Policymakers, investors, and the public may assign responsibility in various ways for greenhouse gas emissions and subsequent changes in climate and associated impacts. Assigning responsibility for climate change is a societal judgment, one that can be informed by but not determined through scientific analysis.
The UNFCCC process, focused on the allocation of climate responsibilities among emitting nations, is a well-established approach to addressing this challenging problem. But, society can attribute climate responsibilities in other ways as well, including to individuals and major emitting industries. Shareholder resolutions and calls for institutional divestment from the primary producers of coal, oil, and natural gas are now contributing to growing public, investor, and scholarly discourse on the particular climate responsibilities of major investor-owned producers of fossil fuels (Lubber 2012; Oreskes 2013; Rockefeller Brothers Fund 2014). This is a reflection, in part, of growing attention to the actions that companies took and could have taken in light of the scientific evidence of climate change (Frumhoff et al. 2015).
The tools of attribution science are being applied to characterize specific damages resulting from anthropogenic climate change (Mitchell et al. 2016). Policymakers in several jurisdictions are now considering whether fossil fuel producers might bear some responsibility for such climate damages potentially traceable to emissions from their products (van Renssen 2016).
The present analysis provides a first step to inform such considerations by characterizing several global consequences linked to these emissions. One source of uncertainty in characterizing the contribution of emissions to global climate change is the equilibrium climate sensitivity (ECS) and transient climate response (TCR) to an increase in atmospheric CO2(IPCC 2013). Parameters were tested through their published range of values. The best-estimate parameters with full historical forcing yield results that most closely aligned with historical observations over recent decades.
A secondary source of uncertainty is the short-term effects of fossil fuel aerosols. Typically, nations set the policies regarding aerosol emissions from fossil fuel combustion; hence the aerosols linked to a given product could vary under different regulations governing combustion. If policymakers decide to factor in aerosols traced back to carbon producers when considering their climate responsibilities, steps could then be taken to require reporting of production, processing, and combustion region to better estimate resulting aerosol emissions. Policy in this area should be cognizant, however, of the danger of providing corporations with an incentive to emit aerosols to offset the climate responsibilities implied by their products or “private geoengineering.” Over the long term, the legacy of long-lived heat-trapping emissions outlive the temporary offset from the associated short-lived aerosols from fossil fuel combustion, biomass burning, and other anthropogenic sources (Shindell and Faluvegi 2010; Solomon et al. 2010).
With respect to the question of climate responsibility, a third source of uncertainty is the policy relevance of different time periods of historical carbon emissions. Researchers exploring the allocation of responsibilities among nations for greenhouse gas emissions typically consider cumulative emissions over the historical period since 1880, when GMST data are sufficiently abundant and biases in the early records are understood and can be corrected (Matthews et al. 2014; Karl et al. 2015).
The attribution of climate responsibility among non-state actors such as investor-owned fossil fuel companies might also take account of the timing by which companies should have reasonably been expected to respond to evidence of the climate risks of their products by, for example, investing in low-carbon energy technologies, supporting climate policies and legislation, or communicating these risks with consumers and shareholders (Frumhoff et al. 2015). Strikingly, more than half of all emissions traced to carbon producers over the 1880–2010 period were produced since 1986, the period in which the climate risks of fossil fuel combustion were well established. Consistent with this, our results show that bounding consideration of cumulative emissions to recent decades (1980–2010) only modestly reduces the global climate impact of major carbon producer-traced emissions with regard to global mean surface temperature. Global sea level rise response to climate forcing operates on slower times scales, so emissions from 1980 to 2010 have less of a relative contribution for GSL compared with GMST.
Further research might inform assessments of responsibility for the costs of adaptation for future climate change by considering the legacy consequences of historical emissions on global climate. The warming effect of CO2 emissions is largely realized within a decade after release but persists for centuries to millennia. Other impact-relevant effects, such as sea level rise, will not be fully manifested for a century or longer after emissions (Joos et al. 2013; Ricke and Caldeira 2014; Strauss et al. 2015). In short, calculating only the historical contribution underestimates the total contribution (i.e. historical plus the legacy going forward plus any additional warming associated with the near-immediate removal of the partial offset by aerosols).
As a first approximation, one can estimate the impact of historical emissions traced to major carbon producers on near-term future sea-level rise by assuming that no major volcanic eruptions occur and recent historical emissions drive constant rates of sea-level rise for several decades (Joos et al. 2013). Projecting the best estimate full forcing reference case for the average annual rate of sea level rise over 2000–2010 of ∼0.43 cm/year would mean another ∼17 cm above 2010 level by 2040. Without the 90 carbon producers, sea level would rise ∼5.7 cm above 2010 level by 2040.
Our study demonstrates that the proportional increase in atmospheric carbon dioxide, GMST, and GSL—key indicators of human impact on the global environment—from emissions traced to major carbon producers is quantifiable and substantial. The analyses presented here could be extended to examine the contribution of emissions traced to major carbon producers to other impacts, such as historical increases in ocean acidification (Ekstrom et al. 2015) or the mortality impacts from extreme heat and other extreme events (Otto et al. 2012; Mote et al. 2015; Mera et al. 2015; Mitchell et al. 2016).
Size of contribution, such as calculated in this study, is one factor to consider in assessing responsibility for climate change consequences associated with atmospheric CO2 and CH4, radiative forcing, GMST, and GSL. Other factors include consideration of differences among carbon producers in how they responded to the scientific evidence of the climate risks of their products (Frumhoff et al. 2015). These factors coupled with ethical, legal, and historical considerations may further inform discussions about carbon producer responsibilities to contribute to limiting climate change through investment in mitigation, support for adaptation, and compensation for climate damages.