past to projecting the future. This is because the past projections include, for example, a representation of “natural forcing” associated with volcanoes. This type of natural variability is not known for the future.

Lean and Rind derive relations that represent the historical variation associated with the solar cycle, volcanic aerosols, El Nino, and anthropogenic influences (This is the orange curve.). This model does represent the lack of warming in the past 4 - 5 years. Looking to the future, the variability in the solid orange curve shows the wave-like variability of the solar cycle and the temperature increase due to increasing greenhouse gases. Lean and Rind anticipate, in the next decade, periods when the observations are both warmer and cooler than the IPCC projections. Still in the average, the temperature will be rising. Furthermore, when Lean and Rind consider the potential variability associated with a volcanic eruption or a large El Nino, any discrepancies in the near term projections and the observations are more that accounted for. This variability is represented by the dashed lines.

Lean and Rind, in fact, are making a courageous statement here. They take a position on predictability of a measure of “global warming” on a decadal time scale. This is based on an analysis of past natural variability and the assumption that that variability extends into the future. Another recent paper by Keenlyside et al., 2008 in Nature, examines the impact of the variability in the Atlantic Ocean on regional and global climate. Keenlyside et al. project that based only on the projection of the observed Atlantic variability into the future, natural cooling will act counter to the projected human-made warming. Lean and Rind assert that their analysis suggests warming even in the presence of this projected cooling. These are both statements that will be subject to “validation” with observations.

Prediction on decadal timescales is emerging as one of the next big challenges in model-based prediction. Papers like the ones referenced here are amongst the first setting the foundation for such predictions. Decadal prediction is a stunningly difficult challenge because it will require better representation of the variability associated with the ocean, and the difficult problem of the interaction of the atmosphere and the ocean.

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