My name's Charles Vörösmarty and I direct the Environmental Initiative, at the Advanced Science Research Center, which is part of the graduate center located at the City University of New York. I have a bit of a daunting task ahead of me, which is to introduce this entire MOOC, with regard to water and the many roles that it plays in the earth system and in society. And in fact, that's where the whole problem begins. Water does have so many roles in the earth system it exists in: gaseous, liquid, and solid forms. It changes phase, it evaporates, it precipitates out of the sky, it exists as snow or ice, it melts and refreezes, and it really defines the character of the earth system, it’s energy balance. It's part and parcel of huge transports of heat from the world's lower latitudes up towards the poles and radiates back out into space, at least the energy that it carries towards the poles. It really defines much of the climate and climate change. Water is also the essential link between land and the oceans. It wears away the rocks that formed the continental land mass. It takes the sediments that's worn away from those rocks and mixes it with organic matter and other constituents from the land and it delivers them to the world's coastal oceans. Water is also essential to all life forms and ecosystems on the planet, and it also has interestingly a very cosmic dimension. And the search for life is often the search for water and the extra-terrestrial biologists are always looking for hints and signs that water exists on far-off celestial bodies. Life, of course, for humans with would be impossible without water, our economy would not run without water. Our social systems would be stressed to a very, very dire endpoints. Clean water is essential to human health. Water is absolutely necessary to feed humankind through our growing of crops, our aqua culture systems, our fisheries. Water in concert with the economy is also a very interesting way to rebalance water shortages through something called virtual water, where areas of the planet that are water-rich and let's say grow crops that you could normally not grow in much drier parts of the world, they enter the international trade system. And so virtually, a country can import water in terms of the foods that it's consuming and getting from the far corners of the earth. Water also supports our energy systems, for example, providing enormous cooling potential for electric power generation. It's necessary for the industrial sector to run. It's necessary for transportation and it's also necessary to sustain the growing number of people in the growing number of cities that is currently transforming the planet from a rural planet to an urban planet. Many of our perceptions and our understanding of hydrologies come about by what I would term very local perspectives and understandings, and if we talk about issues having to do, for example, with sanitation and access to clean water, we're often talking about delivery systems that serve small communities, serve particular households, very localized in terms of the benefits that we see from sanitation and clean water. If we talk about water that's engineered to help us secure a reliable water supply, we’re talking about building reservoirs. We're talking about creating irrigation systems. We're talking about water treatment facilities that are often very localized in their design and in their character to respond to the very localized conditions and problems that we see, that we're trying to overcome. Urban water supplies are very urban-centric and customized for particular cities that they're serving. So we're facing as a planet a global climate crisis, even though this is a global scale phenomenon, many of the implications of climate change play themselves out through the water system in terms of too much flooding or too much drought, which often are played out again, at very, very local scales. So after literally millennia of coping with water issues on these very local scales, the last 10 to 15, maybe 20 years, we've seen a bit of a revolution in the way we view water systems on planet Earth. We're talking about relying on a operational satellite-based monitoring systems of various aspects of what we call the hydrosphere, looking at patterns of rainfall and snowfall and evaporation and trying to understand how these patterns interact with the global climate system at large. We've been able to assemble what we call geo-referenced, geospatial data sets that bring together information about population distributions, the distribution of different kinds of terrestrial ecosystems, from which we can infer the condition of watersheds, and therefore, how much clean water can actually flow off of these systems. And it's really been a bit of a revolution and a transformation for us to really have perspectives that are far from just the local. And when you begin to put all of this together, you begin to see patterns that you never saw before. And you begin to create a mindset about water that we didn't have 20 or 30 years ago. Looking at the system or realistically looking at the system with the revolution of all these new data sets, we're able to make the statement that between 1.5 and 2 billion people, at the turn of the century, were experiencing severe water stress. That came as a surprise to people. When we put all of this together, we can also begin to ask the question, what condition are these resources in? A few years back, we started to assemble these data sets. We assembled data sets having to do with poor land and watershed management. Lands that were eroding too quickly, lands that were over-fertilized for agriculture. We put these data sets together with data sets, for example, for pollution loads coming out of cities. We made assessments of maladaptive water management, the overuse of water, in particular in areas where you don't have very much water to begin with and you're having to tap into non-renewable water supplies. We also looked at biotic stress ages, like the introduction of exotic species that are taking over many of the world's waterways. And when you put all of this information together, all of this information having very well known negative impacts on the condition of these systems, and you put it into a water system context, you find that there's a very, very dramatic number of ecosystems and dramatic numbers of people who are exposed to very, very high levels of threat to their inland water systems. As best that we can tell, this is a simple byproduct of bad environmental management and it plays itself out through the pervasive element of the earth system called water. I think that this is a very big challenge for us sailing into the 21st century as water becomes more and more relied upon to sustain a society.