Space Climate into the Twenty-First Century

Variations in solar output cause temperature changes throughout the atmosphere, but the most variable regions of the solar spectrum, the ultraviolet, extreme-ultraviolet, and X-ray ranges, are absorbed well above the tropopause. Nevertheless, the theory that increased solar output has caused the observed increases in temperature in the troposphere and at the surface, during the past half-century, still has some popular support. That theory is refuted by the curious but well-established fact that the upper atmosphere is cooling, above the stratopause and into the thermosphere, even as the lower atmosphere warms. This is due to increases in the same anthropogenic gases, particularly carbon dioxide, that absorb infrared radiation in the troposphere. With ascending altitude, as the atmosphere becomes more transparent and less collisional, the balance of infrared radiation transfer swings from absorption toward emission, so gases that are active in the infrared cause radiational cooling of the upper atmosphere. This has caused a small negative trend in mean stratospheric temperature over the last several decades, and more dramatic temperature decreases in the thermosphere and ionosphere above 100 km, which also change their density profiles, especially at the low-Earth orbit altitudes generally considered to be “space.” Many types of observations have contributed to quantifying these trends, and global numerical models of increasing sophistication have been used to simulate the rate of change and to understand the mechanisms. Recent model accomplishments include global dynamical simulations throughout the atmosphere, from the surface to the exobase, using the Whole Atmosphere Community Climate Model - eXtended, which is a component of the NCAR Community Earth System Model (e.g., Solomon et al., GRL, 2018; JGR, 2019). This work demonstrated that a consistent formulation of physics, chemistry, and dynamics can describe short- and long-term variation in the entire atmosphere system, and showed how the complicating effects of solar activity variation are almost negligible near the surface, but increase with increasing altitude to become extreme, especially in the ionosphere. Here, we present the results of long-term simulations, for the first time including a fully-coupled ocean model (rather than observed sea-surface temperatures). This enables us to conduct a whole-atmosphere climate forecast for the twenty-first century, calibrated using simulations and observations of the past half-century, and include scenarios for solar-cycle progression.