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The Intergovernmental Panel on Climate Change (IPCC) has consistently predicted expanding world wide drought as one of the horrors of global warming. New research suggests that things may not be quite as arid as they predicted. According to scientists, a commonly used technique for estimating drought severity may actually overestimate the effects of dry spells. Revisiting historical data for the period 1980-2008, a new assessment technique found that the global area stricken by drought grew by approximately 0.08% per year—less than one-seventh (14%) the increase previously reported. Moreover, researchers found that drought causes higher temperatures, not the other way around. Once again, the horrendous effects of anthropogenic global warming have been found to be over hyped by the self-serving savants of the IPCC.
The IPCC has predicted a wide range of calamities linked to rising global temperatures deriving from human activity. Melting icecaps, rising ocean levels, increased storm activity in some areas and drought in others. These horror stories for the uneducated masses have been swallowed hook, line and sinker by the vacuous intellects of Hollywood, the press and politicians, who's watchword is “never waste a good disaster.” We have recently witnessed the rush of scientific ignoramuses proclaiming the results of Hurricane Sandy as nature's just revenge for human indiscretions.
Drought is one of the horrors of global warming according to the IPCC.
Among the scourges predicted to wreak havoc on mankind is drought. The 2007 IPCC Fourth Assessment Report's (AR4) concluded that the area affected by droughts was “likely” to have increased in many regions since the 1970s and, indeed, previous studies have suggested that droughts have increased over the past 60 years. Until recently, researchers have measured dry spells using the Palmer Drought Severity Index (PDSI). The PDSI was developed originally as an agricultural monitoring tool in the United States in the 1960s and since then it has become widely used for monitoring and increasingly in studies of climate change. It is one component of the US National Drought Monitor, recently in the news regarding widespread severe drought in the Southern and Midwestern states.
But researchers have long known that the simplicity that make the PDSI so popular is also its greatest weakness. In a new study, “Little change in global drought over the past 60 years,” published in the journal Nature, Justin Sheffield, Eric F. Wood and Michael L. Roderick take a closer look at measuring drought and what is really happening to global climate. From the article's abstract:
Drought is expected to increase in frequency and severity in the future as a result of climate change, mainly as a consequence of decreases in regional precipitation but also because of increasing evaporation driven by global warming. Previous assessments of historic changes in drought over the late twentieth and early twenty-first centuries indicate that this may already be happening globally. ...
Here we show that the previously reported increase in global drought is overestimated because the PDSI uses a simplified model of potential evaporation that responds only to changes in temperature and thus responds incorrectly to global warming in recent decades. More realistic calculations, based on the underlying physical principles that take into account changes in available energy, humidity and wind speed, suggest that there has been little change in drought over the past 60 years.
The PDSI measures the balance between precipitation in a region and the loss of soil moisture. This is done either by direct evaporation or via plants that suck water out of the soil and then lose it to the atmosphere through transpiration. The method, which typically estimates the potential for evaporation using only the temperature and latitude at a site, is popular largely because it is easy to use. “[PDSI] is simple, but it's relatively crude,” notes lead author Justin Sheffield, a hydrologist at Princeton University.
Researchers have long known that evaporation also depends on factors such as wind speed, relative humidity, and the amount of sunlight reaching the ground at various wavelengths, factors not explicitly included in PDSI estimates. In a News & Views article in the same issue of Nature, Sonia I. Seneviratne, from the Institute for Atmospheric and Climate Science, ETH Zurich, provides some additional perspective and background information: “Soil-moisture drought, which is of most relevance to agriculture, is induced by a deficit in the land water balance and is caused by lack of precipitation and/or excess evapotranspiration. Evapotranspiration refers to the moisture loss from soils, either through plant transpiration (water extracted by the plants and lost through the leaves' stomata) or by direct evaporation from moist surfaces (such as bare soils, lakes, rivers, or water stored on top of leaves).”
Drivers of Drought. Source S. Seneviratne/Nature.
In the illustration above, red arrows indicate factors that contribute to drought, and blue arrows show factors that counteract it. Unfortunately, global databases of those parameters haven't been easy to come by, according to Sheffield et al. Using satellite data gathered in recent decades, Sheffield and his colleagues reconstructed climate history and estimated those parameters as far back as 1948. The researchers then reassessed trends in drought severity between 1948 and 2008.
