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Table SPM-2. (WGI) Recent trends, assessment of human influence on the trend, and projections for extreme weather events for which there is an observed late 20th century trend.
Phenomenon a and direction of trend Likelihood that trend occurred in late 20th century (typically post 1960) Likelihood of a human contribution to observed trend b Likelihood of future
trends based on
projections for 21st
century using SRES
scenarios
Table notes:
a See Table 3.7 for further details regarding definitions.
b See Table TS-4, Box TS.3.4 and Table 9.4.
c Decreased frequency of cold days and nights (coldest 10%).
d Warming of the most extreme days and nights each year.
e Increased frequency of hot days and nights (hottest 10%).
f Magnitude of anthropogenic contributions not assessed. Attribution for these phenomena based on expert judgement rather
than formal attribution studies.
g Extreme high sea level depends on average sea level and on regional weather systems. It is defined here as the highest 1%
of hourly values of observed sea level at a station for a given reference period.
h Changes in observed extreme high sea level closely follow the changes in average sea level {5.5.2.6}. It is very likely that
anthropogenic activity contributed to a rise in average sea level. {9.5.2}
i In all scenarios, the projected global average sea level at 2100 is higher than in the reference period {10.6}. The effect of
changes in regional weather systems on sea level extremes has not been assessed.
Warmer and fewer cold days and nights over most land Very likely c Likely d Virtually certain d
Warmer and more frequent hot days and nights over most land areas Very likely e Likely (nights) d Virtually certain d
Warm spells / heat waves. Frequency increases over most land areas Likely More likely than not f Very likely
Heavy precipitation events. Frequency (or proportion of total rainfall from heavy falls) increases over most areas Likely More likely than not f Very likely
Area affected by droughts increases Likely in many regions since 1970s More likely than not Likely
Intense tropical cyclone activity increases Likely in some regions since 1970 More likely than not f Likely
Increased incidence of extreme high sea level (excludes tsunamis) g Likely More likely than not f, h Likely i

Source: IPCC Climate Change 2007:  The Physical Science Basis, Summary for Policymakers (2007), p9