Global Warming and the Water Cycle
Earth's average global surface temperature is increasing. It has risen about 1ºF in the last century, and is expected to go up another 3 -11ºF in the next 100 years. This climate change has been dubbed "Global Warming". Although the magnitude of warming may seem insignificant, many natural processes have been profoundly affected. For example, a number of changes in the water cycle have been documented, and many more are under investigation. Global warming intensifies the movement of water through the cycle, because many of the processes involved are temperature-dependent.
The images and questions below address some of the interdependent relationships between global warming and the water cycle. These include the long-term redistribution of water, accelerated water movement between reservoirs, and shifting weather and climate patterns. Study the images and read the information below, then answer the questions. If you want more information, many web sites contain more detailed data on global warming and the water cycle. These two are good places to begin:
Instructions: Study the images and read the information below. Then click on the Question and Answer button to answer the questions that correspond to each section.
Sea Level: A common concern is that global warming will melt the planet’s ice caps and drown the land. Look at the satellite images of recent changes in polar ice, and the map and table describing potential changes in ice amounts and sea level.
- Global sea level has risen approximately 10" in the last 100 years. It is predicted to rise another 3" to 37" by 2100. Even more dramatic increases are possible over several centuries. Explain the relationship between sea level and ice based on your knowledge of the water cycle.
Water evaporates from the oceans and falls as precipitation on land, then runs off back into the ocean. Currently, about 2% of the water in the water cycle is temporarily out of this loop, because it is trapped in ice. When glaciers and ice fields on the continents melt, more water flows into rivers or infiltrates into the ground, and eventually drains into the ocean, which raises sea level. When glaciers grow, like they did during the ice ages, runoff and groundwater discharge decline and sea level drops.
Also, water expands as it gets hotter, so as the oceans warm up, the water in them takes up more space. Since there is so much water in the oceans, thermal expansion could raise sea level significantly.
- Speculate on the effects of sea level rise on people and property.
Obviously, bigger oceans mean less land, and less room for people and cities. Low-lying areas on the edges of the continents and islands will be inundated, and the people living there now will either have to move or spend a great deal of money on dikes and barriers to try and keep the sea at bay. Even areas that remain above water will suffer - storm flooding and beach and coastline erosion will reach further inland. Groundwater, bays, and estuaries will become increasingly contaminated by salt water. As shallow water and coastal habitats shift or disappear, plants and animals that people eat may also decline, causing food shortages.
Weather Events: Study the map showing the recent record of billion dollar weather disasters in the United States.
Individual storms, heat waves, or other weather phenomena can't be blamed on global warming - hurricanes, droughts, blizzards, and the like have been occurring for millennia. However, the increasing temperature has been accompanied by an increase in extreme weather events. Floods and droughts at the end of the 20th century were significantly more widespread, severe, and frequent than they were in the early 1900s.
- Global warming may be making severe weather more common. Can you think of other reasons why weather disasters are becoming increasingly frequent and costly?
A weather event is only considered a disaster when there is significant loss of life and property. As the population grows, there are simply more people and more property that can be harmed. And over time, people have been spreading out, into remote areas that were previously unpopulated, and into areas more vulnerable to extreme weather, like seashores.
Drought: Examine the drought images. One type of unpleasant weather is drought, a time of unusual water scarcity that often stunts and kills both plants and animals. Drought can occur when less rain falls or soils dry out faster than normal, or when rain or snowmelt occurs at the wrong time to meet agricultural needs.
- Explain how higher temperatures could locally influence the water cycle and lead to drought. (Hint: Think about which water cycle process happens fastest when it's hot.)
Hotter temperatures would cause more evaporation from both open water and the soil. As a result, river and lake levels would drop, and soils would dry out. Plants would transpire more in the heat, drawing even more water from the ground. There would be less water on and in the ground just when hot weather was increasing water demand.
- How would drought and drought's impacts on the water cycle affect people and property?
Less runoff means less river flow, and less water for generating electricity and providing transportation and recreation. Drier soils would kill or reduce plant growth, leading to food shortages and making wildfires more frequent and intense. Most seriously, decreases in groundwater and surface water supplies and increases in demand due to the heat mean that there may not be enough clean, fresh water to meet the need for irrigation and even drinking water.
Floods: Examine the flood images. Floods happen when normally dry areas are covered in water. Floods often occur when rain falls or snow melts faster or in greater amounts than the ground can absorb water or rivers can carry it away, or when storm winds push seawater onshore.
- Explain how higher temperatures could locally influence the water cycle and lead to flooding. (Hint: Think about the processes that put water in the air and take it out again.)
Big wet storms seem more likely than small storms to overwhelm the rate of infiltration and the capacity of rivers to carry runoff, and cause flooding. Heating encourages the development of big storms. Hotter temperatures put more water vapor in the atmosphere - the supply of vapor would increase due to increased evaporation, and the warmer air would allow greater amounts of vapor to build up. When all that water finally condenses, enormous clouds will form, and produce similarly enormous amounts of rain and snow.
Also, global warming raises sea level, because ice melts and liquid water expands when temperature rises. So areas that used to be far enough inland or high enough above sea level to escape flooding may now be drowned when hurricanes force seawater onshore.
- How would flooding and the associated impacts on the water cycle affect people and property?
During floods, buildings and crops are washed away and people are drowned. In the aftermath, famine strikes because fields and livestock were destroyed. Pests and diseases may proliferate in waterlogged areas. Paradoxically, flooding often leads to water shortages - debris, toxic chemicals, and sewage picked up by the flood runoff into and contaminate rivers, and some of these pollutants can be injected into the groundwater during infiltration. Increased runoff causes greater riverbank and soil erosion.
Critical ThinkingScientists would like to be able to pinpoint the magnitude and consequences of global warming in the years to come, but they cannot. Precise forecasts are impossible because global warming depends partially on future human behavior, which is uncertain, and partially on the response of planetary climate, which is not completely understood. The tangled interrelationship between climate and the water cycle is just one example of the complexity of the systems involved.
You have seen how global warming can influence the water cycle. Describe ways in which you think the water cycle could affect global warming, and reinforce or diminish temperature change.
Scientists seek to understand and explain how the natural world works. Many of the questions raised in this endeavor have no absolute answers.
There are several ways that the water cycle could accelerate global warming. For example, snow and ice have a cooling effect, because their white and shiny surfaces reflect sunlight away from the surface. If global warming reduces the amount of ice, more solar energy will hit the ground and warm the surface up even more.
Also, if extended droughts decrease the water flowing through rivers, less electricity will be generated hydroelectrically. If this "clean" power source is replaced by sources that emit more of the gases that contribute to global warming, then there will be more warming.
On the other hand, warmer temperatures dry out soils and kill plants. That could cause more dust and dirt to blow into the air, once the plant cover is gone. All those particulates could cool the surface by shielding the ground from sunlight.
And transpiration and evaporation increase when temperature rises. These processes absorb heat, and cool the surface.