Background
The authors write that "analytic climate models have provided the means to predict potential impacts on future climate by anthropogenic changes in atmospheric composition." However, they indicate that "future climate development will not only be influenced by anthropogenic changes, but also by natural variations." And they say that knowledge of these variations is incomplete.

What was done
Humlum et al. developed a new technique for identifying the character of natural climatic variations by means of Fourier and wavelet analyses that decompose climate series into time-frequency space, in order to extract information on periodic signals that may be embedded in the series and to determine their amplitudes and variations over time, after which they employed this technique to analyze two climate series: the Svalbard (78°N) surface air temperature series of 1912-2010, and the last 4000 years of the reconstructed GISP2 surface temperature series from central Greenland.

What was learned
The four Norwegian researchers demonstrated that "the present warm period following the Little Ice Age [LIA] since about AD 1800 can be reproduced by a simple three input period only approach, based on the Greenland GISP2 temperature record." And they say that the application of this technique suggests that "the present period of warming since the LIA to a high degree may be the result of natural climatic variations, known to characterize at least the previous 4000 years."

What it means
In terms of what comes next, Humlum et al. note that "natural cycles that have remained strong over several decades or centuries are likely to continue without major changes into at least the near future, and will therefore be essential for forecasting any future climatic development." And when the results of their findings are projected into the future, they say that such forecasting "suggests that the present post-LIA warm period is likely to continue for most of the 21st century, before the overall Late Holocene cooling may again dominate," leaving a little wiggle room near the end, however, for superimposed potential effects of anthropogenic greenhouse gases.