Scenarios used by the Intergovernmental Panel on Climate Change (IPCC) are central to climate science and policy. Recent studies find that observed trends and International Energy Agency (IEA) projections of global CO2 emissions have diverged from emission scenario outlooks widely employed in climate research. Here, we quantify the bases for this divergence, focusing on Kaya Identity factors: population, per-capita GDP, energy intensity (energy consumption/GDP), and carbon intensity (CO2 emissions/energy consumption). We compare 2005-2017 observations and IEA projections to 2040 of these variables, to "baseline" scenario projections from the IPCC's Fifth Assessment Report (AR5), and from the Shared Socioeconomic Pathways (SSPs) used in the upcoming Sixth Assessment Report (AR6). We find that the historical divergence of observed CO2 emissions from baseline scenario projections can be explained largely by slower-than-projected per-capita GDP growth—predating the COVID-19 crisis. We also find carbon intensity divergence from baselines in IEA's projections to 2040. IEA projects less coal energy expansion than the baseline scenarios, with divergence expected to continue to 2100. Future economic growth is uncertain, but we show that past divergence from observations makes it unlikely that per-capita GDP growth will catch up to baselines before mid-century. Some experts hypothesize high enough economic growth rates to allow per-capita GDP growth to catch up to or exceed baseline scenarios by 2100. However, we argue that this magnitude of catch-up may be unlikely, in light of: headwinds such as aging and debt, the likelihood of unanticipated economic crises, the fact that past economic forecasts have tended to over-project, the aftermath of the current pandemic, and economic impacts of climate change unaccounted-for in the baseline scenarios. Our analyses inform the rapidly evolving discussions on climate and development futures, and on uses of scenarios in climate science and policy.

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IPCC 基线情景高估了二氧化碳排放量和经济增长

政府间气候变化专门委员会 (IPCC) 使用的情景是气候科学和政策的核心。最近的研究发现，观察到的趋势和国际能源署 (IEA) 对全球二氧化碳排放的预测与气候研究中广泛采用的排放情景展望不同。在这里，我们量化了这种差异的基础，重点关注 Kaya 身份因素：人口、人均 GDP、能源强度（能源消耗量/GDP）和碳强度（二氧化碳排放量/能源消耗量）。我们将 2005-2017 年的观察结果和 IEA 预测与这些变量的 2040 年、IPCC 第五次评估报告 (AR5) 中的“基线”情景预测以及即将发布的第六次评估报告 (AR6) 中使用的共享社会经济途径 (SSP) 进行比较. 我们发现，观察到的二氧化碳排放量与基线情景预测的历史差异在很大程度上可以解释为人均 GDP 增长低于预期——早于 COVID-19 危机。我们还发现，在 IEA 到 2040 年的预测中，碳强度与基线存在差异。IEA 预测的煤炭能源扩张少于基线情景，预计这种差异将持续到 2100 年。未来经济增长不确定，但我们表明，过去与观察的差异使得不太可能人均GDP增长将在本世纪中叶之前赶上基线。一些专家假设，到 2100 年，经济增长率足够高，可以使人均 GDP 增长赶上或超过基准情景。然而，我们认为，这种赶超幅度可能不太可能，因为：老龄化和债务，意料之外的经济危机的可能性、过去的经济预测往往过度预测的事实、当前大流行的后果以及在基线情景中无法解释的气候变化的经济影响。我们的分析为关于气候和发展未来以及情景在气候科学和政策中使用的快速发展的讨论提供了信息。