KS: Climatology uses what are called General Circulation Models to project temperatures, based on a complex valuation and manipulation of quite a few mathematical variables drawn from observations. These are run on supercomputers, show internal consistency and appear to be able to forecast broad trends in global temperature readings fairly accurately. True or False?

KR: This appears to be true. We have not had the luxury of being able to run a model and then wait several decades for the results to verify. So we are forced to somewhat guess how good these forecasts might be, and part of that confidence stems from the fact that the projections are all very similar to each other. So either they have a good chance of being right, or they are all wrong for similar reasons, which at this point seems unlikely.

KS: However, "downscaling" these same models to smaller regions -- such as Southern California -- produces results that aren't as reliable. Why is this?

KR: As one very well grounded forecaster that I know put it: "there is no substitute for an actual forecast." There is in general somewhat less confidence in downscaled information, in part because we have not had a chance to watch an actual forecast verify, which it does in slow motion.

KS: A study (pdf) by a climatologist named Anthony Westerling for the California Climate Change Center, trying to get a handle on the possibility that climate change could mean a greater risk of wildfire in California, uses two different climate models to estimate (speaking in broad terms) changes in temperature and precipitation for California. He ran each model in two different modes: first, a "business-as-usual" mode, in which greenhouse gas emissions continued to rocket upward, and second, a mode in which they were moderated.

Westerling reported, "...within California, the [first model's] A2 and B1 scenarios produced wetter conditions over southern California and drier conditions over northern California when compared with the 1961-1990 reference period, while the [second model's] A2 and B1 scenarios produced drier conditions in both the north and the south of the state."

Of course different climate models are going to produce different results, but is it common to have results that differ so dramatically?

KR: This is not uncommon. I'm neither terribly surprised or too much concerned, and I read no cosmic significance into such differences.

I think this more or less reflects the state of climate modeling. I have seen other studies that compare many more models (done for the same IPCC Fourth Assessment, now in the works), about a dozen, and the consensus is trending toward less precip along the US-Mexico border (5-10 percent less than present value), more precip (maybe 10 percent more than now) along the US-Canada border, and a dividing line roughly from northern California eastward across to the Mississippi.

Within California, there is a lot of scatter in these projections, mostly within 10-30 percent of present values, on both plus and minus sides.

A number of studies have shown that consensus projections obtained from collections of models, with their usual similarities but with many different formulations of details, provide better and more accurate information than any of their individual constituent inputs. This applies to groups of human forecasters as well as to models.

Consensus is the hardest forecast to beat.

Furthermore, precip is much harder to forecast, and forecast well, than is temperature. This is definitely true on the daily weather scale, and there are lots of reasons to believe it holds up on the annual and decadal scale too.

Precipitation is a hard beast to forecast, because it is actually a two-part forecast: First one has to forecast whether a precipitation situation will occur or not, and secondly, one has to then forecast how much will fall, given that precipitation is expected to occur.

Another way of putting this is that it is not just the total annual precipitation, but rather the character of this precipitation (frequency, seasonality, intensity, temperature, etc) and its timing and type (rain/snow).

Also quite important for fire and many other things is the seasonal distribution of precipitation. The annual amount could remain the same, but if it is distributed differently within the year, that could help or hurt fire. The general trend of many of these models is toward increased winter (late winter, early spring) precip, and toward decreased warm season precip. This probably hurts from fire standpoint, with a winter of good growing conditions, and a summer to dry out. Wet winters often help suppress summer fires by giving trees a good moisture supply to venture out into summer, but dry winters set up a long, dry, vulnerable, situation for trees. If summer is really long, the effects of a wet winter can be "forgotten."

KS: Westerling goes on to talk about "considerable uncertainty assessing climate change impacts in Southern California." Must we then conclude that this sort of study simply isn't going to help us in Southern California for some time?

KR: I think the precipitation picture is going to take its sweet time becoming more clear. There are other things we are more certain about, like temperature, which by just about any measure of uncertainty seems very likely to go up. This helps raise evaporative demand and to dry things out during the warm part of the year.

But, another fly in the ointment, the coast is special, climate-wise, and things happen here, or near here, that do not happen just a short distance inland. Models are not so great at simulating small scale effects like upwelling over periods of years to decades, and have a very tough time with marine stratocumulus and fog, important ingredients to coastal climate. These have proven to be very difficult problems to address in the rather coarse climate models widely in use. It's really important that we get these to work correctly.

Ventura County [and much of Southern California] has its climatic future staked in many ways on what the near-shore marine environment decides to do.

KS: A report for the California Climate Change Center suggested that the state will see substantially more extreme heat days this century, as much as 80 in a medium emissions scenario in the latter quarter of the century. Is it misleading to look that far into the future? Or is this something that should concern us now?

KR: We've been talking with a lot of people about this issue, particularly with respect to water resources. I think that most of our attention should be on the more foreseeable future, within living people's lifetimes, approximately the next 10-40 years or so. Otherwise, it's all too abstract, and is somebody else's problem down the generational road.

