Nowhere will the impacts of climate change and the need for adaptive responses be more apparent than in our vast, complex transportation system.
The great preponderance of scientific evidence suggests that the planet is warming at an accelerating rate due in large measure to the use of fossil fuels. Heretofore, most of the discussion about global warming has focused on limiting or mitigating climate change by reducing emissions of greenhouse gases (GHGs), most notably carbon dioxide, but also methane, nitrous oxides, and other gases.
No matter what policies are eventually adopted and implemented by the world community, however, the impacts of increasing levels of GHGs will continue to be felt for decades to come. These include the northward migration of pests and disease vectors; changes in local and regional weather patterns, such as decreasing precipitation in the already arid Southwest and increasingly intense storms in the Midwest and Northeast; extended heat waves with concomitant effects on air quality, health, and material longevity; and sea level rise that will impact coastal communities and ecosystems.
Nowhere will the impacts of climate change be greater and the need for adaptive responses more apparent than in the built infrastructure, especially the vast network of highways, bridges, tunnels, railroads, transit systems, airports, ports and harbors, and pipelines. Although the focus of this paper is on transportation, most of these concerns will also affect other infrastructure segments, such as power generation and transmission facilities and water and wastewater distribution and treatment systems.
Impacts of Climate Change
Based on the climate-science literature, one can conclude that in the next 50 to 100 years five specific impacts of global warming will have potentially serious implications for the built infrastructure: sea level rise (virtually certain, > 99 percent probability); heat waves (very likely, > 90 percent); rising Arctic temperatures (virtually certain, > 99 percent); changes in precipitation patterns (very likely, > 90 percent); and increasingly intense, strong hurricanes (likely, > 66 percent) (IPCC, 2007; NRC, 2008).
Sea Level Rise
As a result of melting glaciers and the expansion of the ocean as water temperatures rise, sea levels will continue to rise throughout this century. Globally, the sea level is projected to rise 7 to 23 inches. However, in some regions, such as the U.S. Gulf Coast, the relative sea level rise will be exacerbated by land subsidence.
A sea level rise of 2 to 4 feet along the Gulf Coast, which is well within the range of possibility over the next century, would inundate major portions of the coastline from Mobile to Houston, particularly in Louisiana and East Texas (Figure 1). To put this in perspective, a 4-foot rise would inundate 2,400 miles of roadway, 9 percent of rail lines, and 72 percent of ports in the region
If storm surges are combined with higher sea levels, the damage can be much more severe and extend much farther inland. The 25-foot storm surge during Hurricane Katrina literally lifted the deck of the Bay St. Louis Bridge off its piers (Figure 2). Figure 3 shows the impact of a 23-foot storm surge on the region: 64 percent of Interstates, 57 percent of arterial roads, 41 percent of freight rail lines, 99 percent of ports, and 29 airports could be affected.
Even though it is highly unlikely that a single storm surge will flood the entire area, it is important that we understand the risks to the Gulf Coast and to the nation. Seven of the 10 largest freight ports in the United States are located on the Gulf Coast, and approximately two-thirds of the nation’s oil imports pass through Gulf Coast facilities. Adaptation to the rise in sea level will require that those ports and harbors be reconfigured or reconstructed to accommodate higher seas.
But the danger would go beyond transportation systems and affect much of the built infrastructure. Water and wastewater treatment facilities and distribution and collection systems could be out of service for weeks, or even months, as they were in 2005 after Hurricanes Rita and Katrina. Electric power generation and distribution systems and critical service facilities such as hospitals will also be threatened. In addition, higher sea level and storm surges will accelerate the destruction of the barrier islands that protect the mainland. Without them, sections of the Inland Waterway will become unprotected ocean and unsuitable for barge traffic.
Although the Gulf Coast is the “poster child” for the impact of rising seas and storm surges, many other coastal areas are just as vulnerable. More frequent disruptions and damage to much of the infrastructure near all of our coasts can be expected. Many of the nation’s busiest airports—(e.g., in Fort Lauderdale, New Orleans, Boston, and New York)—are in coastal, flood-prone zones. Tunnels and other low-lying infrastructure will also come under assault. Studies in New York have shown that heavy storm surges could inundate major portions of the lower Manhattan subway system.
High temperatures and heat waves are very likely to become more intense and more frequent and to last longer than they do today. Climate scientists have developed numerous models that forecast future temperature levels. Absent effective reductions in GHG emissions, projected temperature rise by the end of this century ranges from a low of 4 to 7°F to a high of 7 to 11°F.
