AR4 Working Group I report:
"Climate Change 2007: The Physical Science Basis"
Issued February 2, 2007

Download the full report by the IPCC's Working Group I here:
http://www.ipcc.ch/SPM2feb07.pdf.

Key passages:

A warming, changing climate
"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 mean sea level."

"At continental, regional, and ocean basin scales, numerous long-term changes in climate have been observed. These include changes in Arctic temperatures and ice, widespread changes in precipitation amounts, ocean salinity, wind patterns and aspects of extreme weather including droughts, heavy precipitation, heat waves and the intensity of tropical cyclones."

Human causation
In IPCC terminology, "very likely" means a probability greater than 90 percent and "likely" means a probability greater than 66 percent.

"Most of the observed increase in globally-averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations."

In 2001, the IPCC said, "most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations."

Projections of future changes
"Continued greenhouse gas emissions at or above current rates would cause further warming and induce many changes in the global climate system during the 21st century that would very likely be larger than those observed during the 20th century."

Compared to 1980-1999, "best estimates" for "projected globally averaged surface warming" by 2090-2099, range from 3.2 - 7.2 degrees F warmer for six scenarios with varied emissions of greenhouse gases. In 2001, the projected range was 2.5 - 10.4 degrees F warmer. "The new assessment of the likely ranges now relies on a larger number of climate models of increasingly complexity and realism, as well as new information regarding the nature of feedbacks from the carbon cycle and constraints on climate response from observations."

The report gives no "best estimates" for projected sea level rise. For six emissions scenarios, it provides six projected ranges, extending from 0.59 feet to 1.94 feet between 1980-1999 and 2090-2099. "The projections include a contribution due to increased ice flow from Greenland and Antarctica at the rates observed for 1993-2003, but these flow rates could increase or decrease in the future." In the 2001 IPCC report, sea level was projected to rise by 0.29 feet to 2.89 feet between 1990 and 2100.

"It is very likely that hot extremes, heat waves, and heavy precipitation events will continue to become more frequent."

"Based on a range of models, it is likely that future tropical cyclones (typhoons and hurricanes) will become more intense, with larger peak wind speeds and more heavy precipitation associated with ongoing increases of tropical SSTs (sea surface temperatures). There is less confidence in projections of a global decrease in numbers of tropical cyclones. The apparent increase in the proportion of very intense storms since 1970 in some regions is much larger than simulated by current models for that period."

Improved scientific understanding since the 2001 IPCC report Climate observations: Since 2001, "progress in understanding how climate is changing in space and in time has been gained through improvements and extensions of numerous datasets and data analyses, broader geographical coverage, better understanding of uncertainties, and a wider variety of measurements."

The human role: "This assessment considers longer and improved records, an expanded range of observations, and improvements in the simulation of many aspects of climate and its variability based on studies since the (2001 IPCC report)."

Future changes: "A major advance of this assessment of climate change projections compared with the (2001 report) is the large number of simulations available from a broader range of models. Taken together with additional information from observations, these provide a quantitative basis for estimating likelihoods for many aspects of future climate change."