In its periodic assessments, the Intergovernmental Panel on Climate Change has expressed growing certainty that global warming is underway and that human activity is a principal cause. The panel’s language has become progressively stronger over time to reflect its growing certainty.
In 1990, the IPCC said that emissions from human activities were “substantially increasing” greenhouse gas concentrations in the atmosphere, which would lead to warming.
By 2013, the panel had concluded that "It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century."
The chart below traces how the IPCC’s conclusions have strengthened over time.
|Year||Assessment||Statement on Human Involvement in Climate Change|
|1990 ||First Assessment|
“…emissions resulting from human activities are substantially increasing atmospheric concentrations of greenhouse gases…
These increases will enhance the greenhouse effect, resulting on average in an additional warming of the Earth’s surface”
“Most of these studies have detected a significant change and show that the observed warming trend is unlikely to be entirely natural in origin…
…the balance of evidence suggests that there is a discernible human influence on global climate.
…the average rate of warming [in projections for the 21st century] would probably be greater than any seen in the last 10,000 years, but the actual annual to decadal changes would include considerable natural variability.”
“There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities.
…the projected rate of warming is much larger than the observed changes during the 20th century and is very likely to be without precedent during at least the last 10,000 years, based on paleoclimate data.”
“Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level.
Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.”
"Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases have increased.
…It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century."