Andrew Lim, of Dunedin, asks :-
There seems to be conflicting statements as to whether or not humans are producing climate change. What is the situation?
Adrian McDonald. an atmospheric physicist and climate scientist at the University of Canterbury, responded.
It is a fact that the 'natural' greenhouse effect (this is the effect of the mixture of gases in the atmosphere unaffected by man) keeps the Earth's surface warmer than would otherwise occur. [They transmit the visible light from the Sun but abosrb the infrared light ] reradiated by Earth.] Basically, the greenhouse gases (the important ones being water vapour, carbon dioxide, methane and a few others) insulate the Earth's surface. Without an atmosphere the Earth's surface would be considerably colder. This fundamental idea can be tested using data from other solar system bodies (planets) as well as Earth.
Man-made emissions have increased the abundance of the greenhouse gases above their natural levels and would thus increase the insulating effect. Without some other process occurring these changes would lead to increased surface temperatures. The climate system is obviously not simple and many processes are still poorly understood. However, given the current set of uncertainties, the changes in surface temperature observed seems very likely to be related to changes in the concentration of greenhouse gases based on a large amount of scientific research.
When models of the climate system are run, with both natural and man-made drivers included, the models match the observations very well in general. The match is much poorer when only the natural drivers of change, such as solar variations, are included. While these models are not perfect, the pattern of change they produce matches well with measurements at the global and continental scale also.
There is definitely uncertainty in the models and in the measurements. But these uncertainties are simply not large enough to dismiss the fact that man is effecting a fundamental balance (CO2 is increasing) and without some counterweight (perhaps some unknown or poorly understood process) we can expect changes to the Earth's climate. The fact that we are observing changes suggests the hoped for counterweight does not exist.
The IPCC is a huge organisation which examines other people's research rather than does it's own research. The sheer numbers of scientists involved (many 1000's) in examining the current knowledge and condensing it into 2-3 very thick summary documents (1000 pages or so each) means it is very difficult to not get a consensus view. The fact that governments are also involved in the review of the document as well means that the document is pretty conservative. This goes as far as having definitions for words which identify the percentage for the level of agreement. For example, 'very likely' means 90%. Now in any document that size you might expect some errors and after two years of effort a number of these errors have come to light - though it is only a handful which suggests the general soundness of the documents to me as they have been very critically reviewed by groups outside IPCC. But, I think the following statement from IPCC stands:
'Greenhouse gas forcing has very likely caused most of the observed global warming over the last 50 years. This conclusion takes into account observational and forcing uncertainty, and the possibility that the response to solar forcing could be underestimated by climate models. It is also robust to the use of different climate models, different methods for estimating the responses to external forcing and variations in the analysis technique.'
The IPCC is a very strong organisation and that the key claims from IPCC are very well supported. If you want to gain some knowledge about climate change and understand those 'key' statements then I suggest you read the IPCC summary for Policy makers which is available at http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-spm.pdf
One note on specifics, we all know that weather is variable and the USA might have very cool temperatures one year, but climate is the general increase or decrease observed over the last 50 or 100 years or more over the whole planet. So pointing at a particular region of cooling and indicating this means global warming isn't happening is bad science. Not to be biased, I should also note that it would be equally bad if a scientist pointed at a strong warming over Australia (or some other country) and said this was proof of global warming. Climate is global in nature and the specific effects of climate change are much more difficult to predict and/or understand at smaller scales. Basically, predictions of temperature in Dunedin in 50 years are much more uncertain than predictions of the global average and understanding observations at a particular point is also much more difficult.