The Climate Mechanisms of World Deserts and Limitations in Allan Savory’s thesis.

Guest post by Dr. Tim Ball – a response to this WUWT post on Allan Savory

Dr Allan Savory proposes stopping desertification and controlling climate change. His focus is a large natural vegetation area called grasslands. His idea of raising cattle to maintain grasslands is founded on the grazing and fertilizing cycle provided by herbivores. Bermuda Grass is an example of a grassland plant species that thrives on being constantly cropped. It grows thick and dense the more it is cut, making it ideal for golf greens. Savory’s ideas all sound attractive and ‘green’ and not without some merit, but are riddled with problems. It is not clear, indeed unlikely, that his proposals would measurably alter natural climate change.

Watching his presentation I imagined all the ‘environmentalists’ recoiling at his suggestions. It is not long since radical environmentalists like Jeremy Rifkin were blaming cattle for most of the evils of western society in his 1992 book, Beyond Beef: The Rise and Fall of the Cattle Culture. True, Savory showed sheep, but he should also have introduced the idea of restocking some areas with natural herds, such as bison in North America. These areas would become world funded preservation areas of natural species as George Perkins Marsh proposed in his 1864 book Man and Nature. Marsh was also among the first in modern times to idenitfy the relationship between removal of vegetation and desertification.

The major conflict is between domesticated and wild herbivores and the production of foodstuffs. This included growing grains to feed the cattle or overgrazing. Presumably, Savory is suggesting domesticated animals to also expand the food supply. The problem is expansion of the food supply usually creates an increase in the human population, which Savory says is at the heart of the world’s problems.

Savory’s Assumptions

He makes three major assumptions, all arguable. First is the claim the world is overpopulated. It is not! People, apparently including Savory, believe it is because of the neo-Malthusian claim underlying the alarmism of the Club of Rome in the 1970s. Claims of overpopulation primarily came from Paul Ehrlich’s work, but his predictions were so inaccurate it’s a wonder he retains any credibility. The reason the ideas remain is probably because supporters of his ideas are in positions of power today. For example, Ehrlich’s co-author of a truly frightening book Ecoscience: Population, Resources, Environment is President Obama’s Science Tsar, John Holdren. In addition, graduates of 1979s and 1980s environmental studies programs are now running the bureaucracies using those ideas.

The second error is his identification of land ‘suffering’ from desertification. Savory identifies five regions on a world map (Figure 1). He is using the term desertification as it evolved back in the 1970s, that is as an environmental problem caused by humans. The problem is almost all the regions he identifies are natural climatic regions of desert and grasslands. He says there is “no other cause” than humans for desertification, which is only true because of his definition. In a 2005 work, “The causes and progression of desertification,” Geist identified more than 100 definitions. Any region that loses vegetation becomes a desert, which happens all the time as climate changes. If you don’t know how much change is due to natural causes you can’t determine the human portion. It is the same as the Intergovernmental Panel on Climate Change (IPCC) examining only human causes of climate change without knowing how much it changes naturally.

The third error he makes, is to assume climate change is new and caused by humans. It isn’t. The amount of change is well within natural variability, but the IPCC and its proponents persistently work to prove it is outside and therfore unnatural. Savory is apparently vulnerable to the “human cause” claim because he blames humans for desertification.

Basic Arid Zone Pattern

The trouble is it appears Savory lacks some basic understandings including;

• how deserts are formed and change with climate change,

• how or why the major hot deserts are generally located within 15 to 35° of latitude each side of the Equator and,

• how grasslands are a transitional area of slightly higher precipitation that surround the deserts and lie between the deserts and the forests. Grassland names differ from Steppe in Russia; Great Plains in the US and their northern extension the Prairies in Canada; Llanos in northern South America; Pampas in southern South America; to Savanna and Veldt in Africa.

Figure 1: Areas of desertificcation identified by Allan Savory

Source: Screen Capture from his presentation

The Sahel is just such a transitional region between the rainforest on the coast of west Africa and the true desert of the Sahara. Alarmist stories appeared about the expanding Sahara desert associated with the cyclical Sahelian drought that visited the region between 1968 and 1974. Famine accompanied the drought and overgrazing was blamed. It, and another drought in 1984-85, launched the environmental career of Bob Geldof.

