Growing emissions from 1997 to 2007

Between 1997 and 2007, US emissions increased by 7.3% (Fig. 1, black curve). Our analysis shows that the main factor behind this increase was an increase in consumption volume caused by growth in per capita consumption of goods and services in the United States. Indeed, increases in such consumption volume correspond to a contribution of a 21.8% increase in emissions over this decade (Fig. 1, red curve). The next most important factor influencing CO 2 emissions over the same period was population growth. Immigration and natural growth have resulted in steady population growth at a rate of ∼1% per year since 1997. These population gains contributed to an 8.9% increase in emissions between 1997 and 2007 (Fig. 1, yellow curve).

Figure 1: Contributions of different factors to changes in the US CO 2 emissions between 1997 and 2013. Using 1997 as base year, the solid black line shows the percentage change in total CO 2 emissions. The other lines show the contribution to the change in emissions from consumption volume (red), population (yellow), consumption patterns (green), production structure (blue), energy intensity (purple) and fuel mix (orange). Full size image

However, other factors slowed the growth of emissions between 1997 and 2007: decreases in the energy intensity of GDP; changes in the consumption patterns of US consumers; shifts in production structure; and decreases in the use of coal as an energy source. For instance, over this period, the energy used per dollar of economic output decreased by 17% (Fig. 2a, black curve), the share of consumer spending on manufactured goods decreased by ∼4% (Fig. 2b), the share of imported inputs to the US industry sectors increased (for example, imports to petroleum and coal products sector increased by 6.7%, and imports to the chemical products, primary metals and textile sectors increased by 2.7%, 2.5% and 2.1%, respectively)11, and the share of US electricity generated from coal decreased by ∼5% while the share generated from natural gas increased by 8% (Fig. 2c). All of these trends exerted a downward influence on emissions. Between 1997 and 2007, changes in energy intensity, consumption patterns, production structure and fuel mix contributed to retarding emissions of 7.4, 6.9, 4.9 and 3.6%, respectively (Fig. 1, purple, green, blue and orange curves, respectively).

Figure 2: Trends underlying the decomposed factors. (a) Per cent changes in the energy intensity (energy used per dollar (US$) of output) of key sectors in the US economy, (b) shares of final demand made up of manufactured goods (that is, food, clothing, agriculture, paper and printing, chemical manufacture (manufact.), petroleum refining, metal manufacturing, machinery and equipment, utilities and construction) and services (that is, retail, hotel, transport, shipping, real estate, public administration, defense, education, health, community and social work, and household employment.) and (c) changes in the fuel mix of the US electricity sector. Full size image

Declining emissions from 2007 to 2013

US CO 2 emissions stopped growing in 2007, and decreased by ∼11% between 2007 and 2013 (Fig. 1, black curve). Looking at this time period in aggregate, the only factor which acted to increase emissions over the period was continued and steady population growth (+3.7%) (Fig. 1, yellow curve). However, the upward influence of population growth was overwhelmed by the downward influence of changes in production structure (−6.1%), fuel mix (−4.4%), consumption volumes triggered by per capita consumption (−3.9%), energy intensity of GDP (−0.5%) and changing consumption patterns (−0.4%; Fig. 1, blue, orange, red, purple and green curves, respectively).

Although all of the analysed factors except population contributed to the decrease in emissions during 2007–2013, different factors dominated over shorter periods. Figure 3 subdivides 2007-2013 into 2-year periods, showing that emissions fell by 9.9% from 2007 to 2009, increased by 1.3% between 2009 to 2011 and decreased again by 2.1% between 2011 and 2013.

Figure 3: Contributions of different factors to the decline in US CO 2 emissions 2007–2009 and 2009–2011 and 2011–2013. Between 2007 and 2009, decreases in the volume of goods and services consumed during the economic recession (red) was the primary contributor to the nearly 10% drop in emissions. But between 2009 and 2011, consumption (consump.) volume rebounded, population grew and the energy intensity of output increased, driving up emissions by 1.3% against modest decreases in the carbon intensity of the fuel mix and shifts in production structure and consumption patterns. Between 2011 and 2013, increases in population and consumption volume again pushed emissions upward, but overall emissions decreased by 2.1% due to further changes in production (prod.) structure, consumption patterns, decreasing use of coal and decreases in energy intensity of output. Not shown here, emissions increased by 1.7% between 2012 and 2013, driven primarily by increases in consumption volume. Full size image

