GECO

Climate ModellingGECO

Overview

The environmental crisis circa 42000 years ago has been discovered from the paleo archives and described by Cooper et al. (2021) but not fully explained. They proposed that it might be caused by the enhanced ionization rate during the geomagnetic field weakening event, known as the Laschamp excursion, and have simulated the related atmospheric chemistry changes with a potential to impact surface ultraviolet dozes and atmospheric circulation. However, they could not reveal any major climatic shifts, necessary to explain the various changes in the biosphere, claimed to be found for this period. To address this problem, a new unconventional approach is suggested based on detailed climate simulations testing another potential climate driver – a connection between the atmospheric ionization rates and autoconversion/aggregation of the cloud particles, which regulates the cloud life cycle. A coupled ocean-atmosphere climate model with the added parameterization of the cloud properties dependence on the ionization rates will be used to perform the experiments. The ionization rates will be calculated using models driven by the typical configuration of the geomagnetic field for geomagnetic excursion periods. With the modified model version,   three 500-year long runs consisting of a 500-year spin-up and 500-year reference run for a normal geomagnetic field and a 500-year experiment run with enhanced ionization rate caused by geomagnetic field weakening will be performed. The comparison of the reference and experiment runs will show whether obtained differences can mimic some features of the environmental crisis, 42000 years ago.

Introduction

The environmental crisis circa 42000 years ago was discovered in the paleo archives and described by Cooper et al. (2021). They have presented evidence of many archaeological, paleoanthropological, and genetic transitions observed around that period, and proposed their relation to the geomagnetic field weakening, known as the Laschamp excursion, that was happening in parallel and triggered intense discussions in the geomagnetic, climate, and paleo-anthropological science communities about the environmental crisis itself as well as about the proposed connection between the geomagnetic field variations and the Earth’s climate.

Figure 1. . Cloud thickness for the low and high ionization rates. The enhancement of the ionization is caused here by energetic particle precipitation. Adopted from Harrison and Lockwood (2020).

Some other drivers have also been proposed, such as super-powerful volcanic eruption clusters, exceptionally lower solar activity, or a series of extreme solar proton events (SPE) during this period. However, the solar radiative forcing is limited by 2 W/m2  and cannot dramatically affect the climate system. The short-term climate effects of powerful volcanic eruptions or SPE can be substantial on a short (from month to several years) timescale, however, very massive clusters of these events have been not observed, and their appearance is very questionable. From the list of possible forcings, the effects of ionization rate enhancement have not been properly evaluated, as only their chemical effect has been tested with respect to the considered event. At the moment, there are several other hypotheses about how the increase in ionization rates caused by weaker geomagnetic field can influence clouds. It was suggested many years ago that galactic cosmic rays (GCR) can affect the aerosol formation process, leading to changes in cloud condensation nuclei and cloud modification. In the case of the Laschamp excursion, the substantial increase in GCR would lead to an increase in cloud cover and probably cool down the climate. However, the importance of this mechanism was not confirmed in IPCC AR5 and by recent results from the CERN CLOUD project. A positive correlation between GCR and cloud cover was found only in the intertropical convergence zone and only in the free troposphere. The second mechanism of GCR influence on clouds is related to the influence of electrically induced droplet changes in the cloud layer, involving different microphysical mechanisms. Some consequences of this process have been identified in the observations on short time scales and for limited areas . Harrison and Lockwood (2020) discovered robust thickening of low-level clouds by approximately 1 km during high solar wind speed events and recommended including this mechanism in numerical climate or weather prediction models. The schematic of the event is shown in Figure 1.

Model

SOCOL-MPIOM short description (Stenke et al., 2013)

References

Cooper et al., 2021
Harrison and Lockwood, 2020
Stenke et al, 2013
For further information please contact: Dr. T. Egorova