Abstract
Anthropogenic aerosol emissions are predicted to decline sharply throughout the 21st century, in line with climate change and air quality mitigation policies, causing a near-term warming of climate that will impact our trajectory towards 1.5 °C above pre-industrial temperatures. However, the persistent uncertainty in aerosol radiative forcing limits our understanding of how much the global mean temperature will respond to near-term reductions in anthropogenic aerosol emissions. We quantify the model and scenario uncertainty in global mean aerosol radiative forcing up to 2050 using statistical emulation of a perturbed parameter ensemble for emission reduction scenarios consistent with three Shared Socioeconomic Pathways. We then use a simple climate model to translate the uncertainty in aerosol radiative forcing into uncertainty in global mean temperature projections, accounting additionally for the potential correlation of aerosol radiative forcing and climate sensitivity. Near-term aerosol radiative forcing uncertainty alone causes an uncertainty window of around 5 years (2034–2039) on the projected year of exceeding a global temperature rise of 1.5 °C above pre-industrial temperatures for a middle of the road emissions scenario (SSP2-RCP4.5). A correlation between aerosol radiative forcing and climate sensitivity would increase the 1.5 °C exceedance window by many years. The results highlight the importance of quantifying aerosol radiative forcing and any relationship with climate sensitivity in climate models in order to reduce uncertainty in temperature projections.
Highlights
The Paris Agreement of the United Nations Framework Convention on Climate Change aims to restrict global mean temperature change since the preindustrial era to well below 2 ◦C and pursue efforts to limit global mean temperature change to 1.5 ◦C [1]
Quantifying uncertainty in near-term aerosol radiative forcing Here we examine the spread in aerosol radiative forcing in near-term climate projections caused by the effect of uncertain aerosol parameters within our aerosol-climate model (HadGEM3-UK Chemistry and Aerosol (UKCA))
We note that the pre-industrial to present-day aerosol radiative forcing in our perturbed parameter ensemble (PPE) is stronger than in multimodel studies
Summary
The Paris Agreement of the United Nations Framework Convention on Climate Change aims to restrict global mean temperature change since the preindustrial era to well below 2 ◦C and pursue efforts to limit global mean temperature change to 1.5 ◦C [1]. There are many uncertainties associated with the projected exceedance year of a global mean temperature rise of 1.5 ◦C These include definitional ambiguities such as the pre-industrial reference year used for calculating a temperature anomaly, whether all warming or only human-induced warming is included, the future emission scenarios, and how the climate will respond to changes in emissions [3–5]. Due to the short lifetime of atmospheric aerosols relative to greenhouse gases, rapid reductions in anthropogenic aerosol emissions and other short lived climate forcers could be the main drivers of near-term climate change [16]. Changes in anthropogenic aerosol emissions will have a bearing on whether we exceed, and if so by how much, the target to limit global average temperature rise to 1.5 ◦C since the pre-industrial period as set by the Paris Agreement [17]
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