Validating a physics-based back-of-the-envelope climate model with state-of-the-art data (1301.1146v2)

Abstract: An old conceptual physics-based back-of-the-envelope model for greenhouse effect is revisited and validated against state-of-the-art reanalyses. Untraditional diagnostics show a physically consistent picture, for which the state of earth's climate is constrained by well-known physical principles, such as energy balance, flow and, conservation. Greenhouse gas concentrations affect the atmospheric optical depth for infrared radiation, and increased opacity implies higher altitude from which earth's equivalent bulk heat loss takes place without being re-absorbed. Such increase is seen in the reanalyses. There has also been a reduction in the correlation between the spatial structure of outgoing long-wave radiation and surface temperature, consistent with increasingly more processes interfering with the upwelling infrared light before it reaches the top of the atmosphere. State-of-the-art reanalyses further imply increases in the overturning in the troposphere, consistent with a constant and continuous vertical energy flow. The associateion between these aspects can be interpreted as an entanglement between greenhouse effect and the hydrological cycle, where reduced energy transfer associated with increased opacity is compensated by tropospheric overturning activity.

• The paper validates Hulburt's 1931 climate model by comparing it with state-of-the-art satellite and reanalysis observations.
• It uses metrics like optical depth, outgoing long-wave radiation, and atmospheric overturning to assess key climate processes.
• Findings indicate a 23 m/decade rise in emission altitude and enhanced vertical transport, linking greenhouse effects with changes in the hydrological cycle.

Validating an Old Physics-Based Back-of-the-Envelope Climate Model with State-of-the-Art Data

This paper revisits an old conceptual model developed by Hulburt in 1931 to explain the greenhouse effect and validates it against modern reanalyses and satellite data. The purpose is to demonstrate that even simple physics-based models can offer insights consistent with current understanding of atmospheric processes and trends, despite their simplicity.

Objectives and Methodology

The primary objective of this paper is to evaluate the consistency of Hulburt's model with contemporary climate data. The paper uses several state-of-the-art reanalyses including ECMWF’s ERA-Interim and NCEP/NCAR, along with satellite measurements of outgoing long-wave radiation (OLR) to validate the model. The paper focuses on key climate indicators influenced by greenhouse gas (GHG) concentrations, specifically:

1. The Optical Depth: This measures the degree of atmospheric opacity to infrared radiation, which influences the effective altitude of Earth's thermal emission.
2. Outgoing Long-Wave Radiation: Observations from satellites provide empirical data against which model predictions are compared.
3. Tropospheric Overturning: The paper uses vertical velocity data as a metric for atmospheric overturning to understand energy transfer dynamics in the vertical column of the atmosphere.

Results

Numerical analyses yield significant findings:

• Emission Altitude: A consistent upward trend of 23 meters per decade in the 254 Kelvin isotherm suggests a deepening of the optical depth, aligning well with observed surface warming rates. This implies that the altitudes from which Earth emits infrared radiation are rising, consistent with increased atmospheric GHGs.
• Correlation Trends: A decreasing correlation between OLR and surface temperatures indicates that the spatial structure of the radiation is becoming more diffuse. This supports the presence of increased opacity due to elevated GHG levels and/or cloud cover.
• Atmospheric Overturning: The analysis reveals increased vertical transport in the middle troposphere since 1995, escalating with altitude. Though the total column water vapor showed insignificant trends, the intensification of vertical energy flow corresponds with the expectations of an accelerated hydrological cycle and latent heat transport.

Discussion

The research indicates that simple physical models, despite their lack of comprehensive detail, can still effectively describe climate processes. The work also suggests an interconnection between the greenhouse effect and the hydrological cycle, where increased atmospheric opacity is offset by intensified convective activity and heat transport in the atmosphere. Notably, these results challenge previous findings, such as the anticipated slowdown in atmospheric circulation under warming conditions, providing a different narrative for energy dynamics in a warming world.

Theoretical and Practical Implications

This paper underlines the utility of simplified models in furthering our understanding of complex climatic interactions. It provides a straightforward framework that connects traditional greenhouse effect concepts with modern data, potentially aiding in improved communication and comprehension of anthropogenic climate effects among scientists and decision-makers. The findings also support the projection of increased hydrological activity as a response to enhanced greenhouse forcing, impacting rainfall patterns and climate extremes.

Future Directions

Future research should explore the interaction between greenhouse gas concentrations and cloud dynamics more thoroughly for a nuanced understanding of atmospheric energy flows. Additionally, reconciling the disparities between model predictions and real-world data concerning atmospheric overturning and circulation patterns can advance the predictive capability of climate models and contribute to more accurate climate forecasts. Such endeavors would benefit from leveraging both simplified models and complex global climate models (GCMs) to integrate diverse scales of climate dynamics into coherent global projections.