Plus, two things: we are going to learn a lot more in the next 5, 10, 15, 20 years, and the projections will clarify significantly; and, society will re-think these issues several times over, and consensus opinion will evolve toward some new ideas.

The modelers like to show these far-off projections because the signals are stronger. True, but people want to know more about the immediate future than the far future, so this is like answering questions people are not asking, and not answering questions people are asking (what's in the near-term future?).

Tempering the previous statement, though, is the unsettling concern that these genies cannot be called back into the bottle, and there is some element of multi-century commitment, so why do we want to rush so recklessly into the future, when a more deliberate path might be the prudent route that engenders less potential risk, and offers greater allowance for imperfections in our present knowledge of the future.

KS: When I talked to you a couple of years ago, you spent a lot of time explaining some basic facts of climatology to me (for example, why it's so much easier to accurately project temperature than precipitation). When I thanked you, you indicated that you felt it was your responsibility to get the word out, because you said climatology was not at all well understood. Is that still true?

KR: It's slowly improving. But there still remain the typical number of misconceptions. I think this is true of most technical subjects, not sure that climate is "picked on" or any worse than others.

KS: If so, and if you could wave a magic wand and make Americans in general understand one central fact about climatology, what would it be?

KR: That the way that climate unfolds is always in accord with physical principles, on the one hand, and that on the other hand the number of processes that are interacting is such that an incredible variety of possibilities are consistent with the aforementioned constraint, so that there are inherent limits to how well it can be predicted.

KS: Bill Patzert, who studies these issues for the Jet Propulsion Lab in Pasadena, points out that California is considerably warmer than it was a century ago -- 5F -- but that at least some of this warming is due to our reconstruction of the landscape, with buildings, roads, parking lots, and so on, reflecting more heat. Could this be misleading us about atmospheric warming?

KR: In places that is true, but I don't think it can explain all of whatever warming is seen. The impervious area of the United States is about the size of the state of Ohio, if we consolidated all our pavement and buildings.

KS: Many researchers prefer the term "climate change" to "global warming," because they know that the globe will not warm uniformly. In fact, when I discussed this with you last time, you said that the consensus projections for the East Coast showed a slight winter cooling of about 2F in the not-so-distant future. Is this still projected?

KR: And, because warming puts the emphasis on temperature, and climate has other things such as precipitation, wind, humidity, evaporation, snowfall, and so forth, and these things matter as well, and can change too.

The model consensus is for a little greater warming in the West than in the East, and on the order of 1-2 degrees F in the next 30-40 years. The actual difference in warming between the eastern and western U.S. (right now much more in the West) is quite a bit greater than the east-west difference in the models.

KS: Can you talk a little about consensus projections for climate change in broad regions of the United States? For example, when I talked to you last time, you expected that the Southwest might face an increased likelihood of drought. Is that still true?

KR: I'd say so. Consensus is a little stronger than before for less annual precipitation. There are suggestions that the jet stream has shifted poleward about a degree of latitude over the last 30 years, according to a study at the University of Washington. Also, warmer temperatures help make precipitation less effective, making climate more "droughtlike."

KS: We've seen a lot of atmospheric warming in the Arctic. Where else have we seen it?

KR: In western North America, central Asia, and in the world's oceans.

KS: A "climatology," I am told, is thirty years worth of weather. This explains why single-year events, such as the big storms California experienced in the winter of 2005, and the big storms the Mid-Atlantic region experienced this spring, don't change or even influence climatological forecasts. But do these events fit the description of an "intensified hydrological cycle" often foreseen in our climate in a global warming scenario?

KR: They could be. Since there are multiple possible sources, I think the safer terminology is "not inconsistent with."

KS: In the scientific community, but less so in the press, there has been a good deal of discussion of the potential for abrupt climate change, which is often linked to the possibility of shifts in ocean currents -- shutdowns or even reversals of major currents, such as the California Current. Are these oceanic shifts really possible?

KR: They don't seem to have very high probabilities within the next 50-100 years (but do not have zero probability). The California Current is driven by the winds around the subtropical high pressure system to the West out over the Pacific, and a shutdown of this seems unlikely. The Gulf Stream is another major concern, because a shutdown would have drastic effects on Atlantic climate. One main concern about abrupt change has more to do with the potential beginning of disintegration of the large Greenland and Antarctic ice caps.

KS: Alarming studies that suggest the possibility of big changes, such as recent studies on the possibility of methane releases from permafrost, and levels of CO2 in the atmosphere not found for 800,000 years, receive a lot of publicity ...

KR: We have to keep a close eye on this.

KS: Yet there are dozens of other less-dramatic studies that don't get worldwide publicity. Are worst-case possibilities for our climate in the future distracting us from small-but-real shifts in our latitudes right now? Or do you think they deserve the attention the media gives them, as "wake-up calls"?

KR: We have a range of projections, from high probability / low consequence change, on up to low probability / high consequence change. We have to average out over all these possibilities.

I feel that we should focus mostly on the most likely shifts (as well as we can assess this), within a certain range, modest but not negligible, but because the consequences are so severe from the less likely but larger changes, we have to keep this in our background mental imagery.