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Lookup NU author(s): Emeritus Professor Nick Cowern
This work is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0).
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, TGS , 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 TGS (written as T'GS ) is shown to agree closely with a novel temperature metric θ that combines uncorrected TGS with scaled OHC. This agreement rules out a substantial 21st-century contribution to TGS from ocean-atmosphere heat exchange. In contrast to TGS , 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.o
Author(s): Cowern NEB
Publication type: Article
Publication status: Published
Journal: Earth System Dynamics
Online publication date: 30/11/2018
Acceptance date: 29/11/2018
Date deposited: 18/12/2018
ISSN (print): 2190-4979
ISSN (electronic): 2190-4987
Publisher: Copernicus Publications
Notes: Non-technical summary
This paper shows that recent accelerated global warming is the initial part of an approximately 0.5C warming transition caused by changes in global pollution levels. The transition is superimposed on the steady global warming trend from greenhouse gas emissions, and could bring forward the time when global warming reaches 1.5C to the late 2020s, significantly sooner than estimated in the recent IPCC climate change report.
The last 5–6 years have seen a rapid rise in global temperature, 2–4 times faster than the expected rate based on known increases in greenhouse gas concentrations. Up to now it has not been clear whether this recent acceleration is a natural variation or the consequence of a change in some unidentified constituent of the atmosphere. In this study, global surface and ocean temperature measurements have been used to explore variations in human-made climate forcing since the beginning of the 21st century. The analysis points to a near constant level of climate forcing during the 2000s and a steep rise since 2012, in contrast to the steady slow rate of increase that would be expected from known increases in greenhouse gas concentrations over the last century.
This discrepancy is attributed to a peak and subsequent steep decrease in global cooling by short-lived tropospheric aerosols, whose formation is enabled by the presence of atmospheric SO2. Compellingly, the timing of this decrease coincides closely with a steep fall in global sulfur dioxide emissions from large sources, particularly coal-fired power plants, observed in satellite measurements. This fall, a result of evolving policy measures to reduce health-harming pollution, includes a global shift from unmitigated coal plants to clean coal and gas-fired plants during the present decade. The recent changes, together with further projected reductions in sulfur emissions, lead to a projected rise in global temperature to around 1.3C above pre-industrial background by 2022 and potentially 1.5C before 2030. These estimates do not account for ‘slow’ climate feedbacks such as Arctic sea-ice and permafrost melt, which may accelerate warming still further.
The paper concludes with a discussion of measures to offset the loss of cooling aerosols, including geoengineering to re-inject sulfur dioxide into higher layers of the atmosphere where it is more persistent and less polluting, and stringent measures to cut emissions of short-lived greenhouse gases and their precursors, specifically methane and non-methane hydrocarbons (NMHCs), by the oil, gas, petrochemicals, agricultural and waste industries. The latter approach is a good match to scenarios for rapidly reducing CO2 emissions, which call for a transition from fossil fuel energy to zero-carbon energy generation, but it will only succeed in slowing warming to the 1.5C level if major reductions in global methane and NMHC emissions are achieved in the next few years. A successful campaign to slow near-term global warming will require a multifaceted approach, likely including geoengineering.
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