Although the sun provides nearly all the energy needed to warm the planet, its contribution to climate change remains widely questioned. Many empirically based studies claim that it has a significant effect on climate, while others (often based on computer global climate simulations) claim that it has a small effect.
The Intergovernmental Panel on Climate Change (IPCC) supports the latter view and estimates that almost 100% of the observed warming of the Earth’s surface from 1850–1900 to 2020 was caused by man-made emissions (AR6 WG1, pages 63, 425, and 962). This is known as the anthropogenic global warming (AGWT) theory.
I addressed this important paradox in a new study published in Geoscience Frontiers. The conundrum appears to arise from two sets of uncertainties: (i) the historical decades and long-term variations in solar activity are unknown; (ii) the sun may affect Earth’s climate through various physical mechanisms many of which are not fully understood and are not incorporated into the global climate models (GCMs).
It is important to notice that the AGWT is based solely on computer global climate model simulations that use total solar irradiance (TSI) records with very low multidecadal and long-term variability. The models also assume that the sun affects the climate system only through radiative forcing, although there is evidence that other solar processes related to solar magnetic activity (solar wind, cosmic rays, interplanetary dust, etc.) also affect the climate.
Over the past two decades, solar activity has been characterized by an extended solar minimum spanning two solar cycles, known as the Clilverd Minimum. This phenomenon is currently affecting the climate, but before we can understand its impact, we must address the significant discrepancy between the solar effects observed in paleoclimate proxy records and modern observations. The relationship between solar signals and climate response is complex and not fully understood. However, there is substantial evidence from models and reanalyses that the relationship exists. A recent hypothesis is that the solar signal modulates heat and moisture transport to the Arctic, which explains its relatively small effect during a single solar cycle. However, when an anomaly in solar activity persists over several cycles, as it did during the 70-year modern solar maximum, its effect accumulates and has a large impact on the planet’s energy budget. Understanding this mechanism is critical to understanding the overall impact of solar activity on our climate.
Current Solar Activity
The monthly sunspot number for June 2023 reached 163.4. While this figure may be revised slightly, it’s likely to stand as the highest number seen in over two decades, since September 2002. Solar Cycle 25 is relatively young, only three and a half years old, which means there are ample opportunities over the next three years to surpass this month’s 20-year record. Based on recent data, it seems very likely that Solar Cycle 25 will surpass Solar Cycle 24 in terms of activity.
In direct contradiction to the official forecast, a team of scientists led by the National Center for Atmospheric Research (NCAR) is predicting that the Sunspot Cycle that started this fall could be one of the strongest since record-keeping began.
In a new article published in Solar Physics, the research team predicts that Sunspot Cycle 25 will peak with a maximum sunspot number somewhere between approximately 210 and 260, which would put the new cycle in the company of the top few ever observed.
The cycle that just ended, Sunspot Cycle 24, peaked with a sunspot number of 116, and the consensus forecast from a panel of experts convened by the National Aeronautics and Space Administration (NASA) and the National Oceanic and Atmospheric Administration (NOAA) is predicting that Sunspot Cycle 25 will be similarly weak. The panel predicts a peak sunspot number of 115.
Changes in space climate driven by long-term changes in solar activity have a significant impact on Earth’s atmosphere and climate. Understanding the complex system requires cooperation between space physics and climate science.
On the right, a picture of the Sun taken at the wavelength of visible light, i.e. like a regular camera at very short shutter speed, visible sunspot groups. The time series in the image illustrate a few long series of data used in space air research.
On green: approximately 40 years of direct satellite measurements, a combination of energetic electrons coming into the Earth’s atmosphere.
In red: from geomagnetic measurements reconstructed estimate of the speed of the solar wind in the last hundred years.
With purple: the longest unified time series for geomagnetic activity (the so-called AA index), starting from 1868 and continuing to the present day.
In blue: 400 year series of sunspots. This set of data is the longest indicator of solar activity based on direct measurements.
In the last few years, hundreds of peer-reviewed scientific papers have been published linking changes in solar activity to Earth’s climate (2016, 2017, 2018). The evidence for a robust Sun-Climate connection continues to accumulate in 2019.
When it comes to the Sun’s influence on climate, one conclusion is certain: there is no widespread scientific agreement as to how and to what extent solar activity and its related parameters (i.e., galactic cosmic rays, geomagnetic activity, solar wind flux) impact changes in the Earth’s temperature and precipitation.
The disagreement is so chasmic and the mechanisms are so poorly understood that scientists’ estimates of the influence of direct solar irradiance forcing between the 17th century and today can range between a negligible +0.1 W m-2 to a very robust +6 W m-2 (Egorova et al., 2018; Mazzarella and Scafetta, 2018).
“There is no consensus on the amplitude of the historical solar forcing. The estimated magnitude of the total solar irradiance difference between Maunder minimum and present time ranges from0.1 to 6 W/m2 making uncertain the simulation of the past and future climate.” (Egorova et al., 2018)
“According to the IPCC (2013), solar forcing is extremely small and cannot induce the estimated 1.0–1.5 °C since the LIA. However, thesolar radiative forcing is quite uncertain because from 1700 to 2000 the proposed historical total solar irradiance reconstructions vary greatly from a minimum of 0.5 W/m2 to a maximum of about 6 W/m2 (cf..: Hoyt and Schatten 1993; Wang et al. 2005; Shapiro et al. 2011). Moreover, it is believed that the sun can influence the climate also via a magnetically induced cosmic ray flux modulation (e.g.: Kirkby 2007) or via heliospheric oscillation related to planetary resonances (e.g.: Scafetta 2013, 2014b; Scafetta et al. 2016, and others). Since solar and climate records correlate quite significantly throughout the Holocene (cf: Kerr 2001; Steinhilber et al. 2012; Scafetta 2012, 20104b), the results shown herein may be quite realistic, although the exact physical mechanisms linking astronomical forcings to climate change are still poorly understood.” (Mazzarella and Scafetta, 2018)”
We discuss the issues of primary importance to understand the nature of climate changes in the 20th century and main physical processes responsible for these changes and present a physical model for the solar activity (SA) effect on climate characteristics. A key concept of this model is the heliogeophysical disturbance effect on the Earth climate system parameters driving the long-wave radiation flux moving away from the Earth out into space in high-latitude regions. We address the solar activity effect on the changes in the temperature of the atmosphere and of the World Ocean. The aa–index of the geomagnetic activity (GA) was used as an SA proxy index. We discuss the results of analyzing the regularities and peculiarities of the tropospheric and sea surface temperature (SST) responses to both separate heliogeophysical disturbances and long-term changes in solar and geomagnetic activity. The structure of the tropospheric and SST temperature responses was shown to feature a spatial time irregularity. We revealed the regions, where long-term SST changes are determined mainly by SA variations.
La géologie, une science plus que passionnante … et diverse