Statement regarding “Long-term natural variability and 20th century climate change” by Kyle Swanson, George Sugihara, and Anastasios Tsonis

During the 20th century, the climate varied not only due to forced factors (increasing greenhouse gases, anthropogenic aerosols, volcanism, and changes in solar output), but also due to natural variability within the climate system, primarily resulting from changes in oceanic circulation. This paper seeks to understand and highlight the significance of this natural variability. The premise is relatively straightforward; basically we ask how the global mean temperature would behave in an unforced climate model simulation if that simulation exactly reproduced the observed 20th century time evolution of the gradients in sea surface temperature (SST), i.e., the spatial distribution of SST relative to it global mean value. This work shows that certain important aspects of the 20th century temperature evolution, namely the enhanced warming from roughly 1915-1940, the cooling from 1945-1975, and a portion of the post 1980 warming, are consistent with the signature of natural variability in the climate system. Removing this natural variability-associated global mean temperature signal leaves a consistently increasing global mean temperature signal over the 20th century, with the rate of warming accelerating from roughly .05°C/decade early in the 20th century to roughly .12°C/decade at the end of the 20th century.

This work indicates a significant role for natural variability in the climate system on decadal-to-century time scales, marked by relatively large amplitude (±0.15°C) swings about a consistently increasing global mean temperature. However, the rate of temperature change of the residual signal at the end of the 20th century remains consistent with the range of early 21st century temperature change reported by the IPCC, albeit at the lower end of that range.