Committed Warming Due to Earth’s Radiative Imbalance Using a Simple 5-Factor Energy Balance Model

Abstract

The concept of committed warming has been neglected or understated in the climate change conversation, although climate scientists acknowledge that some additional global warming is “baked in” no matter how quickly the world switches to a clean-energy economy. The Earth’s climate is not equilibrated with anthropogenic climate forcing. Ignoring thermal inertia of the oceans, this study shows that stabilizing CO2 concentrations at today’s levels would still commit the earth to at least 0.5oC of additional warming in the next century. Due to past anthropogenic emissions the earth currently absorbs more heat than it emits, which induces a yet unrealized commitment. Even if the CO2 levels stopped increasing, the planet would have to warm further in order to balance solar energy absorbed with energy radiated back to space. This study incorporates a simplified, five-factor model (5FEBM) that uses only known or well-understood factors that drive global warming. These five key factors come close to accounting for the earth’s current radiative imbalance, as well as future temperatures predicted by global climate models. The factors include human greenhouse gas emissions, human generated aerosol emissions, human caused land surface alteration, water vapor feedback, and Planck radiation feedback. Published research on the topic of committed warming mostly corroborates the results of the 5FEBM model. Multiple sources predict a committed warming on the order of 1.5oC relative to pre-industrial conditions. This is particularly relevant to the IPCC’s recent call for more aggressive emission reduction targets to keep total warming below 1.5oC by the end of the 21st century. The evidence herein suggests that may not be possible. Ignoring other feedbacks that will surely worsen, the five factors listed above commit the earth to a global mean surface temperature (GMST) anomaly slightly more than 1.5oC in 2100. A numerical method is developed from the 5FEBM to estimate committed warming under constant concentration conditions. The method is validated by integrating a differential equation involving temperature and time at constant CO2 concentration. The effect of this warming commitment on the year-to-year GMST anomaly is graphed. A sensitivity analysis demonstrates the solution is robust with respect to assumed initial conditions. There is some divergence among estimates of committed warming based partly on assumptions about emissions. Most studies assume constant composition commitment (CCC), but some envision zero emissions. Others assume continued emissions from current fossil fuel investments that would persist through the end of the investments’ useful life. Another scenario admits to an ongoing emissions commitment due to certain energy technologies for which there is currently no commercially available replacement. Presumably, the rationale for estimating committed warming is to portray an idealized but still reasonable picture of the current climate crisis. Calling attention to committed warming seeks to motivate prompt action on climate change by showing both the possibilities and the urgency of mitigating its impacts. The CCC scenario balances these two objectives.