February 24, 2010

Update on Global Drought Patterns (IPCC Take Note)

Filed under: Droughts, Precipitation

We are sure you have heard that global warming is causing more frequent and intense droughts throughout the world. Right? The claim is easy to make – higher temperatures increase evaporation rates, soil moisture is depleted, and drought conditions result. Indeed the Technical Summary of the most recent IPCC assessment includes “More intense and longer droughts have been observed over wider areas, particularly in the tropics and subtropics since the 1970s. While there are many different measures of drought, many studies use precipitation changes together with temperature. Increased drying due to higher temperatures and decreased land precipitation have contributed to these changes”. Further, they write “Although precipitation has increased in many areas of the globe, the area under drought has also increased. Drought duration and intensity has also increased. While regional droughts have occurred in the past, the widespread spatial extent of current droughts is broadly consistent with expected changes in the hydrologic cycle under warming. Water vapour increases with increasing global temperature, due to increased evaporation where surface moisture is available, and this tends to increase precipitation. However, increased continental temperatures are expected to lead to greater evaporation and drying, which is particularly important in dry regions where surface moisture is limited.” The bottom line in the table below from the IPCC’s Technical Summary leaves little doubt that the IPCC thinks that droughts have become more frequent, they have been caused in some part by humans, and they will become more frequent in the decades to come.

A major article on global-scale drought has appeared recently in the Journal of Climate by drought experts from Princeton University and the University of Washington; the work was supported by the U.S. Department of Energy the National Oceanic and Atmospheric Administration. We saw an interesting sentence in their abstract as Sheffield et al. wrote “Globally, the mid-1950s showed the highest drought activity and the mid-1970s to mid-1980s the lowest activity.” That does not seem consistent with the story coming from the IPCC.

Sheffield et al. begin stating “Drought is a naturally occurring climate phenomenon that impacts human and environmental activity globally. It is among the costliest and most widespread of natural disasters. One of the reasons for this is the usually large spatial extent of droughts and their lengthy duration, sometimes reaching continental scales and lasting for many years. Drought is generally driven by extremes in the natural variation of climate, which are forced by the internal interactions of the atmosphere and feedbacks with the oceans and land surface. These are modulated by external forcings such as variations in solar input and atmospheric composition, either natural or anthropogenic.” Fair enough, they reveal that drought could be impacted by anthropogenic changes to atmospheric composition.

Sheffield et al. note that “Soil moisture is a useful indicator of drought because it provides an aggregate estimate of available water from the balance of precipitation, evaporation, and runoff fluxes.” Accordingly, they used a popular hydrologic simulation model to estimate soil moisture levels at the 1º latitude by 1º longitude resolution for land areas of the globe for the period 1950 to 2000.

Their results seem completely at odds with the conclusions of the IPCC. Sheffield et al. note with respect to global and continental droughts “The longest duration drought was 49 months (4 yr) in Asia from 1984 to 1988, closely followed by the 1950–53 North American drought (44 months). The most spatially extensive was the African drought of the early 1980s, which reached its peak extent in April 1983 when it covered over 11 million square kilometers.” Their time series plot for the globe and for various continents shows no upward trend whatsoever (below).

Figure 1. Monthly time series of area-averaged soil moisture percentile, percentage area in drought, and percentage contiguous area in drought (from Sheffield et al., 2009)

The table below from the article is amazing … note that the longest duration drought record-breakers generally occurred early in the record with four of the six from the 1950s. Similarly, the maximum spatial extent record-breaking droughts also tended to occur early, not late, in the time series.

Table 1. Summary of large-scale drought occurrence for the six continents. For the last column, the extent as a percentage of total area and the date when the maximum spatial extent was attained are given in brackets. Oceania is defined as Australia, New Zealand, Papua New Guinea, and the Pacific Islands.

The article contains no end of comments indicating no upward trend on drought conditions. The authors state “The mean number of global droughts > 500 000 km2 occurring in any month is about 4.5 (or 55 yr-1) with a standard deviation of 1.6. This time series is quite variable and indicates several periods of increased global drought activity: the mid-1950s, 1960s, late 1980s to early 1990s, and late 1990s. The mid-1970s to mid-1980s are characterized by the lowest number of droughts, apart from a short burst of activity around 1976–77. The year with most drought months is 1992”.

The authors note that most of the last droughts are well-documented and have been analyzed by climate scientists for years. Sheffield et al. note “Other droughts are ranked highly in terms of severity and spatial extent yet are not well documented or analyzed, such as the 1965 Australian and 1963–64 South American droughts.”

The IPCC and the global warming alarmists continue to insist that droughts are becoming more frequent, more intense, more spatially extensive, and of longer duration. However, Sheffield et al. analyzed drought patterns at the global scale for the period 1950 to 2000, and found no evidence to support claims of increasing drought activity.

Enough said.

Reference:

Sheffield, J., K.M. Andreadis, E.F. Wood, and D.P. Lettenmaier. 2009. Global and Continental Drought in the Second Half of the Twentieth Century: Severity–Area–Duration Analysis and Temporal Variability of Large-Scale Events. Journal of Climate, 22, 1962-1981.




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