Publication reports on methods for adjusting global temperature trends for unforced variability

The potential existence of internal oscillations in the Earth’s climate system may complicate attempts to estimate temperature trends and climate sensitivity. For example, multidecadal oscillations are not well captured by General Circulation Models and are not causally understood. As a result, it is not easy to subtract these oscillations from the anthropogenic component of climate trends. Multidecadal climate oscillations, such as the Atlantic Multidecadal Oscillation (AMO), can occur due to feedbacks in the climate system (e.g., persistent ocean warm pools, salinity changes) that tend to maintain systems of winds and ocean currents.

While others have used the El Niño-Southern Oscillation as an index of internal oscillations, Dr. Craig Loehle of NCASI recently proposed using the Atlantic Multidecadal Oscillation (AMO) as a direct index and evaluated the validity of this supposition.

Dr. Loehle found a global temperature data set adjusted with an AMO-based index was highly correlated with the anthropogenic forcing history from the Intergovernmental Panel on Climate Change’s Fifth Assessment Report. He reports that, although the AMO is likely not unique in capturing internal variability, this analysis opens a line of enquiry to potentially reducing the uncertainty due to internal variability.

The publication presenting Dr. Loehle’s methods and results recently appeared in the Universal Journal of Geoscience. The abstract follows.

“Multidecadal climate variability has proven difficult to deal with when estimating temperature trends. This possible unforced internal oscillation of the climate system provides an opportunity to correct temperature trends. The Atlantic Multidecadal Oscillation (AMO) is proposed as a potential index for this unforced variability. The AMO pattern does not appear to correspond to forcing histories used by the IPCC. Subtracting a scaled version of the AMO from the Hadley global temperature data produced damped decadal-scale fluctuations in the temperature data. The adjusted dataset is highly correlated with the anthropogenic forcing history from IPCC AR5. The linear post-1970 temperature trend is 0.83°C/century vs. 1.63°C/century for the raw data. Thus almost exactly half of the post-1970 warming is possibly natural. The use of the AMO as an index of unforced variability is supported by the fact that subtracting it simplifies the temperature response by damping the peaks and troughs consistently.”

  

Contact information  

 

Reference 

Loehle, C. 2015. Global temperature trends adjusted for unforced variability. Universal Journal of Geoscience 3(6):183-187. http://dx.doi.org/10.13189/ujg.2015.030601