These are enormous increases! Currently, for example, Dallas has a 30-percent probability of having one day per year with a temperature higher than 110°F. By 2100, the 30-percent probability rises to 19 days with temperatures of 110°F and an 80 percent chance of at least 8 days of 110°F.
These temperature increases will affect thermal expansion joints on bridges, increase stresses and buckling on rail tracks, and cause more rapid degradation of pavements, especially asphalt. In addition, construction workers will have to operate on reduced schedules or at night as summer temperatures consistently rise above 90°F in much of the country.
Higher temperatures, decreases in ice cover, and increases in evaporation are forecast to lower water levels in the Great Lakes and the Saint Lawrence Seaway thereby necessitating reductions in the cargo-carrying capacity of freighters on the Great Lakes and oceans. Similarly, elevated temperatures will lead to more severe droughts, especially in the Southwest and perhaps in the Southeast. Water shortages, which are already a critical problem in the Southwest, will become even more acute, leading to feuds between agricultural interests and municipal and industrial users and fighting among neighboring states over a diminishing water supply.
Pests, such as the pine bark beetle in Colorado and the spruce bark beetle in Alaska, are already decimating lodgepole pines and spruces and creating a tinder box for forest fires. These insects are spreading because winter temperatures no longer stay cold long enough to kill the beetle larvae. Large forest fires seem to be breaking out more frequently causing disruptions to air and ground traffic and damage to infrastructure of all types, as well as to residential buildings. Fires are often followed by landslides on denuded slopes.
Increasingly Intense Precipitation
Over the past 50 years, there has been a significant increase in the frequency and intensity of heavy precipitation events across the country. The small increase in total precipitation over this time period is the result of these more frequent, heavier downpours.
In simple terms, warmer temperatures lead to more evaporation, hence drought in the Southwest, lower water levels in the Great Lakes, and greater moisture-carrying capacity in the atmosphere, which leads to more intense storms in the Midwest and Northeast. Severe storms create delays and disruptions to almost all types of transportation, and overwhelmed drainage systems for roads, airports, tunnels, and neighborhoods cause localized flooding. Generally, the intensity of precipitation is expected to increase in northern latitudes, such as Alaska and the Northeast, while the West and Southwest become drier (Figure 4).
The probability of a particular storm event is called the return frequency. A one in twenty (1:20) year event equals a 5 percent probability of a storm of a specified intensity occurring in a given year. Return frequencies are based on historical weather data going back 100 years or more. However, because the climate is changing relatively rapidly, historical statistical computations do not reflect the upturn in precipitation events. In other words, the 1:100 year storm of yesterday may now be a 1:20 year event.
Inland waterways may experience higher and perhaps more frequent floods. Over the past 17 years, there have been two major floods in the upper Mississippi, both of them 1:300 to 1:500 year return frequency events. Is this just statistical probability at work, or are our return frequency curves hopelessly out of date because of the effects of climate change?
Hydrologic models and computations must be revised to reflect tomorrow’s precipitation intensities. Updated models would immediately alter the designation of flood plains and the design of virtually all hydraulic structures, including storm drains.
Increasingly Intense Hurricanes
Some evidence shows that rising temperatures in the ocean, notably in the Gulf of Mexico, fuel stronger hurricanes—not necessarily more frequent storms, but deadlier storms with higher wind speeds and heavier precipitation. The science of hurricanes leaves many unanswered questions, but storms packing higher winds and coming further inland on higher sea levels will be a recipe for disaster.
The damage to oil and gas facilities from Hurricanes Katrina and Rita exemplifies the impact of major storms on the nation’s economy of shutting down off-shore production, refineries, and pipelines. Reportedly, 15 percent of U.S. refining capacity was shut down in anticipation of Hurricane Gustav in 2008, and virtually all off-shore oil and gas production was halted in the Gulf. In an attempt to avoid the evacuation tragedies of 2005, more than three million people fled the coast ahead of Hurricane Gustav.
Global warming is most apparent at high northern latitudes, that is, in the Arctic. Temperatures in Alaska have already risen 3 to 5°F, twice as much as in the contiguous 48 states. We anticipate thawing of as much as 90 percent of the permafrost, which will result in the displacement of pavements, runways, rail lines, pipelines, and buildings. Bridges and pipelines, which are especially vulnerable to the heaving of permafrost, are typically difficult to protect and repair.
