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.

Now that many countries have set goals for reaching net zero emissions in mid-century, it is important to clarify the role of each country in achieving the 1.5°C target of the Paris Agreement. Here, we evaluated China’s role by calculating the global temperature impacts caused by different national emission pathways toward the net zero target. Our results showed that China’s contribution to global warming since 2005 is 0.17°C on average in 2050, with a range of 0.1°C to 0.22°C. The peak contributions of these pathways vary from 0.1°C to 0.23°C, with the years reached distributing between 2036 and 2065. The large difference in peak temperatures arises from the differences in emission pathways of carbon dioxide (CO2), methane (CH4), and sulfur dioxide (SO2). We further analyzed the effect of the different mix of CO2 and CH4 mitigation trajectories from China’s pathways on the global mean temperature. We found that China’s near-term CH4 mitigation reduces the peak temperature in the mid-century by 0.02°C whereas it plays a less important role in determining the end-of-the-century temperature. Early CH4 mitigation action in China is an effective way to shave the peak temperature, further contributing to reducing the temperature overshoot along the way toward the 1.5°C target. This further underscores the necessity for early CO2 mitigation to achieve the long-term temperature goal ultimately.

There is a substantial gap between the current emissions of greenhouse gases and levels required for achieving the 2 and 1.5 °C temperature targets of the Paris Agreement. Understanding the implications of a temperature overshoot is thus an increasingly relevant research topic. Here we explore the carbon cycle feedbacks over land and ocean in the SSP5-3.4-OS overshoot scenario by using an ensemble of Coupled Model Intercomparison Project 6 Earth system models. Models show that after the CO2 concentration and air temperature peaks, land and ocean are decreasing carbon sinks from the 2040s and become sources for a limited time in the 22nd century. The decrease in the carbon uptake precedes the CO2 concentration peak. The early peak of ocean uptake stems from its dependency on the atmospheric CO2 growth rate. The early peak of the land uptake occurs due to a larger increase in ecosystem respiration than the increase in gross primary production, as well as due to a concomitant increase in land-use change emissions primarily attributed to the wide implementation of biofuel croplands. The carbon cycle feedback parameters amplify after the CO2 concentration and temperature peaks due to inertia of the Earth system so that land and ocean absorb more carbon per unit change in the atmospheric CO2 change (stronger negative feedback) and lose more carbon per unit temperature change (stronger positive feedback) compared to if the feedbacks stayed unchanged. The increased negative CO2 feedback outperforms the increased positive climate feedback. This feature should be investigated under other scenarios.

The hotter the climate is, the higher the demand for cooling is, leading to more electricity consumption and CO2 emissions. To understand the effect of future regional warming on the electricity demand and CO2 emissions in the Arabian Peninsula region, we selected a representative country, Qatar, and developed a model that relates daily electricity demand with temperature. By combining this model with temperature projections from of 1 23 the CMIP6 database (bias adjusted and statistically downscaled), as well as GDP and population projections from four SSP scenarios, we calculated Qatar’s demand for electricity until the end of the century. We found an average sensitivity of 1.7 GWh/°C for the electricity demand, equivalent to 0.4 MtCO2/°C for CO2 emissions. The electricity demand is projected to increase by 5 to 35% due to warming alone at the end of this century. Under SSP1, the warming-induced CO2 emissions could be offset by improvements of carbon intensity. Under SSP5, assuming no improvement of carbon intensity, future warming could add 20 to 35% of CO2 emissions per year by the end of the century, with half of the electricity demand related to extremely hot days becoming more frequent in the future. Our findings suggest that it is important to consider additional CO2 emissions arising from future warming in future temperature projections.

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.

Part of the economic recovery plans implemented by governments following COVID-19 is directed towards the energy transition. To understand the potential effects of these post-COVID green recovery packages on reductions of greenhouse gases emissions, we investigated three different approaches. Firstly, we analysed simulation results of Integrated Assessment Models (IAMs) to infer the change in CO2 intensity of GDP that could result from post-COVID low-carbon investment plans. Secondly, we investigated the scenarios provided by the International Energy Agency (IEA) based on a bottom-up energy system model. By combining the two approaches, we found that green recovery packages implemented and planned globally can lead to an emission reduction of merely 1%-6% from 2030 baseline levels at most. Thirdly, we looked into the results of the Adaptative Regional Input-Output model, which simulates the dynamic effects of economic crisis and fiscal stimuli through supply chains following labour shortage. The third approach shows that the increase of activity driven by fiscal stimuli leads to a rebound of CO2 emissions even if they do not target carbon-intensive sectors. We conclude that green recovery packages targeting low-carbon technologies have a limited impact on near-term CO2 emissions and that demand-side incentives, as well as other policy efforts to disincentivise the use of fossil fuels, are also important for scaling up climate mitigation.

As the COVID-19 virus spread over the world, governments restricted mobility to slow transmission. Public health measures had different intensities across European countries but all had significant impact on people’s daily lives and economic activities, causing a drop of CO2 emissions of about 10% for the whole year 2020. Here, we analyze changes in natural gas use in the industry and built environment sectors during the first half of year 2020 with daily gas flows data from pipeline and storage facilities in Europe. We find that reductions of industrial gas use reflect decreases in industrial production across most countries. Surprisingly, natural gas use in buildings also decreased despite most people being confined at home and cold spells in March 2020. Those reductions that we attribute to the impacts of COVID-19 remain of comparable magnitude to previous variations induced by cold or warm climate anomalies in the cold season. We conclude that climate variations played a larger role than COVID-19 induced stay-home orders in natural gas consumption across Europe.