Question 22:

What are the key climate change science findings in the IPCC Fourth Assessment Report?

In early 2007 Working Group 1 of the IPCC released its contribution to the fourth assessment report. The report, Climate Change 2007: The Physical Science Basis, found:

There is now no question that the climate system has warmed.
It is very likely that greenhouse gas emissions related to human activity caused most of the warming that has been observed since the mid-20th century. In their third assessment report in 2001, the IPCC had only considered it likely.
Global climate change over the past 50 years is extremely unlikely to have been caused by natural variability alone.
If greenhouse gas emissions continue at or above the current rate there will be further warming, and the changes that we will see during the 21st century would very likely be larger than those observed in the 20th century.
Temperatures are projected to increase from 1.1oC-6.4oC by the end of the 21st century, depending on future greenhouse gas emissions.
Global warming is expected to be greatest over land masses and in the Arctic, and least over the Southern Ocean and North Atlantic. As a result, snow cover is projected to contract, and sea ice is expected to shrink in both the Arctic and Antarctic. Late summer sea ice in the Arctic will disappear almost entirely by the latter part of the 21st century.
Australia is expected to warm at a similar rate to the global average, with higher levels of warming expected in the north of the country.
Global sea level rose during the 20th century by 12-22 cm. By the end of the 21st century, sea level is projected to rise by 18-59 cm but this does not include possible changes in big ice sheets in Greenland and Antarctica that could lead to more rapid sea level rise.
The increases in greenhouse gases since 1750 are due primarily to emissions from fossil fuel use, agriculture, and land-use changes.
Ice cores spanning the last 650,000 years indicate that current atmospheric concentrations of carbon dioxide and methane far exceed pre-industrial values.
Since the 1970s droughts have been more intense and longer, and have been observed over wider areas, particularly in the tropics and subtropics. Higher temperatures and lower rainfall have contributed to these changes.
Extreme climate events such as heatwaves and heavy rainfall are very likely to become more frequent.
Although the number of tropical cyclones is expected to decrease, they are projected to be more intense, with larger peak wind speeds and more intense rainfall.
Storm tracks are projected to move towards the poles. Rain-bearing storms over south-west Australia have already moved off the continent and reduced the once-reliable rainfall supply to south-west Western Australia.

The full report and summary for policymakers are available from the IPCC website at www.ipcc.ch/