Multiple regression analysis of anthropogenic and heliogenic climate drivers, and some cautious forecasts

TL;DR

This study uses multiple regression analysis to evaluate the influence of CO2 and solar activity on climate change, providing cautious forecasts and highlighting the need for more data to refine climate sensitivity estimates.

Contribution

It introduces a regression-based approach combining CO2 and geomagnetic data to assess climate sensitivity and offers preliminary climate forecasts based on planetary synchronization ideas.

Findings

Reproduces historical sea surface temperatures with 87% accuracy.

Identifies sensitivity of regression results to recent data and El Niño events.

Predicts mild temperature rise or potential future cooling depending on climate sensitivity.

Abstract

The two main drivers of climate change on sub-Milankovic time scales are re-assessed by means of a multiple regression analysis. Evaluating linear combinations of the logarithm of carbon dioxide concentration and the geomagnetic aa-index as a proxy for solar activity, we reproduce the sea surface temperature (HadSST) since the middle of the 19th century with an adjusted $R^2$ value of around 87 per cent for a climate sensitivity (of TCR type) in the range of 0.6 K until 1.6 K per doubling of CO$_2$. The solution of the regression is quite sensitive: when including data from the last decade, the simultaneous occurrence of a strong El Ni\~no on one side and low aa-values on the other side lead to a preponderance of solutions with relatively high climate sensitivities around 1.6 K. If those later data are excluded, the regression leads to a significantly higher weight of the aa-index and a correspondingly lower climate sensitivity going down to 0.6 K. The plausibility of such low values is discussed in view of recent experimental and satellite-borne measurements. We argue that a further decade of data collection will be needed to allow for a reliable distinction between low and high sensitivity values. Based on recent ideas about a quasi-deterministic planetary synchronization of the solar dynamo, we make a first attempt to predict the aa-index and the resulting temperature anomaly for various typical CO$_2$ scenarios. Even for the highest climate sensitivities, and an unabated linear CO$_2$ increase, we predict only a mild additional temperature rise of around 1 K until the end of the century, while for the lower values an imminent temperature drop in the near future, followed by a rather flat temperature curve, is prognosticated.