News + Media

By Kerry Emanuel, Special to CNN
August 25, 2011 11:45 p.m. EDT

Editor's note: Kerry Emanuel is a professor of meteorology at the Massachusetts Institute of Technology.

(CNN) -- At this moment, Hurricane Irene poses a risk to almost everyone living along the Eastern Seaboard, from Florida to the Canadian Maritimes. Where will Irene track? Which communities will be affected and how badly? Millions of lives and billions of dollars are at stake in decisions made by forecasters, emergency managers and all of us who live in or own property in harm's way.

It is natural to wonder how good the forecasts are likely to be. To what extent can we trust the National Hurricane Center, local professional forecasters and emergency managers to tell us what will happen and what to do? Undeniably, enormous progress has been made in the skill with which hurricanes and other weather phenomena are predicted. Satellites and reconnaissance aircraft monitor every hurricane that threatens the U.S., collecting invaluable data that are fed into computer models whose capacity to simulate weather is one of the great wonders of modern science and technology.

And the human effort and taxpayer funds that have been invested in this endeavor have paid off handsomely: A three-day hurricane track forecast today is as skillful as a one-day forecast was just 30 years ago. This gives everyone more time to respond to the multiple threats that hurricanes pose.

And yet there are still things we don't know.

For example, we do not know for sure whether Irene will make landfall in the Carolinas, on Long Island, or in New England, or stay far enough offshore to deliver little more than a windy, rainy day to East Coast residents. Nor do we have better than a passing ability to forecast how strong Irene will get. In spite of decades of research and greatly improved observations and computer models, our skill in forecasting hurricane strength is little better than it was decades ago. Why is this so, and how should we go about making decisions in the context of uncertain forecasts?

Since the pioneering work of Edward N. Lorenz in the early 1960s, we have known that weather, including hurricanes, is an example of a chaotic process. Small fluctuations (Lorenz's "butterfly effect") that cannot be detected can quickly amplify and completely change the outcome in just a few days. Lorenz's key insight was that even in principle, one cannot forecast the evolution of some kinds of chaotic systems beyond some time horizon.

In the case of weather, meteorologists think that time horizon is around two weeks or so. Add to this fundamental limitation that we measure the atmosphere imperfectly, sparsely and not often enough, and that our computer models are imperfect, and you arrive at the circumstance that everyone knows from experience: weather forecasts are not completely reliable, and their reliability deteriorates rapidly the further out in time the forecast is made. A forecast for a week from today is dicey at best, and no one even tries to forecast two weeks out. But in the past decade or two, meteorologists have made another important advance of which few outside our profession are aware: We have learned to quantify just how uncertain any given forecast is.

This is significant, because the degree of uncertainty itself varies greatly from one day to the next. On one occasion, we might be able to forecast a blizzard five days out with great confidence; on another, we might have very little faith in tomorrow's forecast.

We estimate the level of confidence in a particular forecast by running many different computer models many times, not just once. Each time we run it, we feed it a slightly different but equally plausible estimate of the current state of the atmosphere, given that our observations are few, far between and imperfect. In each case, we get a different answer; the differences are typically small to begin with but can grow rapidly so that by a week or so, the difference between any two forecasts is as great as the difference between any two arbitrary states of the weather at that time of year. No point in going any further!

But we observe that sometimes and in some places, the differences grow slowly, while at other times and places, they may grow much more rapidly. And by using different computer models, we can take into account our imperfect understanding of the physics of the atmosphere. By these means, we can state with some accuracy how confident we are in any particular forecast for any particular time and place. Today, one of the greatest challenges faced by weather forecasters is how best to convey their estimates of forecast confidence to the public.

Ideally, we would like to be able to say with full scientific backing something like "the odds of hurricane force winds in New York City sometime between Friday and Sunday are 20%." We have far to go to perfect these, but probabilistic statements like this are the best for which we can hope.

