Abstract
In order to assess the merits of national climate change mitigation policies, it is important to have a reasonable benchmark for how much human-caused global warming would occur over the coming century with “Business-As-Usual” (BAU) conditions. However, currently, policymakers are limited to making assessments by comparing the Global Climate Model (GCM) projections of future climate change under various different “scenarios”, none of which are explicitly defined as BAU. Moreover, all of these estimates are ab initio computer model projections, and policymakers do not currently have equivalent empirically derived estimates for comparison. Therefore, estimates of the total future human-caused global warming from the three main greenhouse gases of concern (CO2, CH4, and N2O) up to 2100 are here derived for BAU conditions. A semi-empirical approach is used that allows direct comparisons between GCM-based estimates and empirically derived estimates. If the climate sensitivity to greenhouse gases implies a Transient Climate Response (TCR) of ≥ 2.5 °C or an Equilibrium Climate Sensitivity (ECS) of ≥ 5.0 °C then the 2015 Paris Agreement’s target of keeping human-caused global warming below 2.0 °C will have been broken by the middle of the century under BAU. However, for a TCR < 1.5 °C or ECS < 2.0 °C, the target would not be broken under BAU until the 22nd century or later. Therefore, the current Intergovernmental Panel on Climate Change (IPCC) “likely” range estimates for TCR of 1.0 to 2.5 °C and ECS of 1.5 to 4.5 °C have not yet established if human-caused global warming is a 21st century problem.
Highlights
Since the late-1960s and early-1970s, computer model simulations of the Earth’s climate have been predicting that increasing concentrations of “greenhouse gases” in the atmosphere from human activity should be causing substantial global warming at the Earth’s surface and in the lower atmosphere [1,2]
We suggest that unless the scientific debate over all four of these questions are simultaneously considered, it is unlikely that truly satisfactory answers will be achieved to the over-arching question, “How much human-caused global warming should we expect with Business-As-Usual (BAU) climate policies?” With that in mind, we will discuss all four of these sub-questions in turn, including a brief review of the key relevant literature, before attempting to answer the main question
In order to estimate how much human-caused global warming the projected BAU emissions in Section 2 could potentially cause, we need to estimate what fraction of the emitted gases will remain in the atmosphere, i.e., what will the “airborne fraction” of the emitted gases be? Given the many uncertainties associated with the natural sources and sinks for each of the three gases, we argue that currently the best way to estimate this is to consider how the airborne fractions have behaved since continuous records of atmospheric concentrations began
Summary
Since the late-1960s and early-1970s, computer model simulations of the Earth’s climate have been predicting that increasing concentrations of “greenhouse gases” (carbon dioxide or CO2) in the atmosphere from human activity should be causing substantial global warming at the Earth’s surface and in the lower atmosphere [1,2]. The computer models have continued to predict that increasing greenhouse gas concentrations should be causing substantial global warming. We suggest that unless the scientific debate over all four of these questions are simultaneously considered, it is unlikely that truly satisfactory answers will be achieved to the over-arching question, “How much human-caused global warming should we expect with Business-As-Usual (BAU) climate policies?” With that in mind, we will discuss all four of these sub-questions in turn, including a brief review of the key relevant literature, before attempting to answer the main question. [1,2], and as described, this led the UN to argue that the international community needed to dramatically reduce emissions of greenhouse gases in order to minimize any future (human-caused) global warming that would occur if the world continued business-as-usual.
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