Published in Volume 98, Number 40, Page 1-5

The Cheyenne Star, June 1999

DOWNPOURS ON THE RISE  

Right Here in River City

J. Peter Thurmond and Allison L. Thurmond

Well, back by popular demand (and you know who you are), here's something new to mull over regarding the latter 20^th century climate of Roger Mills County, and its ties to global climatic trends. In September of 1995, a team of climatologists led by Thomas Karl of the National Oceanographic and Atmospheric Administration published an article in the British scientific journal Nature demonstrating that both the frequency and intensity of one-day downpours was increasing globally. Even the Wall Street Journal picked up on the article, and summarized its findings that same month. The scientists analyzed temperature and rainfall records back to 1910 from the U.S., China, Russia, and Australia. They found that average temperatures had slowly risen during the 20^th century, and that the incidence of "extreme" or heavy downpours had increased "substantially", especially in the Northern Hemisphere. An extreme downpour was defined as rainfall of two inches or more in 24 hours. Given the parallel rise in temperatures, this rainfall trend was attributed to global warming.

The Oklahoma Climatological Survey (OCS) sponsors a competition among Oklahoma primary school students each year in weather/climate related science projects. As a project topic for the 1998 OCS competition, the second author looked to see if there was local evidence in Roger Mills County of an increase in sudden downpours. She obtained a copy of the daily record of rainfall kept by the Cheyenne Cooperative Association from 1948 to 1993. The OCS installed a Mesonet remote weather station five miles southwest of Cheyenne in 1993, so she had daily rainfall records from 1993 forward as well. Ms. Thurmond looked at the incidence of rainfall in excess of 2 inches in a 24-hour period and above 3 inches in an associated 72-hour period. Her findings, updated through April 1999, are presented in the graph on page 2 and the table appended to this article. Ms. Thurmond won first place in the state with this.

The results are fairly astounding, and exceeded our expectations. The graph shows 2"+ rainfalls in 24 hours as cross-hatched bars. Any associated 72-hour total above 3" is shown as a solid bar. Each couplet of a cross-hatched bar and a solid bar represents a single event over a three day period. There were 41 such events from 1950 to 1989, averaging a remarkably consistent 10 downpours per decade, or one per year. The total rainfall received per downpour event averaged 2.75" over that four decade period. So from 1950 to 1989, we typically received one downpour of 2.75" per year.

The increase since 1990 in both frequency and intensity of downpours in Roger Mills County is fairly staggering. There were 20 downpour events from January 1990 through April 1999, averaging 4.17". That is an increase of 210% in frequency and 152% in intensity. Looking at the data in the table and graph, it is clear that the major change occurred from 1995 forward. The period 1995-1998 saw 14 downpour events averaging 4.84", a 350% increase in frequency and a 176% increase in intensity over the period 1950-1989. So we are now receiving 3-4 downpours a year, roughly four times as many as we used to, and they are nearly twice as intense on average. That seems like a pattern, does it not?

Average annual rainfall has been gradually increasing here through most of the 20^th century. Roger Mills County annual rainfall through the 1940s averaged around 20". By 1989, the average had risen above 25" (the official OCS mean annual rainfall for the county is 24.3"). Local annual rainfall has averaged 34" since 1990, and ranged from 46-52" 1995-1997, a 245% increase over the early 20^th century average. Even in 1998, which because of the hot, dry summer  we locals considered to be a drought, we ended up with 31.17". The second author's grandfather, who lived from 1872 to 1953, would have said "some drought". A drought in his time was a year you got 15" or less. As recently as 1963, the county received only 12.7". The total for 1970 was 15.37". Annual rainfall has not fallen below 20" since the 18.67" year of 1976. A "drought" since then is a year that annual rainfall drops down around 20". That used to be our annual average.

This ties in to a larger pattern, and may be a result of global warming driven by the burning of fossil fuels (oil and coal) and the resultant increase in the atmosphere of the greenhouse gas carbon dioxide (CO₂). The function of CO₂ in trapping solar energy in Earth's atmosphere is well understood. Earth is survivably warm only because of the CO₂ content of its atmosphere; we would not otherwise be here today. The surface of Venus is at oven cleaning temperature because its atmosphere contains an excess of CO₂. So we're pretty much just right.

It is known that Earth's atmosphere has been slowly cooling for 55 million years, a process related to the collision of India with Asia and the uplift of the Himalayas. It's complicated, but the erosion of the Himalayas and the deposition of the resultant sediment in the Indian Ocean has nearly halved the CO₂ content of Earth's atmosphere, and temperatures have fallen in concert. It a much cooler world than the one the dinosaurs inhabited. So, it stands to reason, if we increase the CO₂ concentration of the atmosphere, temperatures will rise. Warmer air can hold more water vapor (also a greenhouse agent), and rainfall tends to increase globally as temperatures rise. Much of the world was considerably drier on average at the height of the last glaciation, around 20,000 years ago, when temperatures were much lower than today.

