Environmental Health Center
People in Kansas City and St. Louis, Missouri, still shudder to remember the flood of 1993, which killed more than 50 people and caused tens of billions of dollars in damage across half a dozen states. Even more deadly, though, were the drought and heat wave that gripped the Midwest in 1988, which ruined crops and were blamed for killing more than 5,000 people.1
Some media accounts were quick to pin blame for the 1988 drought on global warming — a premature judgment,2 as it turned out. The 1993 floods have been linked, with more reason, to El Niño. Both events stand as reminders of some stark realities.
The Mississippi basin is especially vulnerable to floods and drought. These and other weather extremes could well be worsened by global warming and the swings of the El Niño cycle. The corn, soybeans, and wheat grown in the Mississippi basin feed not only the United States, but also much of the world, and weather can mean the difference between a bumper crop and a ruined one. The volume and power of the river can make floods, when they occur, disastrous for the millions of people who live in the way of rising water. The Mississippi system is also a major route for the transport of coal, grain, and other bulk cargoes — and flood, drought, or freeze can cripple shipping for months at a time.
These are only some of the ways weather extremes affect the Mississippi River basin. They can also cost lives, damage property, affect the supply of safe drinking water, create or destroy habitat for migratory waterfowl, and disrupt rail and highway travel.
Although the vulnerabilities of the Mississippi are easy to see, they may be harder to do something about. Extreme weather is still hard to predict in the Mississippi basin, despite recent advances in weather and climate prediction. Further study about basic climate processes is under way and will improve predictions.
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| Source: U.S. Geological Survey |
It reaches from Minnesota to Louisiana and from New Mexico to West Virginia. Yet the great bulk of the Mississippi basin encompasses overlapping regions called the Midwest, the Great Plains, the prairie, the corn belt, and the grain belt.
The Missouri River and Ohio River basins, which could be thought of as regions in and of themselves, drain into the Mississippi. Even the main stem of the Mississippi River changes character as it flows south from Minnesota to Mississippi. The upper and lower Mississippi can often seem like two different worlds.
The Mississippi basin may be too large to think of as one region in terms of climate, agriculture, industry, population, or culture. Yet the people and states upstream are linked inextricably to those downstream in the common destiny of a single water system.
The waters overwhelmed the normal river channel, and serious flooding hit Illinois, Iowa, Minnesota, Missouri, and other states in the Missouri and upper Mississippi basins, an area 500 miles long and 200 miles wide. Fifty-two people died in the flood. At least 70,000 others were forced from their homes, and 50,000 homes were damaged or destroyed. The water knocked out sewage plants, destroyed crops on 12,000 square miles of farmland, and blocked road, rail, and barge traffic. Total damage was put between $15 billion and $20 billion. 3
An El Niño cycle was already a year old when the flood of 1993 hit. 4 (For background on El Niño, see http://www.pmel.noaa.gov/toga-tao/el-nino-story.html.) Many climate scientists did blame El Niño for the 1993 flood. But that does not mean that every El Niño will produce Mississippi flooding, nor does it mean every Mississippi flood can be pinned on El Niño. The year of the previous “Great Flood,” 1927, when seemingly endless winter and spring rains caused even more devastating flooding in the lower Mississippi, was not an El Niño year. Both of these extreme floods can be explained as expressions of the “normal” variability of weather in the Midwest.
Weather varies. Some of the variation is caused by things we understand, some is caused by things we do not understand, and some appears to be truly random.
Most of the time, weather in any particular place tends to follow average tendencies, which we call climate. Knowing that there is a strong likelihood that the rainfall and temperature in a given county will follow a predictable pattern allows farmers to plant crops and builders to design houses. But knowing that improbable extremes of weather occasionally do occur is also useful. People in Kansas, for example, may choose to build storm cellars even though they personally may never get close to a tornado.
In the last decade, climate scientists have learned much more about the climate system. Especially dramatic have been improvements in the understanding of the El Niño-Southern Oscillation (ENSO), a cycle that repeats every two to seven years. El Niño is a warming of the surface of the tropical Pacific that disrupts atmospheric circulation, causing weather changes around the globe. ENSO includes not only El Niño, but its opposite, La Niña, a cooling of the tropical Pacific surface.