We know from experience that everyone will deal with such probabilistic forecasts in their own way: People have a very broad range of risk aversion. But the next time you are inclined to criticize weather forecasters for assigning probabilities to their forecasts, remember this essay and consider how much better off you are than with other types of forecasters you rely on. Your stockbroker, for example. The opinions expressed in this commentary are solely those of Kerry Emanuel.

Nadya Anscombe

If wind power is going to meet 20% of our predicted energy needs in 2100, millions of wind turbines must be installed around the globe. Modelling performed by researchers at Massachusetts Institute of Technology, US, has shown that these vast wind farms, if installed in offshore regions, could reduce the temperature of the lower atmosphere above the site by 1 °C.

This is in contrast to earlier work that found that a land-based deployment of wind turbines large enough to meet one-fifth of predicted world energy needs in 2100 could lead to a significant temperature increase in the lower atmosphere over the installed regions.

Chien Wang and Ronald Prinn say their findings show how important it is that rigorous scientific assessments are made before deployment of large-scale wind farms. "We were surprised by our findings at first but we soon realized that the cooling we predicted is due principally to the enhanced latent heat flux from the sea surface to the lower atmosphere," Wang told environmentalresearchweb.

Wang and Prinn found that the effect varied depending on the location of the wind farm. In tropical and mid-latitude sites, the temperature of the lower atmosphere was reduced by up to 1 °C, whereas even greater reductions were seen in the high-latitude sites.

The consequences of such a temperature change are not clear but the researchers believe that it will have an effect on temperatures, clouds, precipitation and large-scale circulation beyond the installed regions. "However, these non-local impacts or teleconnections are much less significant than we saw in the land cases," said Wang. "This is likely due to the much lower response of the ocean to the imposed surface-drag changes relative to the response of the land to the imposed changes in both surface roughness and displacement height."

Wang and Prinn also examined the issues of intermittency and hence reliability, of large-scale deployment of wind-driven electrical power generation by seasonally averaging the available wind power in various regions of the world. They found that intermittency would be a major issue for a power generation and distribution system that relies on the harvest of wind power from large-scale offshore wind farms.

"Intermittency presents a major challenge for power management, requiring solutions such as on-site energy storage, back-up generation and very long-distance power transmission for any electrical system dominated by offshore wind power," said Wang.

Wang is also keen to point out that the method he and Prinn used to simulate the offshore wind-turbine effect on the lower atmosphere involved simply increasing the ocean surface drag. "While this method is consistent with several detailed fine-scale simulations of wind turbines, it still needs further study to ensure its validity," he said.

Wang and Prinn published their work in Environmental Research Letters (ERL).
The report is available here.

Dr. Tammy Thompson, postdoctoral associate from the MIT Joint Program for the Science and Policy of Global Change, will be speaking at a press conference held by the Texas Sierra Club at the Houston City Hall on Tuesday, July 19, 10:00am. Dr. Thompson wil be speaking on air pollution in Texas.

For the Sierra Club Press Release, click here.

As the world economy attempts to balance burgeoning energy demand with lower carbon emissions, natural gas has become a central pillar of energy strategy in both the developed and developing world. Advances in exploration and production technology have led to newly abundant sources of “unconventional” natural gas in the United States, with implications for policymakers as they look to reduce dependence on imported oil, promote manufacturing and create jobs. Decisions by Germany and Japan to reduce their reliance on nuclear power also have major implications for the global gas market as governments and businesses look for alternative means of power generation. Sustained rapid economic expansion in emerging economies will continue to drive up demand for natural gas, while increased production capacity for liquefied natural gas production in Australasia and the Middle East is already changing the market dynamics of a resource that has historically relied on the politics of pipelines and proximity.

