Little is known about the contribution of global warming to the rise in global sea level. The amount of water that results from the melting of glaciers and ice sheets cannot be determined by looking at the rise in global sea level alone. Rises in global sea level that occurred before the world's reservoirs were built are difficult to explain. The exact magnitude of the rise in global sea level is in dispute.
Global warming has contributed to a rise in global sea level not only because it causes glaciers and ice sheets to melt, but also simply because when water is heated its volume increases. But this rise in global sea level is less than it otherwise would be, since over the years artificial reservoirs have been built all around the world that collectively contain a great deal of water that would otherwise reach the sea.
Which one of the following can most reasonably be concluded on the basis of the information above?