Scientists have also come to a better understanding of the role of soil moisture in affecting precipitation patterns and climate. In the spring and summer of 1993, moisture from the soils in the southern states evaporated and was transported northward, where it produced more severe storms in the areas of greatest flooding. According to Dr. Alan Betts, the European Center for Medium Range Weather Prediction found that it could improve its rainfall predictions for this period markedly by including these soil moisture effects in its models. Soil moisture anomaly effects over land can last for months, with wetter areas experiencing higher evaporation rates and lower maximum temperatures.
Improved understanding of ENSO has helped scientists move many extreme weather events from the “not understood” category to the “understood” category. What once seemed random has become more predictable. Weather once considered “extreme” can now be seen as normal variability.
Knowledge of weather connections to El Niño is better for some parts of the world (such as the tropics) than for others (such as the Upper Mississippi or other large land masses in northern latitudes). Predictions for El Niño-related winter weather are stronger than predictions for summer weather. And predictions for precipitation are better than predictions for temperature.
An El Niño (the ENSO warm phase) is likely to produce different weather effects in the northern states than it would in the southern ones. Typically, the period from November through March is cooler and wetter in the southern states during an El Niño. In the North, less precipitation and warmer-than-average temperatures are likely during the same period.
Since the Mississippi basin spans both regions, El Niño’s effects on the entire basin can be complex. Higher precipitation in the southern states may be offset to some degree by lower precipitation in the northern states. So floods could be intense in southern states and tributaries, while total discharge from the mouth of the Mississippi would be less extreme.
Data from the Midwest Climate Center at the University of Illinois support this picture. Researchers there gathered data on total winter snowfall in the Midwest during eight recent strong El Niño events (1957–58, 1965–66, 1972–73, 1982–83, 1986–87, 1987–88, 1991–92, and 1994–95), from 3,690 stations in Illinois, Indiana, Iowa, Kentucky, Michigan, Minnesota, Missouri, Ohio, and Wisconsin. They found significant reductions in snowfall compared to average — as much as 10 to 20 inches less in some regions.
“El Niños are a natural part of the variability of the global climate system,” said Ken Kunkel, director of the center. 5 “While there does appear to be some impact of El Niño on the climate of Illinois and surrounding states, a wide spectrum of climate conditions have occurred during past El Niños. This illustrates there are other natural factors influencing our climate.” While these other natural climate variations may currently be unpredictable, the goal of further research is to improve their predictability.
But even “knowing the odds” does not make for perfect predictions. For example, summers overall during an El Niño are generally slightly cooler and wetter than average. But, Kunkel says, out of 10 El Niño summers, 5 were cooler than normal, 2 were warmer than normal, and 3 were near normal. Of the same 10 summers, 5 were wetter than normal, 3 were drier than normal, and 2 were near normal.
So Kunkel’s office could predict “slightly increased chances” of a wet and cool summer and fall and a dry and warm winter in the Midwest as an El Niño was brewing in the summer of 1997. However, Kunkel noted, “... the increased chances for these conditions are only 5 to 10 percent better than a coin flip.” As it turned out, the winter of 1997–98 proved one of the warmest on record over most of the Midwest. Kunkel’s prediction was right.
More experience leads to better predictions. It may not be coincidence that the 1997-98 El Niño was one of the most intense events measured with modern instruments. Scientists now think that more dramatic changes in Pacific surface temperature may make the “expected” long-distance effects on places like the Midwest more likely. In other words, strong El Niños allow more certain predictions.
During the cold phase of ENSO (La Niña), temperature tendencies are generally reversed, with the northern Mississippi basin likely to feel colder winters and the southern basin likely to feel warmer ones.
Extremes of temperature and precipitation can and do occur at any time in the ENSO cycle for reasons largely unrelated to it. But because ENSO swings weather from one end of the scale to another, there is a relationship between ENSO phases and the likelihood of weather extremes.
The effect of ENSO on the probability of flood and drought in the Mississippi basin is somewhat harder to see. These effects are different in different parts of the basin (that is, one area may tend toward drought while another tends toward flood), and they are different according to the time of year. The Mississippi drainage system, in effect, adds up or integrates all these differing tendencies. By the time waters reach the Mississippi delta, some of the effects have canceled each other out, and the uncertainties of predictions are compounded.
