Analysis: Climate Changes in Iowa (August 2011)
Eugene S. Takle, [email protected]
Professor, Department of Agronomy, Department of Geological and Atmospheric Sciences,
Director, Climate Science Program, Iowa State University
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).
Changes in Humidity
The humidity level in Iowa has risen substantially in the last 35 years, as is shown by the measurements of the temperature at which condensation begins (the “dewpoint” temperature; see Figure 1). The closer the dewpoint temperature is to the air temperature, the higher the relative humidity will be. The summertime dew-point temperature increase of about 3.5°F, as measured in Des Moines in the last 35 years, means that the atmosphere has about 13% more moisture (Figure 1). Higher amounts of atmospheric moisture also provide more water to fuel the convective thunderstorms that provide abundant summer precipitation in our region. Higher winter temperatures bring higher probability of rain and lower probability of snow, which may increase the recharging of winter soil moisture.
Figure 1. Thirty-five year trends in summer dew-point temperature for three Midwestern cities. Dew-point temperature is an indication of the moisture content of the air. The rise in summertime dew-point temperature for Des Moines over this 35-year period translates into a 13% increase in atmospheric moisture near the earth’s surface. Winter dew-point temperature also has risen in Des Moines, but not as much as in summer. (Iowa Environmental Mesonet 2010)
Changes in Streamflow and Flooding
In recent years, levels of streamflow have risen in part because of changes in precipitation, leaving areas having relatively modest topographic variation prone to flooding. Iowa’s soils can absorb about 1.25 inches of precipitation in a one-day rain event, but rainfall in excess of this amount initiates runoff and increases streamflow. It is easy to understand that a 5-inch rainfall will contribute about 1 inch more water to runoff, and eventually to streamflow, than would a 4-inch rainfall. Our studies of streamflow (Jha et al. 2004) under climate change show that a predicted 21% increase in precipitation in a future scenario (the 2040s) climate leads to a 50% increase in streamflow in the Upper Mississippi River basin over a 50-year period. From the recent increase in the number of days with extreme precipitation at single stations (Figures 2 and 3), we can logically conclude that enhanced streamflow and the potential for flooding are also rising.
Figure 2. 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.)
Figure 3. 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.)
For a given amount of rainfall, summertime precipitation has a higher probability of creating flood events than does springtime precipitation. This increased probability can be explained by the seasonal change in alignment of storm tracks passing over the state. Springtime (March-May) storms tend to track SW-NE across the state, but summer storms have a more west-to-east orientation, occasionally shifting to a NW-SE track in late summer (Figure 4) (Takle 1995). Tracks of storm cells align more closely with river basins in central and eastern Iowa in summertime than they do in spring. Thus, summertime storms have the potential to dump much larger amounts of rain in a given river basin – and also have a greater probability of causing flooding. Iowa’s recent rise in the number of large summertime rainfall events (those exceeding 1.25 inches, see Figures 2 and 3) further increases the probability of summertime floods.
Figure 4. Tracks of storms (shown by black lines) producing heavy rains across Iowa for 1978, 1981-83, and 1985-87. Note that spring storm tracks (March-May) tend to be oriented from SW-NE (red arrows), while summer storm tracks (June-September) tend to be more E-W or even NW-SE (Takle 1995). Most rivers (blue arrows) in central and eastern Iowa flow from NW to SE, and their drainage basins (blue ovals) are more closely parallel to the late-summertime storm tracks. Thus, mid- to latesummer storms tend to deposit more rain in a given river basin than do springtime storms, which are perpendicular to these river basins and pass over them rapidly. (Sources: Augustine and Howard 1988, 1991)
Higher winter and spring temperatures seem to be causing earlier and more protracted snowmelt, leading to a reduced probability of spring flooding, while changes outlined in the previous paragraph have led to increased summer flooding. This seems to be the new norm for seasonal flood occurrence in the state.
