Global warming in the pipeline (2212.04474v3)

Abstract: Improved knowledge of glacial-to-interglacial global temperature change implies that fast-feedback equilibrium climate sensitivity (ECS) is 1.2 +/- 0.3{\deg}C (2$\sigma$) per W/m$^2$. Consistent analysis of temperature over the full Cenozoic era -- including "slow" feedbacks by ice sheets and trace gases -- supports this ECS and implies that CO$_2$ was about 300 ppm in the Pliocene and 400 ppm at transition to a nearly ice-free planet, thus exposing unrealistic lethargy of ice sheet models. Equilibrium global warming including slow feedbacks for today's human-made greenhouse gas (GHG) climate forcing (4.1 W/m$^2$) is 10{\deg}C, reduced to 8{\deg}C by today's aerosols. Decline of aerosol emissions since 2010 should increase the 1970-2010 global warming rate of 0.18{\deg}C per decade to a post-2010 rate of at least 0.27{\deg}C per decade. Under the current geopolitical approach to GHG emissions, global warming will likely pierce the 1.5{\deg}C ceiling in the 2020s and 2{\deg}C before 2050. Impacts on people and nature will accelerate as global warming pumps up hydrologic extremes. The enormity of consequences demands a return to Holocene-level global temperature. Required actions include: 1) a global increasing price on GHG emissions, 2) East-West cooperation in a way that accommodates developing world needs, and 3) intervention with Earth's radiation imbalance to phase down today's massive human-made "geo-transformation" of Earth's climate. These changes will not happen with the current geopolitical approach, but current political crises present an opportunity for reset, especially if young people can grasp their situation.

• The paper revises fast-feedback equilibrium climate sensitivity (ECS) to 1.2 ± 0.3°C per W/m² using paleoclimate proxies and GCM simulations.
• It highlights that conventional models may underestimate warming due to excessive ocean thermal mixing and misrepresented cloud feedbacks.
• It underscores urgent policy measures, including carbon pricing and geoengineering, to correct energy imbalance and mitigate future warming.

Examination of Climate Change Dynamics: Insights from "Global Warming in the Pipeline"

The paper "Global Warming in the Pipeline" authored by Hansen et al. offers a comprehensive analysis of the climate system's response to both existing and projected greenhouse gas (GHG) forcings, employing insights from paleoclimate data and modern observations. The primary focus is on equilibrium climate sensitivity (ECS) and Earth system sensitivity (ESS), providing a nuanced synthesis of findings regarding climate feedbacks and their implications for future warming trajectories.

Hansen et al. highlight a revised estimate of fast-feedback ECS at 1.2 ± 0.3°C per W/m². This stands in contrast to the historical consensus ECS value of about 3°C per 2× CO₂, as upheld by IPCC assessments. The authors base this finding on a robust analysis of glacial-interglacial transitions, utilizing paleo-temperature proxies and GCM simulations to triangulate their ECS estimate with improved precision. This recalibration of ECS implies potentially more significant warming is "in the pipeline" than current climatic models suggest.

A compelling exploration of the climate response times reveals that while energy imbalance adjusts relatively rapidly, global surface temperatures require a substantially longer timeframe to reach equilibrium. The paper notes that conventional climate models may underestimate ECS due to excessive thermal mixing in ocean models, underscoring an urgent need for improvements in cloud feedback parameterizations.

The Cenozoic era’s climate record serves as a pivotal reference point for understanding ESS. This epoch reveals that atmospheric CO₂ concentrations during previous warm periods (like the Pliocene and Eocene) were significantly lower than current levels, challenging the veracity of certain climate models. This is especially relevant given that arctic and global warmth of these eras is not replicated under current CO₂ levels in models, suggesting a broader issue with feedback representation.

The paper provides a noteworthy examination of aerosols and their compensatory cooling effect, positing that aerosols have historically masked some effects of GHG-induced warming. Hansen et al. argue that aerosol forcing has been underestimated by up to -2 W/m², and this misapprehension might have abetted the apparent anomaly of the 20th-century temperature rise slowing. Extrapolated over recent decades, the waning of "aerosol-induced cooling" heralds an acceleration of global warming rates, implying proximity to crucial climatic thresholds, such as surpassing the 1.5°C and 2°C ceilings by mid-century under current emissions trajectories.

In addressing the implications, the authors advocate for aggressive policy measures: the implementation of a significant global price on carbon, intensified international cooperation integrating concerns from developing countries, and a proactive approach towards rectifying the planet's energy imbalance through innovative geoengineering techniques. These policy prescriptions are situated within the acknowledgment that extant geopolitical strategies are insufficient for the magnitude of the anticipated warming.

In summary, Hansen et al. provide a targeted critique of existing climate sensitivities and model assumptions, emphasizing an urgent need for refined predictive models that better account for feedback complexities. They astutely conclude that, while the climate's response to both emitted and latent GHGs is complex, actionable decisions based on robust scientific inquiry can still mitigate the hardest impacts if timely and proper measures are adopted. The urgency of these matters is underscored by their potential ramifications on policy frameworks and the fate of future generations.