A Different Interpretation Of Issac Held’s View Of Forced Climate Change

Issac Held has a post on his weblog titled

Summer is warmer than winter

Excerpts from his post are

Two common questions that I (and many others) often get are “How can you predict anything about  the state of the atmosphere 100 years from now when you can’t predict the weather 10 days in advance?” and “How do you know that the climate system isn’t far more complicated than you realize or can possibly model?”  I often start my answer in both cases with the title of this post.  It may sound like I am being facetious, but I’m not; the fact that summer is warmer than winter is an excellent starting point when addressing both of these questions.

I don’t think that we have to spend much time on the first question here.  We all successfully and continually predict the state of the atmosphere several months in advance whenever we plan our summer or winter vacations.  Of course, the seasonal cycle is forced; no one can predict the chaotic day-to-day weather months in advance.  More reasonably, we do try to predict whether the temperature averaged over the next summer will be warmer or colder than average in some region, part of the challenge we call seasonal forecasting.

Analogously, when we talk about predicting the trend in the climate over the next 100 years due to a projected increase in carbon dioxide, we are talking about a forced response, fully analogous to predicting the extent to which summer is different from winter on average.  The forcing in this case is through a reduction in the outgoing infrared flux rather than a redistribution of the solar flux, so the details are different.  And the time scales are different.  And –  the biggest difference of all , of course– we have experienced a lot of seasonal cycles and don’t have to rely on theories/models to tell us what the forced response is going to be. But just because we have a lot more observational input into one problem than the other does not change the fact that the two physical problems are very closely analogous.  The analogy to seasonal forecasting, whether next summer will be warmer or wetter than average, is the challenge of predicting the decadal-to-multi-decadal internal variability, generated by the oceans, that will modify the emerging forced signal.

Why is the seasonal cycle in Minneapolis temperatures so simple despite the nonlinear chaotic behavior of the weather making up these averages? Is it because the seasonal cycle is so large compared to internal variability, so that it just overpowers any attempt of the internal variability to couple with it and create more counter-intuitive behavior. 

What kind of internal dynamics might plausibly couple nonlinearly with the response to the anthropogenic carbon pulse?  The decadal to multi-decadal variability typically associated with the thermohaline circulation in the Atlantic seems too fast.  Even if one had a lower frequency candidate, my impression is that it is a lot harder to generate serious nonlinearity when internal variability interacts with pulse-like, as opposed to periodic, forcing.

At the extreme, there is the possibility that the climate system, and climate models, exhibit structural instability — that climate does not vary smoothly as parameters are varied, not just at isolated bifurcations but more generically.   See here and here and here for different perspectives on this issue.  This is not an easy topic, and one that I have a lot to learn about.  But I wouldn’t advise you to cancel your summer vacation plans just yet.

In these excerpts Issac writes

“We all successfully and continually predict the state of the atmosphere several months in advance whenever we plan our summer or winter vacations.”

Issac is incorrect here. This is not a “prediction” but is based on long-term monitoring (i.e. the climatology) of the annual cycle of weather over many years. A “prediction” is a forecast of the actual anomalies (departures) from climatology during a year (or other time period). In terms of multi-decadal climate prediction, the global models must be able to skillfully simulate not only climatology on global, regional and local scales, but also skillfully predict the changes in the climatology over time.

He also writes

“……when we talk about predicting the trend in the climate over the next 100 years due to a projected increase in carbon dioxide, we are talking about a forced response, fully analogous to predicting the extent to which summer is different from winter on average.  The forcing in this case is through a reduction in the outgoing infrared flux rather than a redistribution of the solar flux, so the details are different.  And the time scales are different.  And –  the biggest difference of all , of course– we have experienced a lot of seasonal cycles and don’t have to rely on theories/models to tell us what the forced response is going to be. But just because we have a lot more observational input into one problem than the other does not change the fact that the two physical problems are very closely analogous.”

However,

1. The forcing is different. They are not “very closely analogous“.  This is more than a “detail“.  As he writes, one is an alteration in the outgoing infrared flux and the other (the annual cycle) is from the variations during the year of the solar flux. Moreover, unlike solar forcing, feedbacks from added water vapor are a requirement to obtain the magnitude of warming predicted by the models. This is a much more complex climate effect than solar forcing since water vapor is part of the total hydrologic cycle which includes clouds and precipitation, as well as surface storage of water (in permafrost, continental glaciers, ect).

2.  The time scales are different. One is a more or less monotonic increase in anthropogenic greenhouse gases (with a small annual cycle) with a relatively small spatial variation in atmospheric concentration, while the solar flux varies very significantly over the year as a function of location of latitude.

3. We have considerable real world observational evidence on the climatology associated with the annual solar cycle, but none on added greenhouse gases over the coming decades. All of our understanding is based on unverifiable (until decades from now) model predictions.

Issac also writes

What kind of internal dynamics might plausibly couple nonlinearly with the response to the anthropogenic carbon pulse?

This is a very good question. With respect to the annual cycle, however, we have looked into this with a simple Lorenz model in our paper

Pielke, R.A. and X. Zeng, 1994: Long-term variability of climate. J. Atmos. Sci., 51, 155-159

we addressed the question

“In a nonlinear dynamical system (such as the climate system) can a known short-periodic variation lead to significant long-term variability?”

We concluded that

“…our results lead to our speculation that the seasonal cycle or other prominent periodic variations (such as the 22-year sunspot cycle) may lead to long-term variability due to internal nonlinear dynamics (especially nonlinear coupling processes).”

Thus, it is not much of an extension to conclude that any alteration of the climate can result in a significant non-linear response.

Thus, while I agree with Issac that added anthropogenic greenhouse gases will affect the climate, it is not clear that the response  will  be so close to linear as he has concluded from global climate  models. These models  have yet to skillfully predict years in advance ENSO, the PDO, the NAO ect, and, the changes in the statistics of these major atmospheric/ocean features on this time scale as a result of anthropogenic climate forcings. His weblog post Summer is warmer than winter oversimplifies the complexity of this issue.

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