A Joint Role for Forced and Internally-Driven Variability in the Decadal Modulation of Global Warming
It is widely accepted that the observed long-term warming trend in global mean surface temperature is primarily driven by human-induced increases in atmospheric greenhouse gas concentrations. However, less is known about the decadal fluctuations that are superimposed on this trend. While global warming is largely a forced response to human activities, an external factor, decadal fluctuations in global and regional climates are often thought to be the result of dynamics internal to the climate system. We leveraged a unique dataset of large initial-condition ensembles with Earth System Models to effectively isolate the forced component from the internally-generated component
Understanding the relative role of anthropogenic and internally-driven variability on decadal fluctuations of global temperatures is critical for putting historical climate change in context as well as constraining future projections. These results also have profound implications for decadal climate predictions as the unpredictable nature of future volcanic eruptions suggests that a large portion of decadal-scale variability in global mean temperature might not be predictable.
Using the US Climate Variability and Predictability program’s newly developed data archive of large initial condition Earth System Model ensembles, this study presents evidence that a large portion (~29-53%) of the simulated decadal-scale variance in individual time series of global mean sea surface temperature anomaly over 1950-2010 is externally forced. To further explore which forcing drives the decadal variability, we use the CESM1 single forcing ensembles finding it is largely linked to the representation of volcanic aerosols. Comparison with the future (2010-2070) period suggests that external forcing provides a source of additional decadal-scale variability in the historical period.