Nonetheless, the swings of ENSO and weather extremes are connected, and further research can make the connections clearer and increase the usefulness of predictions.
Most climate scientists have become convinced that the increase in atmospheric concentrations of carbon dioxide (CO2) and other greenhouse gases emitted by human culture will, if it continues, warm the Earth’s climate. They are less certain about how much it will warm and how fast. Most conclude that more precipitation will fall worldwide. The estimates come from computer models that use what is currently understood about physical climate processes to simulate how the Earth might respond to increasing greenhouse gases. But limits on computer power currently restrict the fineness of geographic detail that these models can resolve. As a result, there is considerable uncertainty about how climate change might affect different regions within North America.
Still, there is a lot we can say.
For example, scientists think that global warming is likely to bring sea level rise (although again, timing and magnitude are less certain). Higher sea levels would have drastic consequences in low-lying coastal areas like the wetlands of Louisiana. Those wetlands, which provide valuable wildlife habitat, are already being lost — as the land subsides and human changes to the river prevent sediment from naturally rebuilding the wetlands. A sea level rise of one to three feet could submerge 70 percent of Louisiana’s wetlands. As wetland buffers are lost, many Gulf Coast communities, including New Orleans, will be more vulnerable to storm surges and hurricanes. Sea level rise could have other effects, such as increasing the salinity of estuaries or coastal aquifers.
For another example, warmer temperatures would probably reduce spring ice jams and resultant flooding which currently happen in northern parts of the central plains. Although damage from such floods is costly, they also help recharge wetlands.
Physical principles suggest that global warming will have major effects on the hydrologic system — rainfall, snowmelt, runoff, streamflow, and evaporation. Unfortunately, specific regional details remain fuzzy, and better understanding is crucial.
Most models of a warmer greenhouse world estimate an increase in precipitation over much of North America. Such an increase might be expected to increase soil moisture and streamflow. But higher temperatures will also mean that water will evaporate more readily into the atmosphere (removing it from streams). Some biologists also believe that in a greenhouse world, plants will more readily soak up soil moisture through their roots and breathe it out through their leaves (a process called transpiration).
The big question is how these forces will balance out in any particular region. Will precipitation outweigh evaporation and transpiration — or vice versa? It could go either way, depending on things like prevailing climate, terrain, soil type, and vegetation.
Working Group II of the Intergovernmental Panel on Climate Change (IPCC), in its 1998 Regional Impacts report,6 states:
In general, increases in winter and early spring temperatures under a doubled-CO2 climate could shift hydrological regimes toward greater flows in winter and early spring and lower flows in summer in the mid- and high-latitude regions of North America .... River and reservoir systems that are fed by snowmelt or rely on glacier melt for spring and summer flow during critical periods of high agricultural and municipal demand and low precipitation may have critical supply-demand mismatches. California and the Great Plains and prairie regions of Canada and the United States could be particularly vulnerable.
The Working Group II report also cites studies projecting that the frequency and magnitude of extreme rain and snow storms would increase in a warmer climate, particularly in central North America. The same studies also suggest that dry spells would last longer. Working Group II’s findings have not yet been endorsed by the full IPCC, which will review them by the year 2000.
The weather extremes associated with ENSO provide an instructive example of what life in the Mississippi basin might be like in a greenhouse world. Learning how to cope with these changes, by adapting to them or mitigating them, could be a useful dry run for much bigger changes to come. Improved ability to understand and predict the current “natural” climate variability will be an important tool for dealing with the effects of global warming on the region.
The seriousness of a flood depends on a number of things, especially the intensity and duration of rain or other precipitation and the area it covers. A flood is an accumulation. A cloudburst that dumps inches of rain on an area in an hour can produce a flash flood. But a slower rain, if it goes on long enough over a wide enough area, can bring a major flood to the river into which that water drains. In the case of the Mississippi system, the drainage area is immense, second in area only to the Amazon. So the Mississippi has a running start in accumulating large quantities of floodwater.
