Earth's climate sensitivity has long been subject to heated debate and has spurred renewed interest after the latest IPCC assessment report suggested a downward adjustment of its most likely range 1 . Recent observational studies have produced estimates of Transient Climate Sensitivity, i.e. the global mean surface temperature increase at the time of CO2 doubling, as low as 1.3K 2,3 , well below the best estimate produced by global climate models (1.8K). Here, we present an observation-based study of the time period 1964 to 2010, which does not rely on climate models. The method incorporates observations of greenhouse gas concentrations, temperature and radiation from approximately 1300 surface sites into an energy balance framework. Statistical methods commonly applied to economic time series are then used to decompose observed temperature trends into components attributable to changes in greenhouse gas concentrations and surface radiation. We find that surface radiation trends, which have been largely explained by changes in atmospheric aerosol loading, caused a cooling that masked approximately one third of the continental warming due to increasing greenhouse gas concentrations over the last half-century. In consequence, the method yields a higher transient climate sensitivity (2.0 +/-0.8K) than other observational studies.Atmospheric CO2 concentration is projected to double from preindustrial levels during this century 1 , and constraining Earth's temperature response is a primary objective for designing mitigation and adaptation policies. While substantial attention has been devoted to model estimates of Earth's equilibrium climate sensitivity 4 (i.e., the surface temperature response to CO2 doubling once a new equilibrium climate state has been reached), more relevant to public and policy makers is the temperature change that occurs at the time of CO2 doubling, known as 'transient climate sensitivity' (TCS) 5 . Constraining TCS based on observational records is complicated by the fact that recent climate change was not forced by CO2 changes alone.Downward solar radiation at the surface (DSRS, measured in Wm -2 ) reported at approximately 1300 surface stations over the time period 1964 -2010 ( Fig. 1a and b) display a downward trend in DSRS which is commonly referred to as 'global dimming' (Fig. 1c) 6 . The most plausible explanation for global dimming is increased atmospheric aerosol loading derived from anthropogenic burning of fossil fuels and biomass 7 . The overall effect of aerosols increases Earth's albedo, either by direct interaction with solar radiation, or by increasing the lifetime, areal extent, and/or reflectivity of clouds 8 . For some portions of the world, the appearance of regional trends opposing the global negative DSRS trend ('regional brightening') is observed towards the end of the 20 th century, consistent with a reduction in aerosol emissions in much of the developed world 6 . Atmospheric aerosol loading is broadly reflected in global emissions of sulfur dioxide (SO2), a precurso...

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The global temperature trend observed over the last century is largely the result of two opposing effects-cooling from aerosol particles and greenhouse gas warming. While the effect of increasing greenhouse gas concentrations on Earth's radiation budget is well constrained, that due to anthropogenic aerosols is not, partly due to a lack of observations. However, long-term surface measurements of changes in downward solar radiation (SDSR), an often used proxy for aerosol radiative impact, are available worldwide over the last half century. We compare SDSR changes from ∼1,400 stations to those from the Coupled Model Intercomparison Project Version 5 global climate simulations over the period 1961-2005. The observed SDSR shows a strong early downward trend followed by a weaker trend reversal, broadly consistent with historical aerosol emissions. However, despite considerable changes to known aerosol emissions over time, the models show negligible SDSR trends, revealing a lethargic response to aerosol emissions and casting doubt on the accuracy of their future climate projections.Plain Language Summary Observations of incoming solar radiation, as measured at approximately 1,400 surface stations worldwide, show a strong downward trend from the 1960s to the 1980s, followed by a weaker trend reversal thereafter. These trends are thought to be due to changes in the amount of aerosol particles in the atmosphere, and we find support for that here in the temporal evolution of anthropogenic aerosol emissions. This is expected because aerosol particles reflect and/or absorb sunlight back to space and have a net cooling effect on Earth's climate. However, we find that the current generation of climate models simulates negligible solar radiation trends over the last half century, suggesting that they have underestimated the cooling effect that aerosol particles have had on climate in recent decades. Despite this, climate models tend to reproduce surface air temperature over the time period in question reasonably well. This, in turn, suggests that the models are not sensitive enough to increasing greenhouse gas concentrations in the atmosphere, with important implications for their ability to simulate future climate.

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Significant progress has been made towards mitigating climate change and its impacts across countries. However, the transboundary effect of CO 2 emissions means that excluding the actions and inactions of certain countries and territories that escalate emissions is alarming. On this note, we examined the heterogeneous contribution of immediate and underlying drivers of emissions across 206 countries and territories for the period spanning 1960-2018. We deployed a dynamic panel estimation technique that accounts for cross-sectional dependence, heterogeneous parameters across countries, and dynamic correlated effects-a constraint for socio-economic, consumption-and pollution-based models. A global accounting of economic policy and debt, population structure, density and urbanization, and environmental-related aggregate indicators in a carbon emission function is presented. The empirical results demonstrate that the overarching effect of the instantaneous increase in economic development, population dynamics and energy utilization stimulate global emissions at national, urban and household levels across countries and territories. Industrialization and trade were found to escalate global pollution levels due to the impact of carbonized and energy-intensive economic structure in many developing and developed economies. Urbanization, urban income growth, and urban energy consumption are intertwined, hence, the institution of urban-related policy interventions is likely to negate the trio-impact on environmental sustainability. The triple effect (exploitation of natural resources, production and consumption) of economic development spurs environmental pollution, thus, calls for structural change from a carbonized to a decarbonized economy. The complex interaction highlights diversification of the energy mix by the inclusion of clean and renewable energy sources, fossil fuel-switching, and modern technologies like carbon capture and storage to improve energy efficiency and decline emission intensities. OPEN ACCESS RECEIVED

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