Release of carbon dioxide (CO2) from fossil fuel combustion and cement manufacture is the primary anthropogenic driver of climate change. Our best estimate is that China became the largest national source of CO2 emissions during 2006. Previously, the United States (US) had occupied that position. However, the annual emission rate in the US has remained relatively stable between 2001–2006 while the emission rate in China has more than doubled, apparently eclipsing that of the US in late 2006. Here we present the seasonal and spatial pattern of CO2 emissions in China, as well as the sectoral breakdown of emissions. Though our best point estimate places China in the lead position in terms of CO2 emissions, we qualify this statement in a discussion of the uncertainty in the underlying data (3–5% for the US; 15–20% for China). Finally, we comment briefly on the implications of China's new position with respect to international agreements to mitigate climate change.
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 Fossil fuel combustion and cement manufacture are the principal anthropogenic sources of the greenhouse gas carbon dioxide (CO2), and hence the principal concern in efforts to address anthropogenic climate change. The United Nations Framework Convention on Climate Change (UNFCCC) and its subsequent Kyoto Protocol were adopted in 1992 and 1997, respectively, as a beginning effort to limit the atmospheric increase in greenhouse gases. The Carbon Dioxide Information Analysis Center (CDIAC) database [Marland et al., 2007] shows global emissions from fossil fuels and cement have grown from 6.2 Pg C in 1990, the base year for commitments under the Kyoto Protocol, to 7.2 Pg C in 2001 and 8.4 Pg C in 2006. Rapid growth over the last five years has been dominated by economic growth in developing countries [see, e.g., Raupach et al., 2007], with 54% of the global increase in CO2 emissions over the period 2001–2006 coming from China alone.
 Historically, the United Sates (US) has long been the world's largest emitter of CO2 from fossil fuel combustion and cement production [Marland et al., 2007]. However, a recent press release from the Netherlands Environmental Assessment Agency , based on preliminary analysis of data from the International Energy Agency (IEA), BP, and the US Geological Survey (USGS), suggested that emissions from China had surpassed those from the United States for the first time in 2006. Our analysis concurs with the general observation of the Netherlands Environmental Assessment Agency, and the purpose of this letter is to provide detailed monthly and province-level data on Chinese fossil fuel emissions and to comment on the uncertainties associated with emissions estimates for China. This provides context and perspective on the role of China in global CO2 emissions from fossil fuel.
2. Materials and Methods
 Data on yearly CO2 emissions from China and the USA are taken from CDIAC [Marland et al., 2007] and are based on energy data from the United Nations (UN) Statistics Division (Energy Statistics Database, 2004 edition, http://unstats.un.org/unsd/energy/edbase.htm) and cement production data from the USGS [Kelly and Matos, 2007]. Data from the United Nations extend through 2004 and from the USGS through 2005. Energy data from BP  extend to 2006 and the fractional increases of time series data from 2004 to 2005 and 2006 have been used to extrapolate the CDIAC emissions estimates for two years. To estimate total emissions for 2005 and 2006, we assume that the fractional change in cement production for each country was the same from 2005 to 2006 as from 2004 to 2005. Applying this extrapolation approach to historical years (back casting), we find that using the BP statistics for a one-year extrapolation of the UN data leads to emissions estimates that vary (on average) about 1% from the UN values, while using the BP data for a 2-year extrapolation leads to estimates that vary (on average) about 2% from the UN values. Estimates include emissions from the flaring of gas at oil and gas fields and processing facilities (less than 1% of total emissions) and we assume that these values have remained unchanged since the end of the UN time series in 2004.
 Consistent sets of monthly data were produced by estimating the fraction of total fuel use that occurred during each month and using these fractions to allocate the annual emissions data from CDIAC [see Gregg and Andres, 2008]. For the US, the Energy Information Administration [1981–2007, 1983–2007, 1984–2007] maintains state-by-state monthly data on US coal consumption in the electric utility sector and monthly data on sales of petroleum products (gasoline, aviation fuel, kerosene type jet fuel, distillate fuel, diesel, and fuel oil) and deliveries of natural gas. Monthly data on cement manufacture and natural gas flaring are from Blasing and Hand . Emissions from cement production include only the CO2 resulting from the calcination process; emissions from fossil fuels used by the cement industry are included with fossil fuel CO2 emissions. For China, All China Marketing Research (ACMR) (China Data Online, 2007, http://chinadataonline.org) provides monthly data on thermal electricity generation, steel production, coke production, and value of industrial outputs, which represent 48%, 7%, 7%, and 27% of coal consumption, respectively. These time series were weighted to estimate the monthly emissions from coal consumption (the remaining proportion of coal we assume to be uniformly distributed residential use). This source also provides quarterly data on petroleum product sales (gasoline, kerosene, diesel, and fuel oil) and monthly statistics on travel volume, which we used to estimate monthly consumption of liquid fossil fuels. In addition, ACMR maintains monthly data on natural gas output, and monthly data on cement production. These data were used to subdivide the annual emissions into a monthly time series. Finally, the China National Bureau of Statistics  includes annual consumption data for coal, petroleum products (gasoline, kerosene, diesel, and fuel oil), natural gas, and cement production by province. We used these data to determine the spatial distribution of emissions in China by allocating the national annual total. These analyses do not include the special administrative regions of Hong Kong or Macao, and they also exclude Taiwan, because data are kept separately for these regions.
