Solar Forcing Variability

Climate ModellingSolar Forcing Variability

Overview

Centennial climate evolution strongly correlates with the solar activity proxies before the anthropogenically driven contemporary era. However, this correlation hasn’t yet been proven or disproven as a causal relationship because of the large uncertainties in the amplitude of solar forcing reconstructions from these proxies, as well as uncertainties in other forcings like volcanic eruptions. In our proposal, we suggest tackling this problem in a reversed way by establishing the long-term solar forcing variations amplitude from the climate variables reconstructions with the use of Earth system modelling. For this purpose, we will focus on the 17th-18th centuries and exploit our new Earth system model (ESM) SOCOLv4, which was explicitly designed for simulating the volcanic- and solar-related processes in the atmosphere, forced by the updated estimates for the volcanic activity data and a range of solar scenarios with varying amplitudes. Comparing these experiments with the latest reconstructions and reanalysis of climate variables will allow us to answer the question of how large the solar forcing needs to be in order to reproduce the observations.

Introduction

Modelling the future climate requires knowledge of anthropogenic and natural forcing agents. The main attention in the recent IPCC report (IPCC, 2021) was concentrated on the anthropogenic part because the last 60-70 years were characterised by rather stable incoming solar electromagnetic energy flux and just sporadic major volcanic eruptions. Therefore, natural forcing could not compete with strong radiative forcing changes of anthropogenic origin. However, the reconstructions of the past and some predictions of the future solar magnetic activity show that the solar activity during 1950-2020 is characterised as grand maximum (e.g., Usoskin, 2013, Figure 20; Steinhilber and Beer, 2013) and potentially can reach much lower values in the past as well in the future.

Figure 1.

Studying the connection between the past climate and solar activity variations can help to reduce this uncertainty, especially given that there is a strong correlation between the climatic and solar proxy records during the previous periods of reduced solar activity, e.g., the Maunder and Dalton minima. This topic, however, is also still highly debated, because of no established agreement on how to convert the cosmogenic isotopes’ variability to the solar irradiance forcing, uncertainty and limited spatial availability of the reconstructed climate variables, as well as numerous uncertainties in climate models that have been used to address this question. The current state of the climatic and solar irradiance research in this area has been recently reviewed by Schmutz (2021) and formulated in three key statements:

  1. During the 13-19th centuries, there were substantial long-term climate variations;
  2. There are no known climate feedback mechanisms that could substantially amplify the global mean response to  direct solar forcing;
  3. The physics-based models suggest that the solar forcing cannot be lower by more than 2 Wm-2 below the measured present-day total solar irradiance (TSI) value during the solar cycle minimum, which is too low to explain observed cooling.

Thus, the past centennial climatic variations have to be either due to other factors or the solar forcing needs to be revised.

 Model

Recently, we have developed the fourth version of the SOCOL model (SOCOLv4) as the combination of our recently improved chemistry and aerosol microphysics modules of SOCOL-AERv2 and the MPI-Met Earth System Model, which includes an updated general circulation model ECHAM6 and interactive ocean, land surface, and marine biogeochemistry modules (Sukhodolov et al., 2021). This model considers the majority of processes responsible for the behaviour of the Earth’s climate system including gas-phase/heterogeneous chemistry and bin-resolved stratospheric sulphate aerosol and is able to simulate direct forcings and feedbacks important for climate, ozone, and volcanic aerosol evolution. The default vertical and horizontal resolution of the model has also been increased. We applied SOCOLv4 to simulate the climate, ozone, and stratospheric sulphate aerosol historical evolution from 1980 to 2018 and to evaluate the model against available observations.

Figure 2.

References

Schmutz, 2021
Sukhodolov et al., 2021
For further information please contact: Dr. T. Sukhodolov