The connection between rainfall and riverflow is not always simple. Soil moisture is a key variable. When rain falls on parched and porous soil, it soaks in, and little of it enters the river immediately. When soil is already soaked (or frozen), however, less water can soak in, so rainfall runs off over the land into streams and rivers. Many rivers have a baseflow that comes from groundwater, but even this baseflow can vary seasonally. When vegetation blooms in the spring, plants pull large amounts of water out of the soil, and when plants die in the fall, base streamflows may increase noticeably. Not only does weather affect soil moisture, but over an area as vast as the Mississippi basin, soil moisture can also affect weather. The amount of water evaporated or “breathed” out of soil and plants constitutes an immense input of water and energy into the atmospheric weather system. It is not surprising, then, that hydrologists define drought not in terms of streamflow, but in terms of soil moisture.
In fall of 1992, soil in the upper basin of the Mississippi was already very moist. Winter rain and snow further saturated the soil to the point where it could absorb almost no more water. Then the rains started in earnest. 7 A particular rain-producing pattern of atmospheric circulation stalled over the Midwest, and from June through August 1993, storm after storm drenched the Mississippi basin. In some places nearly 30 inches fell — about twice the normal amount.
What caused the June-through-August rains? The jet stream had shifted much farther south than normal and was making storms out of a stream of moist air coming from the Gulf of Mexico. But why had the jet stream been pushed south? Kevin Trenberth of the National Center for Atmospheric Research and others 8 concluded that El Niño conditions in the tropical Pacific were probably responsible for the shift. Other researchers, 9 however, say a lee trough (that is, a low pressure area created by the wind-shadow of the mountians)created by the Rocky Mountains was enough to explain the shift.
Trenberth raises the intriguing possibility that another mechanism was also at work in the 1993 flood — soil moisture. 10 He suggests that once a wet period gets started, soil moisture actually contributes to further rainfall — a positive feedback loop which perpetuates the condition.
The lower Mississippi basin escaped the ravages that typified the 1993 flooding in the upper Mississippi and lower Missouri basins — even though the discharge of water at the mouth of the Mississippi reached highs not seen for half a century or more. 11 Much of the credit for the lower basin’s escape goes to the various flood control works (usually levees and floodways) built by the U.S. Army Corps of Engineers and others over more than a century. But it was also the result of where and when the rain fell.
In 1927, the lower basin was not so lucky. 12 That flood inundated some 27,000 square miles, lands where 930,000 people lived. An estimated 330,000 people had to be rescued from roofs, trees, or high ground. Government agencies estimated the number of people killed variously at 246 or 313, but other estimates put the death toll at more than 1,000. Many of the dead were poor African Americans whose deaths were not documented. Direct and indirect economic losses were as high as $1 billion in 1927 dollars.
The 1927 flood was catastrophic because a rising river overwhelmed the system of levees meant to contain it. Those levees didn’t simply contain floods; they “reclaimed” floodplains and wetlands for agricultural enterprises such as huge cotton plantations. By closing off floodplains, the levees eliminated a natural buffer which absorbed the force of floods. Because levees allowed people, farms, and buildings to move into the floodplain, they increased the potential damage from a catastrophic flood.
As long as the levees held, everything was fine. The levees had been progressively raised and strengthened over many decades of economic growth and flooding along the Mississippi. In 1927, authorities such as the Mississippi River Commission publicly expressed confidence that the levees were adequate — but of course they were not. Starting in 1928, efforts to strengthen levees were redoubled. Engineers also began designing a series of floodways — vast tracts of floodplain farmland into which an overflowing river could be spilled to minimize economic damage.
Today, there is much more reason to be confident than in 1927. But with the Mississippi, there are no guarantees. In the 1993 flood along the upper Mississippi, about 1,000 out of 1,300 levees failed to hold back flood waters. 13
Tourists in New Orleans eating beignets at the Café du Monde can have the unsettling experience of looking up at boats on the Mississippi River. Residents have grown used to it. Even during normal flow, the surface of the river is many feet above city sidewalks. In New Orleans, the water is raised not only by the levees containing it, but by sediment buildup that raises the river’s bottom. Should the river ever breach the levees, the damage, and potentially the loss of life, would be enormous in New Orleans. For protection from catastrophic floods, New Orleans relies on the upstream diversion of the Mississippi through the Atchafalaya River spillway.
