Background
Nearly a quarter-century ago, Idso (1988) published a brief paper in which he concluded that "a comparative analysis of long-term (several-hundred-year) temperature and carbon dioxide (CO2) trends suggests that the global warming of the past century is not due to the widely accepted CO2 greenhouse effect but rather to the natural recovery of the earth from the global chill of the Little Ice Age, which was both initiated and ended by some unrelated phenomenon."

What was done
Akasofu addressed the same subject in a similar manner, but with the benefit of nearly a quarter-century of additional temperature and CO2 data, plus a greater variety of other pertinent data, employing "openly available data on sea level changes, glacier retreat, freezing/break-up dates of rivers, sea ice retreat, tree-ring observations, ice cores and changes of the cosmic-ray intensity, from the year 1000 to the present."

What was learned
The founding director of the International Arctic Research Center of the University of Alaska Fairbanks (USA) was able to demonstrate that earth's recovery from the Little Ice Age (LIA) "has proceeded continuously, roughly in a linear manner, from 1800-1850 to the present," with the rate of recovery being about 0.5°C/century. Thus, he suggests that the earth is "still in the process of recovery from the LIA," which is being brought about by whatever was responsible for the mean linear warming of the 20th century, as modulated by a "multi-decadal oscillation of a period of 50 to 60 years" that is superimposed upon it and which "peaked in 1940 and 2000, causing the halting of warming temporarily after 2000."

What it means
Extending these two phenomena into the future, Akasofu predicts the temperature increase over the 21st century to be 0.5°C ± 0.2°C, rather than the much greater 4°C ± 2°C that is predicted by the Intergovernmental Panel on Climate Change.

Reference
Idso, S.B. 1988. Greenhouse warming or Little Ice Age demise: A critical problem for climatology. Theoretical and Applied Climatology 39: 54-56.