Here we report on the outcome of three of these initiatives:
- “Towards a more complete assessment of the impact of solar variability on the Earth’s climate” (TOSCA), a project that uses a network of European scientists from 20 countries that met from 2011 to 2015 to assess contributions of solar variability to Earth’s climate
- Solar Irradiance Data Exploitation (SOLID), a European-funded project dedicated to merging all exploitable spectrally resolved solar irradiance records into one single composite data set
- An international team of scientists that met at the International Space Science Institute (ISSI) to produce a comprehensive data set that includes solar radiative forcing and contributions from energetic particles
These initiatives have culminated in the production of two public data sets to assist with the scientific analysis of solar forcing: a composite data set of all irradiance observations and a comprehensive data set containing different solar forcings (radiative and by particles) since 1850.
How Does Solar Variability Affect Climate?
Solar variability affects Earth’s climate in many intricate and nonlinear ways. Most effects are ultimately driven and modulated by the solar magnetic field and its conspicuous solar cycle, which repeats approximately every 11 years.
The effect of solar variability on climate is mostly hidden in the natural variability of the climate system; thus, careful statistical analysis is required to extract it from a noisy background. Such analyses require records that extend over a long period of time, but the paucity of observations in existing records poses a serious challenge. For example, scientists have been making direct measurements (from space) of the total solar radiative input into Earth’s atmosphere only since 1978, although there had been earlier attempts to measure it from the ground.Although solar radiation represents more than 99.9% of the energy entering Earth’s system, radiation is not the only means by which solar variability affects climate. Another source of variability comes from energetic particles, some of which originate from the Sun.
The most energetic particles, known as galactic cosmic rays, have an extragalactic origin; their role in cloud formation has attracted strong media attention. However, recent experiments at the European Organization for Nuclear Research (CERN) suggest that these cosmic rays have a limited impact on the microphysics of clouds. Energetic protons produced during solar flares and energetic electrons that originate from the Earth’s magnetosphere have received much less attention, yet they may play a role by contributing to catalytic ozone loss in the polar atmosphere [Andersson et al., 2014]. Such ozone depletion primarily affects the upper layers of the atmosphere (60–80 kilometers) but eventually it affects the lower layers and climate as well.
For many years, a single quantity, total solar irradiance (TSI), which describes the total solar radiated power incident on Earth’s upper atmosphere, was used to summarize the solar contribution into climate models, neglecting other contributions. The assumption was that solar radiation would mainly act on Earth’s environment by directly heating the oceans, continents, and lower atmosphere.
The discovery of the effects of radiation in the ultraviolet (UV) wavelength band shattered this simple picture. Researchers have shown that UV radiation affects climate through direct heating and the production and destruction of ozone in the stratosphere, which then leads to regional effects at Earth’s surface through a complex chain of mechanisms. All these effects, however, are found to have a minor impact on climate in comparison to recent man-made global warming.
What is now the way forward? Clearly, improving our understanding of the physical mechanisms on the Sun that drive irradiance variations, particularly those that may lead to long-term climate variations, should be a priority. Several teams are actively working on this issue.
We are still missing an international framework that enables a critical comparison of irradiance models with the aim of improving them. The highest priority, however, is to continue simultaneous total and spectral irradiance observations by different instruments. Our ultimate aim is to quantify more precisely the role of the Sun in the natural forcing of climate variability and climate change.
In short: The Sun still has a lot to tell us.