Climate and carbon budget implications of linked future changes in CO ₂ and non-CO ₂ forcing

Abstract

The approximate proportional relationship between cumulative carbon emissions and instantaneous global temperature rise (the carbon budget approximation) has proven to be a useful concept to translate policy-relevant temperature objectives into CO 2 emissions pathways. However, when non-CO 2 forcing is changing along with CO 2 forcing, errors in the approximation increases. Using the GCAM model to produce an ensemble of ∼3000 scenarios, we show that linked changes in CO 2 forcing, aerosol forcing, and non-CO 2 greenhouse gas (GHG) forcing lead to an increase in total non-CO 2 forcing over the 21st century across mitigation scenarios. This increase causes the relationship between instantaneous temperature and cumulative CO 2 emissions to become more complex than the proportional approximation often assumed, particularly for low temperature objectives such as 1.5°C. The same linked changes in emissions also contribute to a near-term increase in aerosol forcing that effectively places a limit on how low peak temperature could be constrained through GHG mitigation alone. In particular, we find that 23% of scenarios that include CCS (but only 1% of scenarios that do not include CCS) achieve a temperature objective of 1.5°C without temperature overshoot.

Trade and Climate Mitigation Interactions Create Agro‐Economic Opportunities With Social and Environmental Trade‐Offs in Latin America and the Caribbean

“…This simulates a level of ambition consistent with achieving net-zero CO 2 emissions shortly after mid-century. We price LUC CO 2 emissions at 10% of the level applied to fossil fuel and industrial (FFI) CO 2 , consistent with previous studies (Feijoo et al, 2019). The resulting global CO 2 emissions pathway results in less than 700 GtCO 2 of cumulative CO 2 emissions by 2050 and is consistent with limiting end-of-century temperature increase to well below 2°C (Figure S1 in Supporting Information S1; Huppmann et al, 2019).…”

Deep mitigation of CO2 and non-CO2 greenhouse gases toward 1.5 °C and 2 °C futures

“…Second, alternative assumptions in modeling structure, non-CO 2 mitigation parameterization, and climate modeling choices that characterize the physical effects of climate forcers, can certainly affect the ultimate temperature changes 21 , 32 , 35 . Although other climate forcers such as black carbon, aerosols, and ozone, can also contribute to temperature change 36 and interact with CO 2 and non-CO 2 GHG forcings through complex atmospheric processes 5 , 37 , their emissions are primarily affected by the level of CO 2 mitigation 24 instead of specific non-CO 2 GHG mitigation measures. Third, while our climate results are unaffected by the GWP choices (Methods), the GWP assumptions can affect the relative contributions of different GHG species to the total GHG reductions when presenting.…”

“…Thus, cuts in their emissions could potentially lessen future climate forcing 11 . The present literature indicates that the ultimate level of surface temperature warming depends both on driving CO 2 emissions to zero but also on the residual level of non-CO 2 emissions 5 , 9 , 12 – 20 . Although these studies have in some way accounted for the climate benefits for non-CO 2 mitigation, different representations and non-CO 2 mitigation options, as well as the economic structure of the models, can lead to a fairly large variation in the reported remaining carbon budget or net-zero commitment years that aim to achieve the same 1.5 °C or 2 °C goals 21 – 23 .…”

Balancing a budget or running a deficit? The offset regime of carbon removal and solar geoengineering under a carbon budget

“…In estimating carbon budgets, non-CO 2 forcing is often assumed to be constant or declining (Rogelj et al 2019a). But this assumption could be wrong: net non-CO 2 forcing might increase over the course of the twenty-first century (Feijoo et al 2019;Mengis and Matthews 2020 The important point from the above discussion is that the uncertainty over how net non-CO 2 forcing (a total of positive and negative forcing from non-CO 2 GHGs and aerosols) will evolve over time complicates the work of estimating a carbon budget. The carbon budget concept puts the central focus on limiting cumulative CO 2 emissions within an allowable budget.…”