I think Revkin and Hoerling deserve a shout-out for this piece in terms of explaining how the IPCC reached their conclusion. Revkin quoted from a note from Martin Hoerling, a NOAA scientist, so I guess Dr. Hoerling has a gift for explaining things at what I consider the right level of detail (I know this is in the eye of the beholder). Here’s a link to the Revkin blog post. Below is part of the quote from Hoerling’s note.
Concerning the “debate” highlighted by the above exchanges between Pielke and Holdren, the issue isn’t about analogues to past droughts (which, by the way, the California resource managers were more interested in), but about the scientific evidence that California droughts have become more severe due to climate change.
To the extent that precipitation is key, it can be said with high confidence that there is no trend toward either wetter or drier conditions for statewide average precipitation since 1895, so that has not likely been a player. But there are other indicators, and aspects of rainfall behavior that could be conducive to drought, even if the mean seasonal rainfall isn’t changing. What is the evidence there?
The argument hinges mostly on temperature and how it may be affecting water resources. (Never mind, by the way, that the farmers and water managers are praying to the heavens for rain, not for cooler temperatures, to bust their drought!).
A way of integrating the effects of temperature on drought is to examine soil moisture time series. These have been assessed (based on simulations with sophisticated land models), the results of which are summarized by the IPCC (2012) report on extreme events (for which this drought qualifies). The Palmer Drought index and simple counts of consecutive dry days have also been diagnosed. That latest 2012 report, (the so-called SREX report) in their Table 3-2 examines the evidence for regional changes since 1950, and makes the following assessment of these various indicators for western North America:
“No overall or slight decrease in dryness since 1950; large variability; large drought of the 1930s dominate.”
The team of 42 authors assigned a “Medium Confidence” to that assessment. The report’s team in Table 3-3 then goes on to assess the scientific evidence for how drought in this region will change in the 21st century. They write:
“Inconsistent signal in consecutive dry days and soil moisture changes,” to which they assign a low confidence.
It is quite clear that the scientific evidence does not support an argument that this current California drought is appreciably, if at all, linked to human-induced climate change.
This is not to say that a warmer climate can’t and won’t act to decrease soil moisture. It simply reminds us that the current drought event, like its historical ancestors, continues to be strongly driven by the vagaries of storm tracks and the manner in which rains are delivered to the narrow stripe of the U.S. West Coast.