Drought and flooding

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Ancient lakebed in the Kalahari Desert, Botswana, where megadrought began 150,000 years ago as a major river changed course. Source: C. Michael Hogan


Drought and flooding can be associated with a variety of predecessor causes, including (a) ocean circulation; (b) atmospheric conditions; (c) air pollution and aerosols; (d) land use change; (5) river course changes. El Nino events are of tropical ocean origin in the Pacific Ocean, and arise from ocean atmospheric interaction. A megadrought is said to have occurred if a severe drought persists for a century; however, megadroughts have not been seen for many hundreds of years. Drought events occur naturally since ancient time, and have a profound effect on droughts and flooding over the entire planet. (McPhaden et al, 2020) Correspondingly El Nina events also occur with frequency between two and seven years, and have dramatic effects on marine and terrestrial weather. Using coral cores, it has been established that El Nino events have been occurring naturally for at least 400 years.

Present day trends or frequencies of droughts and floods cannot be differentiated from ancient and historic levels, except that droughts in the last five centuries are less severe and shorter in duration than centuries ago. Studies of tree rings and of coral reefs give the best insight to these facts, (Cook, 2001); in fact there is no evidence that claims of climate change are having any measurable impact on either droughts (severity or duration) or flooding. In fact, flood impacts as measured by human deaths show a decided declining trend, although much of that can be ascribed to improved preparedness and flood mitigation.

While trends in flooding show essentially no change since historical records have been kept, the number of deaths due to flooding has fallen dramatically over the last 170 years; this is predominantly due to the wealth effect and improved preparedness for flood events. In any case there is no adverse correlation between the small amount of temperature change and the extent of floods.

Land Use Change and Water Management

Land use and population change are two of the most significant causes of drought, especially as measured by human impact . First of all, this article is chiefly about actual physical drought, or the prolonged absence of precipitation; on the other hand drought is evidenced chiefly by its impacts on agricultural production and water availability for the human population. As the human population has grown exponentially, the perception of droughts has increased accordingly. For example, in California, water supplies of the present would have been adequate to support the human population and agricultural needs that existed a century ago.

An important worldwide land use change that affects reporting of temperature trends is the urban heat island effect, a well known phenomenon where urban areas are much warmer than the surrounding area; this not only causes a perception of warmer temperatures beyond mean data, but also compromises many of the thermal sensors, leading to an upward bias of reported temperatures.

Ancient Perspective

There are numerous examples of ancient drought conditions; some of these, such as the Danger Cave Native American site in Utah, are based upon stalagmite studies do document time history of droughts. The Great Basin Desert Culture peoples of Danger Cave prospered over a period dating to at least as early as 7000 BC. Their ability to adapt to megadroughts is evident from their cultural remains in this important site. Stalagmite data shows that the most prolonged drought in the Great Basin since the start of the Holocene lasted for thousands of years, surpassing the minor droughts of current time and even the last five centuries. (Snow, 2011)

Another proxy ancient drought study at Lake Tahoe in California demonstrates that the era of greatest megadroughts occurred in the mid-Holocene, applying data from submerged preserved ancient tree stumps. (Lindström, 1990) This drought, of course, was before any increase in carbon dioxide starting in the Industrial Revolution.

Similar findings of megadroughts are observed in Europe and West Asia. Generally, droughts have been correlated with periods of global cooling. Tree ring studies show that the most severe modern droughts are associated with the time periods circa 1315 AD (chiefly West Asia and northeast Mediterranean); 1540 AD (central Europe); 1616 AD (central Europe); 1741 AD (northern Europe, Scandinavia, British Isles); 1893 AD (northern and western Europe); 1921 AD (western Europe). In summary, there is no correlation in Europe and West Asia between anthropogenic forcing, industrial activity, greenhouse gases and drought.

Weather Event Causation

USA hurricane frequency by decade, showing hurricanes in decline. Source: US National Oceanic and Atmospheric Administration (NOAA)
An obvious contributor to some flooding events is an extreme weather event such as a hurricane. A subject of frequent discussion is time trends of extreme weather events such as hurricanes. The attached table shows hurricane frequency in the USA by decade over recorded history. As one can see, the frequency of hurricanes in North America has been in a declining trend over all of recorded history. There is no correlation between industrialization and fossil fuel burning with hurricane frequency, other than an inverse correlation, in which hurricane frequency is declining in the last 170 years.

Air Pollution and Aerosols

Increases in aerosols can affect cloud development. Increases in air pollution and particulate matter in the atmosphere can affect cloud development in ways that may reduce precipitation in dry regions or seasons; sometimes air pollution can increase rain, snowfall and the intensity of storms in wet regions or seasons. Aerosols--soot, dust and other particulates in the atmosphere--may affect weather. The findings have implications for the availability, management and use of water resources in regions across the USA and around the world.

