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Global Warming and Extreme Weather:
Impacts on Agriculture (Italy)

Volume 5, Number 23: 5 June 2002

Climate alarmists say rising temperatures produce more frequent and aberrant weather extremes and that some of these things - such as droughts, floods and storms, as well as the higher temperatures themselves - negatively impact agriculture.  This is a great hypothesis, since it can be readily tested with real-world data.  Mann et al. (1999), for example, have compiled considerable evidence suggesting that the Northern Hemisphere warmed throughout the 20th century at a rate that was unprecedented over the past millennium; while Esper et al. (2002) have demonstrated that this unprecedented warming actually began nearly two centuries ago.  Long-term weather records, therefore, should readily reveal whether or not climate alarmists are justified in their pessimism about agricultural productivity being negatively impacted in a warming world.

The study of Moonen et al. (2002) is an excellent example of what can be done in this regard.  It is based on 122 years of real-world data collected from 1878 to 1999 at the weather station of the Department of Agronomy and Agroecosystem Management of the University of Pisa, Italy, which is located at the edge of the city.  Meteorological parameters routinely measured over this period were daily maximum, minimum and mean air temperature, along with daily rainfall amount; while agrometeorological parameters included the date of first autumn frost, date of last spring frost, length of growing season, number of frost days, lengths of dry spells, potential evapotranspiration, reference evapotranspiration, soil moisture surplus, theoretical irrigation requirement, number of days with soil moisture surplus, and number of days with soil moisture deficit.

What did the scientists find?  With respect to temperature, they note that "extremely cold temperature events have decreased and extremely warm temperature events have remained unchanged."  They suggest that both of these observations may be attributed to the increase in cloud cover that would be expected to occur in a warming world.  More clouds would reduce midday heating, for example, and thereby offset much - if not all - of the impetus for global warming during the hottest part of the day.  At night, however, the increased cloud cover would enhance the atmosphere's greenhouse effect, thereby adding to the long-term warming trend.  Consequently, Moonen et al. say that "no negative effects can be expected on crop production from this point of view."  In fact, they find a real "silver lining" in the latter of these cloud feedback phenomena, reporting that "the number of frost days per year has decreased significantly resulting in a decrease in risk of crop damage."  Hence, they say the time of planting spring crops could be safely advanced by many days, noting that the length of the growing season increased by fully 47 days over the period of their study.

With respect to rainfall, Moonen et al. found a somewhat analogous situation.  On an annual basis, extremely high rainfall events seem not to have changed over the period of their study; but there was an increase in very low rainfall events.  The one exception to this rule on a seasonal basis was a decrease in high rainfall events in the spring, which might be expected to increase drought risk at that time of year.  However, when the maximum length of dry spells was assessed on a seasonal basis, the only significant change observed was a lengthening of this parameter in the autumn.  But autumn is the wettest season of the year in Pisa.  Hence, the authors concluded that "no increased drought risk is to be expected."

The bottom line with respect to water in agriculture, however, is the balance that exists between what is received via rainfall and what is lost via evapotranspiration, as this difference is what determines the soil moisture balance.  Hence, even though there was a downward trend in yearly rainfall at Pisa over the past 122 years (due to the decrease of high rainfall events in the spring), there was a nearly offsetting downward trend in evapotranspiration (possibly induced by enhanced daytime cloud cover), such that there were "no significant changes in soil water surplus or deficit on an annual basis."  In the autumn, however, the scientists noted a significant decrease in the number of surplus soil moisture days; but because autumn is the wettest season of the year, the scientists say "this indicates a reduced flooding risk in autumn, which could have positive effects on workability of the soil and imply a reduction of erosion."

Considering their several observations in total, Moonen et al. conclude that concomitant with the significant, if not unprecedented, Northern Hemispheric warming of the past 122 years, "extreme events in Pisa have not changed in a way that is likely to negatively affect crop production."  More often than not, in fact, the changes that have occurred seem to have positive impacts on agriculture.  And as the authors state in concluding their study, "there is no doubt regarding the reality of the observed changes."

Dr. Sherwood B. Idso
Dr. Keith E. Idso
Vice President

Esper, J., Cook, E.R. and Schweingruber, F.H.  2002.  Low-frequency signals in long tree-ring chronologies for reconstructing past temperature variability.  Science 295: 2250-2253.

Mann, M.E., Bradley, R.S. and Hughes, M.K.  1999.  Northern Hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations.  Geophysical Research Letters 26: 759-762.

Moonen, A.C., Ercoli, L., Mariotti, M. and Masoni, A.  2002.  Climate change in Italy indicated by agrometeorological indices over 122 years.  Agricultural and Forest Meteorology 111: 13-27.