A similar desertification situation was identified in the Thar desert on the Indian-Pakistan border in the 1970s, with claims the area wasn’t totally ‘natural’ but created by overgrazing, especially by the ubiquitous goat. University of Wisconsin climatologist Reid Bryson theorized that removal of vegetation cover increased surface temperatures, which caused increase convection and advection (wind). Resulting soil erosion and winds carried dust to altitude. Here it absorbed sunlight directly, raising upper air temperatures while reducing surface heating. Warm air over cold is an inversion, a very stable situation that prevents cloud formation, thus perpetuating the aridity. As I recall, much money was spent on bringing water into the region to plant grasses and stabilize the surface to break the cycle. The grass promoted was Marram, a well known sand dune stabilizer.

World Hot Deserts and Grasslands

It is impossible to get even crude estimates of the percentage of land surface that is grassland or desert. Land is 149 million km² of the Earth’s total surface and hot deserts make-up an estimated 15 to 30 percent (Figure 2). The Sahara provides a scale because it is 9.1 million km², almost identical to the land area of the US. The hot deserts of the world in order (millions of km²⁾ are;

Sahara – 9.1

Central Asia – 4.5

Australian – 3.4

North American – 1.3

Patagonian – 0.7

Indian – 0.6

Kalahari – Namib 0.57

Atacama – 0.36

The word ‘hot’ is in bold because, as Koppen (Figures 4 and 5) recognized in his climate classification system, there are vast cold deserts. The North and South poles are among the driest places on Earth.

Figure 2. Major hot deserts generally straddling the Tropics of Cancer and Capricorn.

In his system, Koppen identified “B” climates primarily as those with insufficient ‘effective’ precipitation to support trees (BS for Steppe climate) then grass (BW desert climate). He further divided BW climates into BWh (coldest month average above 0°C) and BWk (at least one month average below 0°C). When doing a Koppen classification you begin by eliminating the B climates. Savory lumps them together as shown in Figure 1.

Estimates for grassland are more variable than for deserts varying from 15 to 40 percent of the land surface, excluding Antarctica and Greenland. Savory showed, unknowingly, why defining grasslands is so difficult. He showed clumps of grass with bare ground in between, implying they were examples of desertification. The problem is such conditions are natural and exist over very large areas with grasses known as tussock.

The sun is directly overhead the equator twice a year and is never more than approximately 23.5° from the vertical. This results in maximum heat energy and therefore high year round temperatures. It creates what was known as the “heat equator”, which, because of land water differences is not coincident with the actual Equator. Belem on the Amazon in the interior of Brazil has a range of 1.6°C from the warmest to the coolest month.

High temperatures result in high evaporation and rising warm air. The vertical air currents mean very little horizontal surface wind, a problem in sailing days. English sailing ships recorded the conditions and from their records George Hadley, in 1753, figured out his circulation cell (Figure 3). Clouds develop daily and result in heavy rainfall almost daily. Duitenzorg, Java, averages 322 days a year with thunderstorms.

The warm air rises to the tropopause where it is now cold, dense and dry. Deflected away from the Equator it descends. As it descends increasing pressure creates adiabatic warming. By the time it reaches the surface it is hot and dry. The amount of moisture is the same but chances of condensation and cloud formation is virtually zero. Average relative humidity for the Sahara is approximately 19%. Water vapour is the most important greenhouse gas and low levels in desert region mean the ground and air heat and cool very rapidly. Cloud cover in the Sahara varies from about 10% in winter to 4% in summer.

The highest shade temperatures in the world occur such as 58°C in Libya and 56.7°C in Death Valley, California. At In-Salah, Algeria, the temperature dropped from an afternoon high of 52.2°C to an overnight low of –3.3°C, a range of 55.5°C in about 12 hours. These conditions mean the air holds less water vapour, but the air temperature drops well below the dew point temeprature thus creating condensation.

Figure 3. Hadley Cell circulation has air rising at the equator and descending between 15 and 30° latitude. A similar cell exists for the Southern Hemisphere.

Heated air at the equator creates low pressure, the Equatorial Low, while descending air creates high pressure in the subtropics, the Subtropical Highs.

The pattern of high rainfall at the Equator and deserts in the Low Latitudes is disturbed by the land/water distribution and influence of ocean currents. The greatest disturbance occurs in eastern Africa and Asia so the desert zone extends through Saudi Arabia, Afghanistan, Pakistan and western China. Low latitude landmass in the southern hemisphere is restricted but includes southern Africa, Australia and South America. In South America the Andes Mountains block the extension of the deserts across the entire continent. However, where they exist on the coast they are among the driest on earth.

