The Earth warms both when fossil fuel carbon is oxidized to carbon dioxide and when greenhouse effect of carbon dioxide inhibits longwave radiation from escaping to space. Various important time scales and ratios comparing these two climate forcings have not previously been quantified. For example, the global and time-integrated radiative forcing from burning a fossil fuel exceeds the heat released upon combustion within 2 months. Over the long lifetime of CO2 in the atmosphere, the cumulative CO2-radiative forcing exceeds the amount of energy released upon combustion by a factor >100,000. For a new power plant, the radiative forcing from the accumulation of released CO2 exceeds the direct thermal emissions in less than half a year. Furthermore, we show that the energy released from the combustion of fossil fuels is now about 1.71% of the radiative forcing from CO2 that has accumulated in the atmosphere as a consequence of historical fossil fuel combustion.

The Earth is heated both when reduced carbon is oxidized to carbon dioxide and when outgoing longwave radiation is trapped by carbon dioxide in the atmosphere (CO2 greenhouse effect) [Washington, 1972; Nordell, 2003; Block et al., 2004; Chaisson, 2008; Flanner, 2009; Ma et al., 2011; G. J. Zhang et al., 2013; X. Zhang et al., 2013]. The purpose of this study is to improve our understanding of time scales and relative magnitudes of climate forcing increase over time from pulse, continuous, and historical CO2 and thermal emissions. We aim to (1) improve our understanding of time scales and relative magnitudes of the forcing increase over time due to pulse fossil fuel combustion thermal and CO2emissions; (2) identify for a pulse emission the crossover time when warming from CO2 exceeds warming from thermal; and (3) understand how this affects cumulative forcing from thermal and CO2 emissions since the Industrial Revolution.

In converting energy from chemical/physical energy to thermal energy and from thermal energy to electrical energy, the electricity generation largely dissipates energy as thermal emission or heat [United States Environmental Protection Agency (USEPA), 1997; Chen et al., 2009; Zhan et al., 2009;Zhang et al., 2009; Ma et al., 2010, 2012; Zhan et al., 2012; Zhang et al., 2012a, 2012b]. Even the part that goes into useful energy is eventually dissipated as heat. Thermal emission from fossil fuel combustion is a forcing on the climate system [Washington, 1972;Nordell, 2003; Block et al., 2004; Chaisson, 2008; Flanner, 2009;Myhrvold and Caldeira, 2012; G. J. Zhang et al., 2013; X. Zhang et al., 2013] (see also Text S1 in the supporting information). Thermal emissions are generally small compared with climate forcing due to CO2 and other greenhouse gases [Washington, 1972] but could have important climate effects [Nordell, 2003; Block et al., 2004; Chaisson, 2008; Flanner, 2009; G. J. Zhang et al., 2013; X. Zhang et al., 2013]. G. J. Zhang et al. [2013] and X. Zhang et al., [2013] points out that 42.2% of worldwide fossil fuel energy combustion occurs within 1.27% of the Earth surface, suggesting that thermal emissions may be more important to local and regional climates (and the urban heat island effect) than to global climate. For additional literature review, please see Text S1 in the supporting information.