Ultimately, flood protection is limited by the money available, if not by natural laws. Engineers may design flood works to withstand a hypothetical “100-year flood” — the greatest flood statistically likely to happen in 100 years — or even a 500-year flood. Those benchmarks are not infallible. Not only do climatic conditions change over the decades, but natural processes and human activity also change the river. The “iffy” nature of probability means that there is no way of telling whether the “500-year flood” will come next year, 499 years from now, or not at all during the 500-year period.
People in some places have found themselves facing “100-year floods” two or three years running. They end up wondering about the value of 100-year flood design criteria. Probability calculations based on averages may also miss a weather/climate phenomenon known as persistence. The 1993 flood resulted from just such an abnormally persistent weather pattern. 14 The weather for any particular day or month is influenced in various ways by weather that has preceded it, through soil moisture effects. Wet weather may encourage more wet weather at locations downwind. Because flooding is a cumulative thing, this raises the possibility of floods more catastrophic than averages alone would suggest.
On the other hand, the long period of flooding on the upper Mississippi and lower Missouri in the summer of 1993 also played havoc with transportation. The flood damaged or destroyed many bridges. Between July 16 and July 20, no bridges were available for river crossings on a 212-mile stretch of the Mississippi between Burlington, Iowa, and St. Louis, Missouri. Barge traffic stopped between St. Paul, Minnesota, and Cairo, Illinois, from late June through early August. Some 5,000 loaded barges were unable to move, resulting in lost revenues of about $3 million a day.
Heavy rains can also increase the sediment carried by the Mississippi, which makes it harder for deep-draft boats to navigate river-mouth ports and makes millions of dollars worth of additional dredging necessary. Winter ice usually stops most winter barge traffic on the Missouri and upper Mississippi. Global warming could actually extend the navigation season.
The Mississippi system plays a key role in the waterborne commerce of the United States. In 1996, the latest year for which figures are available, 15 the Mississippi system carried 702 million of the total 2.3 billion short tons of cargo in U.S. waterborne commerce — just under a third. The Port of South Louisiana handles more tonnage (190 million short tons) than any other port in the United States, and if you add to that the traffic at other Mississippi-mouth ports like New Orleans, Baton Rouge, Plaquemines, and Lake Charles (all in South Louisiana), the total is much higher. Disruptions of the Mississippi system as a result of weather cause big bottom-line impacts on domestic and foreign commerce.
Better medium-range regional climate predictions could help shippers and carriers anticipate, and even avoid, delays caused by an unnavigable river system — or even to take advantage of an extended shipping season. Better predictions would also help the federal agencies that manage locks and dams on the Mississippi system to optimize its navigability.
For example, bird species from many parts of the world use the seasonal wetlands (“potholes”) on the Great Plains and prairie ecosystems as breeding areas and as temporary shelters during their migrations. Some experts 16 predict that global warming could bring drier conditions to the prairies. If these predictions are correct, ducks, geese, and migrating shorebirds could suffer from loss of habitat, according to University of Toronto forestry professor Jay Malcolm.
Timing is everything. A warmer climate could cause the marshes on farmland to dry up earlier in the year, tempting farmers to cultivate them and forcing birds to fly farther north for nesting areas. Many plants and animals have adapted to the predictable timing of climate events with finely timed behaviors. When climate is disrupted, their survival and reproductive strategies may be less likely to succeed.
Humans have already changed the Mississippi basin ecosystems radically. Planters and farmers cleared the bottomland hardwood forests of the Mississippi alluvial plain (the Delta) to grow crops such as cotton, sugar cane, and soybeans. Such forests in the Delta dwindled from 21 million acres in the late 1700s to fewer than 5 million acres today, according to the Delta Land Trust. 17 All up and down the river, over the years, people have converted the river’s natural floodplain to farmland, dwellings, industry, and other uses. This has meant less habitat for many species. Increased flooding caused by climate change could lead to the river’s reclaiming some of these areas.