On June 27, the Energy Security Initiative at Brookings hosted the Washington, D.C. launch of the Massachusetts Institute of Technology’s “The Future of Natural Gas,” an examination of the complex and rapidly changing prospects for the global natural gas market, and the role of natural gas in meeting global energy and environmental challenges. Ernest Moniz, director of the MIT Energy Initiative (MITEI) and co-chair of the report, and Melanie Kenderdine, executive director of MITEI, presented their findings on the extent of global natural gas reserves, production costs and potential end uses, as well as the geopolitical and environmental implications of a gas-fueled global economy. Following a presentation of the report’s findings, David Goldwyn, nonresident senior fellow with the Energy Security Initiative, moderated a panel discussion. Gerald Kepes, partner and head of Upstream and Gas at PFC Energy and Phil Sharp, president of Resources for the Future, joined the discussion. After the program, the speakers took audience questions.

At MIT, former Michigan governor touts new bipartisan initiatives to make the U.S. the world’s clean-energy champ.

David L. Chandler, MIT News Office

In a spirited talk at MIT, former Michigan governor Jennifer Granholm presented a plan for a bipartisan initiative that she said could help the United States regain a world leadership role in the creation of new clean-energy technologies — and the thousands of new jobs that those technologies could provide.

Introduced by her “old pal,” Massachusetts Gov. Deval Patrick, and MIT President Susan Hockfield, Granholm spoke at Tuesday’s reception on clean-energy innovation. The event was hosted by the MIT Energy Initiative and the Joint Program on the Science and Policy of Global Change, a program that its co-director, TEPCO Professor of Atmospheric Science Ronald Prinn, described as a “unique collaboration between the natural and social sciences.”

“At MIT, we’re bullish on clean energy,” Hockfield said in her introduction. In fact, she said, “bullish is an understatement. We’re maniacs about it!” She added that she sees the clean-energy domain as a major area in which to rebuild the nation’s economy.

Patrick said his attendance was intended “to celebrate the leadership of MIT” in clean-energy technology. He said the Institute “has gone so far beyond the basic science … to commercialize so many great ideas” in clean energy, and that in today’s climate of volatile oil prices, “all the elements align for moving ourselves rapidly to a clean-energy future.” He added that in Massachusetts, there has been a 60 percent increase in energy-related employment “during the worst economy in living memory.”

Granholm, who now represents the Pew Charitable Trusts’ Clean Energy Program, said other countries have been “much more aggressive” than the United States in pushing for clean energy, while this country has “a patchwork” of state policies and no strong national program to promote such technologies. In searching for what Granholm called “pragmatic energy policies that can get bipartisan support” even in the current highly polarized political debate, her organization has identified four specific policy priorities, she said.

First, “a national renewable energy standard” would call for at least 20 percent of the nation’s energy to come from renewable sources by 2020, she said. Such a policy “sends a market signal” that would help businesses focus on developing needed technologies.

A second priority, she said, is encouraging more energy efficiency in industrial facilities. She pointed to the example of a French company called Veolia Energy, which develops combined heat and power systems that can be up to 90 percent efficient in using natural gas, the cleanest of all fossil fuels, compared to typical fossil-fuel powerplant efficiencies of around 50 percent. Granholm pointed out that so much energy is wasted in U.S. powerplants in the form of heat that “if you could just capture that waste heat, you could power the entire nation of Japan.”

Third, she said, is to push for more electrification of the transportation system — including a 25 percent market share for new electric cars by 2020 — and improved efficiency for non-electric vehicles. That would help spur the growth of companies such as the MIT-spinoff A123 Systems, which is already “hiring hundreds of people” for its new battery factories.

And fourth, she said, is to “increase the amount of money we, as a nation, invest in energy development.” ARPA-E, the U.S. Department of Energy’s agency for investment in innovative energy technology, currently has a budget of $3.8 billion per year. “If we boost that to $16 billion, we could really be on the map” as a major producer of energy systems, she said.

Granholm pointed out that since 2004, there has been a 630 percent increase in private-sector investment in clean energy worldwide. In 2008, the United States was number one in production of clean-energy technology, but by 2009 China had surged ahead, and in 2010 both China and Germany were ahead of the United States. “Every day, businesses make decisions about where to locate,” and without a strong clean-energy policy, the country’s competitive position “will continue to ratchet down,” she said.