More than half (53%) of the initial and most substantial decrease in emissions, between 2007 and 2009, was due to a sharp drop in the volume of consumed goods as a result of reduction in per capita consumption during the global economic recession (Fig. 3, red bar). In particular, Fig. 4 shows that sharp decreases in the volume of capital expenditures and exported goods between 2007 and 2009 drove down associated emissions by 25% and 18%, respectively. Changes in the production structure of the US economy (that is, the volume and type of intermediate goods demanded) and the fuel mix of the energy sector contributed 30% and 17% of the initial (2007–2009) decrease in emissions, respectively, while increases in the energy intensity of the US economy and changing consumption patterns exerted modest upward influences on emissions during the same period.

Figure 4: Contributions of different factors to changes in US CO 2 emissions specific to different final demand components 1997–2013. Shown are changes in emissions related to household expenditures (a), government expenditures (b), capital investment (c) and exports (d). In each panel, the solid black line shows the percentage change in CO 2 emissions triggered by changes in the corresponding final demand component, and the other lines show the contribution to the change in emissions from consumption volume (red), population (yellow), consumption patterns (green), production structure (blue), fuel mix (orange) and energy intensity (purple). Full size image

As the US economy had slowly recovered from the global economic recession, between 2009 and 2013, the average annual change in US emissions was small: a 0.2% decrease. Economic recovery is reflected by the upward influence of the volume of goods consumed on emissions during both 2009–2011 and 2011–2013. Between 2009 and 2011, rising consumption volume, population growth, and increasing energy intensity urged emissions up by a combined 4.0% (2.2%, 1.5% and 0.3%, respectively), which was only partly offset by the changes in consumption patterns (−1.1%), production structure (−1.0%) and fuel mix (−0.6%), resulting in an actual increase in emissions of 1.3% (Fig. 3). However, between 2011 and 2013, the upward influence of consumption volume and population on emissions was less (+1.2% and +1.2%, respectively) and the energy intensity of the economy decreased (−2.1%). When combined with changes in the fuel mix of the energy sector (−1.2%) and shifting consumption patterns (−0.2%), the net effect was a 2.1% decrease in emissions during 2011–2013 (Fig. 3).

Increases in the supply of natural gas affect two of the factors in our analysis: the fuel mix of the energy sector and, to a lesser extent, the energy intensity of the US economy. By decreasing gas prices, abundant gas encourages a shift in the fuel mix from more carbon-intensive coal to gas. In turn, a shift to gas may contribute to decreased energy intensity because gas-fired power plants are on average 20% more efficient at converting fuel energy to electricity than coal plants12.

The boom of natural gas from breakthroughs in hydraulic fracturing of shale deposits had only just begun to affect US gas supplies in 2009 (ref. 5). Thus, the decrease in emissions from changes in the fuel mix of the energy sector prior 2009 reflects an independent and longer-term trend of the declining use of coal in the US energy sector (see, for example, Fig. 2c). However, as seen in Fig. 3, changes in the US fuel mix from 2007 to 2009 alone would not have caused a decrease in US emissions.

Although the decreases in emissions since 2009 have been relatively small, the influence of shale gas is visible. For example, about half of the 2.1% decrease in emissions during 2011–2013 is related to changes in the fuel mix of the energy sector (−1.2%, orange bar in Fig. 3). Yet the decrease in the energy intensity of the US economy was nearly twice as strong an influence on emissions over the same period (purple bar in Fig. 3).

Although a drop in the energy intensity (exajoule per dollar output) of the energy sector in 2013 accounts for roughly a third of the observed decrease in US energy intensity in 2011–2013, the remaining two-thirds relate to changes in energy used by the transport and service sectors (Fig. 2a). Three unrelated trends underlie the decreasing energy intensity of these sectors. First, high gasoline prices during 2011–2013 (the average price of gasoline had remained above $3.40 per gallon during this period, in contrast to the average price of $2.50 per gallon in 2005) have contributed to both a reduction in per capita miles driven (Supplementary Fig. 1a) and an increase in average fuel efficiency of vehicles (Supplementary Fig. 1b), and thus a 33% decrease in US gasoline consumption during 2011–2013. Second, a mild winter in 2012 meant less energy was used for heating and thus reduced energy intensity of the service sector (households also used less energy for home heating, which accounts for part of the drop in consumption volume)13 (Supplementary Fig. 2). Last, there is evidence that manufacturing in the United States became more energy efficient: energy use by manufacturing was nearly constant 2011–2013 despite average annual growth in GDP of 2.3% per year over the same period.

