When "scientific consensus" is referred to in relation to climate change, it's referring to the following points:
•The earth is getting warmer
•The warming is mostly due to human activity
•If greenhouse gas (GHG) emissions continue, the warming will accelerate
Numerous survey studies have been done, and the results are overwhelmingly in favor of scientific consensus that the earth is warming and human activity is the cause. Surveys done by reputable organizations find that around 97% of climate scientists agree with the statements above. The following are results from a few of these surveys, plus a resolution from a very distinguished group of scientists.
Expert Credibility in Climate Change (NAS)
This study compiled a list of 1,372 climate scientists, and then looked at those who are "actively publishing" in the science of climate. They categorized the scientists as either "convinced" or "unconvinced" by the evidence. The results were that 97% of actively publishing climate scientists are convinced by the evidence of anthropogenic climate change. They also found that those scientists that were unconvinced had significantly fewer publications (in any science) than those that were convinced. This suggests that the (vocal) "unconvinced" group actually has done a lot less research. (Read this study in full.)
Examining the Scientific Consensus on Climate Change (AGU)
This study was done in order to address the broader question of public opinion versus scientific opinion. It asked two questions, one about whether temperature is increasing, and one about whether or not human activity is contributing to any change. Here are the results:
Question #1: When compared with pre-1800s levels, do you think that mean global temperatures have generally risen, fallen, or remained relatively constant?
About 90% of all the scientists and 97% of the climate scientists said temperatures had risen.
Question #2: Do you think human activity is a significant contributing factor in changing mean global temperatures?
About 82% of all the scientists and 97% climate scientists agreed that human activity is a significant contributing factor.