The last several years have left California facing a series of water emergencies, as the usual winter rainfall hasn't materialized. The drought has been associated with a ridge of high-pressure air off the Pacific Northwest coast, which prevents storms from the Western Pacific and Alaska from reaching California. That ridge, in turn, has been associated with warm sea surface temperatures in the area.

Beyond the immediate causes, however, it's reasonable to ask whether the drought is a symptom of a warming climate, and thus whether we should expect more of them in the future. Several papers have already looked into the matter, with mixed results. But now, NOAA has weighed in with a report that pins the blame on natural variability. But the report has come under criticism from some scientists, and it may have been finalized before some recent, relevant papers.

There are a number of ways to ask whether a particular event (or, in this case, a series of events) is natural or driven by human influences. Events should not be viewed as conclusive on their own, but collectively, they can build a case. The NOAA report tries a number of these.

One of the simplest is to ask whether something like this has happened in periods before humanity's influence on the climate was as large as it is presently. The NOAA report's authors do this and conclude that it has. Although it's been excessively dry for several years, "these events are not without precedence," they write, highlighting other dry periods on the record. They also note that, when the 120-year historic precipitation record is considered, there's no trend toward drier conditions.

That would seem to suggest that natural variability could be a sufficient explanation for the recent drought. But a separate study that has been posted in advance of publication has looked at the same problem using tree ring data. It finds that, while three-year droughts are common in the record, the current drought has put trees under water stress that they've never experienced in the last 1,200 years. So, the drought may be historically unprecedented after all.

But the majority of the NOAA report focuses on the use of climate models to explore the conditions that produced this drought. As in previous studies, the models suggest that the lack of rainfall can be traced back to warm surface waters in the Eastern Pacific, which set up the high-pressure ridge that blocks California from the source of its rain systems. This accounts for a bit less than half of the tendency to drought, however; the rest they ascribe to natural variability in atmospheric conditions.

A key question is whether this variability—or at least three consecutive years of it—is normal. It's a bit hard to say, given that the models that were used did not fully couple the atmosphere and the oceans, such that sea surface temperatures couldn't respond to atmospheric conditions. Things are also complicated by the fact that the sea surface temperatures in each of the years look somewhat different in terms of their location and extent, which makes it hard to ascribe them all to a single underlying cause.

What finally seems to have pushed the authors over the edge on their conclusion that the drought is natural is the fact that models don't predict that warming temperatures will produce the pattern of rainfall seen during these drought years. They do predict a dry spring, but the warm sea surface should also produce an early winter burst of rain that partly offsets that. Since that burst never materialized, the authors conclude that these winters haven't been the product of warming.

That conclusion, however, has been criticized by other scientists (a typical example of the criticism comes from Michael Mann, who elaborated on his criticism in a blog post). California, they point out, has been suffering through unusually warm weather during the drought, which has accelerated evaporation. The full impact of the drought, they contend, can't be captured simply by looking at the lack of rain.

In the end, the NOAA report leaves things almost exactly where they were when we looked at the topic last. Some of the conditions that contributed to the drought are what you might expect to see in a warming world—warmer sea surface temperatures, hotter atmospheric temperatures at the site of the drought—but models don't predict a simple fall off in the amount of rain California receives. Until we can get into the causes that are driving the sea surface changes in more detail, we're not likely to be able to fully ascribe the drought to either natural or human causes.

A copy of the NOAA report has been posted online.