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Soil Erosion -- Summary
Almost from the beginning of scientific interest in the subject, study after study had concluded that soil erosion by both wind and water was a major environmental problem. In fact, in an article published in Science a decade and a half ago, Trimble and Crosson (2000) noted that "some sources have suggested that recent erosion is as great as or greater than that of the 1930s," just as some sources were suggesting that global temperatures were greater at that time than they were in the 1930s (Crowley, 2000; Mann 2000).

The remarkable feature of this long-held belief in continued high, or even increasing, soil erosion, in the words of Trimble and Crosson, was that "it was based mostly on models," just as the global warming scare was (and still is!) based mostly on models. Enlarging on this thesis, the two researchers indicated that little physical field-based evidence other than anecdotal statements had been offered to verify the high soil erosion estimates, noting that "it is questionable whether there has ever been another perceived public problem for which so much time, effort, and money were spent in light of so little scientific evidence," which almost begs one to suggest that the perceived public problem of CO2-induced global warming is no different. In fact, the "problem" of anthropogenic CO2 emissions has long since outstripped the soil erosion problem in terms of notoriety, even surpassing global terrorism and nuclear warfare in the minds of some politicians and climate alarmists. But the good news, according to Trimble and Crosson, was that "available field evidence suggests declines of soil erosion, some very precipitous, during the past six decades."

So what confused the issue for so many years? The problem was largely a failure to realize that most of the soil particles removed from one part of the land, by either wind or water, were later deposited in nearby areas, so that the net loss of soil was only a very small portion of that which was moved about by the forces of nature.

Over the course of the data-driven shift in the public's perception of U.S. soil erosion history, it has become evident that our perceptions of several ancillary phenomena also need adjusting. Trimble and Crosson noted, for example, that certain studies once warned that "increasingly eroded soil profiles will allow less rainfall to be infiltrated and stored," leading to "increased overland flow, erosion, and flooding." However, as they further noted, detailed hydrologic studies had indicated that just the opposite was occurring: "runoff is decreasing, flood peaks are smaller, and in some places, the base flow is greater." In addition, they said that "these field studies show that more water is infiltrating into the soil and, in some cases ... significantly more water is being transpired by plants."

These real-world observations are what one would have expected on the basis of Idso's 1989 prediction. With gradually increasing atmospheric CO2 concentrations gradually enhancing plant water use efficiencies, more plants should have gradually been spreading over the surface of the land, reducing rates of surface runoff and allowing more water to infiltrate the soil, thereby providing more water to be extracted from the soil by more plants for subsequent transpiration. And these hydrologic improvements, in turn, should have improved the status of still other aspects of the planet's natural resource base, such as by increasing the stability of streams; and a visual testament to the reality of this phenomenon was provided by a pair of photographs in the Trimble and Crosson article. Both photos showed the same view of a portion of Bohemian Creek, La Crosse County, Wisconsin, USA. The first of the pictures, taken in 1940, showed an "eroded, shallow channel composed of gravel and cobbles, with coarse sediment deposited by overflows on the floodplain." The second, taken 34 years later in 1974, indicated that the stream channel was "narrower, smaller, and more stable." In addition, Trimble and Crosson noted that "the coarse sediment has been covered with fine material, and the flood plain is vegetated to the edge of the stream." What is more, they said that conditions improved even more over the following quarter century.

In reviewing these many real-world manifestations of the benefits of the ongoing rise in the air's CO2 concentration for the world's important soil and water resources, one would hope that the lesson taught by Trimble and Crosson - about the contrasting "myth and reality" of U.S. soil erosion history - would not be lost on those currently struggling with the reality of rising atmospheric CO2 concentrations and the myriad myths that have been associated with this phenomenon; for as Trimble and Crosson wrote, "no problem of resource or environmental management can be rationally addressed until its true space and time dimensions are known," which is something that has yet to be achieved in the climate change arena. In addition, their conclusion that "the uncritical use of models is unacceptable as science and unacceptable as a basis for national policy" is also right on the mark. Unfortunately, many nations have yet to learn this lesson; and if they do not learn it soon, we will all very shortly find ourselves in a world of economic hurt, brought on by our own naivety.

With this broad introduction to the problem of soil erosion, its similarity to the "problem" of anthropogenic CO2 emissions, and the potential relationship between the two subjects, it is instructive to review the results of a number of scientific studies that have focused on specific impacts of rising temperatures and atmospheric CO2 concentrations on soil erosion.