The potential ecological effects of changes in the hydrology of the Mississippi basin go beyond just the basin. For example, to protect New Orleans during extreme floods, engineers need to open the Bonnet Carre spillway, diverting water from the river into Lake Ponchartrain and the Mississippi Sound. The flood of freshwater — enough to fill the New Orleans Superdome every 14 seconds — replaces the salt water that is normally in the sound. The result can be major damage to commercial oyster, brown shrimp, crab, and fish populations, according to Dr. Ed Cake, an oyster biologist from Ocean Springs, Mississippi. 18
That’s just the beginning. Climate-induced increases in Mississippi flood flows would bring more of everything the river carries to the Gulf of Mexico: sediment, nutrients, pesticides, and other pollutants. Experience in recent years shows that greater loads of these pollutants worsen the oxygen-starved “dead zone” that the river causes in the gulf. During the 1993 flood, unusual wind and water current patterns drove the plume of muddy, low-salinity water as far as the Florida Keys. So accurately predicting the effects of extreme weather involves more than just knowing how much water the Mississippi is discharging into the gulf. It requires a thorough understanding of larger oceanic and atmospheric systems and the ways in which they respond to Mississippi River outflow.
The “Corn Belt” region, Illinois, Indiana, Iowa, Minnesota, Missouri, Nebraska, and Ohio, lies squarely in the middle of the Mississippi basin. Illinois, Iowa, Minnesota, and Nebraska alone produce more than 50 percent of the corn grown in the United States. Other major corn-growing states are Indiana, Kansas, Kentucky, Michigan, Missouri, Ohio, South Dakota, and Wisconsin. The United States produces more than twice as much corn by volume as any other grain crop. Corn is especially sensitive to temperature and moisture — not only the amount of moisture or intensity of heat, but also the timing of it. Weather must be right for the farmers to get into the fields, for the seed to germinate, for the silks to be pollinated, and for the corn to grow to harvestable size.
Soybeans are another key U.S. crop produced primarily in the Mississippi basin. Soybeans are, in fact, a major cash crop in the alluvial lands along the river itself below Cairo, Illinois, and in the Mississippi Delta. The United States produces 49 percent of the world’s soybeans and 69 percent of the soy-based product exports.
Wheat, too, is widely grown in the basin, especially the Missouri subbasin. Many hungry developing nations depend on imported wheat as a staple. Much of the wheat exported by the United States goes to nations like China, Egypt, Indonesia, Pakistan, and the Philippines, as well as more developed nations. The United States in recent years has typically produced about one-third of the world’s wheat exports. Fortunately, the world has other major wheat producers, principally Australia, Canada, and Western Europe. Wheat is more often grown in arid regions with poor soil quality, because it can be grown without irrigation. Most of the major U.S. wheat-producing states are in the Mississippi basin. Kansas, Montana, Nebraska, Ohio, and South Dakota are among them.
The Mississippi system is an important source of irrigation water. Of the 45.8 million acres irrigated in the lower 48 states (1984 figures), 11 million were in the Missouri basin, 4.8 million were in the Arkansas-White-Red basin, 3 million were in the lower Mississippi basin, 0.6 million were in the Upper Mississippi basin, and 0.08 million were in the Ohio basin. Irrigation gives a crucial boost to many crops, including alfalfa, corn, wheat, and soy. Changes in runoff patterns that are likely to follow from climate change will disrupt the current irrigation regime. Arid areas in the southern and western parts of the basin, which already depend on irrigation to make agriculture viable, could experience the biggest shortfalls.
But the adaptability of agriculture in the Mississippi system to greenhouse warming may be improved by engineering works already in place — most notably the vast system of dams and reservoirs on the Missouri River basin — that supply agriculture with water held back to prevent flooding farther downstream.
Corn is very sensitive to temperature and rainfall. A July drought can virtually ruin a corn crop. The ENSO cycle has significant effects on temperature and rainfall, so it would be reasonable to expect its swings to affect corn harvests. But research has yet to find simple and direct connections between the ENSO cycle and corn harvests. 20
This is a reminder that there are other forces besides the ENSO cycle causing droughts and floods. Scientists also suspect that processes “local” to the Midwest play a key role as well — for example, that soil moisture can prolong rainfall and lack of it can prolong drought. So until much more is understood about the climate of the Mississippi basin, it may be hard to pick the ENSO effects out from all the other things going on. Researchers are focusing their attention on this very issue.
Most experts agree that greenhouse warming will likely cause a northward shift in cropping patterns, as zones of temperature and moisture suitable for particular crops shift northward. Climate change will not necessarily cause a drop in overall production, since Canada may gain harvests lost by the United States.