Savory refers to the rock paintings of herd animals in the central Sahara. They occur there because of climate change when increased rainfall supported grasslands. During the last Ice Age the Polar climate zones expanded pushing the mid latitude temperate climates toward the Equator. Traditional climate referred to the wetter periods in the desert zones that were coincident with Glacials as Pluvials. When the Earth warmed to Interglacials, as now, the desert regions experience Interpluvials.

Swings between Pluvial and Interpluvial are macro climate changes, however smaller changes are occurring all the time. As a result, the pattern of climates shown in Figures 4 and 5 are averages and constantly changing.

Figure 4: Koppen classification The Americas.

Figure 5: Koppen classification Africa, Eurasia and Australia

The Sahel is shown as BSh or hot grassland right across Africa on the south side of the Sahara (BWh). In addition to the longer term climate changes, cyclical changes in precipitation cause drought cycles such as the one from 1968 to 1974. Australia is another large classic region of desert (BWh) surrounded by semi-arid grassland (BSh).

Importance of Condensation

Savory draws attention to the potential of condensation moisture in the semi arid areas. This is not new, as people for centuries have gathered condensation moisture. I grew up near the dry chalk lanscape of Salisbury Plain and learned early about “dew ponds”. Gilbert White, a renowned 18th century English naturalist, described the ecology around Selbourne. He described a dew pond near the village as “…only 3 feet deep and 30 feet in diameter, that contained some 15,000 gallons of water which supplied 300 sheep and cattle every day without fail.”

In many dry regions people put xerophytic plants close to large rocks, which provide sufficient overnight condensation to maintain the plant. On a larger scale, ancient Greeks built large pyramids of rock from which condensation trickled down to a network of clay collection pipes. Called air wells, they are a well known technique. There were 13 such pyramids up to 12 meters high near the ancient Greek city of Theodosia on the Black Sea.

Savory is correct, condensation is the forgotten moisture, as I described a few years ago and more recently repeated here. The issue was the difference between official predictions of poor yields and the actual average or better yields on the Canadian Prairies. “In the late summer of that 1980s year, daytime temperatures were high, generally 27-28°C, which meant it could hold lots of moisture. At night, temperatures dropped to record lows around 3-5°C and moisture deposition was heavy. In a three-week period this yields upward of 50 mm of precipitation equivalent. Farmers know that amount of moisture can be critical to “fill out” a crop. It has several advantages over normal precipitation. It occurs at night when heat stress on the plant is reduced. Evaporation is reduced. Distribution is more even and widely distributed than rainfall. Unfortunately, it is not moisture counted in the weather statistics used by all the experts. Ironically, it’s moisture farmers know about because, until it evaporates, it can delay harvesting.”

Savory’s method can take advantage of the moisture, but it will only produce grasses in the natural grassland regions he defines. To change true desert (BWh) to grassland requires much larger volumes of water than condensation provides.

It is not clear how his proposal will stop climate change. Presumably, he assumes changing the surface will change the albedo, which will change the energy balance. The problem is there is not much difference in albedo between desert, which ranges from 15 to 45, and grassy fields with ranges10 to 30. The desert range is wide because deserts are only partially sand dunes. The dune areas known as Erg are higher albedo, but are a small percentage of a desert. The much larger, lower albedo, area is the hamada or rock strewn areas that are 70 percent of the Sahara.

Savory’s comment about the importance of microclimates is more critical than he realizes. Most vegetation, and certainly the grasses, grow in the 1.25 m below the Stevenson Screen, the official weather station. The climate below that level is markedly different, as Geiger identified in his marvelous 1950 book, The Climate Near the Ground. Any attempt at planning or changing conditions in this portion of the Biosphere requires far more information than is currently available.

Change is the norm. Climate change is normal and current changes are well within natural variability. Allan Savory’s proposal to stabilize grassland areas has some merit, but requires much more understanding and context, especially about climate patterns and climate change mechanisms. Of course, as the world cools in the next few decades the colder climate zone will expand and the desert zone will shrink naturally. The grasslands will benefit from cooler wetter conditions as the natural cycles continue.