Storm monitored by cloud radars provides key data. Credit: Z. Li, University of Maryland.

"Using a 10-year dataset of atmospheric measurements, we have uncovered the long-term, net impact of aerosols on cloud height and thickness and the resulting changes in precipitation frequency and intensity," says Zhanqing Li, an atmospheric scientist at the University of Maryland. (Li, 2011)

"Aerosols' effects on cloud and precipitation development are key questions for the scientific community," says Chungu Lu, program director in the National Science Foundation's (NSF) Division of Atmospheric and Geospace Sciences. "The question is not only important for our understanding of the effects of natural processes and human activities on climate change, but for addressing issues in air pollution, disaster relief, water resource management and human weather modification." In addition to the scope and timeframe of the research team's observations, the scientists matched their results with a cloud-resolving computer model.

"Understanding interactions among clouds, aerosols and precipitation is one of the grand challenges for climate research in the decade ahead," says Tony Busalacchi, a scientist at the University of Maryland and chair of the Joint Scientific Committee of the World Climate Research Program: "Findings from this study are a significant advance in our understanding of such processes, with implications for both climate science and sustainable development."

"We have known for a long time that aerosols impact both the heating and phase changes As condensing and freezing of clouds, and that they can either inhibit or intensify clouds and precipitation," says Russell Dickerson, an atmospheric scientist at the University of Maryland. "What we have not been able to determine until now is the net effect," says Dickerson. "This study shows that fine particulate matter, mostly from air pollution, impedes gentle rains while exacerbating severe storms. It adds urgency to the need to control sulfur, nitrogen and hydrocarbon emissions."

According to Steve Ghan of the Pacific Northwest National Laboratory, "This work confirms what previous cloud modeling studies had suggested: that although clouds are influenced by many factors, increasing aerosols enhances the variability of precipitation, suppressing it when precipitation is light and intensifying it when it is strong. "This complex influence is completely missing from climate models, casting doubt on their ability to simulate the response of precipitation to changes in aerosol pollution."

Aerosols are tiny solid particles or liquid particles suspended in air. They include soot, dust and sulfate particles and are what we commonly think of when we talk about air pollution. Aerosols come, for example, from combustion in industrial processes, from some agricultural processes and from the accidental or deliberate burning of fields and forests. They can be hazardous to human health and the environment.

Aerosol particles also affect the Earth's surface temperature by reflecting some sunlight back into space. The variable cooling and heating that results is, in part, how aerosols modify the stability that dictates atmospheric vertical motion and cloud formation. Aerosols also affect cloud microphysics because they serve as nuclei around which water droplets or ice particles form. Both processes can affect cloud properties and rainfall. Different processes may work in harmony or offset each other, leading to complex yet inconclusive interpretations, regarding their long-term net effect.

Greenhouse gases and aerosol particles are two agents affecting climate variability. The mechanisms of climate warming effects of increased greenhouse gases are clear: they trap solar energy absorbed at the Earth's surface and prevent it from being radiated as heat back into space. The climate effects of increased aerosols are much less certain.

"This study demonstrates the importance and value of keeping a long record of continuous and comprehensive measurements to identify and quantify the important roles of aerosols in climate processes," says Steve Schwartz, a scientist at Brookhaven National Laboratory. "While the mechanisms for some of these effects remain uncertain, the well-defined relationships discovered demonstrate their significance," says Schwartz. "Controlling for these processes in models remains a future challenge, but this study clearly points to important directions."

"The findings from ground measurements of long-term effects are consistent with the global effects revealed from satellite measurements reported in our separate study." (Li,2011)

Relation to Wildfires

Sometimes there are discussions of links between drought and wildfire; however, the chief causations of wildfire frequency are lack of forest fuel reduction (Agee and Skinner, 2005), rising wildfire arson frequency (and associated lack of consequences for arsonists, especially in California and Oregon); recent 2020-2021 reductions in aerial combat capability and emergency response forces (especially in California); and increasing numbers of homeless encampments in forests (again, especially in California, where 40% of USA wildfires occur. (Insurance Information Institute, 2021) Essentially, there is little historic correlation between drought and wildfires.. Part of this reason is that lightning strikes cause ten percent of fires, and lightning is correlated with rainfall.

Drought Cyclicity

Even though there is no historic or present time trend with drought frequency or severity, there are some recognized cyclicities; chiefly there are natural cycles that have occurred through recorded history, having nothing to do with greenhouse gas concentration or absolute surface temperature. The main cyclical elements are the El Nino and El Nina events. For example, a study of drought in Zimbabwe showed strong correlation with El Nino occurrences, showing striking periodicities. (Nangombe, 2018)


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National Science Foundation, C. Michael Hogan (2011, updated 2021). Drought and flooding. ed. S. Draggan. Encyclopedia of Earth. National Council for Science and Environment. Washington DC.