Allen et al. (1999) analyzed sediment cores extracted from a lake in southern Italy and from the Mediterranean Sea, deriving thereby a high-resolution climate and vegetation data set for this region that covered the last 102,000 years. This information indicated that rapid changes in vegetation were correlated with rapid changes in climate, such that complete shifts in natural ecosystems sometimes occurred over periods of less than 200 years. Throughout the warmest portion of this record, i.e., the current interglacial or Holocene, the total organic carbon content of the vegetation reached its highest level, more than doubling values experienced over the rest of the record; and other proxy indicators revealed that during the more productive woody plant period of the Holocene, the increased vegetative cover was typically associated with less soil erosion.

Rillig et al. (2000) examined several characteristics of beneficial arbuscular mycorrhizal fungi (AMF) associated with the roots of plants growing for at least 20 years along a natural CO2 gradient near a CO2-emmitting spring in New Zealand. In doing so, they determined that enriching the air's CO2 concentration from 370 to 670 ppm increased percent root colonization by AMF in a linear fashion - and by nearly 4-fold! Similarly, fungal hyphal length experienced a linear increase of over 3-fold along the same CO2 gradient; while total soil glomalin (a protein secreted by fungal hyphae that increases soil aggregation and stability) experienced a linear increase of approximately 5-fold. Consequently, as the air's CO2 concentration continues to rise, the positive responses of AMF identified in this study will likely become ever more pronounced, causing soil losses via wind and water erosion to be significantly reduced, due to CO2-induced glomalin-mediated increases in soil aggregate stability, which should benefit terrestrial ecosystems throughout the world.

In a closely allied free-air CO2-enrichment (FACE) study of adequately-fertilized sorghum, where daylight atmospheric CO2 concentration was increased by approximately 50%, Rillig et al. (2001) studied plants grown under both well-watered and water-stressed irrigation treatments, focusing on the effects of elevated CO2 on the hyphal growth of AMF, two fractions of glomalin, and the production of water-stable soil aggregates. This work revealed that fungal hyphae lengths were dramatically increased by the 50% increase in the air's CO2 concentration: by about 120% in the wet irrigation treatment and by 240% in the dry treatment. The mass of water-stable soil aggregates was also increased by the biological effects of the extra CO2 in the air: by 40% in the wet treatment and 20% in the dry treatment. In addition, the researchers found that the "two fractions of glomalin and AMF hyphal lengths were all positively correlated with soil aggregate water stability." Hence, they said it was "demonstrated for the first time that elevated CO2 can affect soil aggregation in an agricultural system," where "a soil stabilizing effect of CO2 would be clearly advantageous."

Knox (2001) determined how the conversion of the United States' Upper Mississippi River Valley from prairie and forest to crop and pasture land by settlers in the early 1800s influenced subsequent watershed runoff and soil erosion rates. This work revealed that conversion of the region's natural landscape to agricultural uses boosted surface erosion rates to values three to eight times greater than those characteristic of pre-settlement times. In addition, the land-use conversion increased peak discharges from high-frequency floods by 200 to 400%. Since the late 1930s, however, surface runoff has been decreasing; but this decrease was "not associated with climatic causes," according to Knox, who said that "an analysis of temporal variation in storm magnitudes for the same period showed no statistically significant trend."

It is important to note that the decreases in soil erosion rates and extreme streamflow conditions that began in the late 1930s in the Upper Mississippi River Valley were the exact opposite of climate-alarmist predictions, which suggested that these phenomena should have been increasing as a result of unprecedented CO2-induced global warming. However, they likely were not related to things climatic, in the opinion of Knox, who attributed them to the introduction of soil conservation measures, such as contour plowing, strip-cropping, terracing and minimum tillage, to which list could be added the concomitant rise in atmospheric CO2 concentration and its impacts on the various beneficial phenomena discussed in this summary.

Olafsdottir and Gudmundsson (2002) studied spatial and temporal patterns of land degradation in northeastern Iceland over the past 7500 years based on data obtained from excavations of 67 soil profiles, comparing their results with climatic variations known to have occurred over the same time period. These activities revealed, in their words, that "the deterioration in vegetation and soil cover noted coincides with the recorded deterioration in climate." In fact, during every major cold period of their entire record, land degradation was classified as "severe." During every major warm period, on the other hand, this condition was reversed, and soils were built up, as vegetation cover expanded. The primary implication of these findings, according to the two researchers, is that "climate has a significant role in altering land cover per se and may trigger land degradation without the additional influence of men." Hence, they concluded that "in Iceland severe land degradation could commence without anthropogenic influence - simply as a result of the cold periods."

