AER Contributes to Landmark Study on Earth’s Greenhouse Effect

Eli Mlawer
February 25, 2015

Scientists who research thermal radiation in the Earth’s atmosphere are confident that we have a very detailed and accurate understanding of the nature of the greenhouse effect of carbon dioxide (CO2), including the impact of rising CO2 levels. 

Recently I participated in a study that provided the first direct observation at the surface of the Earth’s increased greenhouse effect due to rising CO2 levels. This represents an important milestone in the detection of the increased greenhouse effect due to fossil fuel emissions.

Another key result of our study was that the measured trend agreed with theoretical calculations of AER’s highly accurate Line-By-Line Radiative Transfer Model (LBLRTM).

The study “Observational Determination of Surface Radiative Forcing by CO2 from 2000 to 2010",  published this week in the journal Nature, was led by Dr. Daniel Feldman of the US Department of Energy’s Lawrence Berkeley National Laboratory. (Please see press release "First Direct Observation of Carbon Dioxide’s Increasing Greenhouse Effect at the Earth’s Surface" summary of this research from Berkeley Lab.)

Thermal radiation exiting from the top of our atmosphere cools the planet, balancing out the radiant energy received from the sun. Rising CO2 levels lower this outgoing thermal energy (i.e. increasing the greenhouse effect), thereby affecting our planet’s energy balance and causing an overall rise in global temperatures. This same greenhouse effect physics also causes an increase in thermal radiation at the surface when CO2 rises.

In our study, an analysis of thermal radiation measurements at two surface locations, one in Oklahoma and one on the North Slope of Alaska, determined that increases in CO2 between 2000 and 2010 led to a rise in the observed thermal radiation, as expected. As each gas in the atmosphere has a particular “fingerprint” with regards to its absorption and emission of different wavelengths of thermal radiation, we were able to distinguish the impact of changes in CO2 from other effects. 

The accuracy of AER’s LBLRTM was critical in this analysis.  The foundation of the LBLRTM’s accuracy are laboratory measurements and theoretical calculations by scientists in the field of molecular spectroscopy, as well as extensive comparisons with spectral radiation measurements, such as in the recent study led by Dr. Matthew Alvarado of AER.  Many improvements to LBLRTM have been made as a result of such studies, and the model is widely utilized and relied on by atmospheric scientists.  Our fast radiation code RRTMG, which is incorporated in many global models used for climate prediction, is also based on LBLRTM. 

Our Nature article provides further confirmation that today’s climate models correctly represent the impact of CO2 on Earth’s radiation balance.  Although the impacts of rising greenhouse gases on Earth’s climate are very complex, it is important to understand that the driving force behind climate change is the simple concept of energy balance.  This study confirms that we have an excellent understanding of the direct impact of rising greenhouse gases like CO2 on Earth’s radiation energy.