While some people worry that implementing any national policy on clean energy may be difficult right now given the polarized atmosphere in Washington, Granholm said, a recent national survey gives reason for hope. “Eighty-four percent of Americans want to see a national energy policy that encourages renewable energy and efficiency,” a number that includes 74 percent of Republicans, and even a majority of Tea Party members, she said.

Patrick said fostering clean-energy technologies “is good for us, it’s good for the environment, it’s good for the economy, it’s good for jobs. So let’s get on with it!”

The cleanest of fossil fuels, it is far more abundant than previously thought and can have significant impact at little cost, MIT study finds.David L. Chandler, MIT News Office

Natural gas is important in many sectors of the economy: for generating electricity, as a heat source for industry and buildings, and in chemical feedstock. Given the abundance of natural gas available through extensive global resources and the recent emergence of substantial unconventional supplies in the United States, worldwide usage of the fuel is likely to continue to grow considerably and contribute to significant reductions of greenhouse gas emissions for decades to come, according to a comprehensive, multidisciplinary study carried out over the last three years by MIT researchers.

The study — managed by the MIT Energy Initiative (MITEI) and carried out by a team of Institute faculty, staff and graduate students — examined the scale of U.S. natural gas resources and the potential of this fuel to reduce greenhouse gas emissions. Based on the work of the multidisciplinary team, with advice from a board of 18 leaders from industry, government and environmental groups, the report examines the future of natural gas through 2050 from the perspectives of technology, economics, politics, national security and the environment.

An interim report with some of the study’s major findings and recommendations was released in June 2010. The full report, including additional data and extensive new analysis, was released by MITEI this week.

Because it has the lowest carbon content of all fossil fuels, natural gas can play a critical role as a bridge to a low-carbon future. The study’s economic analysis of the effects of a national policy calling for a 50 percent reduction in greenhouse gas emissions shows that such a policy would result in widespread substitution of natural gas for coal in electricity generation. However, in order to achieve even greater reductions in carbon emissions — which may be mandated in coming decades — natural gas will in turn need to make way for other low- or zero-carbon sources of energy. It is in this sense that natural gas may be seen as a “bridge” rather than as the ultimate long-term solution itself.

The report says that it is important to continue a robust program of research and development on other energy alternatives, which can be used to take the place of natural gas later in the century if and when emissions regulations become stricter. Henry Jacoby, MIT professor of management and co-chair of the study, said that such research is crucial because “people speak of [natural] gas as a bridge to the future, but there had better be something at the other end of the bridge.”

The study found that, contrary to best estimates of a decade or so ago, natural gas supplies are abundant and should be ample even for greatly expanded use of the fuel in coming decades. This is largely the result of the development of “unconventional” sources, such as shale gas. Because of its abundance, widespread distribution and advantages in cost and emissions, use of natural gas is expected to increase substantially under virtually all scenarios involving national policies, regulations and incentives, the study notes.

“Shale gas is transformative for the economy of the United States, and potentially on a global scale” because it has so dramatically increased the amount of gas that can be economically produced domestically, Anthony Meggs, a visiting engineer at the MIT Energy Initiative and co-chair of the study, said at Thursday’s press conference introducing the report.

Concerns have been raised about the possible environmental effects of developing shale gas using a controversial process called “fracking” (for hydro-fracturing), which involves injecting fluids into deep horizontal wells under pressure. The ultimate disposal of those fluids after they are pumped back out, and the possibility that they could contaminate water supplies, have been the subject of lawsuits and legislative attempts to limit the practice. The study found that “the environmental impacts of shale development are challenging but manageable,” and that some cases of the gas entering freshwater tables were “most likely the result of substandard well-completion practices by a few operators.”