But things are even more complicated. The first author and archaeological colleague Don G. Wyckoff, at the Oklahoma Museum of Natural History in Norman, Oklahoma, have been studying ancient buried soils around the county in recent years. These soils are a proxy for past climate, and a record back some 28,000 years has been documented. Think about how much time that is. Two hundred and sixty centuries, or roughly a thousand human generations. The specific objective is to study how average rainfall has changed over time. Wetter periods when vegetation was lush formed dark, organically enriched soils, and by radiocarbon dating the organic carbon in those soils, the actual age of these events can be determined. Drier periods formed humus-poor sediments that lie between and separate these soils.

A surprisingly regular 400-year cycle has been detected in past local rainfall. For at least the last 28,000 years, rainfall has cycled up and down, in an on-again/off-again manner. It's wetter for 200 years, then it's drier for 200 years. These are averages, and there has been a lot of variation, but the actual duration of each wet or dry episode has been surprisingly close to the 200-year mean. In an article in the 1998 issue of the journal Current Research in the Pleistocene, a series of dated soils were reported from sand dunes around Cheyenne dating from 26,000 to 17,200 years ago that reflect this cycle, in synch with the rise and fall of glacial lakes in eastern New Mexico and western Texas. It was much wetter on average in the south-central United States during this time, but the swings from wet to dry were severe beyond our experience. The local environment apparently gyrated from woodland to desert every 200 years, and the transitions were abrupt.

These severe climate changes appear to have continued locally until around 10,000 years ago. Climate moderated, the swings from wet to dry became much less severe, and the average was on the wet side. Evidence for the presence of people first appears in the Western Hemisphere at least 14,000 years ago. The first well-defined culture on the Southern Plains dates to around 12,000 years ago. There are many archaeological sites in Roger Mills County dating from roughly 10,000 to 7,000 years ago, indicating that our area was capable of supporting habitation at that time. From 6800 to 2300 years ago (a period climatologists and archaeologists term the Altithermal), the area turned to desert, and people all but disappeared. In the August 1999 issue of Plains Anthropologist, the evidence is summarized that the 400 year climate cycle continued to operate on the Southern Plains even during this 4500 year period of desertification, and that a singular culture called Calf Creek (after a distinctive type of spear point) flourished during one relatively rainier period from 5000 to 4800 years ago.

People returned to Roger Mills County after 2300 BP, particularly during the first millenium AD. A total of 62 radiocarbon dates has been assayed on soils from the last two millennia. Episodes of higher effective precipitation (termed pluvials by climatologists, after the Latin word for rain, pluvia) occurred locally from 50 BC to AD 100, AD 400-650, AD 775-1000, AD 1150-1300, and AD 1450-1650. The soils produced by the last four of these pluvials have been found stacked one above the other at a cutbank exposure on Brokenleg Creek, southwest of Cheyenne (see soil profile drawing at right). It appears from the accounts of the 19^th century, and 20^th century rainfall records, that a sixth pluvial began on the Southern Plains around 1900, and that we are roughly a century into a period of higher rainfall that should, if we don't muck things up, last roughly another century.  The stable carbon isotope data from the soils we have studied indicate that each pluvial since the time of Christ gradually increased in intensity, then terminated in a drought far more severe than the Dust Bowl. In other words, be real glad you won't still be here in 2100. Our descendants might want to think about moving on before that time.

A review of the paleoclimate ("paleo-" means ancient, after the classical Greek palaois) literature has revealed multiple examples of the same 400 year climate cycle from Antarctica to Tibet, the North Pacific, North and South America, Greenland, the North Atlantic, Scandinavia, and Western Europe. The periods of higher rainfall we see documented in the buried soils of Roger Mills County are contemporaneous with times of higher temperatures, particularly from the North Atlantic to Western Europe. There is an extensive literature suggesting that this is a function of a regular 400-year cycle in solar output.

So, the local soils record for the last 28,000 years, and for the last 2000 years in particular, suggests that it rains more here when the atmosphere heats up moderately. The soils record might indicate that a little atmospheric warming would not be so bad for us here in River City. It is likely that the atmospheric warming documented for most of the 20^th century is a function of the natural 400 year solar cycle. But recall that we are apparently well into a pluvial, and presumably approaching the upper range of the solar cycle-driven temperature variation of the last two millennia. Most computer models generated by climatologists predict strong warming of Earth's atmosphere above current values over the next century, and greater aridity for the U.S. Southwest and Southern Plains. The CO₂ we are pumping into Earth's atmosphere could take us back to the climate of the Altithermal of 6800-2300 years ago. Our descendants may have to move on to gentler climes well before the end of the 21^st century. Ireland, anyone?

24 Hour Rainfall Totals in Excess of 2.00"

And Associated 72 Hour Totals Above 3.00"

For Roger Mills County, Oklahoma Since 1950

Data 01/50 - 12/93 per Cheyenne Cooperative Association

From 01/94 forward per Oklahoma Climatological Survey

24 hour totals above 3.00" and 72 hour totals above 4.00" shown in bold print.