What Caused the mid-1970s Climate Anomaly? Evaluating the Role of Humans Versus Nature


Observations indicate there was a significant shift in the mid-1970s from cooler to warmer tropical Pacific sea surface temperatures (SSTs). The cause of these basin-wide SST anomalies with global impacts has been a topic of scientific debate. In a recent paper, DOE-funded scientists at the National Center for Atmospheric Research in Boulder, Colorado, and the Program for Climate Model Diagnosis and Intercomparison at Lawrence Livermore National Laboratory analyzed observations and climate model simulations to investigate the 1970s climate anomaly. Results show the 1970s climate shift in the Pacific SST was a combination of warming due to human-produced greenhouse gases superimposed on what was likely a natural decadal fluctuation of the Pacific climate system. Determining the relative roles of human influence versus naturally occurring internal variability is important for understanding climate fluctuations that have already been observed. This knowledge is also important for the new field of decadal climate prediction with the challenge of estimating regional climate anomalies over the next several decades with contributions from both inherent climate variability and external forcing from human activities.


Meehl, G. A., A. Hu, and B.D. Santer, 2009:  The mid-1970s climate shift in the Pacific and the relative roles of forced versus inherent decadal variability, J. Climate, 22, 780–792.