In truth, it’s far easier to find what now looks like excessive caution in IPCC reports. For instance, the 1990 report stated that increases in greenhouse gases were causing global warming but added that, because of natural climate variability, this warming could not be clearly detected in the observed record. As warming has continued at about the rate projected by the reports, each subsequent report has in general shown increasing confidence in its conclusions. Let me give you another example: the 2007 report declined to estimate the possible effect of accelerated melting of ice caps, as it considered no reliable estimates were available at that time.There are two separate issues here. One is the trend over time which could easily be shown to contain more 'alarming' statements. Early reports were about detection and attribution, centred around climate sensitivity. Later reports included extreme events much more prominently. So Sir Houghton, unwittingly perhaps, confimrs that the trend over time has become more alarming indeed.
The more difficult question is how to measure alarmism (or complacency) in one time period. What is the yardstick? Is it at all possible to measure? If the debate is characterised by uncertainty, we only ever know with hindsight that something was 'too alarmist' (or the opposite) -- after we have collected reliable observations.
The recent examples of alarmism in the IPCC AR4 (glaciers, draughts in Africa, disaster losses, etc) were either the result of faulty sources, of faults in the peer review process, or of faults in the editing of the SPM documents (this is my reading of the situation). In all cases the consensus among practitioners seems to be the yardstick for assessing excessive alarmism. At this point in time the practitioners do not agree among themselves how to assess the IPCC record with regard to erroneous statements (see other post about IPCC Open letter and comments). A nice example of social construction of knowledge.
You will not be surprised to see this statement coming from a sociologist. Guess how Sir John ends his article: "We scientists have facts on our sides — we must not be afraid to deploy them."