Observed climate change in Tennessee

By | February 2, 2008

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There is no observational evidence of unusual long-term climate changes in Tennessee. No emissions reductions by Tennessee will have any detectable regional or global effect whatsoever on climate change.

>Annual temperature: Long-term temperature records began in Tennessee in 1895. Over the entire record, there is no statistically significant trend. Instead, the record is dominated by annual and decadal-scale variability. Temperatures during the past decade or so have been similar to those of 1920-1940. They follow 40 years from the late 1950s through the mid-1990s when statewide average temperatures were cooler than average. Therefore there is nothing unusual about recent temperatures when set against the state’s long-term climate history:

>Tennessee annual temperatures, 1895-2007

>Annual mean temperatures and secular trend

>Figure 1. Annual statewide average temperature history for Tennessee, 1895-2007. Source: US National Climatic Data Center:


>Seasonal temperatures: Likewise, there are no long-term seasonal trends. Instead, year-to-year and/or decade-to-decade variability is most evident. In no season do recent temperatures appear unusual compared with observed temperature history. There is no evidence of “climate change”:

Tennessee seasonal temperatures, 1895-2007

>Seasonal mean temperatures and secular trends

Figure 2. Seasonal mean temperatures in Tennessee, 1895-2007. Source: US National Climatic Data Center:

>Precipitation: There is also no statistically-significant trend and, again, the record is dominated by large interannual variations—ranging from as much as 66.68 inches of rain in 1979 to as little as 35.67 inches in 1941. Certainly, 2007 stands out as a very dry year, but there is no drying trend: 2007 merely reflects natural variability:

>Tennessee annual precipitation, 1885-2007

>Total annual precipitation and secular trend

Figure 3. Total annual precipitation in Tennessee. Source: National Climatic Data Center:

>Drought: Since 1895, there has been no long-term trend of drought in Tennessee. Instead, annual and decadal variability prevail:

>Tennessee drought severity, 1885-2007

>Palmer drought-severity index

Figure 4. Monthly mean Palmer Drought Severity Index (PDSI) in Tennessee, 1895-2007.

>Source: National Climate Data Center, www.ncdc.noaa.gov.

>Monthly mean Palmer Drought Severity Index values—a standard measure of moisture conditions that reconciles inputs from precipitation and losses from evaporation—show no trend during the past 113 years. The period of record is dominated by short-term variations, although there was a drought in the mid-1950s and a period of high rainfall in the mid-1970s. There were more droughts before 1950 than after.

>Tornadoes: Tennessee lies close to “tornado alley” and experiences some tornadoes. However, an apparent increase is probably an artifact of better technology, a larger organized network of tornado-spotters, and a growing population:

> Figure 5. Tornado activity in the United States per 1,000 square miles. Source: National Climatic Data Center, http://lwf.ncdc.noaa.gov/oa/climate/severeweather/tornadoes.html.

>During the super-tornado outbreak of April 3-4, 1974—the worst tornado outbreak in U.S. history—at least 28 tornadoes passed through Tennessee, leaving 50 people dead, 635 injured and $30 million in damages. It has been suggested that the frequency of extreme weather events such as tornadoes will increase as a result of “global warming”.

>However, in Tennessee, as in the United States as a whole, the recent increase in tornado observations can be explained by non-climate factors. Small tornadoes that were once missed are now being detected by Doppler radar and the larger network of observers. A report by the Nashville Tennessee Office of the National Weather Service found:

> “Population growth and warning coordination and awareness efforts have dramatically increased the number of documented tornadoes—especially weak tornadoes—in recent years, while simultaneously lowering the number of tornado-related fatalities.”

>The number of major tornadoes across Tennessee—those less likely to have passed unnoticed owing to the damage they cause—shows no long-term increase, further indicating that the apparent increase in the occurrence of all tornadoes is probably a reflection of our growing ability to detect them.

> Figure 6. Top: Total number of tornadoes observed in Tennessee, 1950-2007. Middle: annual number of major (F3-F5) tornadoes observed. Bottom: annual number of fatalities resulting from tornadoes. Source: NOAA Storm Prediction Center: http://www.spc.noaa.gov/climo/historical.html)

>Hurricanes: Hurricanes play a largely beneficial part in Tennessee’s summer climate. The remnants of tropical cyclones that pass near Tennessee have usually lost the ferocity of their winds and deliver late-summer rainfall when it is most needed:

> Figure 7. Proportion of total June-November rainfall from tropical systems. Source: Knight et al., 2007.

> When a strong tropical cyclone strikes the U.S. coastline, the result can be devastating. However, further inland, the remnants of tropical systems produce good rainfall which is often helpful to agriculture, forestry, and other water interests in the state. Recent research shows that Tennessee, on average, receives 4-8% of its total June-November rainfall, and 6-15% of its normal September rainfall, from tropical storms.