In a FACE study conducted on the North Island of New Zealand, Newton et al. (2003) measured the water repellency of a grassland soil - which contained about 20 species of legumes, C3 grasses, C4 grasses and forbs, and was grazed periodically by adult sheep - after five years of exposure to an extra 100 ppm of CO2. This work revealed there was a significant reduction in the water repellency of the soil in the elevated CO2 treatment. In fact, the researchers noted that "at field moisture content the repellence of the ambient soil was severe and significantly greater than that of the elevated [CO2] soil."

In discussing their findings, Newton et al. said that water repellency "is a soil property that prevents free water from entering the pores of dry soil (Tillman et al., 1989)," and they reported that it "has become recognized as a widespread problem, occurring under a range of vegetation and soil types (agricultural, forestry and amenity; sand, loam, clay, peat and volcanic) (Bachmann et al., 2001) and over a large geographical range (Europe, USA, Asia, Oceania) (Bauters et al., 1998)." Specifically, they noted that water-repellency-induced problems for land managers include "increased losses of pesticides and fertilizers, reduced effectiveness of irrigation, increased rates of erosion, and increased runoff," and they also reported that there are water-repellency-induced problems "in the establishment and growth of crops (Bond, 1972; Crabtree and Gilkes, 1999) and implications for the dynamics of natural ecosystems, particularly those subject to fire (DeBano, 2000)." Hence, it is clear that the CO2-induced reduction of soil water repellency discovered in this study portends a wide range of very important benefits for both agro- and natural ecosystems as the air's CO2 content continues to rise in the years and decades ahead.

Prior et al. (2004) noted that "enhanced aboveground crop growth under elevated CO2, leading to more soil surface residue and greater percent ground cover (Prior et al., 1997), coupled with positive shifts in crop root systems (Prior et al., 2003), may have the potential to alter soil structural characteristics." Hence, they decided to see if this inference was indeed true, and if it was true, to see if elevated atmospheric CO2 concentrations tended to enhance or degrade soil physical properties, by growing plots of soybean and sorghum plants from seed to maturity for five consecutive growing seasons within open-top chambers maintained at atmospheric CO2 concentrations of either 360 or 720 ppm. The soil in which the plants grew had been fallow for more than 25 years prior to the start of the study and was located within a huge outdoor bin. There, at the end of each growing season, aboveground non-yield residues (stalks, soybean pod hulls and sorghum chaff), including 10% of the grain yield, were allowed to remain on the surfaces of the plots to simulate no-tillage farming. Measurements of various soil properties made at the beginning of the experiment were then compared with similar measurements conducted at its conclusion.

This work revealed that elevated CO2 (1) had no effect on soil bulk density in the sorghum plot, but lowered it in the soybean plot by approximately 5%, (2) had no effect on soil saturated hydraulic conductivity in the sorghum plot, but increased it in the soybean plot by about 42%, (3) increased soil aggregate stability in both plots, but by a greater amount in the soybean plot, and (4) increased total soil carbon content by 16% in the sorghum plot and 29% in the soybean plot. Consequently, the soils of both plots experienced some improvements in response to the experimental doubling of the atmosphere's CO2 concentration, although there were more and greater improvements in the soybean plot than in the sorghum plot. Prior et al. thus concluded that their results "indicate potential for improvements in soil carbon storage, water infiltration and soil water retention, and reduced erosion," which valuable positive consequences they rightly described as "CO2-induced benefits."

In a paper published one year later, Zhang and Liu (2005), reported how they used the general circulation model of the UK Meteorological Office's Hadley Centre to calculate expected changes in temperature and precipitation throughout the Chinese Loess Plateau over the next century, which exercise yielded 2.3-4.3°C increases in daily maximum temperature, 3.6-5.3°C increases in daily minimum temperature, and 23-37% increases in annual precipitation. They then used a stochastic weather generator to downscale these monthly projections to daily values, after which the Water Erosion Prediction Project model of Flanagan and Nearing (1995), as modified to account for CO2 effects on evapotranspiration and biomass production by Favis-Mortlock and Savabi (1996), was finally run for a wheat-wheat-corn rotation utilizing either conventional or conservation tillage.

