TWISTA (The Wide-ranging Impacts of STratospheric smoke Aerosols)

Lead Research Organisation: University of Exeter
Department Name: Mathematics

Abstract

There is much new evidence of stratospheric intrusions of smoke from intense pyro-cumulonimbus (pyroCb) events from wildfires. These events appear to be increasing in frequency, intensity and plume height in both the northern and southern hemispheres. The impacts of smoke aerosol on climate may be disproportionately larger than volcanic aerosols that are sporadically injected into the stratosphere because i) they strongly absorb sunlight which may influence stratospheric dynamics and ii) their surface characteristics may enhance their role in heterogeneous chemistry and ozone depletion. Given that the record persistent Antarctic ozone hole and record Arctic ozone depletion of 2020 were preceded by unprecedented wildfires, there is an urgent need to understand their role in the climate system.

In the northern hemisphere, the strongest stratospheric smoke events have occurred over the past five years. In 2017, a range of remote sensing observations showed that smoke from the Pacific Northwest event (PNE) persisted at 18-22 km altitude for over 5 months. In 2019-2020, stratospheric aerosol loading was anomalously high owing to sulfate and ash from the Raikoke eruption but there is evidence of a possible significant contribution from smoke from the August 2019 Siberian wildfires (SIB). Record halogen-catalysed Arctic ozone depletion was observed in spring of 2020 with a strong Arctic polar vortex cold enough for polar stratospheric clouds (PSCs) to form until well until springtime. Research has suggested that, in the absence of the Montreal Protocol which has reduced humanity's emissions of ozone depleting substances (ODSs), we would have had a northern hemisphere ozone hole, similar to that of the Antarctic.

In the southern hemisphere, the January 2020 Australian 'Black Summer' (ABS) wildfires were unprecedented in scale and intensity with millions of tonnes of smoke aerosol and associated gases being injected into the upper troposphere and lower stratosphere. Initial injection altitudes reached 16 km and the smoke eventually reached altitudes of up to 36 km (three times higher than the operating altitude of commercial trans-Atlantic jet aircraft. The ascent to remarkably high altitudes resulted from the self-lofting caused by the presence of black carbon (BC) within smoke which absorbs sunlight and heats the air surrounding it. Just like a hot air-balloon, this absorption of sunlight causes the smoke and the surrounding air to rise. Significantly, it appears that this single event caused the largest global mean stratospheric temperature perturbation for three decades. The Antarctic ozone hole in 2020 was extremely deep and persistent, with record low polar stratospheric temperatures and a strong polar vortex.

Several chemical fingerprints determined from satellite observations that suggest that the severe ozone depletion in the Arctic, and the record ozone hole in the Antarctic are linked to these wildfire events. This becomes even more worrying when one considers that wildfire frequency, intensity, and plume altitude are all forecast to increase under future global warming scenarios. It could be postulated that all of the hard work that has been performed very successfully under the Montreal Protocol might be undone not through lack of adherence to ODS reductions, but through global warming. If an ozone hole opens up over the northern hemisphere, this could pose a further existential threat to the delicate ecosystem balance that humanity relies upon. Similarly, the dynamical impacts of absorbing aerosols in the stratosphere may directly impact the Earth's surface climate: an enhanced positive phase of the North Atlantic Oscillation has been modelled in idealised studies which could lead to enhanced flooding in northern Europe and potentially devastating drought over the Iberian Peninsula. It is therefore critical to include these factors and feedbacks in global climate simulations at the earliest opportunity.

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