>Heatwaves: Heatwaves affect people less than before, thanks to air-conditioning and social programs to protect high-risk individuals, despite rising urban temperatures:

> Figure 8. Annual heat-related excess deaths per million. Bars (left to right) indicate

>1970s, 1980s, 1990s. Source: Davis et al., 2003b.

> Several studies (e.g. Davis et al., 2003ab) show that U.S. urban populations are better adapted to heatwaves than formerly. In Figure 8, each bar represents the annual number of heat-related deaths in 28 major U.S. cities. For nearly all cities, heat-related deaths are declining. Though no cities from Tennessee were included in the studies by Davis et al., in neighboring cities the number of heat-related deaths in the 1990s was negligible. This successful adaptation is the result of improvements in medical technology, air-conditioning, better public awareness, and proactive responses by municipalities to extreme weather events.

>In the southern states, where heatwaves are more common, heat-related mortality is much lower than in regions where heatwaves are rarer, such as the north-eastern U.S – further evidence that populations adapt to their prevailing climate. If heatwaves become more common, adaptations will occur without difficulty.

>Vector-borne diseases: Malaria, dengue fever, and West Nile Virus, which have been erroneously predicted to spread owing to “global warming”, are not tropical diseases. Climate change will accordingly have a negligible effect on their transmission rates. These diseases are readily controlled by well-known public health policies.

>Malaria epidemics occurred as far north as Archangel, Russia, in the 1920s, and in the Netherlands. Malaria was common in most of the United States until the 1950s (Reiter, 1996). In the late 1800s, when the United States was colder than today, malaria was endemic east of the Rocky Mountains—a region stretching from the Gulf Coast all the way up into northern Minnesota, including the non-mountainous counties of Tennessee.

>In 1878, 100,000 Americans were infected with malaria, and some 25,000 died. Malaria was eradicated from the United States in the 1950s not because of climate change (it was warmer in the 1950s than in the 1880s), but because of technological advances. Air-conditioning, the use of screen doors and windows, and the elimination of urban overpopulation brought about by the development of suburbs and automobile commuting were largely responsible for the decline in malaria (Reiter, 1996).

> Figure 9. In the late 19th century malaria was endemic in shaded regions, including

>the western two-thirds of Tennessee. Source: Reiter, 2001.

>The effect of technology is also clear from statistics on dengue fever outbreaks, another mosquito-borne disease. In 1995, a dengue pandemic hit the Caribbean and Mexico. More than 2,000 cases were reported in the Mexican border town of Reynosa. But in the town of Hidalgo, Texas, located just across the river, there were only seven reported cases (Reiter, 1996). This is just not an isolated example. Data collected over the past decade have shown a similarly large disparity between incidence of disease in northern Mexico and in the southwestern United States, though there is very little climate difference.

Another “tropical” disease that is often wrongly linked to climate change is the West Nile Virus. The claim is often made that a warming climate is allowing the mosquitoes that carry West Nile Virus to spread into Tennessee. This reasoning is incorrect. West Nile Virus, a mosquito-borne infection, was introduced to the United States through the port of New York in summer 1999. Since its introduction, it has spread rapidly, reaching the West Coast by 2002. Incidence has now been documented in every state as well as most provinces of Canada. This is not a sign that the U.S. and Canada are progressively warming. Rather, it is a sign that the existing environment is primed for the virus. In the infected territories, mean temperature has a range more than 40ºF. The virus can thrive from the tropics to the tundra of the Arctic – anywhere with a resident mosquito population. The already-resident mosquito populations of Tennessee are appropriate hosts for the West Nile virus—as they are in every other state.

>Since there are no climate trends in Tennessee, climate changes cannot have been responsible for the establishment of West Nile Virus. Mean annual temperature in Tennessee could change by many degrees in either direction, or the precipitation regime could vastly change, without affecting distribution of the West Nile Virus.

>Impacts of climate-mitigation measures in Tennessee

>Globally, in 2003, humankind emitted 25,780 million metric tons of carbon dioxide (mmtCO2: EIA, 2007a), of which Tennessee accounted for 120.1 mmtCO2, or only 0.47% (EIA, 2007b). The proportion of manmade CO2 emissions from Tennessee will decrease over the 21st century as the rapid demand for power in developing countries such as China and India outpaces the growth of Tennessee’s CO2 emissions (EIA, 2007b).

>During the past 5 years, global emissions of CO2 from human activity have increased at an average rate of 3.5%/yr (EIA, 2007a), meaning that the annual increase of anthropogenic global CO2 emissions is more than 7 times greater than Tennessee’s total emissions. Even a complete cessation of all CO2 emissions in Tennessee will be undetectable globally. A fortiori, regulations prescribing a reduction, rather than a complete cessation, of Tennessee’s CO2 emissions will have no effect on global climate.

