Abstract

The climate of the past millennium was marked by substantial decadal variability. Model simulations can be used to attribute this decadal variability to both external forcings and internal variability; where external forcings include anthropogenic drivers of climate change in addition to natural forcings such as solar fluctuations and volcanic eruptions. In this talk I will present results from an analysis of the last millennium which found an important role for volcanic eruptions and changes in greenhouse gas concentrations but found that the effect of solar variations was likely small. This talk will also discuss the recent “hiatus” period, placing it in the context of this long term decadal variability. Finally I will discuss how last millennium climate simulations can be used to calculate an estimate of pre-industrial global temperature. This is important because at the Paris climate conference 2015, nations agreed to holding increases to well below 2 degrees above pre-industrial levels and to pursue efforts to limit temperature increase to 1.5 degrees. Given that in this context the “pre-industrial” baseline was not defined I will show the implications on the likelihoods of temperatures exceeding these two thresholds given different assumptions about pre-industrial temperature levels.