Background
The authors state that "most of northern China has experienced severe and prolonged dry periods since the late 1990s," and that "in some of the areas, the extreme drought situations were unprecedented in the latter half of the twentieth century," further reporting that "during 1972-1997, there were 20 years when the Yellow River experienced drying-up (zero streamflow) episodes," including "the severe drought of 1997 [that] resulted in a period of 226 days with no streamflow in the Yellow River."

What was done
In an attempt to put these recent disturbing events in a longer historical context, Gou et al. constructed a 593-year history of Yellow River annual streamflow based on tree-ring width chronologies of cypress (Juniperus przewalski) trees growing on six sites of the headwaters region of the Yellow River above the Tangnaihai hydrological station in the northeast Tibetan Plateau.

What was learned
With respect to their findings, the six researchers report that "several severe droughts and low-flow events are recognized in the decades 1920-1930, 1820-1830, 1700-1710, 1590-1600, and 1480-1490." More recently (since the 1980s), however, they report that "Yellow River streamflow has decreased significantly." Nevertheless, they indicate that "it is not yet out of the range of earlier streamflow fluctuations reconstructed for the past several centuries," noting that their reconstruction demonstrated that 1480-1490 was the driest decade of all.

What it means
Many times the overpowering immediacy of current climate catastrophes leads one to think they are truly unprecedented and must indeed be due to CO2-induced global warming, as climate alarmists such as Al Gore and James Hansen regularly declare to be the case. However, when viewed in a multi-century perspective, as in the case described here, they are generally found to not be unprecedented, and to have often been surpassed during cooler periods when there was much less CO2 in the atmosphere than there is today.