Satellite photography clearly shows that the sea ice is retreating above the Alaskan North Slope, and as it does, the protection afforded by the ice sheet is being lost. Strong winds, which are prevalent in the Arctic, are creating waves that erode coastlines endangering whole villages and transportation systems. Alaska projects that the cost of maintaining public infrastructure will increase by $4 to $6 billion by 2030 as a result of addressing the effects of climate change.
A positive consequence of Arctic warming is the possible opening of the Northwest Passage, which would permit shipping and tourism (cruise ships) above the Arctic Circle. Last summer, two German freighters accompanied by a Russian icebreaker navigated the northern sea route off the coast of Siberia, shortening the shipping journey by thousands of miles (Box 1).
Adaptation: Engineering Challenges and Opportunities
The best scientific studies clearly indicate that threats from global warming are real, not illusory. Moreover, we can no longer use historical weather analyses to predict the future environment in which we will have to function. Indeed, climate data collected by the National Oceanic and Atmospheric Administration shows that global temperatures from January through June 2010 were the warmest on record (NOAA, 2010). Weather extremes will become increasingly severe and destructive to infrastructure, and mitigation measures, however effective, will not appreciably change the trajectory of global warming for decades (Box 2).
How should we respond or adapt to the anticipated impacts of global warming, especially as they impinge on infrastructure? Note that the question is not whether we should adapt to climate change, because we necessarily will adapt. The question facing the engineering profession is whether adaptation will be a planned, studied response or a haphazard reaction to events as they unfold.
In many ways, adaptation is classic risk management, but it is complicated by the inherent uncertainties associated with climate change. The terms of the risk analysis include: the hazards of concern (sea level rise, stronger storms, and heat waves); vulnerable assets (transportation infrastructure and its value to the economy); potential consequences (direct and indirect); and the likelihood or probability that a hazard will occur.
These are the questions engineers must address to balance risk and benefits. The answers will range from very low-probability, high-consequence events (e.g., Hurricane Katrina’s impact on New Orleans) to high-probability, low-consequence events (e.g., annual flooding of agricultural flood plains). The focus then is on the direct and indirect cost implications within the range of probabilities (Figure 4).
The Special Challenge of Uncertainty
The very words “climate change” and “global warming” immediately raise questions about the certainty of science and the actions to be taken. Indeed, there are three broad areas of uncertainty in addressing climate projections. First, natural variations occur in climate systems even when there are no external forcing factors, such as GHG emissions or major volcanic eruptions. Natural variations include large-scale phenomena, such as the El Niño/La Niña-Southern Oscillation.
Second, there is uncertainty about the level of GHG emissions, which may change with technological breakthroughs (e.g., carbon sequestration), political agreements, and social and economic drivers. However, given the level of geopolitical disagreement between industrialized and developing nations and internal debate in the United States and elsewhere about the risks of climate change, it seems unlikely that this kind of uncertainty can be reduced.
Finally, there is uncertainty about the response of the climate to various perturbations, especially increasing levels of GHGs. Most global climate models are based on radiative forcing mechanisms, and virtually all of them predict increased warming of the globe, albeit at different rates. Models based on lower GHG emissions predict temperature rises of 4 to 6°F by the end of this century in the United States. Models based on higher emissions predict rises of 7 to 11°F.
The Issue of Scale
These complex analytical challenges go beyond the uncertainties surrounding basic climate science. Consider the issue of scale, for example. Virtually all current climate change models are created at the global scale (i.e., the global temperature will rise by some amount over a set period of time). As the spatial scale is reduced, however, confidence in the predictions decreases.
Unfortunately, engineers can do little with data on temperature change on a global level. For the information to be useful, it must be on the regional or local level. Even though confidence in our understanding of the changes in climate increases as the spatial scale increases, the practical value of the information to owners and operators of the nation’s infrastructure diminishes as the scale increases. This means that developing a finer-scale understanding of climate change will be essential to developing a better understanding of the risks and, hence, the adoption of better, more cost-effective adaptation measures. Fortunately, climate scientists are becoming more confident in down-scaling their models to regional levels.
As we look more closely at the five impact areas identified above, it becomes apparent that some relate to gradual changes, such as sea level rise, while others relate to extreme events. For example, the gradual increase in total precipitation in the Midwest is of far less concern than the projected increase in the return frequency of heavy storms and flooding. Analytical data on the size and frequency of extreme events will be necessary for the development of effective response mechanisms.
