UC San Diego

Extreme land temperatures are among the most direct and consequential expressions of climate change, with impacts on human health, ecosystems, infrastructure, and the habit- ability of hot regions. Yet the physical controls on Earth’s hottest surface and near-surface air temperatures remain incompletely understood, especially in the dry subtropical environments where record-setting heat most often occurs. This dissertation addresses that problem by linking climate-model projections, radiative-transfer theory, and reduced-order land–atmosphere modeling.First, I analyze CMIP6 simulations to evaluate how the hottest land temperatures change under greenhouse forcing. The results show that annual hottest daily-mean and daily-maximum temperatures over tropical land warm about 30–40% faster than the tropical land mean. The geography of record-like heat remains concentrated in subtropical arid and semi-arid regions, particularly the Middle East and South Asia, while the likelihood of exceeding a fixed historical hottest-land benchmark increases by about 35% per 1 ◦C of tropical land warming. Second, I develop a daily-mean radiative framework for Earth’s warmest dry states using a simplified surface–atmosphere energy balance constrained by line-by-line radiative-transfer calculations. The hottest present-day daily-mean states occur in hot, dry columns that remain close to a top-of-atmosphere radiative balance. Even when the lower atmosphere is strongly infrared opaque, outgoing longwave radiation continues to increase with temperature. Doubling CO2 shifts this balance toward warmer conditions by reducing longwave emission at fixed temperature, but it does not remove the radiative constraint.Third, I examine daily-maximum temperatures with a diurnal-cycle model for the sur- face skin layer and a deep convective boundary layer. The model shows that the surface skin can become tens of kelvin warmer than both near-surface air and the bulk boundary layer during the afternoon maximum. This separation arises from the contrast between the shallow thermal inertia of the land surface and the much larger heat capacity of the boundary layer. Together, these results clarify why Earth’s warmest temperatures occur in hot, dry desert environments, why satellite-observed skin temperatures can greatly exceed standard air-temperature measurements, and how greenhouse forcing alters both daily-mean and daily-maximum heat extremes.