Short-term effective radiative forcing trends are difficult to verify, so caution is required in predictions/policy judgments that depend on them, such as the time remaining to, or the outstanding carbon budget consistent with, 1.5C warming

Is Anthropogenic Global Warming Accelerating? Stuart Jenkins et al. Journal of Climate, Vol 35, Issue 24, Pages 4273–4290, Nov 22 2022. https://doi.org/10.1175/JCLI-D-22-0081.1

Abstract: Estimates of the anthropogenic effective radiative forcing (ERF) trend have increased by 50% since 2000 (from +0.4 W m−2 decade−1 in 2000–09 to +0.6 W m−2 decade−1 in 2010–19), the majority of which is driven by changes in the aerosol ERF trend, as a result of aerosol emissions reductions. Here we study the extent to which observations of the climate system agree with these ERF assumptions. We use a large ERF ensemble from the IPCC’s Sixth Assessment Report (AR6) to attribute the anthropogenic contributions to global mean surface temperature (GMST), top-of-atmosphere radiative flux, and we use aerosol optical depth observations. The GMST trend has increased from +0.18°C decade−1 in 2000–09 to +0.35°C decade−1 in 2010–19, coinciding with the anthropogenic warming trend rising from +0.19°C decade−1 in 2000–09 to +0.24°C decade−1 in 2010–19. This, as well as observed trends in top-of-atmosphere radiative fluxes and aerosol optical depths, supports the claim of an aerosol-induced temporary acceleration in the rate of warming. However, all three observation datasets additionally suggest that smaller aerosol ERF trend changes are compatible with observations since 2000, since radiative flux and GMST trends are significantly influenced by internal variability over this period. A zero-trend-change aerosol ERF scenario results in a much smaller anthropogenic warming acceleration since 2000 but is poorly represented in AR6’s ERF ensemble. Short-term ERF trends are difficult to verify using observations, so caution is required in predictions or policy judgments that depend on them, such as estimates of current anthropogenic warming trend, and the time remaining to, or the outstanding carbon budget consistent with, 1.5°C warming. Further systematic research focused on quantifying trends and early identification of acceleration or deceleration is required.

4. Consequences of alternative aerosol forcing trends

Successive IPCC reports have given assessments of the level of anthropogenic global warming, but no equivalent assessment of the rate of human-induced warming has been made. This is despite the rate of warming offering arguably more policy relevance at the present day in determining the time remaining to key goals of the Paris Agreement. This study highlights the key contributors to a perceived acceleration in the anthropogenic rate of warming since 2000 and uses observations of the climate system to constrain the forced response. Section 2 breaks down an ERF dataset that suggests anthropogenic ERF accelerated between 2000 and 2020, shown to arise because of aerosol forcing trends becoming positive around 2010 in the ERF ensemble. In section 3 we then step through three observational records to search for evidence supporting this assessed trend change.

Global temperatures show a clear change in trend between 2000 and 2020, characterized by temperatures remaining stable at around +1.0°C above preindustrial levels in the first decade, whereas in the second decade temperatures increase rapidly (with the rate of warming peaking at over +0.3°C decade−1). The reduced warming trend around 2000 has been discussed in the context of ocean heat uptake and natural variability (Masson-Delmotte et al. 2021; IPCC et al. 2013), but less research has focused on a possible warming acceleration in the following decade induced by aerosol ERF trend changes. By attributing the temperature trends to anthropogenic and natural ERFs we show that the forcing time series from Fig. 2 capture the broad warming contributions over the previous two decades (Fig. 4). However, significant variations around the anthropogenic best fit are still present (e.g., see the period of early Arctic warming in Fig. 4a), and alternative forcing trend change assumptions can be applied with little indication of a worse fit. A three-way regression isolates the aerosol contribution over history in Fig. 5, fitting the mid-twentieth-century GMST more successfully by downscaling the aerosol contribution. Hence, the aerosol ERF trend change contribution since 2000 is also downscaled, with best-estimate anthropogenic warming trend change around +0.05°C decade−1 between 2000–09 and 2010–19 (the 5th–95th-percentile range spans 0.01°–0.08°C decade−1). The aerosol contribution to this is +0.03°C decade−1 between 2000–09 and 2010–19 (the 5th–95th-percentile range spans 0.00°–0.07°C decade−1).

