Unsettled: What Climate Science Tells Us, What It Doesn’t, and Why It Matters

by Steven E. Koonin

The failure of even the latest models to warm rapidly enough in the early twentieth century suggests that it’s possible, even likely, that internal variability—the natural ebbs and flows of the climate system—has contributed significantly to the warming of recent decades.20 That the models can’t reproduce the past is a big red flag—it erodes confidence in their projections of future climates. In particular, it greatly complicates sorting out the relative roles of natural variability and human influences in the warming that has occurred since 1980. FIGURE 4.5 Global mean surface temperature anomalies from twenty-six CMIP6 models. Individual model runs are shown as light gray traces, while the dark lines are three different observational data sets. Anomalies are relative to an 1880–1910 baseline, and the curves are smoothed over eleven-year intervals.19