Syun-Ichi Akasofu of the International Arctic Research Center takes issue with IPCC Report.

There seems to be a roughly linear increase of the temperature from about 1800, or even much earlier, to the present. This warming trend is likely to be a natural change; a rapid increase of CO2 began in about 1940. This trend should be subtracted from the temperature data during the last 100 years. Thus, there is a possibility that only a fraction of the present warming trend may be attributed to the greenhouse effect resulting from human activities. This conclusion is contrary to the IPCC (2007) Report, which states that
“most” of the present warming is due to the greenhouse effect. One possible cause of the linear increase may be that the Earth is still recovering from the Little Ice Age. It is urgent that natural changes be correctly identified and removed accurately from the presently on-going changes in order to find the contribution of the greenhouse effect.