Furthermore, it is instructive to compare the conclusions of this assessment report with previous ones and see how increased evidence has led to increased certainty about both present-day climate change, but also its significance in the historical record. For example, in 2001, the IPCC noted that the last few decades were likely the warmest in the northern hemisphere for 1,000 years; six years later, that had been extended to 1,300 years; in the most recent report, it now stands at 1,400 years.
Additionally, the IPCC's projections of sea-level rise are now higher than they were in 2007, there is an understanding than the Greenland ice sheet is less stable than was presumed six years ago, and projections of the earliest ice-free Arctic Ocean summers have been moved forward from the end of the century to the middle of it.
And the IPCC feels able to make its statements with a greater degree of confidence as time goes on. For example, in 2007, it stated that "Most of the observed increase in global average temperatures since the mid-20th century is very likely (90 percent confidence) due to the observed increase in anthropogenic greenhouse gas concentrations"; six years later, the relevant paragraph reads that, "It is extremely likely (95 percent confidence) more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in greenhouse gas concentrations and other anthropogenic forcings together." (Emphasis added.)