Climate Changes in Iowa - Part 1

AgMRC Renewable Energy & Climate Change Newsletter
July 2011

Eugene TakleEugene S. Takle,
Professor, Department of Agronomy,
Professor, Department of Geological and Atmospheric Sciences,
Director, Climate Science Program,
Iowa State University

The following article is part one of a two part series on climate changes in Iowa.  Part two will appear in next month’s newsletter.  All references appear at the end of part two.

Climate change is much more than simply a rise in temperature. Other climate factors, specifically the frequency of extreme precipitation events, rise in humidity, and length of the growing season, are having much more impact on Iowa than changes in annual mean temperature. We know from geological records that the climate of Iowa, like other regions of the Midwest and even the entire planet, has always been changing, although at a slower rate than today. Current climate changes in Iowa are linked, in very complex and sometimes yet unknown ways, to global climate change. Independent evidence of a warming global climate comes from temperature trends at both the surface and in the upper atmosphere, trends in the melting of continental glaciers and arctic and sea ice, rising ocean temperatures, and increases in atmospheric moisture. Biological evidence consistent with climate trends points to a decline of coral resulting from warmer ocean water, earlier blooming of widely observed plants such as lilacs, altered seasonal migration patterns, and changes in plant hardiness zones (Jackson and Berendzen 2011).

A vast number of scientific studies using physically based methods for analysis and simulation of climate processes show that past climate changes over the globe have not been uniform and will not be uniform in the future. Although global averages pose overall constraints on climate changes, the regional changes that have high impact on communities and economic activities will be quite diverse.

The following paragraphs provide examples of recent trends and variability of a few climate factors in Iowa and projections of their future changes. These trends and projections are based primarily on my analyses of Iowa’s past climate, global modeling studies reported by the Intergovernmental Panel on Climate Change (IPCC 2007), and the reports of the US Climate Change Science Program (e.g., Backlund et al. 2008).

*   Reprinted with permission from Climate Change Impacts on Iowa

Changes in Storms and Precipitation

Climatological patterns of precipitation for Iowa consist of an east-west gradient, with drier conditions to the west and wetter to the east, and a somewhat wetter pattern to the south and slightly drier to the north. These continental patterns generally are created by transport of Pacific Ocean moisture from the west (including the North American Monsoon in the southwest) and Gulf of Mexico moisture in the east (including hurricane-derived precipitation in the south and southeast). Because Iowa sits in the transition zone among these competing sources of moisture, its changes in precipitation will be strongly influenced by changes in the continental-scale processes that control its moisture supply.

Figure 1. Iowa annual state-wide precipitation in inches from 1873-2008. Note that the state has had an 8% increase in annual average precipitation over this 136-year period. (Data from Iowa Climatology Bureau 2010.)

Total Annual State-Wide Average precipitation

Figure 2. Cedar Rapids precipitation records for the period 1896-2008. Over this 113-year period, this city (with its 32% increase) and eastern Iowa in general have had a steeper upward trend in precipitation than the state as a whole. (Data from Iowa Climatology Bureau 2010.)

total annual precipitation 

Precipitation in Iowa has gradually increased over the last 100 years, although year-to-year variability is high (Figure 1). Eastern Iowa (Figure 2) has a higher upward trend than the statewide average. A notable trend, however, is that there has been a change in “seasonality”: most of the precipitation increase has come in the first half of the year, and less in the second half, leading to wetter springs and drier autumns. Trends toward more precipitation and changed seasonality, as well as higher increases in eastern Iowa, are projected to continue (IPCC 2007). The central US also has been experiencing more variability of summer precipitation (Karl et al. 2008), with more intense rain events and hence more episodes of higher runoff (see Figure 3). Records for Iowa also show a higher tendency for more intense rain events, as shown in Figure 4 for Cedar Rapids, which has experienced a five-fold increase in number of years having eight or more days with daily total precipitation exceeding 1.25 inches (3.0 cm). Des Moines also has experienced an increase in days with total precipitation exceeding 1.25 inches (Figure 5).

Figure 3. Increase in very heavy precipitation in the US. Climate change has regional implications for Iowa and the Midwest. Shown here is the increase in very heavy precipitation in different regions of the US from 1958 to 2007. Very heavy precipitation is defined as the heaviest 1% of all events. (Karl et al. 2009)

percent chagne in very heavy precipitation 

Growing evidence points to stronger summer storm systems in the Midwest, although no studies have been done on recent trends in severe storms, lightning, hail, or tornadoes for Iowa, in part because trends in extreme events such as tornadoes are difficult to quantify. With summer temperatures becoming warmer (more heat energy) and humidity levels increasing (more moisture), both in summer and winter, it is plausible to speculate that this rise in the ingredients needed for severe weather will increase the likelihood of these extreme events. However, more studies are needed to explore this relationship.

Figure 4. Annual number of days when daily total precipitation exceeded 1.25 inches (3 cm) in Cedar Rapids. Note the increase in number of years having more than 8 days with daily total precipitation exceeding 1.25 inches. (Data from Iowa Climatology Bureau 2010.)

number of days with total precipitation greater than or equal to 1.25 inches 

Figure 5. Annual number of days when daily total precipitation exceeded 1.25 inches (3 cm) in Des Moines. Note the increase in number of years having more than 8 days with daily total precipitation exceeding 1.25 inches. (Data from Iowa Climatology Bureau 2010.)

number of days with more than 1.25 inches 

Pinning down causal effects for changes in storms and patterns of precipitation is more complex than tracing such causes for changes in temperature. Global climate models are not in good agreement on future trends of precipitation for Iowa and the Midwest. It is generally agreed that the western US will dry substantially in the future, and regions to the north and east of Iowa will become wetter. To our south, a drying trend is most likely.

