Greenhouse gas (GHG) emission metrics, that is, conversion factors to evaluate the emissions of non-CO2 climate forcers on a common scale with CO2, serve crucial functions upon the implementation of the Paris Agreement. While different metrics have been proposed, they have not been investigated under a range of pathways, including those significantly overshooting the temperature targets of the Paris Agreement. Here we show that cost-effective metrics that minimize the overall cost of climate mitigation are time-dependent, primarily determined by the period remaining before the eventual stabilization, and strongly influenced by temperature overshoot. Our study suggests that flexibility should be maintained to adapt the choice of metrics in time as the future unfolds, if cost-effectiveness is a key consideration for global climate policy, instead of hardwiring the 100-year Global Warming Potential (GWP100) as a permanent feature of the Paris Agreement implementation as is currently under negotiation.

Radiation management (RM) has been proposed as a conceivable climate engineering (CE) intervention to mitigate global warming. In this study, we use a coupled climate model (MPI-ESM) with a very idealized setup to investigate the efficacy and risks of CE at a local scale in space and time (regional radiation management, RRM) assuming that cloud modification is technically possible. RM is implemented in the climate model by the brightening of low-level clouds (solar radiation management, SRM) and thinning of cirrus (terrestrial radiation management, TRM). The region chosen is North America, and we simulate a period of 30 years. The implemented sustained RM resulted in a net local radiative forcing of -9.8 Wm and a local cooling of -0.8 K. Surface temperature (SAT) extremes (90 and 10 percentile) show negative anomalies in the target region. However, substantial climate impacts are also simulated outside the target area, with warming in the Arctic and pronounced precipitation change in the eastern Pacific. As a variant of RRM, a targeted intervention to suppress heat waves (HW) is investigated in further simulations by implementing intermittent cloud modification locally, prior to the simulated HW situations. The intermittent RRM results in most cases in a successful reduction of temperatures locally, with substantially smaller impacts outside the target area, compared to the sustained RRM.

Stratospheric Aerosol Geoengineering (SAG) is proposed to offset global warming; the use of this approach can impact the hydrological cycle. We use simulations from Coupled Model Intercomparison Project (CMIP5) and Geoengineering Model Intercomparison Project (G3 simulation) to analyze the impacts of SAG on precipitation (P) and to determine its responsible causes in West Africa and Sahel region. CMIP5 Historical data are firstly validated, the results obtained are consistent with those of observations data (CMAP and GPCP). Under the Representative Concentration Pathway (RCP) scenario RCP4.5, a slight increase is found in West Africa Region (WAR) relative to present-day climate. The dynamic processes especially the monsoon shifts are responsible for this change of precipitation. Under RCP4.5, during the monsoon period, reductions in P are 0.86%, 0.80% related to the present-day climate in the Northern Sahel (NSA), Southern Sahel (SSA) respectively while P is increased by 1.04% in WAR. Under SAG, 3.71% of P change (decrease) was associated with a -3.51 value of efficacy in the West African Region (AR). Under G3, a significant decrease of P is found in the West African region. This decrease in monsoon precipitation is mainly explained by changes in dynamics, which leads to weakened monsoon circulation and a shift in the distribution of monsoon precipitation. This result suggests that SAG deployment to balancing all warming can be harmful to rainfall in WAR if the amount of SO2 to be injected in this tropical area is not taken into consideration.

It has been claimed that COVID-19 public stimulus packages could be sufficient to meet the short-term energy investment needs to leverage a shift toward a pathway consistent with the 1.5 °C target of the Paris Agreement. Here we provide complementary perspectives to reiterate that substantial, broad, and sustained engagements beyond recovery packages will be needed for achieving the Paris Agreement long-term targets. Low-carbon investments will need to scale up and persist over the next several decades following short-term stimulus packages. The required total energy investments in the real world can be larger than the currently available estimates from Integrated Assessment Models (IAMs). Existing databases from IAMs are not sufficient for analyzing the effect of public spending on emission reduction. To inform what role COVID-19 stimulus packages and public investments may play for reaching the Paris Agreement targets, explicit modelling of such policies is required.

Aerosols interact with radiation and clouds. Substantial progress made over the past 40 years in observing, understanding, and modeling these processes helped quantify the imbalance in the Earth’s radiation budget caused by anthropogenic aerosols, called aerosol radiative forcing, but uncertainties remain large. This poster presents the outcome of an international workshop and subsequent review paper, which quantify the likely range of aerosol radiative forcing over the industrial era based on multiple lines of evidence, including modelling approaches, theoretical considerations, and observations. Improved understanding of aerosol absorption and the causes of trends in surface radiative fluxes narrow the range of the forcing from aerosol-radiation interactions compared to the latest assessment by the Intergovernmental Panel on Climate Change (IPCC). A robust theoretical foundation and convincing evidence constrain the forcing caused by aerosol-driven increases in liquid cloud droplet number concentration. However, the influence of anthropogenic aerosols on cloud liquid water content and cloud fraction and on mixed-phase and ice clouds remains poorly constrained. Observed changes in surface temperature and radiative fluxes provide additional constraints. These multiple lines of evidence lead to total aerosol radiative forcing ranges that are of similar width to the last IPCC assessment but more clearly based on physical arguments.