Very broadly speaking, the UN's International Panel on Climate Change holds the following position:
- Use of fossil fuels adds ominously to the CO2 in the atmosphere, as well as to other 'greenhouse gases'.
- These anthropogenic additions 'force' temperatures further up — to deleterious levels.
- There follow various consequences to (eg) marine life, wildlife, daily weather, agriculture, health, etc., etc.
(One example, from 'Climate Change 2007,Impacts, Adaptation, Vulnerability': "Many millions more people are projected to be flooded every year due to sea-level rise by the 2080s.Those densely-populated and low-lying areas where adaptive capacity is relatively low, and which already face other challenges such as tropical storms or local coastal subsidence, are especially at risk.The numbers affected will be largest in the mega-deltas of Asia and Africa while small islands are especially vulnerable.")
Politicians & their advisors then draw the obvious conclusions: Reduce the use of fossil fuels now.
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To reach their position, the IPCC rely on work using extremely complicated global climate models, that need supercomputers to run them.
Furthermore, to arrive at the IPCC result:-
(A) Besides meteorology — itself a multidisciplinary study, a variety of related disciplines have to be drawn upon. E.g.:- atmospheric physics; palaeoclimate studies — from/through geology; the study of ice-cores (trapped CO2); & of tree rings (their widths are held to indicate annual temperatures, inter alia); studies of 'natural' sources of CO2 in the atmosphere — this last involves (inter alia) oceanography: CO2 is both absorbed & released by, deep/surface ocean currents; etc., etc. Solar radiation varies over time, so its effect must also be studied: what impact it might (or might not) have on climate & hence temperatures (or on temperatures directly), in the medium- to long-term. — And all this is only a lay inquirer's list; a knowledgeable scientist would come up with a more accurate & far longer list.
(B) The conclusions of all these disciplines/studies have all to point towards (1) & (2) of the IPCC position above. E.g., oceanographic studies must (on balance) at least be consistent with the proposition that human activities are far more significant in raising the levels of CO2 in the atmosphere; and so on.
The IPCC go through a fairly complicated process of commissioning surveys of findings, that are then vetted by 'IPCC reviewers', written up by 'Lead Authors' & others, re-written, etc., etc. For public consumption, they cut the Gordian knot by referring to the number of scientists involved: '3000' or whatever. It is only after some inquiry that the layperson finds out the complexities above.
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The real issues are (1) & (2) above:- Not just a 'rise in CO2' or 'global warming' tout court, but an ominous rise in CO2 (& other'greenhouse gases'), & a deleterious rise in temperature throughout the globe. This is where the differences lie between those scientists who subscribe to the IPCC 'consensus', & those scientists who are outside this 'consensus'. Some of the issues involved: distinguishing 'natural' from 'anthropogenic' changes in CO2; the extent/causes of 'natural' changes; the processes that lead from anthropogenic/other increases in CO2 (& other 'greenhouse gases') to a deleterious rise in temperatures; the effects of other human activities (eg, irrigation, expansion/decrease of area under crops, grass, forest); etc., etc.
In sum: 'Rising CO2' or 'global warming' do not summarise the IPCC position. It is this last which underlies the politics.
