In the second to last paragraph the authors write (emphasis my own):
Our results indicate that global mean temperature for the decade 2000–2009 (34) has not yet exceeded the warmest temperatures of the early Holocene (5000 to 10,000 yr B.P.). These temperatures are, however, warmer than 82% of the Holocene distribution as represented by the Standard5×5 stack, or 72% after making plausible corrections for inherent smoothing of the high frequencies in the stack (6) (Fig. 3). In contrast, the decadal mean global temperature of the early 20th century (1900–1909) was cooler than >95% of the Holocene distribution under both the Standard5×5 and high-frequency corrected scenarios. Global temperature, therefore, has risen from near the coldest to the warmest levels of the Holocene within the past century, reversing the long-term cooling trend that began ~5000 yr B.P. Climate models project that temperatures are likely to exceed the full distribution of Holocene warmth by 2100 for all versions of the temperature stack (35) (Fig. 3), regardless of the greenhouse gas emission scenario considered (excluding the year 2000 constant composition scenario, which has already been exceeded). By 2100, global average temperatures will probably be 5 to 12 standard deviations above the Holocene temperature mean for the A1B scenario (35) based on our Standard5×5 plus high-frequency addition stack (Fig. 3).My intuition is that most people outside of climatology circles don't realize how strong of a statement this paragraph makes. We know from ice core reconstructions of temperature that we should currently be in the middle of an interglacial period and a long cool trend, and a wealth of evidence supports this understanding (e.g. the early 20th century comment above). That current temperatures are warmer than ~70% of Holocene temperatures is certainly concerning in its own right, but that still only references different levels of temperature. When we add in this understanding of the change in trend, it implies that compared to our counterfactual non-anthropogenic climate we are changing things very rapidly and strongly against trend. In other words, when we think of something like the canonical Mann hockey stick graph we should have in our minds that the handle of the stick was inclined down until we hit our recent inflection point. In other words, our mental model should look something like this:
I'm a big fan of referencing history when I teach economics, because I find the sense of perspective helps students understand the universality and importance of social and economic problems today (or lack thereof). I'm beginning to feel similarly about paleoclimatology's role in explaining climate change to nonspecialists.