Recent Changes in the Global Near-surface Energy Balance: the Critical Role of Specific Humidity in Addition to Temperature for Interpreting Global Trends

Abstract:

Global change is a change in the planetary energy balance. It is usually communicated to the public as a change in near-surface air temperatures (T_a), but most of the excess energy enters the oceans and changes to T_a is only part of the near-surface energy balance equation. Changes in atmospheric energy content due to changes in specific humidity (E_SH) may result in a situation where total atmospheric energy content (E) increases without T_a change. (Kinetic energy from wind velocity also enters the energy balance equation, but its changes due to global stilling are on average trivial compared to energy changes from temperature (E_T) and E_SH.) To improve our understanding of recent changes to near-surface energy content, we analyzed MERRA-2 global gridded reanalysis data products and 15 models from the Atmospheric Model Intercomparison Project (AMIP) over the 1980-2014 period. Using the non-parametric Sen’s slope to ascertain significant trends, 39% of near-surface atmosphere pixels had a significant increase in E_T, 34% had a significant increase in E_SH, and 46% had a significant increase in E. The observed average increase in E_T (E_SH) was 23 J year^-1 (27 J year^-1), showing specific humidity contributed 17% more to the global energy increase compared to T_a. AMIP models also estimated 23 J year^-1 for E_SH, but overestimated E_T by 16%. Although the magnitudes of modeled and reanalysis E_SH matched, AMIP models underestimated significant E_SH changes across most seas near the Equator, the Namib Desert and some Arctic regions. AMIP models overestimated E across most of the land area and underestimated it across the equatorial region and some areas in Antarctica. Results demonstrate the critical role that specific humidity plays in our understanding of global changes to E across critical global regions.