In the CERES record, LW TOA flux contributions are explained by recent GMST and forcing trends combined, while there is greater uncertainty in the contributors to the SW and net flux anomalies. In the SW anomaly, unforced variability in temperature and TOA flux time series precludes clear assessments of the aerosol ERF contribution, supporting the assessment that significant contributions from ENSO and PDO are present in recent TOA flux trend changes. Given this, TOA flux trends cannot rule out little to no anthropogenic ERF trend change over the two decades, despite the best-estimate anthropogenic ERF time series agreeing well with both TOA fluxes and temperature anomalies. Figure 6’s middle column (atmosphere-only TOA flux anomalies) and right column (coupled-model TOA flux anomalies) confirm the major role played by natural variability processes in these TOA flux records, demonstrating that the trend change induced in the SW TOA flux anomaly, where we expect to observe a large trend change induced by aerosols (Fig. 6h), is small relative to the trend change caused by unforced variability over the previous two decades (Fig. 6e). Continued funding for new satellites to study the outgoing radiative balance of the Earth system (such as the recently announced FORUM mission; ESA 2021) is vitally important in order to maintain long-term records and constrain radiative feedbacks with greater certainty.

Satellite observations and CMIP6 models agree that a relatively small AOD trend change has occurred over the last two decades, despite significant reductions in anthropogenic aerosol emissions in the Northern Hemisphere. Exploring a linear relationship between AOD and aerosol ERF trends (based on the mean response of CMIP6 models) allows us to estimate the ERF trend change expected in response to the observed AOD trend change since 2000. This analysis again supports the best estimate ERFs in Fig. 2, but also suggests that little-to-no trend change remains a possible assessment for ERF trends since 2000 in observations. The spatial pattern of aerosol emissions also may play a role in determining the aerosol ERF level (Stier et al. 2013), causing nonlinearities in the AOD and ERF responses to globally averaged aerosol emissions reductions. Further research of ERF trends and feedbacks using their full spatiotemporal signal in AMIP GCM experiments where forcing time series are known, and conducting regional aerosol perturbation experiments in coupled models (Wilcox et al. 2022), will provide more insight.

Overall, this assessment suggests that aerosol emissions reductions have contributed to an increase in the rate of anthropogenic warming since 2000, but both to a lesser degree than is suggested in the ERFs presented in Fig. 2, and with a substantial uncertainty range including the possibility of little contribution over two decades. The forced behavior coincides with a period of considerable internal variability, meaning that isolating the aerosol-induced ERF trend change from observations is challenging, and that a wide range of ERF scenarios offer plausible explanations of the past 20 years. Some of these possibilities are not well represented in the ERF ensemble shown in Fig. 2: for example, a zero-trend-change aerosol ERF scenario between 2000 and 2020 (shown in Fig. 2 as an dashed orange line, with the corresponding anthropogenic ERF a black dashed line), is considered possible in the analysis of all three observation datasets but is not represented well in the ERF ensemble of Fig. 2. Using a zero-trend-change aerosol ERF to attribute the anthropogenic contribution to global warming results in a similar quality fit to GMSTs (see dashed orange and black lines in Fig. 5, left panel), but substantially reduces the anthropogenic warming trend change that occurs since 2000 (orange and black dashed lines in Fig. 5, right panel). A comprehensive ERF ensemble of the recent time-history of anthropogenic ERF should offer these alternative scenarios, including scenarios with reduced aerosol ERF trend change since 2000 and with alternative rescalings for all pollutants using global energy balance constraints [e.g., those in Smith et al. (2021a) or Fig. 5].

The importance of continuing to track and constrain the early twenty-first century’s forcing trends is underappreciated, and future work should focus on providing better constraints to near-term forcing trends as well as their levels. Short-term ERF trends are vital to accurately assess this decade’s warming rate, with tangible, real-time impacts for global mitigation policy.