One of the shortcomings of the PDSI is its treatment of potential evaporation (PE, the evaporative demand of the atmosphere), which is calculated from temperature data by using the empirical Thornthwaite equation. Though estimating PE from temperature is fairly effective under many circumstances it fails to yield good results when looking for climate trends.
“When used in the PDSI model, which derives soil moisture from the balance between precipitation, evaporation and runoff, the increase in PE drives an increase in drought globally in addition to the impact of any changes in precipitation,” the authors state. “However, numerous studies based on observations and detailed physical modelling have shown regional declines in evaporative demand over past decades as a result of various combinations of declining radiation, vapour-pressure deficit and/or wind speed, despite generally increasing regional temperatures.”
The researchers, looking to resolve the discrepancies mentioned above, computed the PDSI using two different PE calculations: the Thornthwaite algorithm and a physically based estimate based on the currently accepted Penman–Monteith equation (PM). Using their accumulated global meteorological data set the researchers calculated the PDSI over time and compared the two methods, the results of which are shown below.
PDSI_Th (blue line) and PDSI_PM (red line) and area under drought.
Plotting the global averaged time series of PDSI using the two PE methods a, clearly shows the decreasing trend in the PDSI using Thornthwaite (PDSI_Th in blue) since the 1970s but not for the PDSI using Penman–Monteith (PDSI_PM in red). The uncertainty due to precipitation and net radiation, shown by the shading around the curves, was estimated using four different global precipitation data sets and two different net radiation data sets. Panel b, shows area in drought (PDSI <−3.0) for the PDSI_Th (blue line) and PDSI_PM (red line).
“The results show that previous calculations of the increase in global drought are overestimated,” the authors succinctly conclude, thought they posit that uncertainty due to the data on precipitation and net radiation as estimated from the standard deviation of PDSI trends remains. Another way to visualize the differences in the two methods of calculation are shown in the figure below, also taken from the Nature article.
Trends in the PDSI and PE.
Shown above are, on the left, annual average PDSI_Th (a) and the PDSI_PM (c), and their difference (e) for 1950–2008. On the right, annual average PE from the Thornthwaite equation (b) and the PM equations (d), and their difference (f) for the same time span. Statistically significant trends at the 95% level are indicated by hatching. As can be seen, the traditional PDSI_Th method overestimates the pervasiveness of drought expansion. From this evidence the authors conclude that drought is not increasing as predicted by the IPCC AR4 report.
The spatial distribution of trends in the PDSI_Th shows drying across much of the global land, particularly over Africa and eastern Asia. In contrast, the PDSI_PM shows a mixture of drying and wetting that combines to give a smaller trend globally. The equivalent trends in the PE_Th are increasing everywhere (98% of land area), as expected given the global increase in temperature, but are a mixture of increases (58% of land area) and decreases (42%) for the PE_PM. The two methods disagree in the sign of the trends across much of northern South America, Central America, eastern North America, eastern sub-Saharan Africa, western Russia, southern and southeast Asia, and Australia.
This report clearly contradicts other, older estimates of the spread of drought conditions around the world, but those reports were based on the older, flawed technique. The revised technique for assessing drought severity “gives a much better match to reality,” according to Sonia Seneviratne, in the article cited earlier. But it is not just the spread of drought that is impacted by these new data—temperatures are also tied to evaporation. Here are some of the authors' closing remarks:
Several regional studies have suggested that higher temperatures than normal were the cause for increased drought in recent years through increased evaporation. Yet there is evidence that the direct impact of temperature on drying may actually be a misinterpretation of feedbacks between the land and the atmosphere. It is more plausible that evaporation actually decreases during drought because of less precipitation, and that drought drives increases in temperatures because there is less evaporative cooling and thus a higher sensible heat flux warming the air. Short-term temperature anomalies are likely to be a response to drought, rather than a factor in forcing drought. Of concern is if the perceived influence of warming on drought as quantified by empirical approaches is extrapolated into the future and predictions of the impacts of climate change are likely to be overestimated.
And there we have it, another sign of the eco-apocholypse has been debunked. Those willing to do a little research of their own will find that the 2012 US drought was not outside historical climatic norms nor even anything special in historical terms. And those hot summer months, far from being a sign of global warming, were in part caused by the lack of evaporation—global warming did not cause the drought, the drought helped cause the hotter than normal temperatures. As more and better science is performed, the circumstantial case for catastrophic human caused global warming is running out of evidence. Perhaps it is time for the eco-catastrophists to come up with a new threat to frighten the people of the world.
Be safe, enjoy the interglacial and stay skeptical.