Reversing climate warming by artificial atmospheric carbon‐dioxide removal: Can a Holocene‐like climate be restored?

Abstract

Most climate modeling studies of future climate have focused on the effects of carbon emissions in the present century or the long‐term fate of anthropogenically emitted carbon. However, after carbon emissions cease, there may be a desire to return to a “safe” CO2 concentration within this millennium. Realistically, this implies artificially removing CO2 from the atmosphere. In this study, experiments are conducted using the University of Victoria Earth system‐climate model forced with novel future scenarios to explore the reversibility of climate warming as a response to a gradual return to preindustrial radiative forcing. Due to hysteresis in the permafrost carbon pool, the quantity of carbon that must be removed from the atmosphere is larger than the quantity that was originally emitted (115–180% of original emissions). In all the reversibility simulations with a moderate climate sensitivity, a climate resembling that of the Holocene can be restored by 3000 CE.

Negative emissions physically needed to keep global warming below 2 °C

“…A reduction in future carbon sinks would reduce compatible emissions, and thus increase the need for negative emissions. Permafrost thawing, as a potential future source of carbon, is also not accounted for in our study, and it may similarly reduce compatible emissions 31 . Compatible emissions used here are restricted to the RCP2.6 land-use change scenario (see Methods).…”

The Carbon Dioxide Removal Model Intercomparison Project (CDRMIP): rationale and experimental protocol for CMIP6

“…Little is known about how reversible this relationship is or whether it applies to other Earth system properties (e.g., net primary productivity, sea level, etc.). Investigations of CDR-induced climate reversibility have suggested that many Earth system properties are "reversible", but often with nonlinear responses (Armour et al, 2011;Boucher et al, 2012;MacDougall, 2013;Tokarska and Zickfeld, 2015;Wu et al, 2014;Zickfeld et al, 2016). However, these analyses were generally limited to global annual mean values, and most models did not include potentially important components such as permafrost or terrestrial ice sheets.…”

“…To date, modeling studies of CDR focusing on the carbon cycle and climatic responses have been undertaken with only a few Earth system models (Arora and Boer, 2014;Boucher et al, 2012;Cao and Caldeira, 2010;Gasser et al, 2015;Jones et al, 2016a;Keller et al, 2014;MacDougall, 2013;Mathesius et al, 2015;Tokarska and Zickfeld, 2015;Zickfeld et al, 2016). However, as these studies all use different experimental designs, their results are not directly comparable, and consequently building a consensus on responses is challenging.…”

Irreversible ocean thermal expansion under negative CO<sub>2</sub> emissions

“…The total negative emissions for reverting to the initial CO 2 concentration are higher than the total positive emissions if the permafrost carbon feedback, which accounts for additional carbon emissions from thawing permafrost, is included (MacDougall, 2013). This means that more carbon needs to be artificially removed from the atmosphere than was initially emitted due to the hysteresis behaviour of the permafrost carbon pool.…”

“…This difference would need to be taken into account when setting total allowable emissions for a certain warming target after overshoot (Zickfeld et al, 2016). Previous studies found thermosteric sea level rise to be in principle reversible Zickfeld et al, 2013;MacDougall, 2013), but reversing it on centennial time scales requires large amounts of negative CO 2 emissions, which are 20 likely infeasible with currently discussed technologies (Tokarska and Zickfeld, 2015). By using a 2-layer ocean model (Gregory, 2000;Geoffroy et al, 2013) show that the decline in thermosteric sea level in response to zeroed or negative radiative forcing (with preceding positive radiative forcing) is due to a strong temperature decline in the upper ocean relative to the deep ocean, which enables the release of heat.…”