It is generally believed earth's hydrologic cycle is intensified at higher temperatures; and, in some parts of the world, it is.  Less precipitation in tropical Venezuela, for example, has been inferred from proxy climate records of the cold Younger Dryas and Little Ice Age compared to the warmer Holocene Optimum and Medieval Warm Periods (Haug et al., 2001); and climate alarmists have used such findings to support their predictions of more frequent and severe runoff in response to rising global temperatures.  Although this hypothesis may sound reasonable on the basis of these observations, more extensive real-world data demonstrate it to be globally incorrect.

As a first example of this fact, Winsor et al. (2001) analyzed several hydrographic data sets pertaining to regions bordering the Baltic Sea, including river runoff over the period 1920-1990.  The results of their analysis showed relatively small variations throughout the record, but no long-term trend was identified.

In another study, Cluis and Laberge (2001) utilized streamflow records stored in the databank of the Global Runoff Data Center at the Federal Institute of Hydrology in Koblenz, Germany, to see if there were any runoff changes of the type predicted by IPCC scenarios of global warming, i.e., increases in streamflow and increases in streamflow variability that would lead to more floods and droughts.  The study encompassed 78 rivers said to be "geographically distributed throughout the whole Asia-Pacific region."  The mean start and end dates of the flow records of the 78 rivers were 1936 ± 5 years and 1988 ± 1 year, respectively, representing an approximate half-century time span.

The results of Cluis and Laberge's analysis showed that mean river discharges were unchanged in 67% of the cases investigated; and where there were trends, 69% of them were downward, indicative of less runoff.  Maximum river discharges were even more stable, remaining unchanged in 77% of the cases investigated; and where there were trends, 72% of them were also downward, indicative of less runoff during the rainy season.  Minimum river discharges were unchanged in 53% of the cases investigated; and where there were trends, 62% of them were upward, indicative of more runoff in the dry season.  Thus, runoff in the Asia-Pacific region has become less extreme at both ends of the water-availability spectrum during the past half-century of modest global warming, indicative of less water running off the land when flooding is more likely, and more water being available to both run off the land as well as infiltrate it when drought is more likely, both of which situations must be considered beneficial.

Similar findings have been reported by Knox (2001).  Since the 1940s and early 1950s, the magnitudes of the largest daily river and stream flows in the Upper Mississippi River Valley watershed have been decreasing at the same time that the average daily flow has been increasing, revealing a trend toward more overall water availability but less likelihood of flooding.

Why do the historical real-world data described above run counter to climate-alarmist predictions of more frequent and extreme runoff events in a warming world?  One answer may be that global air temperatures have not risen all that much over the past 70 years (see our Editorials of 15 June, 1 July, 15 July, 2 August, and 9 August 2000), contrary to climate-alarmist claims of "unprecedented" global warming over this period.  A second answer could be that the models upon which the world's climate-alarmists base their predictions are simply wrong in certain important respects, leading them to simulate wildly inflated temperature and precipitation increases in response to the historical increase in the air's CO2 content.  A third answer may be that the aerial fertilization effect of the atmosphere's rising CO2 concentration has progressively enhanced the vitality of earth's vegetation and thereby helped it stabilize the planet's land surfaces, reduce soil erosion, and enable more water to infiltrate the ground.

A striking visual testament to the reality of one or more of these phenomena is provided by a pair of photographs of Bohemian Creek, La Crosse County, Wisconsin (Trimble and Crosson, 2000).  The first of these photos, taken in 1940, shows 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, shows that the stream channel "is narrower, smaller, and more stable."  Also, "the coarse sediment has been covered with fine material, and the flood plain is vegetated to the edge of the stream."  What is more, Trimble and Crosson note that conditions improved even more over the 25 years that elapsed between the time of the second picture and the writing of their paper.

We also note that not all climate models predict disastrous increases in runoff in a warming world.  In the first of two such studies, Hulme et al. (1999) analyzed results from GCM simulations and two environmental response models to estimate the effects of natural climate variability and potential human-induced climate change on river runoff and agricultural wheat yield potential in Europe over the next 50 years.  The results of their analysis showed the impacts of natural climate variability on both runoff and wheat yields to be "as great as, or greater than, the estimated impacts of human-induced climate change."

Finally, Baron et al. (2000) employed a regional hydro-ecological simulation model to evaluate the consequences of a doubling of the air's CO2 content and 2 to 4°C increases in air temperature on a high-elevation Rocky Mountain watershed.  The authors note that the 4°C increase in air temperature did not perturb total runoff very much.  However, it did cause seasonal snow melt to begin four to five weeks earlier than it does currently, allowing the melt water to infiltrate the soil more gradually and for a longer period of time than at present.  According to the authors, this outcome is beneficial, because the gradual release of nitrates that are retained in the snowpack and otherwise released in a large pulse in the spring relieves some of the ecological pressure caused by high nitrate concentrations in typical springtime flows.

In light of the results presented above, it does not appear we have anything to fear in the way of catastrophic increases in runoff to streams and rivers in the years and decades ahead, even if the planet were to warm a bit more.  In fact, at the high end of the runoff spectrum, where floods occur, some observations indicate a tendency for runoff to decline with increasing temperature.

References
Baron, J.S., Hartman, M.D., Band, L.E. and Lammers, R.B.  2000.  Sensitivity of a high-elevation Rocky Mountain watershed to altered climate and CO2.  Water Resources Research 36: 89-99.

Cluis, D. and Laberge, C.  2001.  Climate change and trend detection in selected rivers within the Asia-Pacific region.  Water International 26: 411-424.

Haug, G.H., Hughen, K.A., Sigman, D.M., Peterson, L.C. and Rohl, U.  2001.  Southward migration of the intertropical convergence zone through the Holocene.  Science 293: 1304-1308.

Hulme, M., Barrow, E.M., Arnell, N.W., Harrison, P.A., Johns, T.C. and Downing, T.E.  1999.  Relative impacts of human-induced climate change and natural climate variability.  Nature 397: 688-691.

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

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

Winsor, P., Rodhe, J. and Omstedt, A.  2001.  Baltic Sea ocean climate: an analysis of 100 yr of hydrographic data with focus on the freshwater budget.  Climate Research 18: 5-15.