Comprehensive analyses unambiguously support the presence of substantial global heating. These include temperatures averaged over a large number of regional centres (such as Australia) and globally (Fig. 5). They include multiple aspects of “climate” including surface and atmospheric temperatures, surface ocean temperature and lake temperatures [paper, paper]. This is reflected in the mean temperature as well as the number of hot days and, because warm water expands, rising sea levels. A large body of work documents this, but a comprehensive summary can be found in a “State of the Climate” report by the American Meteorological Society, with over 100 contributing scientific authors [paper], as well as regular reports by Australia’s CSIRO and Bureau of Meteorology [report; see also site].
Figure 5: A snapshot of climate change indices [report], including number of hot days (top left); temperature anomalies (top right), ocean heat (bottom left) and sea levels (bottom right)
Notably, global heating is evident when inferring temperature from other meteorological observables including barometric pressure [paper], and from recent satellite estimates of global temperature in the mid- to upper troposphere [paper]. These studies invalidate spurious claims occasionally made in the public domain that historical surface recordings, or their analyses, are unreliable.
There naturally exist local deviations in all of these indices, such as instances of cold weather. Likewise, in “recent times” (centuries), there have been instances of regional heating and cooling. However, there is no evidence for globally coherent warm or cold periods in the last 2000 years that compare with the widespread heating of the present era (Fig. 6)
Figure 6: No evidence for globally coherent warm periods in the last 2000 years. Colours depict spread of years for local maximum temperatures during the Roman Period (left), the Medieaval Period (centre) and the present period (right) with far greater global coherence [paper].
Next section: Is it due to human activity?