02/25/2015

Observational Determination of Surface Radiative Forcing by Atmospheric CO2

Summary

Scientists have observed carbon dioxide’s (CO₂) greenhouse effect at Earth’s surface for the first time. Although the influence of atmospheric CO₂ on the planet’s energy balance is well established, this effect had not been experimentally confirmed outside the laboratory until now. The researchers, led by the U.S. Department of Energy’s (DOE) Lawrence Berkeley National Laboratory, measured atmospheric CO₂’s increasing capacity to absorb thermal radiation emitted from Earth’s surface over an 11-year period at two locations in North America. They used precise spectroscopic instruments operated by DOE’s Atmospheric Radiation Measurement (ARM) program at the ARM research sites in Oklahoma and Alaska. The instruments measure thermal infrared energy that travels down through the atmosphere to the surface and can detect the unique spectral signature of infrared energy from CO₂. Other instruments detect the unique signatures of phenomena that also can emit infrared energy such as clouds or vapor. The combination of measurements enabled the scientists to isolate the signals solely attributed to CO₂. They found that CO₂ was responsible for a significant uptick in radiative forcing at both locations, about 0.2 Watts/m². They linked this trend to the 22 ppm increase in atmospheric CO₂ between 2000 and 2010. The measurements also enabled the scientists to detect, for the first time, the influence of photosynthesis on the balance of energy at the surface. They found that CO₂-attributed radiative forcing dipped in the spring as flourishing photosynthetic activity pulled more of the greenhouse gas from the air. Their results agree with theoretical predictions of the greenhouse effect due to human activity. The research also provides further confirmation that the calculations used in today’s climate models are on track when it comes to representing the impact of CO₂.

References

Feldman, D. R., W. D. Collins, P. J. Gero, M. S. Torn, E. J. Mlawer, and T. R. Shippert. 2015. “Observational Determination of Surface Radiative Forcing by CO₂ from 2000 to 2010,” Nature 519, 339-43. DOI: 10.1038/nature14240.