Ed. note: This is the first post in the "Dumb" Questions series of posts that TreeHugger is writing to provide answers to some basic questions about the environment and all things green. We realize that many of these questions/answers will seem, well basic, to most of you, but please bear with us: we just want to make sure that everybody is on the right page! After all, what's the fun in having you read our posts if you get stuck on a basic premise?
To answer this question, it helps to first have a rudimentary knowledge of the planet's hydrologic, or water, cycle. The water cycle, in essence, describes the processes by which large quantities of water move continuously through the Earth's oceans, land and atmosphere over short and long time scales. It is primarily dominated by the oceans — which account for 96% of the planet's water and where 86% of global evaporation takes place — though it has no defined starting or ending point.
Being a closed system, the water cycle depends on an equal number of inputs and outputs to function — which can include, but are not limited to, snowmelt (melted water flowing from glaciers and ice caps) and precipitation on one hand and evapotranspiration (water transpired from plants and evaporated from the soil) and surface/subsurface storage of water in lakes or aquifers on the other. In general, more water is often stored in reservoirs — like the oceans — than moves through the water cycle. This is especially true during warmer climactic periods when, as a result, average global sea levels tend to rise. Colder climactic periods, on the other hand, are characterized by the formation of more ice caps and glaciers — which come to accumulate a significant portion of the Earth's water supply.
If this is then the case, how can we account for the fact that we keep on hearing that we're running out of water? For one thing, the tremendous and unprecedented rise in our global population — especially in lesser developed countries — has put significant pressure on our finite supplies of freshwater. Whether it be through our contamination of some of the water supply, our relentless urbanization or our aggressive extraction of its reservoirs for agricultural irrigation, anthropogenic influences have sparked major variabilities in the Earth's water cycle whose implications we do not yet fully understand.
As was exemplified this summer by the unusual weather patterns that afflicted various regions of the world — prompting catastrophic floods in parts of Britain and severe droughts in Africa and the southwestern parts of the United States — climate change will come to play an increasingly important role in shaping the future of the planet and its water cycle. According to the IPCC's latest climate model projections, the water cycle will intensify throughout the next century as annual precipitation continues to rise in the already wet near-equatorial and high latitude regions while continuing to fall in dry sub-tropical regions — trends that can neatly be summarized by the following expression: "the wet will get wetter and the dry drier."
More broadly, the increases in global temperatures we are witnessing — which have already helped trigger the rapid and large-scale melting of ice sheets in Greenland — will continue eliminating valuable freshwater buffers (like glaciers) and contribute to ever rising sea water levels. These rises, in turn, may result in some saltwater bodies encountering once isolated freshwater bodies, contaminating them and making them inaccessible to us for our needs. This will only spur unsustainable practices such as the continued drilling and extraction of water from underground wells.
The likelihood of more unpredictable positive feedback loops arising as a result of our actions and the changing climate could worsen an already dire situation.
Based on projections made by the International Water Management Institute, freshwater shortages are likely to become more prevalent among poorer, rapidly developing countries in Africa and Asia by 2025 — a consequence of overpopulation and poor water infrastructure. Developed countries in Europe and North America will not see a serious threat to their water supply because of their wealth and technology, and because many of their populations are expected to decline over the coming years — thus better aligning them with their needs. These are only projections, of course, so we may end up facing a much harsher reality in 2025 — or, hopefully, a more pleasant one if we do a better job of addressing the underlying problems.
Well, that about does it for this episode of TreeHugger's "Dumb" Questions series. Hopefully this post was of some use to those of you still perplexed about the planet's water cycle and the state of our current and future supply of freshwater. We could've gone on (there's a lot more where that came from) but thought this was enough for a primer.
We're sure that some of you will have more questions about some of the topics we tackled (and, more likely, didn't tackle), so feel free to fire away in the comments or, better yet, in this forum thread!