Meggs said that in the small number of cases where there has been contamination, the problem has stemmed from improper cementing of the well casings. “The quality of that cementing is the area where the industry, frankly, has to do a better job,” he noted. But even so, he said, the study found only 42 documented incidents of such problems, out of tens of thousands of wells drilled. “It is not trivial,” he said, “but neither is it all-encompassing.” And, he added, even where there are problems, it is possible to go back and fix the well casings later.

The study recommends that to address these concerns, “it is essential that both large and small companies follow industry best practices; that water supply and disposal are coordinated on a regional basis and that improved methods are developed for recycling of returned fracture liquids.” Government funding for research on such systems should be “greatly increased in scope and scale,” the report says.

The robust supply situation enhances the opportunities for natural gas to substitute for other fuels. Using very efficient natural gas powerplants to replace coal-fired plants is “the most cost-effective way of reducing CO2 emissions in the power sector” over the next 25 to 30 years, the report says. Natural gas will also play a central role in integrating more intermittent renewable sources — wind and solar — into the electricity system because they can easily be brought in and out of service as needed.

The study also finds important opportunities for cost and emissions reduction in industry by switching to very high-efficiency natural gas boilers, and for more efficient energy use in commercial and residential buildings through new standards that would provide consumers information on end-to-end energy use of space- and water-heating alternatives. Furthermore, the current large price difference between oil and natural gas, if sustained, could lead to increased use of gas as a transportation fuel, either directly or through conversion to liquid fuel.

The study group suggests that U.S. national security interests will be served by policies that encourage integration of the presently fragmented global natural gas markets, and calls for better integration of such issues into foreign policy.

The report includes a set of specific proposals for legislative and regulatory policies, as well as recommendations for actions that the energy industry can pursue on its own, to maximize the fuel’s impact on mitigating greenhouse gas.

In addressing public concerns about the environmental impact of natural gas drilling operations, the industry could be taking a much more active role, said Ernest Moniz, director of MITEI and chairman of the study. The study makes many specific recommendations for improving well-development procedures, including full disclosure of chemicals used in the hydro-fracturing process and regional coordination on water-use issues. “An endorsement [of these policy recommendations] by industry would be very welcome,” Moniz said. “If industry actively promotes them, that can certainly help overcome” these public and legislative concerns.

Ironically, the study found that “public and public-private funding for natural gas research is down substantially,” even as the fuel is being recognized as a major contributor to strategies for lowering greenhouse gas emissions, said Melanie Kenderdine, executive director of the MIT Energy Initiative. The study recommends a substantial increase in such research.

By ANDREW C. REVKIN

The Massachusetts Institute of Technology has released “The Future of Natural Gas,” the latest in its valuable series of reports examining global energy choices. The report lays out a path for exploiting this abundant and relatively clean source of energy in ways that could limit environmental concerns and give the biggest near-term boost to efforts to trim emissions of carbon dioxide.

One of the most important take-home points, to me, was the authors’ endorsement of a rising role for natural gas as a feedstock for producing liquid transportation fuels. This resonated with discussions I had earlier this month with some senior people at Shell. They spoke at length about Shell’s existing push to produce liquid fuels from gas, most notably at the huge new gas-to-liquids plant in Qatar:

Here are a few highlights from the new report, starting with the M.I.T. team’s conclusion on hydraulic fracturing, or “fracking“:

The environmental impacts of shale development are challenging but manageable. Research and regulation, both state and federal, are needed to minimize the environmental consequences.

On gas as a substitute for coal:

…In the U.S. electricity supply sector, the cost benchmark for reducing carbon dioxide emissions lies with substitution of natural gas for coal, especially older, less efficient units. Substitution through increased utilization of existing combined cycle natural gas power plants provides a relatively low-cost, short-term opportunity to reduce U.S. power sector carbon dioxide emissions by up to 20 percent, while also reducing emissions of criteria pollutants and mercury.

