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What is certain when it comes to climate change is that the average world temperature has increased by approximately one degree Celsius since 1970, and that sea levels have also shown a measurable rise over the same period. any of the effects of this trend can also be linked to global warming with certainty. Though the evidence in the summary is compelling, the causal relationship between human activity and climate change is still not entirely certain, nor are many of the projected effects of the current climate change or indeed the projections concerning the future of climate change. Though even the summary refrains from language that claims its conclusions as certain, the information it provides does contend that humans are most likely the source of climate change. Natural global temperature variation is also cited as a possible and even a likely contributor, though its effect is more powerful in skewing overall results.

The likelihood that climate change is related to human activity is not given a precise number in the summary; it is noted that eighty-nine percent of seventy-five different studies' more than twenty-nine thousand observational data series support global warming as a cause for observed effects, but this figure does not link the climate change to human activity taken on its own. The authors of the summary state that the connection can be stated with "high confidence," however, meaning that they approximate the likelihood as an eight-out-of-ten chance (80%). In order to come to this conclusion, the summary largely relied on data syntheses that seem to have predicted effects of anthropogenic warming it then confirmed without necessarily establishing a causal link. Modeling studies that actually separated warming causes are perhaps the most conclusive pieces of evidence supporting this conclusion.

There some rather obvious and large assumption and interpretations made by the authors of this summary in some of their conclusions, namely the anthropogenic cause of climate change. Basically, the studies that the summary used to come to this conclusion laid out a series of effects that they expected to se form warming caused by humans, and then observed those effects. The number of studies that approached this topic from that perspective seems to be the persuasive factor in this study; that is, it is primarily the consensus among scientists rather than the actual data they have collected that suggests an anthropogenic cause of climate change. There have still been no observational studies -- or at least none are referenced in this article -- that can show a direct causal link between human aerosol and carbon emissions and global warming.