PyroStrat

The severity of wildfires has remarkably increased over the last years in both Northern and Southern hemispheres and there is an emerging realization of their effect on climate. Intense wildfires release tremendous amounts of heat into the atmosphere, which gives rise to extreme thunderstorms termed Pyrocumulonimbus (PyroCb). These storms, augmented by the energy of combustion, can generate vigorous convective updrafts injecting smoke into the stratosphere, where the residence time of aerosols is not limited by cloud scavenging and precipitation. A number of recent studies have put in evidence that the effects of strong PyroCb events on the global stratosphere rival those of moderate volcanic eruptions in terms of magnitude and duration whilst exceeding them in terms of radiative forcing. A striking discovery made by the project’s authors after the Australian “Black Summer” wildfires 2019/20 was that PyroCb injections into the stratosphere combined with solar heating of highly-absorptive carbonaceous aerosols leads to generation of synoptic-scale vortices that can persist in the stratosphere for months and loft confined bubbles containing several megatons of combustion products and moisture into the middle stratosphere The ability of PyroCb plumes to self-organize into stable confined structures, lofting solar-absorbing aerosols and climate-altering gases to very high altitudes, warrants a survey of the stratospheric effects of severe wildfires during the available stratospheric observation record. Furthermore, the increasing frequency and intensity of wildfires, occurring in response to global and regional warming trends, urges assessment of their climate-altering potential. Unlike the volcanic effects whose role in the climate system is well recognized, the impact of PyroCb emissions on the stratosphere and climate via stratospheric feedback has never been systematically explored. Such information is of crucial importance for understanding the factors of climate change and PyroStrat is conceived to fill this gap. The overarching goal of our project is to quantify the impact of pyroconvection on the stratospheric composition and radiative balance. We will address five outstanding scientific questions on the subject, namely: 1. What is the composition of the PyroCb emissions into the stratosphere and how large are they? 2. What are the physicochemical properties and evolution of PyroCb plumes in the stratosphere? 3. What is the radiative forcing of stratospheric smoke and how does it compare with volcanic effects? 4. How do the changes in stratospheric composition affect its dynamics? 5. What is the role of PyroCb in the stratospheric water budget and chemical ozone depletion? The project's goal will be achieved through a synergistic use of available and new observations together with simulations by a suite of chemistry-transport, radiative transfer and chemistry-climate models. The first step will be to catalog the PyroCb emissions into the stratosphere using available observations (WP1) and to constrain their physicochemical properties using observations and realistic simulations (WP2). The second step will be to characterise the impacts of PyroCb on the ozone layer, stratospheric particulate and gaseous composition, and to quantify the associated radiative forcing and climate effects (WP3). The consortium consists of research groups from three French institutes with highly complementary and renowned profiles in observations and modeling. The observational component of the project is reinforced by three international partners from U.S., Canada and New Zealand. As a whole, the consortium provides unique expertise covering all scientific aspects and different stages for the realization of the project. Beside the answers to the scientific questions, PyroStrat will deliver new observational and modeling data and software products regarding the composition, dynamics and radiative properties of PyroCb emissions.