 CO2 emissions from China increased nearly 80% from 2000 to 2006. Emissions for 2003 and 2004 saw rates of increase of 17% and 18% respectively. This outpaced the phenomenal 10% annual growth in real gross domestic product (GDP) (World Bank, World Development Indicators Database, 2007, http://siteresources.worldbank.org/DATASTATISTICS/Resources/GDP.pdf), increasing China's carbon intensity (emissions per unit of real GDP). The rate of increase in emissions slowed to 10% and 8% for 2005 and 2006, but even at 10% annual growth, emissions would double again in less than nine years. The recent rate of growth in emissions from China has defied projections made five years prior. The International Energy Agency  had originally projected that China would become the world leader in emissions in 2030, then the following year adjusted that estimate to 2009. Cyranoski  updated the estimate to late 2007. Our best estimates suggest that for the full year of 2006 emissions from the United States were possibly still larger than emissions from China, but the difference was very small and clearly within the uncertainty bounds (Figure 1). However, when considering estimates of monthly CO2 emissions, our best estimate is that China reached US levels of emissions for the first time in November 2005, with both countries emitting 132 TgC/month, and then eventually passed the US in September of 2006, emitting at a rate 142 TgC/month (Figure 2), subject again to the uncertainties in the underlying data on energy consumption, as discussed below. Therefore, our best estimate is that the crossing between the United States and China occurred late 2006.
 Not only has economic growth been very high over the last few years in China, but growth has been powered largely with coal, the primary fossil energy source with the highest emissions of CO2 per unit of useful energy. CO2 emissions from China in 2004 were derived from coal (72%), petroleum (17%), cement (10%), and natural gas (1%) (Figure 3) [Marland et al., 2007]. Coal combustion is used for electric power generation and for other industrial processes. Since 2000, China has been the world's leading producer of coal, crude steel, and cement; and China is second in electricity production (behind the US) [China National Bureau of Statistics, 2000–2007]. Economic sectors that use petroleum have also been rapidly expanding since 2000. For example, between 2000 and 2006 transport volume doubled [China National Bureau of Statistics, 2000–2007]. China is also the world leader in fertilizer production, a process that uses substantial amounts of coal and natural gas as inputs [China National Bureau of Statistics, 2000–2007]. But a significant portion of growth in energy consumption and CO2 emissions has been driven by the globalization of the world economy and China is a major exporter of energy-intensive goods that are consumed elsewhere. Shui and Harris (2006) have estimated, for example, that between 7% and 14% of current CO2 emissions from China are a result of producing goods that will be consumed in the US.
 Estimates of monthly emissions in China show a consistent peak in December with a precipitous drop in January. There is also a slight peak in late summer in emissions from coal combustion (Figure 3). This pattern is consistent throughout the industrial production statistics from China and is also reflected in the quarterly GDP reports (ACMR, China Data Online, 2007, http://chinadataonline.org). Spatially, CO2 emissions from China are concentrated in the provinces around Beijing and along the east coast of China (Figure 4). Liquid fuel consumption is concentrated on the east coast of China, particularly in Guangdong province, which includes the special economic zone of Shenzhen and has been the beneficiary of international investment and manufacturing. Shanxi province, to the west of Beijing, is the leading consumer of coal, primarily for the production of coke, and has the highest per capita emissions in the country. Per capita emissions from China as a whole have risen to 1.1 tonnes C/person, but in Shanxi province, a leading coal producer, this rate is 3.3 tonnes C/person, equivalent to per capita emissions rates in Western Europe. At 1.1 tonnes C/person, the per capita emissions in China are close to the world average.