Iowa’s future precipitation regime therefore will depend on how these dominating patterns are altered by global climate change. For instance, a strengthening of the drying pattern to the west could routinely push drier air eastward to include Iowa. However, a strengthened northward flow of moisture from the Gulf of Mexico, without an eastward shift in its path, might lead to wetter conditions in Iowa. It is plausible, therefore, that Iowa might experience a wider swing in extremes in precipitation from year to year.

Temperature Changes

Iowa’s annual average temperature has increased since 1873 at a rather modest rate (Figure 6), but seasonal and day-night changes are proportionately larger and have higher impacts. Temperatures have increased six times more in winter (0.18°F/decade) than in summer (0.03°F/decade), and nighttime temperatures have been increasing more than daytime temperatures.

Figure 6. Annual average of state-wide daily average temperatures (°F) from 1873-2008. Note the increase in the first half of the 20th century and little change since then. Seasonal and day-night changes are proportionately larger and have greater impacts – with winter and nighttime temperatures increasing more than summer and daytime temperatures. (Data from Iowa Climatology Bureau 2010.)

annual average of state-wide daily average temperatures

Since 1970, daily minimum temperatures have increased both in summer and winter (Figure 7), and while daily maximum temperatures have risen in winter, they have actually declined substantially in summer. Iowa (and the central US) has experienced a declining number of extreme high summer temperatures, a feature of summer climate that seems counter to global and continental trends. Des Moines has had only six days with a temperature of 100°F or above in the past 22 years, in contrast to the individual years of 1988, 1983, and 1977, in which 10, 13, and 8 days of temperatures 100°F or above, respectively, were recorded. A NOAA study (NRDC 2010) of the summer of 2010 has shown that none of the 23 Iowa stations in the NOAA Historical Climatology Network reported 2010 to be among the five hottest summers, but four reported 2010 having the highest average nighttime temperatures on record. And 16 stations had average nighttime temperatures among the five hottest on record.

It is important to recognize the roles that increased summer precipitation and soil moisture have had on suppressing surface heating, and thus daytime summer maximum temperatures (Figure 7d). If Iowa were to experience a severe drought, as has occurred frequently in the past, the slow and steady rise in statewide annual mean temperature, now masked in summer by moist surface conditions, could lead to an abrupt switch to extreme summer heat comparable to the summers of 1983 or 1988.

Figure 7. Iowa state-wide temperature trends over the most recent forty years for (a) winter daily minimum temperature, (b) winter daily maximum temperature, (c) summer daily minimum temperature, and (d) summer daily maximum temperature. Note the decline in maximum summer temperatures, a trend that is counter to continental trends. In recent years, increasing precipitation and soil moisture have suppressed Iowa’s summer daily-maximum temperatures, but a future drought could abruptly return extreme summer heat. (Data from Iowa Climatology Bureau 2010.)

iwoa state wide temperature trends
Iowa now has a statewide average of five more frostfree days per year than 50 years ago, and 8 to 9 more than at the beginning of the 20th century (Figure 8). This provides Iowa with a longer growing season, earlier seasonal snowmelt, and longer ice-free period on lakes and streams. The daily accumulation of Growing Degree Days (GDD) is used to estimate the seasonal physiological development of crops. (GDD is calculated by subtracting 50 from the daily average temperature in °F.) The annual number of GDDs in Iowa has varied from location to location across the state over the last 116 years, with increases in Cedar Rapids and Des Moines and a decrease in Waterloo. The most recent 40 years (Figure 9) also show site-specific trends, with slight increases in Cedar Rapids and Des Moines and slight decreases in Ottumwa and Mason City. The Iowa annual average total Heating Degree Days (HDDs), widely used as a measure of cold-season demand for space heating, has shown a significant decline since 1893 (Figure 10). (The number of HDDs is calculated by subtracting the daily average temperature from 65°F and accumulating these values (positive values only) over the heating season.)

Figure 8. Annual state-wide average of number of frost-free days from 1893-2008. The number of frostfree days has increased by 8-9 days over this 116-year period. This allows the growing season to begin earlier in spring and last later into the fall. (Data from Iowa Climatology Bureau 2010.)

annual state wide average of number of frost fee days

Figure 9. Annual average number of growing degree days calculated for (a) Cedar Rapids, (b) Ottumwa, (c) Mason City, and (d) Des Moines from 1971-2010. Rising summer temperature minimums have offset declining temperature maximums (see Fig 2-6 c and d) to produce little change in both daily mean temperature and GDD. (Data from Iowa Climatology Bureau 2010.)

annual average number of growing degree days

Figure 10. Annual total heating degree days for Ames for 1893-2008. The substantial rise in winter temperatures has significantly reduced HDDs, and with it Iowa’s space-heating requirements. (Data from Iowa Climatology Bureau 2010.)

total annual heating degree days

Global and regional climate models predict that Iowa’s annual average temperatures can be expected to continue to increase as they have in recent years (IPCC 2007). These models – which 20 years ago correctly projected polar regions to warm more than equatorial regions, minimum daily temperatures to rise more than maximum temperatures, and US winter temperatures to increase more than summer temperatures – indicate that the current trends likely will continue. Rates of temperature increases in the near future will be more like the trends of the last 30 years than those of the last 136 years. However, year-to-year variation around the averages of the last 10 years will be high, and decadal averages will not likely return to those that were recorded before 1970.