Imagining the future when the unimaginable already has happened

Guest Column | Gene Takle

The Cedar River at the Highway 151 bridge east of the city reached a discharge peak of 140,000 cubic feet per second on June 13, 2008, almost twice the prior record set in 1961. Does the adage that “if it happened once it could happen again” apply for such a flood? Before the event, could this have been classed as unimaginable, a so-called “black swan” event? A quick look at the historical record of rainfall for the Cedar River Basin, recent trends, projected future scenarios and the science of extreme events will put the June 13, 2008 into context. The science of climate change has advanced substantially since this flood, as has public determination to learn from the event, improve mitigation where possible, and respond more effectively if such events happen again.

In the last 30 years (1991-2020) Iowa’s climate has undergone notable changes, the two most impactful being temperature increases in the cold season, which have extended the growing season, and a 21 percent increase in rainfall in the April-May-June period, compared to the previous three 30-year periods (1901-1990), which has improved crop growing conditions but also exacerbated flooding. Winter precipitation also has increased, with the winter season ending February 2008 being the 6th snowiest in the 115-year history at that time with approximately 50 inches or twice the normal average. Snowmelt in March was followed by April rains that dumped an excess over normal of three to seven inches of rain over large parts of the basin saturated soils and sent river levels to record or near-record highs.

With the basin as a whole being awash, the Cedar Rapids city capstone rain events of 1.4 inches on June 6, followed by 2.4 inches on June 8 and 1.7 inches on June 12 sent the Cedar River discharge rate on June 13 at the Highway 151 bridge to a record 140,000 cubic feet per second. This was nearly twice the previous extreme flow of the location’s streamflow measurements dating back to 1903.

In retrospect we can make the following assessments:

• The climatological increase in precipitation annually, in winter, and particularly in the April-May-June period over Iowa and particularly the Cedar River Basin have been part of a decadelong trend that has continued through 2020.

• The unique timing, magnitude and spatial distribution of hydrometeorological extremes (precipitation, soil water, runoff, subsurface drainage tile flow, streamflow, stream stage) produced a multibillion dollar disaster that, with current techniques at the time, was impossible to predict.

Since 2008 a wealth of scientific research has given us new insight on current trends and future changes in Iowa and Central North America. For example, it is well known from multiple studies that:

• Iowa’s longer growing season, rise in average annual rainfall, and increase in extreme high rainfall events are part of regional trends that are related to global climate change resulting from increases in atmospheric greenhouse gases.

• The notable rise in Iowa’s average April-May-June rainfall, a key underlying factor in the Cedar Rapids flood of 2008, is caused by a warmer Gulf of Mexico and other well-known atmospheric process that are intensified by a warmer climate.

• Annual rainfall in the basin is projected to increase over the Cedar River Basin during 21st century, but at the same time new extremes (both high and low) of precipitation will be likely.

A joint study conducted by Iowa State University, the Iowa Flood Center, and Iowa Department of Transportation, specifically for the Cedar River Basin using 1960-2013 data, found that average rainfall in the basin is projected to increase during 21st century, and new extremes (both high and low) of precipitation will be likely. The 100-year flood event in the Cedar River Basin of the 20th century is projected to be a 25-year flood event in the 21st century.

According to a NOAA summary report on the Iowa 2008 floods, the accumulated statewide damage was estimated to be about $10 billion. The NOAA website on billion-dollar weather disasters shows that the number of $1 billion-plus disasters in Iowa has more than tripled (from 14 to 50) and total cost of billion-dollar-plus disasters has more than doubled in the last 21 years compared to the previous 21 years. Fischer et al. (2021), writing in Nature, reported data calling attention to the global increased probability of observed “record-shattering“ climate extremes.

They show that record-shattering events are more likely if both the warming and rate of warming of a region are increasing. Their analysis focused on extreme temperatures, but their results raise questions about such possibilities for precipitation and weather disasters more generally and should be acknowledged in future weather-related risk assessment. By adopting the line or reasoning used by Fischer et al. we might consider the 2008 Iowa flood event to be at least a “gray swan.”

Gene Takle is an emeritus professor of agronomy at Iowa State University.