On Friday, Kevin Drum put out a post Our Coming Mega-Drought, which briefly commented on a recent scientific review by Aiguo Dai, Drought Under Global Warming: A review. The paper really caught my attention and I read it several times this weekend, as well as reading various references and background materials. This post, as well as a follow-on tomorrow, summarizes my understanding at this point, as well as raising a few issues that I don't understand. I make my standard disclaimer: I am not a climate scientist and may get a few things wrong. Given my alarm level after reading this, I will probably do a bunch more research on the issues in coming days and weeks and develop a better understanding.
To begin to understand this paper, we need some background on something called the Palmer Drought Severity Index. This is an old idea from the 1960s:
The Palmer Index was developed by Wayne Palmer in the 1960s and uses temperature and rainfall information in a formula to determine dryness. It has become the semi-official drought index.So, in particular, the definition of this index predates any modern understanding of global warming (eg the 1975 Broecker paper that coined the term). The full details are available in Palmer's original paper, but for our purposes, it's enough to know that the index is computed based on knowing only meteorological variables (temperature, precipitation, and in more modern variants, things like wind speed). The goal is essentially to estimate both the supply of water to the soil via rainfall, and then the amount being withdrawn via evapotranspiration, and look for places were there is an accumulated deficit in available water. The scale was designed as follows:
The Palmer Index is most effective in determining long term drought—a matter of several months—and is not as good with short-term forecasts (a matter of weeks). It uses a 0 as normal, and drought is shown in terms of minus numbers; for example, minus 2 is moderate drought, minus 3 is severe drought, and minus 4 is extreme drought....
The Palmer Index can also reflect excess rain using a corresponding level reflected by plus figures; i.e., 0 is normal, plus 2 is moderate rainfall, etc...
The advantage of the Palmer Index is that it is standardized to local climate, so it can be applied to any part of the country to demonstrate relative drought or rainfall conditions. The negative is that it is not as good for short term forecasts, and is not particularly useful in calculating supplies of water locked up in snow, so it works best east of the Continental Divide.
latest map of the PDSI for the United States, from NOAA:
You can see that the upper midwest and plains states are pretty wet with values of +2 to +4 or above, and the south east is in rather a drought with a lot of values below -2. It's important to notice that the PDSI is an index relative to normal conditions. It's not the case that Louisiana is drier than Arizona right now, for example, but Louisiana is drier relative to normal Louisiana conditions than Arizona is relative to normal Arizona conditions.
It's well worth your while to click here to watch an animation of the development of the PDSI over the last 12 months, and in particular, the south-east drought starting to develop from July on. This drought has been making headlines. For example, the Wall St Journal reports:
An extreme drought has taken hold in parts of nine states stretching from the Southeast to the lower Midwest, damaging crops, driving up the cost of keeping livestock and putting officials on alert for wildfires.So that's how a PDSI of -2 to -4 will get written up in the WSJ.
Climatologists say the dry weather likely will continue at least until spring, raising the possibility of prolonged drought in some areas next summer.
"Six months from now, we could be in a fairly significant drought situation throughout the Southeast," said Brian Fuchs, a climatologist at the National Drought Mitigation Center, a federally funded center at the University of Nebraska that monitors drought conditions across the U.S. "The general pattern is going to show worsening."
Parts of Alabama, Louisiana, Arkansas, Mississippi, Tennessee, Missouri, Kentucky, Florida and southern Indiana are suffering "extreme drought," according to data expected to be released by the center Thursday.
That means the areas are experiencing the worst prolonged shortage of rain expected in a 25-year period. Montgomery, Ala., has seen just half an inch of rain so far this month. In Memphis, Tenn., the total is 0.01 inch.
Northeast of Memphis in Mason, Tenn., Marvene Twisdale and her husband have enough hay and water for now to feed their 10 cows. But several ponds have dried up on the farm. Ms. Twisdale, 67 years old, said the drought was the worst she has seen in 47 years there.
"I go outside every day, water my flowers and pray over them," she said. "We are waiting for rain to come."
To give you some feeling for what a really nasty drought looks like, here is the PDSI map for 1934, the worst year of the dustbowl:
This caused almost total crop failures, people dying left and right, and large scale human flight from the worst affected areas. So -4 is really bad, and -6 is pretty much old-testament-biblical.
So, now that we have a bit of a feel for the scale of PDSI, let's turn to the Dai paper. Firstly, a quick sanity check on the author reveals a twenty year history of widely cited climate papers, including a number specifically on precipitation, drought, and the PDSI, including developing a global dataset for PDSI from 1900 on. He works as a scientist at the National Center for Atmospheric Research. So an expert in this field, and not a crank.
The paper contains two main sections. The first is an analysis of the historical record, to which we'll turn tomorrow. The second is the really scary part, and describes an analysis in which the PDSI was computed for all 22 computer models that were incorporated into the IPCC AR4 report. Specifically, these were the model runs for the A1B scenario, a fairly fossil-fuel intensive future (which seems to be what humanity is doing). The takeaway is in Figure 11. It's a complex figure, so I'm going to take it in pieces. Firstly, the color key is as follows:
So greens and blues are wet, yellow and pale orange are moderate droughts, and the pink/red colors are extreme droughts. Note that we go up into violet colors that go down to -20 - completely off the historical scale for the PDSI. Next, we get a series of six panels showing the global PDSI at different times. And again, this is an average of 22 climate models, not observations. Here's what the models show back in the 1950s:
So situation fairly normal - some places (eg the south east US, the Ukraine) were in drought, and other places were wet. But the worst case averages for a whole decade were around a PDSI of -1. Now here is 2030-2039:
- This is PDSI's of -3 or worse across much of the world's populated regions. Things are very bad in the US, South America, China, and Australia, and positively biblical around the Mediterranean.
- This is 2030-2039. This is not your grandkid's world, or even your kid's world for a lot of us: if you are under 50 you will likely live to see these years.
- We are not talking 5% tail risks here. This is the "ensemble mean" - the average prediction of climate models.
So now it's extra-biblical across most of the populated world. The US, most of Europe, China, Australia are all in permanent dustbowl conditions. Needless to say, by the 2090 panel (grandkid's world), it's even worse. Basically, if these projections are correct, this is We're All Gonna Die territory. Global food production under these circumstances is going to go in the tank, and there's no way we'll be feeding nine or ten billion people.
So, can this possibly be right? I don't know, but I'll take up some of the considerations in a post tomorrow.
Note: This post is part of the Future of Drought Series on Early Warning.