Earth's energy imbalance more than doubled in recent decades

Global warming results from anthropogenic greenhouse gas emissions which upset the delicate balance between the incoming sunlight, and the reflected and emitted radiation from Earth. The imbalance leads to energy accumulation in the atmosphere, oceans and land, and melting of the cryosphere, resulting in increasing temperatures, rising sea levels, and more extreme weather around the globe. Despite the fundamental role of the energy imbalance in regulating the climate system, as known to humanity for more than two centuries, our capacity to observe it is rapidly deteriorating as satellites are being decommissioned

Efficacy of Solar irradiance, Methane and Black carbon aerosols

In climate science literature, the concept of radiative forcing is used to infer the equilibrium temperature change that would occur as a result of changes in the forcing agents such as greenhouse gases and aerosols. Radiative forcing is defined as the energy imbalance that occurs at the top-of-atmosphere because of the imposed changes in the forcing agents. It would be a useful quantity to compare the importance of various forcing agents if the climate system response depends on the radiative forcing and not on the forcing agents. However, recent studies show that the climate system response depends on the forcing agents to some extent. In order to address this issue, the concept of ‘efficacy’ of forcing agents was introduced. Efficacy is the ratio of the equilibrium global mean surface temperature change per unit forcing from an agent to the equilibrium temperature change per unit carbon-dioxide (CO2) forcing from the same initial climate state. An efficacy of one implies that two forcing agents that induces same radiative forcing would produce same global mean temperature response while a non-unity efficacy would indicate different temperature response for same radiative forcing. Past studies investigated the efficacy of various forcing agents but a detailed assessment of the climate processes responsible for the efficacy to be different from one is lacking. Using a global climate model, Community Atmosphere Model version 5 (CAM5) developed by the National Centre for Atmospheric Research (NCAR), we estimate the efficacy of three climate forcing agents - solar irradiance, methane (CH4) and black carbon (BC) aerosols and investigate the physical mechanisms responsible for a differing efficacy of these forcing agents compared to CO2. We also investigate and compare the hydrological cycle response to CO2 and these forcing agents. To compute the radiative forcing and efficacy, we adopt the Hansen’s prescribed-sea-surface-temperature (SST) and Gregory’s regression approaches. The recent Intergovernmental Panel on Climate Change (IPCC 2013) finds the radiative forcing estimated using these methods to be better predictor of global mean surface temperature change compared to other radiative forcing definitions. This study is the first modelling study which provides a comprehensive assessment of the physical mechanisms responsible for a non-unity efficacy for solar irradiance, methane and black carbon aerosols. It also highlights the importance of including the efficacy factor in the “forcing and response” relationship for an improved estimate of surface temperature change due to a particular forcing agent. Further investigation using a multi-model intercomparison framework would permit an assessment of the robustness of the results shown in this study

Efficacy of black carbon aerosols: the role of shortwave cloud feedback

Using idealized climate model simulations, we investigate the effectiveness of black carbon (BC) aerosols in warming the planet relative to CO2 forcing. We find that a 60-fold increase in the BC aerosol mixing ratio from the present-day levels leads to the same equilibrium global mean surface warming (similar to 4.1 K) as for a doubling of atmospheric CO2 concentration. However, the radiative forcing is larger (similar to 5.5 Wm(-2)) in the BC case relative to the doubled CO2 case (similar to 3.8 Wm(-2)) for the same surface warming indicating the efficacy (a metric for measuring the effectiveness) of BC aerosols to be less than CO2. The lower efficacy of BC aerosols is related to the differences in the shortwave (SW) cloud feedback: negative in the BC case but positive in the CO2 case. In the BC case, the negative SW cloud feedback is related to an increase in the tropical low clouds which is associated with a northward shift (similar to 7 degrees) of the Intertropical Convergence Zone (ITCZ). Further, we show that in the BC case fast precipitation suppression offsets the surface temperature mediated precipitation response and causes similar to 8% net decline in the global mean precipitation. Our study suggests that a feedback between the location of ITCZ and the interhemispheric temperature could exist, and the consequent SW cloud feedback could be contributing to the lower efficacy of BC aerosols. Therefore, an improved representation of low clouds in climate models is likely the key to understand the global climate sensitivity to BC aerosols

Efficacy of black carbon aerosols: the role of shortwave cloud feedback

Abstract Using idealized climate model simulations, we investigate the effectiveness of black carbon (BC) aerosols in warming the planet relative to CO2 forcing. We find that a 60-fold increase in the BC aerosol mixing ratio from the present-day levels leads to the same equilibrium global mean surface warming (∼4.1 K) as for a doubling of atmospheric CO2 concentration. However, the radiative forcing is larger (∼5.5 Wm−2) in the BC case relative to the doubled CO2 case (∼3.8 Wm−2) for the same surface warming indicating the efficacy (a metric for measuring the effectiveness) of BC aerosols to be less than CO2. The lower efficacy of BC aerosols is related to the differences in the shortwave (SW) cloud feedback: negative in the BC case but positive in the CO2 case. In the BC case, the negative SW cloud feedback is related to an increase in the tropical low clouds which is associated with a northward shift (∼7°) of the Intertropical Convergence Zone (ITCZ). Further, we show that in the BC case fast precipitation suppression offsets the surface temperature mediated precipitation response and causes ∼8% net decline in the global mean precipitation. Our study suggests that a feedback between the location of ITCZ and the interhemispheric temperature could exist, and the consequent SW cloud feedback could be contributing to the lower efficacy of BC aerosols. Therefore, an improved representation of low clouds in climate models is likely the key to understand the global climate sensitivity to BC aerosols.</jats:p

The 2000–2012 Global Warming Hiatus More Likely With a Low Climate Sensitivity

The global warming hiatus during the first decade of the 21st century has posed a challenge to the scientific community, though a leading explanation is that it was caused by internal variability overlaying a forced global warming trend. Here, we apply the Winton-Held two-layer model and show that the probability of the observed 2000–2012 hiatus period to arise from internal variability driven by white noise is larger if climate sensitivity is low. This is due to the delayed response of the oceans that cause the forced trend to increase faster with rising climate sensitivity than does natural variability, leading to a decreasing likelihood of observing the hiatus. The results are confirmed with the latest climate models participating in the Coupled Model Intercomparison Project (CMIP6).</p

Effects of local and remote black carbon aerosols on summer monsoon precipitation over India

In this study, we perform idealized climate model simulations to assess the relative impacts of an increase in local black carbon (BC) aerosols (located over the Indian region) and the remote BC aerosols (located outside the Indian region) on the summer monsoon precipitation over India. We decompose the precipitation changes into fast adjustments triggered by the introduction of the forcing agent and slow response that is associated with the global mean temperature change. We find that a 60-fold increase in the 'present-day' global distribution of BC aerosols leads to an increase in precipitation over India, which is mainly contributed by an increase in remote BC aerosols. When remote BC aerosols are increased, the fast adjustments contribute to an increase in precipitation in association with the warming of the northern hemisphere land and the related northward Intertropical Convergence Zone (ITCZ) shift. For an increase in local aerosols too, by enhancing the upper tropospheric temperature meridional gradient in the Indian region, the fast adjustments contribute to an increase in precipitation over India. The slow response contributions in both cases are related to the regional patterns of SST change and the resulting changes to meridional temperature gradient in the Indian region and zonal circulation changes. The net precipitation change over India is an increase (decrease) for an increase in remote (local) BC aerosols. As the interpretation of our results relies on ITCZ shift related to planetary energetics, differing land-ocean response and meridional temperature gradients in the Indian region, the results from our study are likely to be robust across climate models.</p