changes in the zonation of the global climate, temperature, and distribution of the vegetation types (NGS, 1988); and likely changes in the storage patterns of the organic material in soils.
The broad global and continental-scale changes in the environment, as listed in the preceding paragraph, remain distinct from the shorter-term events that may have pronounced effects on the physical environment and the major material transport paths. Catastrophic events, such as volcanic eruptions, earthquakes, or floods, may profoundly change the physical environment on a more or less great regional scale, affecting in the process the weathering releases through changes in the water drainage pattern, type of rock exposure, or vegetation cover. The entire scope of anthropogenic activities includes a long and continuously growing list of environmental perturbations that may affect to a variable degree the background surficial weathering and transport, on scales from global to regional. Such a list may begin with the possible global climate change due to anthropogenic increases in the atmospheric greenhouse gases and the anticipated changes in temperature and continental water discharge; continue to the anthropogenic accumulation on land and in waters of reduced metals (iron, copper, zinc, and others), chemically reactive calcium-silicate phases in constructional cements, and by-products of the mining and organic chemical industries; and go on to the chemical changes due to fertilizers in soils, and a changing residence time of organic carbon on land due to reduction of the forest-covered area of the continents.
The physical environment of the surficial weathering releases that is considered in this chapter is shown schematically in Figure 2.1. Water evaporating from the ocean surface precipitates as snow and rain; meltwater and net atmospheric precipitation (precipitation less evaporation) flow over the land surface, penetrate underground, reside for variable lengths of time in lakes, and continue to the oceans. Chemical interactions between water and mineral or biogenic solids result in net releases to, and transport by, water flow.
The volume flow or discharge of water from land to the oceans varies greatly over the globe. To allow for differences between water discharge from land areas of different sizes, a more meaningful measure of volume flow is the specific discharge or discharge per unit area q:
where Q is water discharge (km3/yr) and A is the surface area (km2) of a drainage basin as measured on a map.
Specific water discharge values for 5°-wide latitudinal zones of the globe have been compiled for water flows from the continents to the oceans, as well as to the internal drainage basins, by Baumgartner and Reichel (1975). Specific discharge is taken equal to the difference between the annual amounts of atmospheric precipitation and evaporation, and as such it includes both the surficial runoff and the groundwater flow. The values of specific discharge are representative of the water volumes flowing to the oceans from each latitudinal zone if the volume storage on the surface and underground remains constant. An increase in the annual amount of net precipitation (i.e., precipitation less evapotranspiration from land) may be expected to be initially distributed in some greater storage of water on the land surface and underground, ultimately finding its way in the flow to the oceans. A large uncertainty associated with estimates of the length of time it would take for the surface runoff to respond to a major change in the amount of water input to land from the atmosphere is related to the knowledge of the volumes and residence times of surficial water and groundwater.
The value of the surficial global water discharge to the oceans is 3.74 x 104 km 3/yr, excluding the discharge from Greenland and Antarctic, and the global drainage area for discharge to the oceans is nearly 1 x 108 km2 (Baumgartner and Reichel, 1975; Meybeck, 1984; Table 2.1). The volume of global continental ice and snow, excluding the Antarctic and Greenland, is about 122,000 km3 (Barry, 1985). Faster melting of the continental snow and ice sheets at a rate of 1 percent per year would initially increase the discharge from land by up to 3 percent. Increased storage capacity of the meltwater in big periglacial lakes, such as the big lakes occurring on the margins of the Canadian shield and the older lakes (Baker, Chapter 6, this volume), might reduce this estimate of an initial increase in global discharge. Ultimately, however, continuous melting would add more water to the continental surface and likely