On gas to provide a flexible source of “fill in” power to complement expanded use of variable wind power:

Furthermore, additional gas-fired capacity will be needed as backup if variable and intermittent renewables, especially wind, are introduced on a large scale. Policy and regulatory steps are needed to facilitate adequate capacity investment for system reliability and efficiency. These increasingly important roles for natural gas in the electricity sector call for a detailed analysis of the interdependencies of the natural gas and power generation infrastructures.

Finally, on gas’s potential as a transportation fuel and a substitute for liquid fuels:

Over the last decade or so, when oil prices have been high, the ratio of the oil price to the natural gas price has been consistently higher than any of the standard rules of thumb. If this trend is robust, use of natural gas in transportation, either through direct use or following conversion to a liquid fuel, could in time increase appreciably.

The evolution of global gas markets is unclear. A global “liquid” natural gas market is beneficial to U.S. and global economic interests and, at the same time, advances security interests through diversity of supply and resilience to disruption. The U.S. should pursue policies that encourage the development of such a market, integrate energy issues fully into the conduct of U.S. foreign policy and promote sharing of know-how for strategic global expansion of unconventional gas production.

With rising energy prices, could coal-to-liquid conversion become an economical fuel option?

Converting coal into liquid fuels is known to be more costly than current energy technologies, both in terms of production costs and the amount of greenhouse gases the process emits. Production of coal-to-liquid fuel, or CTL, has a large carbon footprint, releasing more than twice the lifecycle greenhouse gases of conventional petroleum fuels. However, with the rise in energy prices that began in 2008 and concerns over energy security, there is renewed interest in the conversion technology.

Researchers from the MIT Joint Program on the Science and Policy of Global Change (JPSPGC) recently released an assessment of the economic viability of CTL conversion. The researchers looked at how different climate policies and the availability of other fuel alternatives, such as biofuels, would influence the prospects of CTL in the future.

Coal-to-liquid technology has been in existence since the 1920s and was used extensively in Germany in 1944, producing around 90 percent of the national fuel needs at that time. Since then, the technology has been largely abandoned for the relatively cheaper crude oil of the Middle East. A notable exception is South Africa, where CTL conversion still provides about 30 percent of national transportation fuel.

But will there be a resurgence of CTL technology? To determine the role that CTL conversion would play in the future global fuel mix, researchers examined several crucial factors affecting CTL prospects. Different scenarios were modeled, varying the stringency of future carbon policies, the availability of biofuels and the ability to trade carbon allowances on an international market. Researchers also examined whether CTL-conversion plants would use carbon capture and storage technology, which would lower greenhouse gas emissions but create an added cost.

The study found that, without climate policy, CTL might become economical as early as 2015 in coal-abundant countries like the United States and China. In other regions, CTL could become economical by 2020 or 2025. Carbon capture and storage technologies would not be used, as they would raise costs. In this scenario, CTL has the potential to account for about a third of the global liquid-fuel supply by 2050.

However, the viability of CTL would be highly limited in regions that adopt climate policies, especially if low-carbon biofuels are available. Under scenarios that include stringent future climate policies, the high costs associated with a large carbon footprint would diminish CTL prospects, even with carbon capture and storage technologies. CTL conversion may only be viable in countries with less stringent climate policies or where low-carbon fuel alternatives are not available.

“In short, various climate proposals have very different impacts on the allowances of regional CO2 emissions, which in turn have quite distinct implications on the prospects for CTL conversion,” says John Reilly, co-director of the JPSPGC and one of the study’s authors. “If climate policies are enforced, world demand for petroleum products would decrease, the price of crude oil would fall, and coal-to-liquid fuels would be much less competitive.”

Researchers find that a federal climate policy would lower total U.S. emissions, but without large reductions in the aviation sector.Aviation is one of the fastest-growing industries worldwide, and with this growth comes a rise in greenhouse gas emissions. An economy-wide cap-and-trade policy would decrease carbon emissions for all U.S. industries, including the aviation sector. However, researchers from MIT recently found that such a policy would cause only a moderate decrease in aviation-related emissions by 2050. They released their results in a report, The Impact of Climate Policy on U.S. Aviation.”

