What was done
The authors analyzed "statistical trend tests of hydroclimatological data ["such as discharge and air temperature"] in the presence of long-term persistence (LTP)," in order to determine "what LTP, if present, implies about the significance of observed trends."

What was learned
Cohn and Lins determined that "the presence of LTP in a stochastic process can induce a significant trend result when no trend is present, if an inappropriate trend test is used." In addition, they say that "given the LTP-like patterns we see in longer hydroclimatological records ... such as the periods of multidecadal drought that occurred during the past millennium and our planet's geologic history of ice ages and sea level changes, it might be prudent to assume that hydroclimatological processes could possess LTP." However, they report that "nearly every assessment of trend significance in geophysical variables published during the past few decades has failed to account properly for long-term persistence."

What it means
The two U.S. Geological Survey scientists conclude that their findings "have implications for both science and public policy." With respect to temperature data, for example, they note there is overwhelming evidence that the planet has warmed during the past century. However, as they ask, "could this warming be due to natural dynamics?" Answering their own question, they say that "given what we know about the complexity, long-term persistence, and non-linearity of the climate system, it seems the answer might be yes."

The bottom line with respect to the implications of this subject is that although reported temperature trends may be real, they may also be insignificant, which leads, in the words of Cohn and Lins, to "a worrisome possibility," i.e., that "natural climatic excursions may be much larger than we imagine ... so large, perhaps, that they render insignificant the changes, human-induced or otherwise, observed during the past century."