Prior to that time (and continuing well past it), study after study had concluded (and continued to conclude) that soil erosion by both wind and water was running at a high sustained rate. In fact, in an article published in Science, 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) has 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 us 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 certain politicians and climate alarmists.

But we digress; for the good news, according to Trimble and Crosson, is 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 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 some adjusting. Trimble and Crosson note, 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 note, detailed hydrologic studies indicate that just the opposite is occurring: "runoff is decreasing, flood peaks are smaller, and in some places, the base flow is greater." In addition, as they describe it, "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 also what would be expected on the basis of Idso's 1989 prediction. With gradually increasing atmospheric CO2 concentrations gradually enhancing plant water use efficiencies, more plants should gradually be spreading over the surface of the soil, 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.

These hydrologic improvements, in turn, tend to improve the status of still other aspects of the planet's natural resource base, such as by increasing the stability of streams; and a good 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, 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 a quarter of a century later in 1974, indicated the stream channel was "narrower, smaller, and more stable." In addition, Trimble and Crosson note 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 say 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 our nation's (and, by implication, the world's) important soil and water resources, we hope that the lesson taught by Trimble and Crosson - about the "myth and reality" of U.S. soil erosion history - will 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. As Trimble and Crosson state, "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 of the earth 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, we now review the results of a number of scientific studies that focus 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 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 say 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 "is not associated with climatic causes," according to Knox, who reports 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 are the exact opposite of climate-alarmist predictions, which suggest these phenomena should be increasing as a result of unprecedented CO2-induced global warming. However, they likely are not related to things climatic, in the opinion of Knox, who attributes them to the introduction of soil conservation measures, such as contour plowing, strip-cropping, terracing and minimum tillage, to which list we would add the concomitant rise in atmospheric CO2 concentration and its impacts on the various beneficial phenomena we are discussing 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 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 is classified as "severe." During every major warm period, on the other hand, this condition is reversed, and soils are built up, as vegetation cover expands. 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 conclude 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. say that water repellency "is a soil property that prevents free water from entering the pores of dry soil (Tillman et al., 1989)," and they report 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 note 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 report 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) note 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. 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.

The researchers found 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 air's CO2 content, 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 describe as "CO2-induced benefits."

Last of all, we come to the study of Zhang and Liu (2005), who 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." Consequently, 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, we might add, they outweighed the negative temperature effect in spectacular fashion, while providing evidence for a concomitant significant reduction in soil erosion.

In conclusion, the results of the many studies we have discussed in this brief review would appear to suggest that the historical increase in the air'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.

References
Allen, J.R.M., Brandt, U., Brauer, A., Hubberten, H.-W., Huntley, B., Keller, J., Kraml, M., Mackensen, A., Mingram, J., Negendank, J.F.W., Nowaczyk, N.R., Oberhansli, H., Watts, W.A., Wulf, S. and Zolitschka, B. 1999. Rapid environmental changes in southern Europe during the last glacial period. Nature 400: 740-743.

Bachmann, J., Horton, R. and van der Ploeg, R.R. 2001. Isothermal and non-isothermal evaporation from four sandy soils of different water repellency. Soil Science Society of America Journal 65: 1599-1607.

Bauters, T.W.J., DiCarlo, D.A. Steenhuis, T.S. and Parlange, J.-Y. 1998. Preferential flow in water-repellent soils. Soil Science Society of America Journal 62: 1185-1190.

Bond, R.D. 1972. Germination and yield of barley when grown in a water-repellent sand. Agronomy Journal 64: 402-403.

Crabtree, W.L. and Gilkes, R.J. 1999. Improved pasture establishment and production on water-repellent soils. Agronomy Journal 91: 467-470.

Crowley, T.J. 2000. Causes of climate change over the past 1000 years. Science 289: 270-277.

DeBano, L.F. 2000. The role of fire and soil heating on water repellency in wildland environments: a review. Journal of Hydrology 231-232: 195-206.

Favis-Mortlock, D.T. and Savabi, M.R. 1996. Shifts in rates and spatial distribution of soil erosion and deposition under climate change. In: Anderson, M.G. and Brooks, S.M. (Eds.). Advances in Hillslope Processes. John Wiley, New York, New York, USA, pp. 529-560.

Flanagan, D.C. and Nearing, M.A. (Eds.). 1995. USDA-Water Erosion Prediction Project: Hillslope Profile and Watershed Model Documentation. National Soil Erosion Research Laboratory Report No. 10. NSERL, West Lafayette, Indiana, USA.

Idso, S.B. 1989. Carbon Dioxide and Global Change: Earth in Transition. IBR Press, Tempe, AZ.

Knox, J.C. 2001. Agricultural influence on landscape sensitivity in the Upper Mississippi River Valley. Catena 42: 193-224.

Mann, M.E. 2000. Climate change: Lessons for a new millennium. Science 289: 253-254.

Newton, P.C.D., Carran, R.A. and Lawrence, E.J. 2003. Reduced water repellency of a grassland soil under elevated atmospheric CO2. Global Change Biology 10: 1-4.

Olafsdottir, R. and Gudmundsson, H.J. 2002. Holocene land degradation and climatic change in northeastern Iceland. The Holocene 12: 159-167.

Prior, S.A., Rogers, H.H., Runion, G.B., Torbert, H.A. and Reicosky, D.C. 1997. Carbon dioxide-enriched agro-ecosystems: Influence of tillage on short-term soil carbon dioxide efflux. Journal of Environmental Quality 26: 244-252.

Prior, S.A., Runion, G.B., Torbert, H.A. and Rogers, H.H. 2004. Elevated atmospheric CO2 in agroecosystems: Soil physical properties. Soil Science 169: 434-439.

Prior, S.A., Torbert, H.A., Runion, G.B. and Rogers, H.H. 2003. Implications of elevated CO2-induced changes in agroecosystem productivity. Journal of Crop Production 8: 217-244.

Rillig, M.C., Hernandez, G.Y. and Newton, P.C.D. 2000. Arbuscular mycorrhizae respond to elevated atmospheric CO2 after long-term exposure: evidence from a CO2 spring in New Zealand supports the resource balance model. Ecology Letters 3: 475-478.

Rillig, M.C., Wright, S.F., Kimball, B.A., Pinter, P.J., Wall, G.W., Ottman, M.J. and Leavitt, S.W. 2001. Elevated carbon dioxide and irrigation effects on water stable aggregates in a Sorghum field: a possible role for arbuscular mycorrhizal fungi. Global Change Biology 7: 333-337.

Tilman, R.W., Scotter, D.R., Wallis, M.G. et al. 1989. Water-repellency and its measurement by using intrinsic sorptivity. Australian Journal of Soil Research 27: 637-644.

Trimble, S.W. and Crosson, P. 2000. U.S. soil erosion rates - myth and reality. Science 289: 248-250.

Zhang, X.-C. and Liu, W.-Z. 2005. Simulating potential response of hydrology, soil erosion, and crop productivity to climate change in Changwu tableland region on the Loess Plateau of China. Agricultural and Forest Meteorology 131: 127-142.