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This chapter provides an overview of the factors that influence Earth’s climate. The relation between reconstructions of global mean surface temperature and estimates of atmospheric carbon dioxide (CO) over the past 500 million years is first described. Vast variations in climate on geologic time scales, driven by natural fluctuations of CO, are readily apparent. We then shift attention to the time period 1765 to present, known as the Anthropocene, during which human activity has strongly influenced atmospheric CO, other greenhouse gases (GHGs), and Earth’s climate. Two mathematical concepts essential for quantitative understanding of climate change, radiative forcing and global warming potential, are described. Next, fingerprints of the impact of human activity on rising temperature and the abundance of various GHGs over the course of the Anthropocene are presented. We conclude by showing Earth is in the midst of a remarkable transformation. In the past, radiative forcing of climate represented a balance between warming due to rising GHGs and cooling due to the presence of suspended particles (aerosols) in the troposphere. There presently exists considerable uncertainty in the actual magnitude of radiative forcing of climate due to tropospheric aerosols, which has important consequences for our understanding of the climate system. In the future, climate will be driven mainly by GHG warming because aerosol precursors are being effectively removed from pollution sources, due to air quality legislation enacted in response to public health concerns.

This chapter provides an overview of the factors that will govern the rise in global mean surface temperature (GMST) over the rest of this century. We evaluate GMST using two approaches: analysis of archived output from atmospheric, oceanic general circulation models (GCMs) and calculations conducted using a computational framework developed by our group, termed the Empirical Model of Global Climate (EM-GC). Comparison of the observed rise in GMST over the past 32 years with GCM output reveals these models tend to warm too quickly, on average by about a factor of two. Most GCMs likely represent climate feedback in a manner that amplifies the radiative forcing of climate due to greenhouse gases (GHGs) too strongly. The GCM-based forecast of GMST over the rest of the century predicts neither the target (1.5 °C) nor upper limit (2.0 °C warming) of the Paris Climate Agreement will be achieved if GHGs follow the trajectories of either the Representative Concentration Pathway (RCP) 4.5 or 8.5 scenarios. Conversely, forecasts of GMST conducted in the EM-GC framework indicate that if GHGs follow the RCP 4.5 trajectory, there is a reasonably good probability (~75 %) the Paris target of 1.5 °C warming will be achieved, and an excellent probability (>95 %) global warming will remain below 2.0 °C. Uncertainty in the EM-GC forecast of GMST is primarily caused by the ability to simulate past climate for various combinations of parameters that represent climate feedback and radiative forcing due to aerosols, which provide disparate projections of future warming.

This chapter begins with a description of the Paris Climate Agreement, which was formulated during the 21 meeting of the Conference of the Parties to the United Nations Framework Convention on Climate Change (UNFCCC) in late 2015. The goal of this agreement is to limit future emission of greenhouse gases (GHGs) such that global warming will not exceed 1.5 °C (target) or 2.0 °C (upper limit). Future emissions of GHGs are based on unilateral pledges submitted by UNFCCC member nations, called Intended Nationally Determined Contributions (INDCs). We compare the global emission of GHGs calculated from the INDCs to the emissions that had been used to formulate the various Representative Concentration Pathway (RCP) trajectories for future atmospheric abundance of GHGs. The RCP 4.5 scenario is particularly important, because our Empirical Model of Global Climate (EM-GC) indicates there is a reasonably good probability (~75 %) the Paris target will be achieved, and an excellent probability (>95 %) the upper limit for global warming will be attained, if the future atmospheric abundance of GHGs follows RCP 4.5. Our analysis of the Paris INDCs shows GHG emissions could remain below RCP 4.5 out to year 2060 if: (1) conditional as well as unconditional INDCs are followed; (2) reductions in GHG emissions needed to achieve the Paris INDC commitments, which generally stop at 2030, are propagated forward to 2060. Prior and future emissions of GHGs are graphically illustrated to provide context for the reductions needed to place global GHG emissions on the RCP 4.5 trajectory.

This chapter provides an overview of reductions in the emission of greenhouse gases (GHGs) that will be needed to achieve either the target (1.5 °C warming) or upper limit (2.0 °C warming) of the Paris Climate Agreement. We quantify how much energy must be produced, either by renewables that do not emit significant levels of atmospheric GHGs or via carbon capture and sequestration (CCS) coupled to fossil fuel power plants, to meet forecast energy demand out to 2060. For the Representative Concentration Pathway (RCP) 4.5 GHG emission trajectory to be matched, which is necessary for having a high probability of achieving the Paris target according to our Empirical Model of Global Climate (EM-GC), then the world must transition to production by renewables of 50 % of total global energy by 2060. For the RCP 2.6 GHG emission trajectory to be matched, which is necessary to achieve the Paris upper limit according to general circulation models (GCMs), then 88 % of the energy generated in 2060 must be supplied either by renewables or combustion of fossil fuels coupled to CCS. We also quantify the probability of achieving the Paris target in the EM-GC framework as a function of future CO emissions. Humans can emit only 82, 69, or 45 % of the prior, cumulative emissions of CO to have either a 50, 66, or 95 % probability of achieving the Paris target of 1.5 °C warming. We also quantify the impact of future atmospheric CH on achieving the goals of the Paris Climate Agreement.