Perhaps the only certainty about climate is change. U.S. farmers have met climatic challenges before (witness the Dust Bowl and the soil conservation movement). The effect of climate change on Midwestern agriculture — whether positive or negative — is likely to depend on how quickly and effectively farmers respond. Greenhouse warming may be the biggest challenge yet.
Much of the midwestern farm news that comes with climate change may be good. The IPCC’s Working Group II said in 1998 that experiments “have generally confirmed high confidence in a net beneficial effect” 21 of higher CO2 concentrations themselves on crops. Some plants grow faster in higher concentrations of CO2.
But much of the outlook is still unknown. Working Group II said in the same report, “Extreme events like drought, flooding, hail, hurricanes, and tornadoes also will impact agriculture, but reliable forecasts of such occurrences are not yet regionally available.” 22 The point is that more matters than just average temperature and rainfall. A general warming trend may be good for crops, but one killing frost or severe hailstorm could destroy them. “The risk of losses due to weeds, insects, and diseases is sensitive to temperature and moisture,” 23 the report also noted.
The Mississippi system also serves as the receiving body for the treated (at least theoretically) sewage from many of the communities along the river. Properly treated sewage adds little pollution to the river. But treatment plants do not always work perfectly. In big downpours, stormwater overflows from combined storm-sanitary systems can flush untreated sewage into the river. Because sewer systems are built to exploit the downhill flow of water, sewage treatment plants are often located near the river. This works well most of the time, but floods can knock these plants out, again causing raw sewage discharges. That happened, for example, at Chesterfield, Missouri, during the 1993 flood. A levee break caused the river to engulf a plant handling wastes from more than 75,000 homes and 1,500 businesses.
Obviously, a better crystal ball would help.
If weather catastrophes themselves can’t be prevented, some of their worst effects may be prevented — if they are predicted in time. Better predictions for the Mississippi basin must be built on better understanding of the basic physical processes that drive weather.
Scientists in many fields are working together to study these basics. Much of this work is organized under the Global Energy and Water Cycle Experiment (GEWEX), which was set up in 1990 by the World Climate Research Program. A special program of research specific to the Mississippi basin, the GEWEX Continental-scale International Project (GCIP), has been underway since 1995. National Oceanic and Atmospheric Administration (NOAA) is the lead federal agency for this project.
The GCIP mission is to demonstrate skill in predicting changes in water resources on time scales up to seasonal and annual as an integral part of a climate prediction system. To achieve this mission, the program has involved atmospheric scientists and hydrologists, modelers and observationalists, and academic researchers and operational experts. This interdisciplinary program features research related to energy and water budgets on regional scales, forecast model development, assimilation of both conventional and new satellite data, water resource applications, process studies, and data set development.
Researchers want to understand every aspect of how water moves through the weather cycle, picking up the sun’s energy as it evaporates and releasing energy when it falls as rain or snow. The network of instruments to measure these energy and water flows has never been more extensive — both from satellites and on the ground.
A key research goal is to “model” these meteorological and hydrological processes — that is, to describe them mathematically in fine detail. The “model” is a computer program. If the model is accurate enough in describing how the “real world” works, it can be useful as a predictive tool. Regional-scale weather models are already quite successful in aiding short-term weather forecasts, which look a few days to a week ahead.
One of the exciting frontiers of climate research is “medium-range” forecasting. These medium-range forecasts look at climate changes from season to season or year to year. Recent advances in modeling of ENSO allowed forecasters to predict months ahead of time the storms that battered California in the winter of 1997-98, for example.
The GCIP initiative seeks to represent more accurately the “coupling” between the atmospheric part of the climate system and the land surface. This means taking account of soil moisture, infiltration and runoff, and vegetation, not to mention the accumulation and melting of ice and snow.
GCIP is a large enterprise — it has to be large to pull together so many kinds of measurements and models from so many places. All kinds of agencies participate, from the National Academy of Sciences, the National Aeronautics and Space Administration, and NOAA to universities and agricultural research stations. These agencies will make coordinated observations of many variables over the same areas during the same periods, in an effort to see the whole picture. These “enhanced observing periods” for GCIP will last through the year 2000.
| June 23, 2000 | | Disclaimer/Policy |
