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Analysis documents changing trends in U.S. carbon emissions

The United States was the world's largest source of carbon emissions for more than a century. In a new analysis, researchers document the changing trends in U.S. carbon emissions from the late 19th century to the early 21st century, focusing on emissions from fossil fuels and industry. They discuss the main factors that contributed to the rollercoaster of changes in historical carbon emissions. The authors close by offering lessons from the U.S. experience for developing countries.

The analysis, written by researchers at Carnegie Mellon University, the University of Montreal, and Boston College, has been by the National Bureau of Economic Research as a Working Paper and will appear in the Journal of Economic Perspectives.

"The growth rate of U.S. carbon emissions and carbon emissions per capita shows striking variation over time," explains Karen Clay, professor of economics and public policy at Carnegie Mellon's Heinz College, who co-authored the analysis. "Yet our understanding of this historical variation in carbon emissions is surprisingly limited."

The authors divide their analysis into four periods—the years before 1920, 1920 to 1960, 1960 to 2005, and the years after 2005—and discuss the main drivers of national carbon emissions, as well as trends in carbon emissions in the electricity sector. They conclude with a discussion of implications for decarbonization efforts, particularly in developing economies where energy demand is rising rapidly.

While each country faces unique institutional, political, and technological constraints, historical patterns offer useful guidance on avoiding carbon-intensive development paths and navigating the complex trade-offs between local and global environmental objectives. Among the implications:

• U.S. electricity generation had become the main consumer of coal by the mid-20th century. A similar pattern may emerge elsewhere unless nations adopt cleaner generation technologies.

• Technological progress and coal efficiency gains have played a central role in curbing carbon emissions. In the United States, rapid innovation significantly contributed to gains in efficiency between 1920 and 1960. By adopting proven technologies and investing in emerging innovations, research, and local capacity building, developing economies may be able to expand access to electricity while limiting carbon emissions.

• The U.S. experience shows how geopolitical shocks and policy responses can have unintended and enduring effects on the energy mix. The 1970s energy crisis and efforts to enhance energy security inadvertently deepened U.S. reliance on coal. Similarly, public backlash to nuclear accidents led the United States and other countries to reduce nuclear power in favor of fossil-fuel generation, which had lasting consequences for carbon emissions. These episodes highlight the need to anticipate the long-term environmental and climate implications of energy decisions made in response to crises.

• Designing environmental regulations that address both local and global pollutants is inherently challenging. The U.S. Clean Air Act of 1970 and its 1977 and 1990 amendments led to major reductions in harmful local pollutants, but some of the compliance strategies inadvertently boosted overall coal consumption and thus carbon emissions. While many advanced economies are now transitioning toward renewables that do not emit local or global pollution, developing countries that continue to rely on fossil fuels may face similar trade-offs. As emerging technologies become more widespread, it is essential to evaluate their broader environmental impacts.

"Looking to the future, effective climate and energy policy must draw on historical experience to design frameworks that minimize unintended consequences," suggests Akshaya Jha, associate professor of economics and public policy at Carnegie Mellon's Heinz College, who co-authored the analysis.

"Developing countries can avoid some of the pitfalls encountered by the United States by investing in efficient technologies, avoiding overreliance on coal, responding strategically to geopolitical shocks, and crafting environmental regulations that reflect both local and global pollution objectives.

"By fostering innovation, maintaining regulatory flexibility, and holistically accounting for environmental externalities, policymakers can meet their climate goals more effectively while ensuring access to energy and fostering economic growth."