by Judith Curry
Of the four IPCC assessment reports, I think the first assessment report (FAR) presents the case with the greatest clarity.
Since the FAR was published 20 years ago, it is worth taking a look to see how their conclusions and levels of confidence and uncertainty have stood up to the test of time.
The FAR Summary for Policy Makers makes the following statements:
We are certain of the following:
- there is a natural greenhouse effect which already keeps the Earth wanner than it would otherwise be
- emissions resulting from human activities are substantially increasing the atmospheric concentrations of the greenhouse gases carbon dioxide, methane, chlorofluorocarbons (CFCs) and nitrous oxide These increases will enhance the greenhouse effect, resulting on average in an additional warming of the Earth’s surface. The main greenhouse gas, water vapour, will increase in response to global warming and further enhance it
We calculate with confidence that:
- some gases are potentially more effective than others at changing climate, and their relative effectiveness can be estimated. Carbon dioxide has been responsible for over half the enhanced greenhouse effect in the past, and is likely to remain so in the future
- atmospheric concentrations of the long-lived gases (carbon dioxide, nitrous oxide and the CFCs) adjust only slowly to changes in emissions. Continued emissions of these gases at present rates would commit us to increased concentrations lor centuries ahead The longer emissions continue to increase at present day rates, the greater reductions would have to be for concentrations to stabilise at a given level.
- the long-lived gases would require immediate reductions in emissions from human activities of over 60% to stabilise their concentrations at today’s levels, methane would require a 15-20% reduction
Our judgement is that:
- Global – mean surface air temperature has increased by 0.3°C to 0.6°C over the last 100 years, with the five global-average warmest years being in the 1980s. Over the same period global sea level has increased by 10-20cm. These increases have not been smooth with time, nor uniform over the globe
- The size of this warming is broadly consistent with predictions of climate models, but it is also of the same magnitude as natural climate variability Thus the observed increase could be largely due to this natural variability, alternatively this variability and other human factors could have offset a still larger human-induced greenhouse warming The unequivocal detection of the enhanced greenhouse effect from observations is not likely for a decade or more
- There is no firm evidence that climate has become more variable over the last few decades However, with an increase in the mean temperature, episodes of high temperatures will most likely become more frequent in the future, and cold episodes less frequent
I have left off various climate model projections and a few other things that aren’t directly related to detection and attribution.
How have these statements held up over time? Have there been any serious challenges to these statements? Is the known ignorance associated with these statements substantial? Are these the statements that pretty much everyone can agree with?
Is this a better way of describing the state of our understanding, relative to the confidence and likelihood statements in the TAR, AR3?
Lets see if we can identify the common ground of agreement on the scientific basics.