Shifts in the production structure of the US economy between 2007 and 2013 have consistently exerted a downward influence on US emissions, as the volume and type of intermediate goods used by various industry sectors has evolved and become more efficient (blue bars in Fig. 3). Yet this structural shift also reflects the progressive offshoring of emissions-intensive industries to China and other developing countries over the analysed period14. For instance, between 2009 and 2011, when changes in domestic production structure exerted a downward influence on US CO 2 emissions (−1%, blue bar in Fig. 3), we calculated that the net import of emissions embodied in US trade increased by 32% (Supplementary Fig. 3). Trade data for the 2011–2013 period is not yet available.

Between 2009 and 2013, the share of US consumption of manufactured goods increased relative to services (Fig. 2b), but the net effect of changes in consumption patterns was to decrease emissions (by 1.1% between 2009 and 2011 and by 0.2% between 2011 and 2013; green bars in Fig. 3). This result reveals that changes in the types of goods being consumed over time can have a significant impact on emissions15,16, and that it is not as simple as the balance of manufactured goods and services.

Discussion

Between 1997 and 2007, US emissions grew steadily (0.7% per year) as increases related to population growth and consumption volume (per capita consumption) outpaced the downward influence of improving energy intensity, shifting consumption patterns and production structure and decarbonizing fuel mix.

The large decrease (9.9%) in US CO 2 emissions between 2007 and 2009 was primarily the result of the economic recession, evidenced by large decreases in household consumption, energy-intensive capital expenditures and export (Figs 1, 3 and 4). The recessionary belt-tightening may also have contributed to the significant efficiency gains in production structure.

Since 2009, the slow recovery of the US economy has urged emissions backup, but has been closely balanced by decreases in energy intensity, especially in the transport, manufacturing and service sectors (Fig. 2a), as well as changes in the fuel mix of the energy sector. The net effect has been very little change in emissions; between 2009 and 2013; US emissions have decreased by an average of 0.2% per year. Contrary to conventional wisdom, our decomposition analysis shows that changes in the fuel mix of the energy sector (including those related to the shale gas boom) account for a relatively small portion of this decrease.

In addition to a more robust understanding of the factors influencing US emissions during 1997–2013, our analysis may be helpful in assessing the efficacy of different forces to reduce US emissions in the future. For example, the modest effect of changes in the fuel mix of the energy sector on emissions in recent years suggests that further increase in the use of natural gas may be of limited benefit in decreasing emissions. This is because barring technology-specific policies (for example, Renewable Portfolio Standards), recent studies have shown that gas does not substitute for coal only; growth of emission-free technologies such as solar, wind and nuclear energy is also limited while gas is cheap17,18. In these studies, future increases in natural gas use act to both reduce domestic coal use and slow the growth of renewable energy, resulting in little net change to cumulative CO 2 emissions17,19,20,21. Moreover, CO 2 emissions are not the only consideration; a growing number of studies also show that increased leakage of methane from new natural gas infrastructure can offset CO 2 reductions relative to coal22,23. Third, decreases in residential gas prices (Supplementary Fig. 4) may lead to rebound effects if people spend some of the money they saved heating their home on carbon- and energy-intensive goods24. And finally, decreased domestic demand for coal has enabled an increase in US coal exports to eager and growing overseas markets. The US power sector consumed 170 million fewer metric tons of coal in 2013 than in 2007, during which period coal exports doubled even as coal prices rose (Supplementary Fig. 5). Although CO 2 emissions from US coal burned elsewhere are generally attributed to the country where those emissions occur, the emissions nonetheless contribute to global climate change (and in fact less energy may be produced per unit of CO 2 emissions when the coal is burned in countries with less-efficient power plants). For all these reasons, further increases in the use of natural gas in the United States may not have a large effect on global greenhouse gas emissions and warming.

Similarly, further emissions reductions due to decreases in energy intensity are not inevitable. As can be seen in Fig. 2a, the energy intensity of utilities increased between 2009 and 2013, perhaps because such utilities chose to pass the cost savings related to cheap gas along to their customers25. The energy intensity of other industry sectors also shows no long-term decreasing trend (Fig. 2a). In contrast, any gas-driven recovery of US manufacturing, such as in the production of vehicles and heavy machinery26, will tend to increase the average energy intensity of the US economy.