How much water is on earth? Is that amount changing?

While seemingly basic questions, they are actually a bit complex and there is much research occurring around them right now:

  • 1. The amount of water on earth is probably best divided into five primary categories: the oceans, liquid freshwater, non-seasonal frozen water, groundwater, and water vapor. Currently, the oceans hold about 96.5 percent of the earth’s water (derived from a 1993 book edited by Peter Gleick). Non-seasonal frozen water is the next biggest category (which includes frozen freshwater in glaciers and permafrost but not seasonal snowpack; it also includes frozen fresh- and saltwater at the poles), which totals to about 2 percent. Groundwater is the next biggest category — this is all of the water below the surface on the ground but not including permafrost. Groundwater could be fresh (especially in the soil zone, which is relatively shallow) but often has high mineral concentrates. It totals about 1.6 percent. That leaves a very, very small amount of water for the atmosphere and lakes and rivers -- much less than 1 percent.
  • 2. The amount of water does change on the earth, and it changes in large amounts. A recent paper by Pope and colleagues suggests that there was roughly 26 percent more water on the earth about 3.8 billion years ago (roughly when life began on the planet) than there is now. The authors believe that hydrogen (one of the elements necessary to form water) is slowly lost to space in the high atmosphere, and over very long timescales the amount of hydrogen lost has ultimately reduced the volume of water significant amounts on the earth’s surface. Water is also created de novo as a product of chemical reactions, especially within the earth, and is also consumed by chemical reactions. These amounts tend to be relatively small and probably cancel each other out, at least in this period of earth’s history.
  • 3. Moreover, the relative amount of different types of water shifts, too, a trend that reflects shifts in climate. Near the end of the Pleistocene era, for instance, about 18,000 years ago, massive glaciers covered much of the northern hemisphere — Europe, North America, and Asia — and glaciers were even found in Africa. Sea levels were over 100 meters (about 330 feet) lower than today, and all of that ocean water was locked up in glaciers and polar ice caps. In some cases, as far south as 45 degrees latitude, those glaciers were several kilometers thick. However, beyond the glaciers, rivers and lakes were generally larger (and in many cases became enormous as the glaciers began melting). Paleoclimatologists have even referred to “pluvial” periods, which are characterized by more precipitation and generally wetter terrestrial conditions. Generally, the earth has been trending towards drier conditions (that is, less freshwater) over the past 10,000 years (a great book on this by Pielou is After the Ice). As recently as four thousand years, rivers such as the Nile or Columbia were much larger than today. During the Roman era, for instance, northern Africa was widely populated, and areas that are currently desert supported cities and agriculture and large populations of wildlife.
  • 4. The last point probably begs the question: where are we headed now? This question is not very easy to answer. Climatologists have noted that generally warmer global temperatures mean that the atmosphere can hold more water than in the recent past, resulting in a trend since about 1950 of increasing global mean precipitation quantities. They refer to this as an “intensification” of the global water cycle. But the distribution of that precipitation is changing too. Dry places seem to be getting more dry, and wet places seem to be getter wetter. But variability — the frequency of extreme events such as droughts and floods or very hot or very cold days — is also changing. And often variability is increasing everywhere, so that we see more droughts AND more floods in the same place. This has been called “global weirding,” and my own hunch is that global weirding reflects that rapid flux of the global climate system as new weather and climate patterns evolve. We are likely to have global flux for at least a century.
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