The scientific
evidence for a human ÒfingerprintÓ on global climate has strengthened over time
ÒThe
balance of evidence suggests a discernible human influence on global climateÓ
ÒThere is
new and stronger evidence that most of the warming observed over the last 50
years is attributable to human activitiesÓ
ÒMost of
the observed increase in globally averaged temperatures since the mid-20thcentury is very
likely* due to the observed increase in anthropogenic greenhouse
gas concentrationsÓ
I havenÕt shown you this, but the
Earth is warming. We know this from detailed analyses of thermometric
temperature records.
The best information we have indicates
that the EarthÕs mean near-surface temperature has increased by around 0.5oC
over the last 100 years.
Analyses of ÒreconstructedÓ
temperature data from tree rings, ice cores, corals and boreholes indicate
that the Earth is now warmer than at any time since at least 1400 A.D.
I have shown you one example of a
ÒfingerprintÓ study, in which model-predicted and observed patterns of
temperature change have significant similarities.
These similarities generally increase
with time, and they canÕt be fully explained by the ÒnoiseÓ of natural
climate fluctuations (assuming that model estimates of this noise are
realistic).
Here, IÕve shown you results for
vertical cross-sections of temperature change. We and others have obtained
rather similar results from analyses of geographical patterns of temperature
change at the EarthÕs surface.