By these means, Zhang and Liu determined that the climate-change scenarios they investigated led to 29-79% more water runoff and 2-81% greater soil loss under conventional tillage practices, but that "adoption of conservation tillage could reduce runoff by 18-38% and decrease soil loss by 56-68% as compared to the conventional tillage under the present climate." And so they concluded that "the use of the conservation tillage would be sufficient to maintain low runoff and erosion levels and thus protect agro-ecosystems under projected climate changes." As for all-important crop productivity, they determined that the warmer, wetter and CO2-enriched environment projected to prevail on the Chinese Loess Plateau a hundred years from now would boost yields by rather significant amounts: 15-44% for wheat and 40-58% for corn. At the end of the day, therefore, Zhang and Liu were very upbeat about the future, noting that "the significant increases in predicted wheat and maize yields [that] were results of increased precipitation and CO2 concentration ... outweighed the negative effect of temperature rise on crop growth." And, one might add, they outweighed the negative temperature effect in spectacular fashion, while providing evidence for a concomitant significant reduction in soil erosion.

Jumping ahead a couple years and shifting from the physiological effects of atmospheric CO2 enrichment on groundcover plants to the immediate impacts of certain climatic phenomena on soil erosivity, D'Asaro et al. (2007) began the report of their study by stating that "warmer atmospheric temperatures associated with greenhouse warming are expected to lead to a more variable hydrological cycle, including more extreme rainfall events (IPCC, 1995)," adding that "this change is expected to influence the erosive power, or erosivity, of rainfall and, hence, soil erosion rates (Nearing, 2001)." Therefore, as a test of this "expectation," D'Asaro et al. had as their objective "to assess changes in annual and seasonal rainfall erosivity that occurred in Sicily during the twentieth century," which hundred-year period is typically described by climate alarmists as having experienced an increase in global temperature that was unprecedented over the past two millennia (Mann and Jones, 2003; Mann et al., 2003) or more (Hansen et al. 2006). This they did by first generating long-term series (from 1916 to 1999 in most cases) of a storm erosion index based on storm rainfall amounts and intensities, and then applying that index at 17 different Sicilian locations (representative of different climatic zones) where the latter two parameters were routinely measured. And this effort revealed, in the words of the three Italian researchers, that "the annual erosivity did not increase during the twentieth century." In fact, they found that it actually "decreased at a few locations."

One year later, Diodato et al. (2008) conducted a detailed analysis of Calore River Basin (South Italy) erosive rainfall using data from 425-year-long series of both observations (1922-2004) and proxy-based reconstructions (1580-1921). The more recent of these two series was based on a scheme that employed the Revised Universal Soil Loss Equation, while documentary descriptions provided the basis for the earlier series. The results of this work revealed pronounced inter-decadal variations, with "multi-decadal erosivity reflecting the mixed population of thermo-convective and cyclonic rainstorms with large anomalies." In addition, they reported that "the so-called Little Ice Age (16th to mid-19th centuries) was identified as the stormiest period, with mixed rainstorm types and high frequency of floods and erosive rainfall."

In the concluding section of their paper, therefore, the three researchers were able to write that "in recent years, climate change (generally assumed as synonymous with global warming) has become a global concern and is widely reported in the media." And one of the chief of these concerns - which is trumpeted over and over by the world's climate alarmists - is that extreme weather phenomena, such as droughts and floods, will become both more frequent and more severe as the planet warms, which, of course, would lead to more soil erosion. However, Diodato et al. said that their study indicated that "climate in the Calore River Basin has been largely characterized by naturally occurring weather anomalies in past centuries (long before industrial CO2 emissions), not only in recent years," and that there has been a "relevant smoothing" of such events during the modern era.

Five years earlier, while working in the Myjava Hill Land of Slovakia, which is situated in the western part of the country near its border with the Czech Republic, Stankoviansky (2003) employed topographical maps and aerial photographs, field geomorphic investigation, and the study of historical documents, including those from local municipal and church sources, to determine the spatial distribution of gully landforms and the temporal history of their creation. This work revealed that "the central part of the area, settled between the second half of the 16th and the beginning of the 19th centuries, was affected by gully formation in two periods, the first between the end of the 16th century and the 1730s and the second roughly between the 1780s and 1840s." And Stankoviansky added that the gullies were formed "during periods of extensive forest clearance and expansion of farmland," while noting that "the triggering mechanism of gullying was extreme rainfalls during the Little Ice Age." More specifically, he wrote that "the gullies were formed relatively quickly by repeated incision of ephemeral flows concentrated during extreme rainfall events, which were clustered in periods that correspond with known climatic fluctuations during the Little Ice Age."