 Many analysts have viewed official Chinese statistics with skepticism, and there has been suspicion that politics motivate adjustments in the reported data [Akimoto et al., 2006; Sinton, 2001; Zhang et al., 2007]. In this analysis, we have used a multitude of industrial production and energy consumption statistics in an effort to reduce uncertainty and error. Nevertheless, much of the data on which we depend is ultimately from the Chinese National Bureau of Statistics, and any inaccuracies in those energy data will result in inaccuracies in our estimates of CO2 emissions. One possibility is that the recent growth depicted in the energy consumption statistics may reflect a correction from under-reporting in the late 1990s. According to official statistics, between 1996 and 2000 emissions in China decreased even while electricity generation, industrial output, and GDP continued to increase exponentially [China National Bureau of Statistics, 2000–2007]. Streets et al.  recognized that errors and uncertainties were increasing in the National Bureau of Statistics during this period and further reasoned that the reductions in coal use reported at the time were not as dramatic as the official data suggested. Logan  suggested that restructuring of the coal industry in 1996 ushered forth a black market that resulted in some coal consumption going unreported in the official statistics. When comparing modeled emissions to remotely sensed tropospheric nitrogen oxide (NOx) concentrations over this period, Akimoto et al.  concluded that the National Bureau of Statistics had significantly underestimated coal consumption during this period.
 Others have raised questions about the GDP figures China has released and have suggested, based on other economic indicators, that growth in the economy has not been as large as reported [Rawski, 2001]. On paper, the carbon intensity in China declined sharply in the late 1990s but has recently been increasing as the data indicate a dramatic increase in energy consumption. If it is the case that a large-scale statistical correction is occurring to compensate for underreporting in the 1990s, then our analysis may be overestimating the recent growth rate in emissions.
 It is also possible that our analysis may be underestimating the current rate of growth in emissions for China. According to our analysis, emissions are lower in the beginning of the year and higher in December. The mean slope within years (January to December) from 2001–2006 shows an increase in emissions of 23 Tg C/month, but the average increase in emissions using annual data is only 10 Tg C/month. Statistically, this difference occurs because of the drop in the emissions estimates from December to January. This could be a result of a ramping up of production to meet annual quotas by the end of the year (or data manipulation to make it appear that way). However, if the annual increase in consumption is actually closer to the reported monthly increase, this would result in us underestimating the annual increase in fossil fuel consumption in China.
 Recent adjustments in official Chinese energy statistics have now been incorporated into international energy data sets and have resulted in revisions of estimates of CO2 emissions. Emissions for 2000, for example, were revised upward by 23% for the year 2000 [Marland et al., 2007]. These revisions are in the direction expected but indicate the magnitude of continuing uncertainty in the Chinese data. Based on evaluation of the US data by the U.S. Environmental Protection Agency , and on the magnitude of recent revisions of energy data from China, we estimate that the two-sigma uncertainty associated with the annual total CO2 emissions estimates is 3% to 5% for the US but could be as high as 15% to 20% for China. It is, of course, not possible to independently evaluate the uncertainty of the Chinese data but the recent adjustments and the incompatibility with satellite NOx measurements suggest uncertainties in this range. There is no information to suggest an asymmetry in this uncertainty. Taking this uncertainty into account implies that the Chinese emissions could have passed those from the US as early as 2004, or the passing could be as late as 2010 (assuming the current trajectories) (Figure 1).
 Designated as a developing country, China was not given Annex I status, and thus is not required to meet emission reduction targets under the Kyoto Protocol. Whether we are observing increasing consumption in developing countries or the export of emissions from developed countries [Munskgaard and Pederson, 2001; Rothman, 1998; Shui and Harriss, 2006], global CO2 emissions are in a period of rapid growth that is nullifying the mitigation aspirations of international agreements [Auffhammer and Carson, 2008]. Although there is still concern about energy data from China, it is clear that CO2 emissions are growing very rapidly; over half of the global growth in emissions is occurring in China. Per capita emissions from China are now at global-average values and are reaching European-average values in some rapidly industrializing areas. This is propelling China into the position as the largest national source of CO2.
 Thanks to TJ Blasing for providing data on US cement and gas flaring. Research for this article was conducted at the Library of Congress and was facilitated by the staff of the Asian Reading Room. Research was also conducted at the Chinese Institute for Geographic Studies and Natural Resources Research (IGSNRR), part of the Chinese Academy of Sciences (CAS). Comments and suggestions from two anonymous reviewers have been invaluable in improving this document. Robert J. Andres and Gregg Marland were partially supported by the U.S. Department of Energy, Office of Science, Biological and Environmental Research Programs. Correspondence and requests for materials should be addressed to Jay Gregg (email@example.com).