>Wigley (1998) examined the climate impact of adherence to the emissions controls agreed under the Kyoto Protocol by participating nations, and found that, if all developed countries meet their commitments in 2010 and maintain them through 2100, with a mid-range sensitivity of surface temperature to changes in CO2, the amount of warming “saved” by the Kyoto Protocol would be 0.07°C by 2050 and 0.15°C by 2100. The global sea level rise “saved” would be 2.6 cm, or one inch. A complete cessation of CO2 emissions in Tennessee is only a tiny fraction of the worldwide reductions assumed in Dr. Wigley’s global analysis, so its impact on future trends in global temperature and sea level will be only a minuscule fraction of the negligible effects calculated by Dr. Wigley.

>We now apply Dr. Wigley’s results to CO2 emissions in Tennessee, assuming that the ratio of U.S. CO2 emissions to those of the developed countries which have agreed to limits under the Kyoto Protocol remains constant at 39% (25% of global emissions) throughout the 21st century. We also assume that developing countries such as China and India continue to emit at an increasing rate. Consequently, the annual proportion of global CO2 emissions from human activity that is contributed by human activity in the United States will decline. Finally, we assume that the proportion of total U.S. CO2 emissions in Tennessee – now 2.1% – remains constant throughout the 21st century. With these assumptions, we generate the following table derived from Wigley’s (1998) mid-range emissions scenario (which itself is based upon the IPCC’s scenario “IS92a”):

> Table 1

>Projected annual CO2 emissions (mmtCO2)

> Note: Developed countries’ emissions, according to Wigley’s assumptions, do not change between 2025 and 2050: neither does total U.S or Tennessee emissions.

>In Table 2, we compare the total CO2 emissions saving that would result if Tennessee’s CO2 emissions were completely halted by 2025 with the emissions savings assumed by Wigley (1998) if all nations met their Kyoto commitments by 2010, and then held their emissions constant throughout the rest of the century. This scenario is “Kyoto Const.”

> Table 2

>Projected annual CO2 emissions savings (mmtCO2)

> Table 3 shows the proportion of the total emissions reductions in Wigley’s (1998) case that would be contributed by a complete halt of all Tennessee’s CO2 emissions (calculated as column 2 in Table 2 divided by column 3 in Table 2).

> Table 3

>Tennessee’s percentage of emissions savings

>Using the percentages in Table 3, and assuming that temperature change scales in proportion to CO2 emissions, we calculate the global temperature savings that will result from the complete cessation of anthropogenic CO2 emissions in Tennessee:

> Table 4

>Projected global temperature savings (ºC)

>Accordingly, a cessation of all of Tennessee’s CO2 emissions would result in a climatically-irrelevant global temperature reduction by the year 2100 of no more than three thousandths of a degree Celsius. Results for sea-level rise are also negligible:

> Table 5

>Projected global sea-level rise savings (cm)

>A complete cessation of all anthropogenic emissions from Tennessee will result in a global sea-level rise savings by the year 2100 of an estimated 0.06 cm, or two hundredths of an inch. Again, this value is climatically irrelevant.

>Even if the entire Western world were to close down its economies completely and revert to the Stone Age, without even the ability to light fires, the growth in emissions from China and India would replace our entire emissions in little more than a decade. In this context, any cuts in emissions from Tennessee would be extravagantly pointless.


Davis, R.E., et al., 2003a. Decadal changes in summer mortality in the U. S. cities. International Journal of Biometeorology, 47, 166-175.

Davis, R.E., et al., 2003b. Changing heat-related mortality in the United States. Environmental Health Perspectives, 111, 1712-1718.

>Energy Information Administration, 2007a. International Energy Annual, 2005. U.S. Department of Energy, Washington, D.C., http://www.eia.doe.gov/iea/contents.html

>Energy Information Administration, 2007b. Emissions of Greenhouse Gases in the United States, 2006. U.S. Department of Energy, Washington, D.C., http://www.eia.doe.gov/oiaf/1605/ggrpt/pdf/0573(2006).pdf

>Intergovernmental Panel on Climate Change, 2007. Summary for Policymakers, (http://www.ipcc.ch/SPM2feb07.pdf

>National Climatic Data Center, U.S. National/State/Divisional Data, (www.ncdc.noaa.gov/oa/climate/climatedata.html)

>National Weather Service, Nashville Tennessee, A Tornado Climatology of Middle Tennessee (1830-2003), http://www.srh.noaa.gov/ohx/research/torcli.htm

>Reiter, P., 1996. Global warming and mosquito-borne disease in the USA. The Lancet, 348, 662.

>Wigley, T.M.L., 1998. The Kyoto Protocol: CO2, CH4 and climate implications. Geophysical Research Letters, 25, 2285-2288.