Warming in the lower atmosphere may be more significant than expected

Research conducted at the Lawrence Livermore National Laboratory (LLNL) has found something that may surprise many people. According to climate scientists and contributors on the project, satellite measurements of the troposphere temperature may have underestimated global warming over the last four decades. The troposphere is the lowest region of the atmosphere.

In the new research, scientists studied four separate properties of tropical climate change. The team says each property is a ratio between trends in two "complementary" variables. Complementary variables include things like tropical temperature and moisture, and they are expected to show a correlated behavior. The team says basic and well-understood physical processes govern the correlated behavior.

The first of three properties the team investigated involved the relationship between tropical temperature and tropical water vapor. Water vapor (WV) trends were compared with trends in sea surface temperature (SST), lower tropospheric temperature (TLT), and mid- to upper tropospheric temperature (TMT). The fourth property was the ratio between TMT and SST trends.

Researchers say that all of those ratios are tightly constrained in climate model simulations despite model differences in climate sensitivity, external forces, and natural variability. Model trend ratios between WV and temperature were closest to observed ratios when the latter was calculated using data sets exhibiting more significant tropical warming of the ocean surface and troposphere.

When looking at TMT/SST ratio, model data depended on the combination of observations used to estimate TMT and SST trends. Observational data sets with more significant warming of the tropical ocean surface yielded TMT/SST ratios in better agreement with model results. Scientists say if climate model expectations of these relationships between topical temperature and moisture are realistic, their findings showing systemic low bias in satellite tropospheric temperature trends or an overestimate of the observed atmospheric moistening signal. The team admits that currently, it's difficult to figure out which scenario is more likely.