Present climate and biological change - Climate Change: Biological and Human Aspects - page 230

forecasts and a high-low range for the same. Yet they also wisely included in their
graphs a higher-lower estimate due to uncertainties in land-ice changes, permafrost
and sediment changes. Importantly, even with this extension of range, they did not
allow for uncertainty relating to the stability of the West Antarctic Ice Sheet (WAIS).
This point is often overlooked. This is because the IPCC's 2001 summary for policy-
makers and the technical summary, published together in Climate Change 2001: The
Scientific Basis (IPCC, 2001b) only touched briefly upon this uncertainty, even if this
brief mention is perfectly clear. The WAIS 'dynamics are still inadequately under-
stood', and a part of a lengthy IPCC graph caption states that its 'range does not allow
for uncertainty relating to ice dynamic changes in the WAIS' (IPCC, 2001b). These
uncertainties and caveats are worth noting for just half a decade later we are seeing
unexpected degrees of melting in both Antarctica and Greenland. These suggest that
the IPCC's sea-level rise estimates in 2001 may have been too. In 2007 the IPCC did
indeed raise their lower estimate for overall sea-level rise, but also lowered their high
estimate.
The IPCC's 1990, 2001 and 2007 conclusions for the anticipated change forecast
between 1990 and 2100 are summarised in Table 5.1. We will return to sea-level rise
shortly and then again when discussing possible 'surprises' in the next chapter.
The IPCC 1990 report summarised that it was 'certain' that the forecast emissions
would result 'on average in both an additional warming of the Earth's surface' and
sea-level rise. The 1992 IPCC supplement concluded that the subsequent scientific
understanding 'either confirm[ed] or [did] not justify alteration of the major conclu-
sions of the first IPCC Scientific Assessment'. The IPCC's 1995 second assessment
was if anything more cautious in its conclusion, in part due to the fierce political
debate surrounding the first assessment's conclusions. In 1995 the IPCC concluded
that 'the balance of evidence suggests a discernable human influence on global cli-
mate'. By 2001, the IPCC had regained some confidence and concluded that 'an
increasing body of observations gives a collective picture of a warming world and
other changes in the climate system'. Further, that 'there is now new and stronger
evidence that most of the warming observed over the past 50 years is attributable to
human activities'. Then in 2007 the IPCC said that 'the understanding of anthropo-
genic warming and cooling influence on climate has improved since the TAR [2001
third assessment report], leading to very high confidence that the global average net
effect of human activities since 1750 has been one of warming'. Here the italics is
the IPCC's own emphasis and 'very high confidence' in IPCC parlance has the very
specific meaning of being a probability of 9 out of 10.
The 2007 report also said that that the 'warming of the climate system is unequi-
vocal' and that 'most of the observed increase in global average temperatures since
the mid-20th century is very likely due to the observed increase in anthropogenic
greenhouse gas concentrations'. Again the use of italics is the IPCC's. This was a
development since the IPCC 2001 assessment, which only put it as being 'likely'.
Again here, the use of terms likely and very likely by the IPCC have a specific
meaning: of more than 66 and more than 90% probability, respectively.
The reasons for the IPCC's careful wording and caution (and also note for example
the prudent use of the word 'most') were 4-fold. First, the IPCC were presenting a
'scientific consensus' of a range of views and this range's breadth had to be signalled.