5:15 PM - 7:15 PM
[ACG52-P09] Hysteresis in permafrost response to increase and decrease of CO2 emissions
Keywords:Climate, Permafrost, Hysteresis, Irreversibility, Climate System, Cryosphere
Global warming has intensified since the late 20th century due to increased greenhouse gas (GHG) emissions from human activities. Many elements of the climate system, such as temperature and precipitation, are considered to respond linearly to global warming, but the Earth systems are known to have so-called “tipping elements”, which have rapidly changed the dynamics of the system after a certain level of warming. Understanding the tipping elements is becoming increasingly important not only for climate science, but also for the response of society to global warming. Permafrost, which is mainly in the high latitudes of the Northern Hemisphere, is defined as area where the ground temperature is below 0 degC for two or more years. Permafrost stores more than twice as much carbon underground as the atmosphere, and when permafrost thaws due to global warming, the soil releases CO2 and methane. This change is irreversible and has positive feedback that accelerates global warming. Therefore, permafrost is one of the most important tipping elements. However, the amount of GHG emissions from permafrost thawing has high uncertainty. In addition, if hysteresis exists in permafrost response, once it melts due to warming, its effects persist and GHGs from the soil will continue even if the climate returns to its previous state. Accordingly, it is important to study the hysteresis and reversibility of permafrost during warming and cooling processes to understand the physical properties of permafrost which is one of tipping elements. Previous studies have suggested that hysteresis exists in the response of permafrost area to increases or decreases in global mean CO2 concentration, but the mechanism is not yet understood.
In this study, series of numerical experiments based on an idealized overshooting scenario (a scenario in which after a certain period of CO2 emissions, the same amount of CO2 absorption continues until net emissions reach zero) using an emission-driven earth system model (Earth System Model, ESM) are conducted to investigate responses of permafrost to increase and decrease of CO2 emissions. Using MIROC-ES2L, one of the Coupled Model Intercomparison Project Phase 6 (CMIP6) models, we did 3000 years of integration under pre-industrial conditions and 1000 years of integration under 10 PgC/year CO2 emissions . During the warming period, climate restoration experiments were conducted by branching to -10PgC/year CO2 emission experiments when the warming response reached 2, 4, 6, and 8 degC.
The results show that the response of permafrost area to climate change is reversible and has hysteresis. Additionally, the lower soils in the permafrost region showed irreversibility of properties that the area of soils which are completely frozen and annual maximum ground temperature is below 0°C reduces compared to the initial state. In the deeper soils of permafrost regions, permafrost thawing continues for 250 years even after the climate begins to cool. To examine heat conduction effects, we did sensitivity experiments by changing the heat conductivity of the soil or the latent heat of phase change. As a result, we found that the small latent heat decreases the hysteresis of permafrost area and the irreversibility of its properties, indicating that the phase change process plays a particularly important role in permafrost response to climate change.
This hysteresis also suggests that once permafrost thaws due to warming, the effects will persist for more than a hundred years, which could affect the characteristics of permafrost as a tipping element. In addition, the hysteresis occurs even at global warming levels below 4 degC, suggesting that the irreversibility of permafrost response is important in understanding its tipping behavior in realistic low-emission scenarios.
In this study, series of numerical experiments based on an idealized overshooting scenario (a scenario in which after a certain period of CO2 emissions, the same amount of CO2 absorption continues until net emissions reach zero) using an emission-driven earth system model (Earth System Model, ESM) are conducted to investigate responses of permafrost to increase and decrease of CO2 emissions. Using MIROC-ES2L, one of the Coupled Model Intercomparison Project Phase 6 (CMIP6) models, we did 3000 years of integration under pre-industrial conditions and 1000 years of integration under 10 PgC/year CO2 emissions . During the warming period, climate restoration experiments were conducted by branching to -10PgC/year CO2 emission experiments when the warming response reached 2, 4, 6, and 8 degC.
The results show that the response of permafrost area to climate change is reversible and has hysteresis. Additionally, the lower soils in the permafrost region showed irreversibility of properties that the area of soils which are completely frozen and annual maximum ground temperature is below 0°C reduces compared to the initial state. In the deeper soils of permafrost regions, permafrost thawing continues for 250 years even after the climate begins to cool. To examine heat conduction effects, we did sensitivity experiments by changing the heat conductivity of the soil or the latent heat of phase change. As a result, we found that the small latent heat decreases the hysteresis of permafrost area and the irreversibility of its properties, indicating that the phase change process plays a particularly important role in permafrost response to climate change.
This hysteresis also suggests that once permafrost thaws due to warming, the effects will persist for more than a hundred years, which could affect the characteristics of permafrost as a tipping element. In addition, the hysteresis occurs even at global warming levels below 4 degC, suggesting that the irreversibility of permafrost response is important in understanding its tipping behavior in realistic low-emission scenarios.