Change in Soil Moisture
Soil moisture has not been widely measured over long periods of time to assess trends. However, it is clear that in Iowa, soil moisture has increased dramatically over the last 30 years (Rogovska and Cruse 2011). This is amply demonstrated by the increase in subsurface drainage tile that has been installed in recent years and continues to be installed here, to cope with increased precipitation and excessive soil moisture. It has been speculated but not definitively shown that the large amounts of subsurface water transpired to lower atmosphere by current agricultural crops are strong contributors to the recent increase in summertime atmospheric moisture and related impacts such as greater soil moisture (Changnon et al. 2003).
The IPCC (2007) report suggests that over the 21st century, there will be a weak decline in soil moisture in Iowa, primarily due to higher temperatures. However, inter-annual variation likely will be more significant than long-term trends. Iowa soils generally are deep with good water-holding capacity, so soil moisture is related to total seasonal precipitation and is less sensitive to the exact timing of precipitation than if Iowa had sandy or shallow soils. Variations in soils, slope, and subsurface geological conditions are complicating factors that also influence soil moisture variability.
Changes in Other Atmospheric Conditions
Surface wind speeds (at the standard measurement height of 32 ft) over Iowa and the US have been declining over the last 30 years (Pryor et al. 2009). Reduced surface wind speeds have a negative impact on agriculture (less ventilation of crops and hence more heat stress during intense heat, longer dew periods allowing growth of pathogens,, etc.) and human health (more intense heat waves, build-up of urban air pollutant concentrations, etc.). This trend in surface winds does not necessarily mean that wind speeds at heights of wind turbine generators (250-300 ft) will be declining, since factors such as land use have lesser impact at 250 ft than at 32 ft. Inter-annual variability of wind speed at turbine heights is much more important than long-term trends to the wind power industry.
Surface solar radiation in the Mississippi River basin has declined for the period 1948-2004, due primarily to increased cloud cover (Qian et al. 2007). These authors use a model to estimate hydrological properties not routinely measured, and they find that evapotranspiration has been increasing. Simulations of a future climate based on increased greenhouse gas concentrations (Pan et al. 2004) show that the decline in surface solar radiation is likely to continue.
Ozone in the lowest five miles of the atmosphere (troposphere) is produced by both natural and anthropogenic causes (reactions of emitted hydrocarbons and oxides of nitrogen under ultraviolet light). Being distinctly different in concentration from stratospheric ozone (which is produced naturally and destroyed by human emissions of long-lived chlorine-containing compounds), tropospheric ozone has a negative impact on agriculture and human health (Rogovska and Cruse 2011, Osterberg and Thorne 2011). Annual increases in temperature – and population growth – suggest that there will be future increases in anthropogenic production of tropospheric ozone. Agricultural crops, particularly soybeans, as well as horticultural crops are negatively affected by tropospheric ozone.
Research Gaps
Of all natural hazards, floods, water-logged soils, and droughts have the highest impact on Iowa’s economy. More research is needed to better understand why Iowa’s precipitation extremes are increasing and whether these increases will continue. Improvements in seasonal climate predictions would enable Iowa decision-makers to better prepare for these extremes and to reduce their economic impact when they do occur.
Conclusions
Many changes of Iowa’s climate over the last 30 years have had major economic impacts, some positive and some negative. Most notable are the annual rise in temperature, increase in extreme precipitation, and increase in humidity. The first climate projection for Iowa based on a global climate model was published nearly 20 years ago (Takle and Zhong 1992). This model correctly projected most, but not all, climate changes in the intervening two decades and demonstrated the utility of climate models for adapting to the major impacts of climate change.
The Iowa legislature has taken the first step in assessing climate changes in Iowa. Other states have demonstrated the usefulness of factoring climate change into long-term decisions on infrastructure and resource management. Iowa’s universities have the research capacity to provide guidance to urban, regional, and state agencies and to the private sector. Iowa legislators and administrators who work with the state’s researchers will be better able to address climate-change trends and their impacts on the economy of the state and welfare of its people.
References Cited
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