Water, Water, Water(1):
The observed and predicted changes to the climate that are forced by carbon dioxide and other greenhouse gases is often called “global warming.” Few argue that the Earth has warmed in the past century. There is a large body of evidence that this observed warming is related to increasing greenhouse gases. Greenhouse gases increase because of the burning of fossil fuels. This warming can be distinguished from warming that occurs due to known sources of natural variability. If there is warming due to unknown sources of variability, well – we really can’t say anything about that which we know nothing about. We continue to look for that which we do not know, but the likelihood of us finding a mechanism other than greenhouse gas increases to explain the current warming is very low. The physics of the warming are simple and robust, and the information collected from many sources is consistent to a very high degree. There is some information that is inconsistent or still not satisfactorily explained. I cannot point to any inconsistency that I am aware of that would be a potential smoking gun to refute the basic tenets of “global warming.”
Warming is the most simple and the most sure prediction. An increase in sea level rise due to both the warming of the oceans and melting of ice on land is also quite certain. The change in sea level is an indicator of the role of water in the climate. In a consequential way, climate change is more about water than it is about warming. The difference between ice ages and temperate periods is a difference between water being stored as ice, liquid, or vapor. The temperature of the atmosphere strongly influences the amount of water vapor that can be held in the atmosphere as well as the amount of ice that exists on land. It is the balance between the different phases of water that defines climate regimes like the ice age, temperate periods, and a greenhouse.
People, economies, and ecosystems have evolved or adapted to the balance of water. From a basic biological point of view, people have shown the ability to survive in virtually all types of watery environments. The ability to thrive is perhaps most closely related to the ability to produce the energy of food crops, and this ability is strongly linked to having water available in the liquid phase. In today’s world there are enormous amounts of water used in the production of energy. It’s used to cool and clean and heat for stream. In the productions of bio-fuels such as ethanol, water is used at many stages in the process. Water, therefore, is at the center of it all. It is central to the stability of the physical climate, ecosystems, agriculture, energy production, and, of course, we need water to drink.
An important point is that water is a stressed resource. Water is a stressed resource, and climate change is an additional stress on this resource. Climate change is not the cause of the stresses on water. In most cases climate change will amplify the stress on water resources. There might be some places where climate change improves the availability of liquid water. Even in this case, however, changes in the expected distribution in water will challenge the large engineering projects that we use to manage water. Water is one of the places where climate change, engineering, policy, and litigation intersect most strongly.
The balance of water between vapor, liquid, and gas is not only of central interest to the global climate, but the phase transitions of water also play a central role in weather. When water changes from vapor to liquid or from liquid to ice, energy is released. When ice melts and liquid evaporates energy is absorbed. This energy is in the form of heat. In clouds there is ice, liquid, and vapor, and conversion of vapor to liquid and ice is important not only for the production of precipitation, but for the release of heat. The release of heat then makes the air more buoyant and the clouds rise higher.
The energy released by water is important to developing weather systems. As many WU users know, there are some significant differences between weather systems in the tropics and weather systems in middle and high latitudes. While the release of energy by the condensation of water vapor is important in most weather systems, this mode of energy conversion is more important in the tropics than outside of the tropics.
Water is also a greenhouse gas. In fact, in the Earth’s radiative balance, water is by far the most important greenhouse gas. Its influence on the radiative budget is larger than the influence of carbon dioxide. Carbon dioxide works on the margins; those who own a business know how things are won and lost on the margins. One way to think of carbon dioxide is like having a window cracked. The quirks of the heating system in my apartment make it very warm, and a small crack in the window makes the place habitable. The surface of the Earth cools directly to space in only a small crack in the sky. Carbon dioxide is closing that crack. (Who remembers Crack the Sky?)
The oceans of the world are huge supplies of water. As the atmosphere and the oceans warm, more water vapor is held in the atmosphere. The consequences of this are 1) there is more water to act as a greenhouse gas, which warms the surface some more (a positive feedback effect); 2) there is potentially more energy to be released in developing storms, and hence, the potential for stronger winds; 3) there is more water available for rain and snow, and hence, the potential for heavier rain and more floods; 4) the fundamental modes by which middle latitude storms get their energy might change with time. (Looking forward to the responses from this one! Plus, where do more droughts come from? Of course there is the huge complication of clouds that fascinate meteorologists, climatologists, and the committed blog readers of the world.)
Water, water, water: Just because the globe is warming does not mean that it ceases to snow. In fact, there are many situations and places where the snow might increase. That the planet is warmer does not mean that it no longer gets below freezing. The high mountains near the coast, like the Cascades and the Sierra Nevada would expect more snow. This is also true for the high altitudes parts of Greenland and Antarctica. It is reasonable to expect stronger snow storms in many parts of the eastern U.S. because 1) there is more water in the air, and 2) it still gets below freezing. Near the Great Lakes more snow is possible because the lakes don’t freeze over, and they can keep supplying moisture to the air.
The real challenge of water is how long it stays as ice and snow. If it is ice and snow and it melts slowly, then it is a good supply of water. If it melts rapidly, then it can be a flood and might be lost for summer irrigation. If there is snow on the ground it reflects solar radiation back to space. If there is snow on sea ice it keeps the ice from melting in the summer. For climate an important variable is how long the snow stays around.
Figure 1: The water cycle from the U.S. Geological Survey