According to the IPCC, “climate change” is defined as,
“[A]ny change in climate over time whether due to natural variability or as a result of human activity.”
However, according to the FCCC, “climate change” is something much more specific. It is defined as:
“[A] change of climate which is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and which is in addition to natural climate variability over comparable time periods.”
The FCCC treats “climate change” as being — by definition — human-caused (and mainly due to greenhouse gases). But, according to the IPCC, climate change is simply “any change in the climate” — it could be human-caused (e.g., greenhouse gases, urban development, land-use change, etc.) or it could be natural (e.g., due to solar variability, changes in oceanic or atmospheric circulation patterns, etc.), or perhaps some combination of them all.
The problems started to occur when the IPCC’s 1st Assessment Report (AR1) was published in 1990. The report provided plenty of evidence that the climate was changing, but it also explained that the climate had always been changing.
The IPCC was unable to say whether the recent climate change was human-caused, natural or both. This became known as the “attribution” problem, and the IPCC’s first attempt at it was very inconvenient for the FCCC.
This fundamental conflict between the FCCC and the IPCC put a lot of pressure on the scientific community.