Because of the large natural variability of climate, scientists do not expect temperatures to rise consistently year after year. (giss.nasa.gov)
Some external influences, such as changes in solar radiation and volcanism, occur naturally and contribute to the total natural variability of the climate system. (ipcc.ch)
For instance, the warming that began in the early 20th century (1925 - 1944) is consistent with natural variability of the climate system (including a generalized lack of significant volcanic activity, which has a cooling effect), solar forcing, and initial forcing from greenhouse gases. (niskanencenter.org)