Stresses That Influence Decisions
Global warming is an important part of the changes we are experiencing, but we must consider climate change in a context that includes other stress factors that threaten the human experience and the ecosystem in which we live. These stress factors include worldwide population growth, environmental degradation, wars and political unrest, and economic turmoil. Climate change, added to these stresses, can be the tipping point that moves an ecosystem beyond recovery, such as the loss of biological species.
Similarly, climate change is one of many concerns that must be addressed in planning for improvements in transportation and the built infrastructure. Continued coastal development, for example, brings with it serious risks to transportation and other infrastructure, as well as to the homes, businesses, and economy that may flourish there for a limited time. Land use and development are traditionally jurisdictional matters for local and regional authorities, but their decisions may have cost implications for a large segment of society and the ecology. For example, insurance costs for coastal communities are likely to be spread to individuals and businesses that are far from the danger zones.
Interactions and relationships among geographical regions and social sectors cannot be ignored. Drought in the Southwest and/or increases in water and air temperatures may reduce the efficiencies of power plants, just when more power is needed for air conditioning. Intense storms and floods can impact commerce, as they did following Katrina and the great floods of 1993 on the upper Mississippi River.
Conflicts inevitably arise between the needs of people and the needs of the ecosystem in which they live, and adaptation measures to manage the risks of climate change must incorporate sound sustainability principles. If we plan and act responsibly, we may be able to “have our cake and eat it too.”
Sound Solutions for the 21st Century Transportation System
We must not use the uncertainties and challenges of adapting to climate change as an excuse for inaction. The challenge to the engineering profession is to take into account the inherent uncertainties of climate science, as well as complex technological, social, economic, and environmental interrelationships, and develop sound solutions for transportation systems that will serve us until the end of the century. A large body of work has been done on making decisions on issues that include great uncertainties. Scenario analyses, for example, can provide “envelopes” of possible outcomes (e.g., best case/worst case scenarios and respective probabilities).
To many, climate change is a distant worry, but developers of transportation systems work on a time horizon of 50 to 100 years for new and rehabilitated facilities. Thus they have no choice but to take into account the impacts of climate change. The marginal costs of accommodating climate change impacts in major systems will be dwarfed by the cost of retrofitting systems to meet these same needs decades hence. To engineers, many of the solutions for adaptation are fairly obvious—build robust, resilient systems, protect or move existing assets, and, when necessary, abandon indefensible facilities.
Some adaptations to climate change are listed below:
Sea Level Rise
- Build or enhance levees and dikes to resist higher sea levels and storm surges.
- Elevate critical infrastructure.
- Abandon or relocate coastal highways, rail lines, and bridges.
- Provide good evacuation routes and operational plans.
- Provide federal incentives to reduce the amount of development in at-risk coastal regions.
- Support research on new, more heat-resistant materials for paving and bridge decks.
- Replace and/or reconstruct highway and bridge expansion joints.
- Increase the length of airport runways to compensate for lower air densities.
- Revisit standards for construction workers exposed to high temperatures.
Increased Storm Intensity
- Revise hydrologic storm and flood frequency maps.
- Develop new design standards for hydraulic structures.
- Reinforce at-risk structures, particularly to protect against scouring of bridge piers.
- Encourage better land-use planning for flood plains.
- Move critical infrastructure inland.
- Reinforce and/or build more robust, resilient structures.
- Design for greater storm surges.
- Strengthen and elevate port facilities.
- Identify areas and infrastructure that will be damaged by thawing permafrost.
- Develop new approaches to foundation design.
- Reinforce, protect, or move seaside villages.
We can continue to debate the validity of climate science, but waiting for decades or longer for final “proof” would be foolhardy at best. Fifty or 100 years from now the impact of increasing emissions of GHGs will be firmly established. If the projections of today’s climate scientists are correct and we have failed to take both mitigating and adaptive actions, then much damage will already have been done.
The potential impacts of climate change on the built environment and the implications for transportation infrastructure are sufficiently well defined for us to take action now. If this generation of engineers fails to act, coastal highways and railroads will be under water, bridges will be unusable, tunnels will be periodically flooded, communities in the Midwest, Northeast, and Southeast will be threatened by river flooding, people in the Southwest will face increasing water shortages, and entire villages along the North Slope of Alaska will be swallowed by the sea.
However, if we incorporate climate change into the regular planning processes for transportation and other infrastructure, the marginal costs of building more robust, resilient systems can be readily accommodated. And we will have met our obligations to future generations.
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