Subsequently, from the mid-19th century to the present, Stankoviansky affirmed that "there has been a decrease in gully growth because of the afforestation of gullies and especially climatic improvements since the termination of the Little Ice Age." And these several observations suggest that extreme and destructive rainfall events were much more common throughout the Myjava Hill Land of Slovakia during the Little Ice Age than they have been subsequently, which view, in his words (and in many of the references he cites), "is often regarded as generally valid for Central Europe." And thus it is that this view runs counter to that of most climate alarmists, who tend to equate such destructive precipitation events and the erosive flooding they cause with global warming.

In the course of a long-term field experiment conducted several years later at the Kessler Farm Field Laboratory in McClain County, Oklahoma, USA, Xue et al. (2011) explored how annual clipping for biofuel feedstock production and warming caused soil erosion and accompanying carbon and nitrogen losses in tallgrass prairie, where warming was provided by infrared heaters suspended 1.5 m above the ground, as described by Kimball (2005), leading to air temperatures being raised by an average of 1.47°C and soil temperatures in the clipping plots by 1.98°C. The results of this experiment revealed that the average relative depth of erosion caused by clipping was 1.65 and 0.54 mm/year, respectively, in the warmed and control plots from November 21, 1999 to April 21, 2009, that the soil erosion rate was 2148 g/m2/year in the warmed plots and 693 g/m2/year in the control plots, that soil organic carbon was lost at a rate of 69.6 g/m2/year in the warmed plots and 22.5 g/m2/year in the control plots, and that total nitrogen was lost at a rate of 4.6 g/m2/year in the warmed plots and 1.4 g/m2/year in the control plots. And Xue et al. made a point of noting, in this regard, that "the amount of carbon and nitrogen loss caused by clipping is equivalent to, or even larger than, changes caused by global change factors."

In discussing their findings, the five researchers said their results suggested that "clipping for biofuel harvest results in significant soil erosion and accompanying losses of soil carbon and nitrogen, which is aggravated by warming." And they indicated that "soil erosion is one of the most pressing global environmental challenges facing the world today, causing declining soil productivity and crop yields, which may create difficulties in meeting the rising demand for food and energy (Brink et al., 1977; Brown, 1981, Lal, 2004; MEA, 2005)," which facts lead one to wonder if the biofuel "cure" for the global warming "disease" might not be worse than the malady itself.

Closing out this review is the study of Tape et al. (2011), who wrote that "recent changes in the climate of Arctic Alaska, including warmer temperatures and a lengthened growing season (Chapin et al., 2005; Serreze and Francis, 2006; Shulski and Wendler, 2007), are linked with ... increased vegetation productivity, as measured using time series of satellite vegetation indices such as Normalized Difference Vegetation Index (NDVI)," as documented by Myneni et al. (1997), Jia et al. (2003), Goetz et al. (2005) and Bhatt et al., 2010); and they noted that this phenomenon "has been partly attributed to the expansion of shrubs, which has been documented using time series of aerial photography (Sturm et al., 2001; Tape et al., 2006), plot studies (Joly et al., 2007), and shrub growth ring chronologies (Forbes et al., 2010; Hallinger et al., 2010)." And in light of these observations they asked themselves the question: "Is the current warming and concurrent shrub expansion on older Arctic landscapes associated with increased or decreased erosion?"

Working with time series imagery obtained from Landsat thematic mapper data covering the period 1986-2009, Tape et al. (2011) examined the landscape pattern of tall shrub distribution and expansion in the Arctic foothills, located on the north side of the Brooks Range, Alaska, while they studied sediments obtained from cores of four lakes near the Chandler River on the central North Slope of Alaska (where shrub expansion is occurring), in order to compare relationships among shrub cover, erosion and runoff over the past quarter-century. And in doing so, they found that their results revealed "a background decline in erosion since 1980, superimposed by episodic erosional events," and they stated that "the background decline in erosion is associated with trends of increasing shrubs and declining peak runoff events."

In conclusion, the results of the many studies discussed in this brief review clearly suggest that the historical increase in the atmosphere's CO2 concentration has significantly reduced the erosion of earth's valuable topsoil over the past several decades, and that the continuing increase in atmospheric CO2 has the potential to maintain this trend, and perhaps even accelerate it, throughout the foreseeable future.

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Last updated 19 February 2014