Researchers from MIT’s Joint Program on the Science and Policy of Global Change and the Partnership for AiR Transportation Noise and Emissions Reductions, or PARTNER, combined two computer models to develop a unique analysis of the impacts of U.S. climate policy. First, the Emissions Prediction and Policy Analysis (EPPA) model analyzed the global economy and the greenhouse gases linked to economic activity, including the impact of a cap-and-trade policy on fuel prices, emission allowances and the overall economy. Next, the Aviation Environmental Portfolio Management Tool for Economics (APMT-E) modeled the aviation industry’s response to climate policy, estimating changes in aviation-related emissions and operations.

The researchers used these two models to examine the impact of the American Clean Energy and Security Act of 2009 (H.R. 2454), more commonly known as the Waxman-Markey bill. This bill, recently passed by the House of Representatives, proposed a 17 percent reduction in greenhouse gas emissions from 2005 levels by 2020 and an 80 percent reduction by 2050. Although the Waxman-Markey bill was rejected by the Senate, policies that set reduction targets for greenhouse gas emissions are likely to be implemented in the future. The reduction targets in this bill cover the entire U.S. economy, including the aviation sector.

But researchers found that aviation-related emissions would actually increase by 97-122 percent between 2012 and 2050 under the Waxman-Markey climate policy — a relatively small change from the 130 percent increase expected without climate policy. This result indicates that, while the overall policy effectively reduces total U.S. emissions, more emissions are abated in non-aviation sectors. That is, aviation contributes to reducing national emissions mainly by funding, via the purchase of permits, relatively inexpensive abatement options in other sectors, such as electricity generation.

In fact, there are three main reasons the Waxman-Markey bill has only a small impact on aviation emissions. The first is that some sectors, such as the energy industry, are able to reduce emissions easily (i.e., inexpensively), while other sectors, such as the aviation industry, have more costly emissions-abatement options. Currently, there are limited opportunities for airlines to replace carbon-intensive energy sources. Because airlines already operate closer to the fuel efficiency frontier than many other industries, there are cheaper mitigation options, or “lower-hanging fruit,” in these other sectors.

Second, although rising fuel prices — and thus rising airfares — can impact demand, there is still strong growth in demand for aviation in the policy scenarios. Third, the researchers found that the U.S. aircraft fleet becomes less fuel-efficient under the modeled climate policy. Fleet-efficiency changes are driven by two opposing forces: On one side, by demand reductions (relative to a future without climate policy), which cause airlines to delay purchases of new, more fuel-efficient aircraft; while on the other side, higher fuel prices oppose this incentive, inducing airlines to purchase more fuel-efficient aircraft. In the MIT simulations, the first force (reduced demand) dominates the second force (fuel price incentives), causing fleet efficiency to decrease.

Overall, the study shows that aviation emissions-abatement options are costly relative to mitigation options in other sectors. But the results of this research come with several caveats. Because the study focuses on long-term trends, shorter adjustments associated with business cycles are not considered. For example, in economic downturns, airlines may park old aircraft, which are then replaced by new, efficient aircraft in future high-growth periods. In addition, the model did not consider adjustments airlines could make to their existing fleets, such as retrofitting seat configuration, using slower flight speeds, or reducing cabin weight. Changes in air-traffic management may also improve operations and reduce emissions. Future work, which will include such considerations, may show that climate policy has a larger, positive effect on fleet fuel efficiency.

The authors also caution that the small reduction in aviation emissions does not indicate that the policy would be ineffective at reducing national emissions. Faced with high abatement costs, it is cheaper for the aviation sector to fund abatement in other sectors than reduce their emissions; a cap-and-trade system allows this to occur. Additionally, the study does not consider benefits from the climate damages that may be avoided through such policies, so the results cannot be used to assess the overall effectiveness of climate policy.