April 8, 2005

Climate Perspectives: The Great Early 20th Century Rainy Period in the American West

One of the problems in communicating climate science concerns peoples’ perceptions versus climate reality. For example, most middle-to-slightly-older-agers who grew up in the Mid-Atlantic region will tell you that it just doesn’t snow like it did in their youth (and usually they will blame global warming). Indeed, the 1960s were a very snowy decade. But somehow, we tend to view what we grew up with as “normal,” while everything different in our adult lives is “abnormal.” [Caution: this applies to more than weather and climate.]

Consider what happened in the American West in the early 20th century, when population grew roughly 50% from decade-to-decade, the largest regional growth spurt in post-colonial American history. People were lured by warm temperatures and abundant moisture. For much of that period, believe it or not, the West was a green paradise. Abundant moisture was so much du jour that allocation rights for Colorado River water, which have been contended ever since, were based upon what turns out to be the wettest period in nearly 1200 years. Had early 20th century planners had modern climatological analyses in their hands, it’s doubtful they would have been so profligate with water distribution from what really is the only big river in the Pacific Southwest.

Connie Woodhouse and three co-authors have just published an interesting paper that puts the southwestern moisture picture in long-term perspective. It contains some remarkable findings, which includes an obvious transition from domination by persistent and severe drought to relatively wet conditions some five centuries ago. Woodhouse et al. also show how remarkably wet the 1905-1917 period was.

Use this word in your next presentation: “pluvial.” That’s what she calls the period. Translation: an era when it rains cats and dogs.

Western Precipitation

Figure 1. Annual (13-yr smoothed) precipitation anomalies averaged across a nine state region (AZ, CA, CO, ID, MT, NV, NM, UT, and WY) in the western United States (adapted from Woodhouse et al., 2005).

Figure 1, a simplified version of one in Woodhouse’s paper, shows the smoothed total rainfall for nine western states (AZ, CA, CO, ID, MT, NV, NM, UT, and WY) expressed as a departure from the century-scale average. In order to filter out the substantial year-to-year variability, Woodhouse subjected the data to a 13-year running mean; the first point is actually the average for 1895-1907, the second from 1896-1908, etc…

That big spike in the beginning of the record is when, in percentage terms, the region experienced the decadal doublings of populations. In the absence of any historical climate data, it’s not hard to imaging the glowing letters being sent back east on the Santa Fe: “It’s a green paradise here. Y’all come!”

Over the much longer haul, the early 20th century pluvial is a true outlier. Figure 2 shows nearly 1,200 years of the Palmer Drought Severity Index, a pretty standard climatological metric, as calculated from regional tree ring records from moisture-sensitive trees. When the index is low (negative) conditions are dry, and vice-versa.

Western PDSI

Figure 2. The 1,200 year long reconstructed history of Palmer Drought Severity Index values for the western United States. Negative values indicate dry conditions, positive values indicate wet conditions (from Woodhouse et al., 2005).

There are several ways to interpret the data, but there is no doubt that the modern era is very wet compared to earlier centuries. Whether there’s a “trend” in the data—an overall upward march in the Palmer Index—or whether there’s a step-change switch that took place in the late 1400s, is a statistical and philosophical debate that cannot be adjudicated by these numbers; nonetheless it is clear that the West can be parched for centuries on end.

Nor does there seem to be much relationship to Northern Hemispheric temperatures.

Western PDSI

Figure 3. The nearly 2,000-yr long Northern Hemispheric temperature reconstruction from Moberg et al., 2005.

Taking a look at the long-term record recently published by Moberg et al. in Nature (Figure 3), the following rough comparisons can be made:

Period (AD) NH Temperature Western Moisture
800-1000 cool dry
1000-1100 warm dry
1200-1400 cool moderately dry
1400-1900 coolest wet
1900-2000 warm wet

Think about this the next time some talking head glibly associates western drought with global warming.


Moberg, A., et al., 2005. Highly variable Northern Hemisphere temperatures reconstructed from low- and high-resolution proxy data. Nature, 433, 613-617.

Woodhouse C.A., et al., 2005. The twentieth century pluvial in the western United States. Geophysical Research Letters, 32, L07701, doi:10.1029/2005GL02241

No Comments

No comments yet.

RSS feed for comments on this post. TrackBack URI

Sorry, the comment form is closed at this time.

Powered by WordPress