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.

Climate change is the general increase or decrease observed over the last 50 years or more over the whole planet. Pointing at a particular region of cooling or warming as evidence for global cooling or warming is bad science.

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. Visible light from the Sun passes through these gases, but they absorb infrared light re-radiated by the Earth and thus act like a 'blanket'. These greenhouse gases (the important ones being water vapour, carbon dioxide, methane and a few others) thus insulate the Earth's surface which would otherwise 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.

The uncertainties in the models and measurements are simply not large enough to dismiss the fact that man is effecting a fundamental balance 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 is formed to examine climate research. The sheer numbers of scientists involved (many thousands) means it is very difficult to not get a consensus view. With governments also involved the document can be considered to be pretty conservative statement of the current scientific knowledge. In any document that size you might expect some errors and a number of these 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. Personally I am therfore confident in the following statement from IPCC:

'Greenhouse gas forcing has very likely caused most of the observed global warming over the last 50 years.'

Their summary for policy makers is available at http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-spm.pdf