Judith L. Lean
Emergent in recent decades are robust specifications and understanding of connections between the Sun’s changing radiative energy and Earth’s changing climate and atmosphere. This follows more than a century of contentious debate about the reality of such connections, fueled by ambiguous observations, dubious correlations, and lack of plausible mechanisms. It derives from a new generation of observations of the Sun and the Earth made from space, and a new generation of physical climate models that integrate the Earth’s surface and ocean with the extended overlying atmosphere. Space-based observations now cover more than three decades and enable statistical attribution of climate change related to the Sun’s 11-year activity cycle on global scales, simultaneously with other natural and anthropogenic influences. Physical models that fully resolve the stratosphere and its embedded ozone layer better replicate the complex and subtle processes that couple the Sun and Earth.
An increase of ~0.1% in the Sun’s total irradiance, as observed near peak activity during recent 11-year solar cycles, is associated with an increase of ~0.1oC in Earth’s global surface temperature, with additional complex, time-dependent regional responses. The overlying atmosphere warms more, by 0.3oC near 20 km. Because solar radiation impinges primarily at low latitudes, the increased radiant energy alters equator-to-pole thermal gradients, initiating dynamical responses that produce regions of both warming and cooling at mid to high latitudes. Because solar energy deposition depends on altitude as a result of height-dependent atmospheric absorption, changing solar radiation establishes vertical thermal gradients that further alter dynamical motions within the Earth system.
It remains uncertain whether there are long-term changes in solar irradiance on multidecadal time scales other than due to the varying amplitude of the 11-year cycle. If so the magnitude of the additional change is expected to be comparable to that observed during the solar activity cycle. Were the Sun’s activity to become anomalously low, declining during the next century to levels of the Maunder Minimum (from 1645 to 1715), the expected global surface temperature cooling is less than a few tenths oC. In contrast, a scenario of moderate greenhouse gas increase with climate forcing of 2.6 W m−2 over the next century is expected to warm the globe 1.5 to 1.9oC, an order of magnitude more than the hypothesized solar-induced cooling over the same period.
Future challenges include the following: securing sufficiently robust observations of the Sun and Earth to elucidate changes on climatological time scales; advancing physical climate models to simulate realistic responses to changing solar radiation on decadal time scales, synergistically at the Earth’s surface and in the ocean and atmosphere; disentangling the Sun’s influence from that of other natural and anthropogenic influences as the climate and atmosphere evolve; projecting past and future changes in the Sun and Earth’s climate and atmosphere; and communicating new understanding across scientific disciplines, and to political and societal stakeholders.