Current rapid global temperature rise linked to falling SO₂ emissions

Abstract. It is widely held that global temperature variations on time scales of a decade or less are primarily caused by internal climate variability, with smaller contributions from changes in external climate forcing such as solar irradiance. This paper shows that observed variations in global mean surface temperature, T_GS, and ocean heat content (OHC) during the last 1–2 decades imply major changes in climate forcing during this period. In a first step, two independent methods are used to evaluate global temperature corrected for ocean–atmosphere heat exchange. El Niño/Southern Oscillation (ENSO) corrected T_GS (written as T_GS) is shown to agree closely with a novel temperature metric θ that combines uncorrected T_GS with scaled OHC. This agreement rules out a substantial 21st-century contribution to T_GS from ocean-atmosphere heat exchange. In contrast to T_GS, the time series T_GS (t) provides a clear fingerprint of transient global cooling associated with major volcanic eruptions, enabling a more accurate empirical estimate of the climate response of the global mean surface. This allows more accurate estimation of the net climate forcing by stratospheric aerosols and solar irradiance, which is then subtracted from T_GS (t) to determine the underlying signal of anthropogenic global warming. Key features of this signal are a slowdown from the late 1990s to 2011 – corresponding to the well known climate hiatus – and a subsequent sharp upturn indicating a steep increase in anthropogenic climate forcing. It is argued that the only plausible cause for this increase is a large fractional decrease in tropospheric aerosol cooling. This attribution is supported by satellite-based observations of a > 50 % decrease in SO₂ emissions from large sources during the last six years. It suggests that current clean-air policies and replacement of coal by natural gas are driving a significant human made climatic event, 2–4 times faster than greenhouse driven warming alone. If confirmed, this implies a considerably shortened timescale to meet the IPCC 1.5 °C objective, with major implications for near-term carbon emission policies.