There is a great new article here: http://www.nature.com/news/2010/100120/full/463284a.html regarding uncertainties in climate science.
What is important to take away from this is that much of the uncertainty is on the bad side - in other words, it is more likely than not that our current knowledge of the science underestimates the negative effects of climate change.
As an example, the article talks about precipitation. In the 2007 IPCC report, there are some attempts at estimating the effects of climate change on precipitation in Australia. The table is at: http://www.ipcc.ch/publications_and_data/ar4/wg2/en/ch11s11-3.html#11-3-1. What is says is that rainfall in south and west Australia is predicted to decline by between 0 and 15 per cent by 2020.
Unfortunately, as the Nature article suggests for other regions, the rainfall predictions are already proving somewhat optimistic.
First, it should be pointed out that we obviously only have three years of data to examine since the IPCC report came out - 2007, 2008 and 2009 - and the statistical value of three years is very small indeed. However, the results may be preliminary indicators that the IPCC has been optimistic.
In the south-east of Australia, the rainfall for the last three years was 12 per cent lower than the average for all the years preceeding 2007. Further, it remains on a trend line indicating that rainfall is declining at the rate of 1.5 per cent of the long-term average per year. By 2020, this predicts that rainfall in south-eastern Australia will be 27 per cent below the long-term average, and 15 per cent lower than the long-term average than it is today. So, even if the IPCC figure is measured as a percentage decline from today, the evidence is that the very top of the range for the south of Australia is likely to be hit or exceeded.