level of confidence we place in the Little Ice Age cooling and 20th century warming. Even less confidence can be placed in the original conclusions by Mann et al. (1999) that “the 1990s are likely the warmest decade, and 1998 the warmest year, in at least a millennium” because the uncertainties inherent in temperature reconstructions for individual years and decades are larger than those for longer time periods, and because not all of the available proxies record temperature information on such short timescales.
What do climate models and forcing estimates tell us about the last 2,000 years?
On the basis of satellite-based monitoring, which began in the late 1970s, it is clear that the rapid global warming of the last few decades is not attributable to an increase in the Sun’s emission. The measurements indicate that the Sun’s emission has not changed significantly during this period, apart from small variations in association with the 11-year sunspot cycle. Whether variations in the Sun’s brightness on longer timescales are large enough to constitute a significant climate forcing is still a matter of debate. It has been hypothesized that reduced solar radiation during the so-called Maunder Minimum in the sightings of sunspots from 1645 to 1715 could have contributed to the coldness of the Little Ice Age.
Sulfate aerosols formed from gases injected into the stratosphere during major volcanic eruptions are known to increase the fraction of the incident solar radiation reflected back to space, cooling the lower atmosphere and the uppermost layers of the ocean. Even though most of the particles settle out of the stratosphere within a year or two, the cooling persists because it takes the ocean several years to cool down and a decade or longer to warm back up. Proxy evidence indicates that the period around A.D. 1000, during which warm intervals are evident in many of the proxy records, corresponded to an extended interval of low volcanic activity in which the incoming solar radiation was relatively unobstructed by the presence of stratospheric aerosols.
Reconstructions of temperatures and external forcings during the 2,000 years preceding the start of the Industrial Revolution are not yet sufficiently accurate to provide a definitive test of the climate sensitivities derived from climate models, mostly because the external forcings on this timescale (mainly solar variability and variations in volcanic activity) are not very well known. Climate model simulations forced with estimates of how solar emission, volcanic activity, and other natural forcings might have varied over this time period, however, are broadly consistent with surface temperature reconstructions (see panel D of Figure O-5).
How central are large-scale surface temperature reconstructions to our understanding of global climate change?
Surface temperature reconstructions have the potential to provide independent information about climate sensitivity and about the natural variability of the climate system that can be compared with estimates based on theoretical calculations and climate models, as well as other empirical data. However, large-scale surface temperature reconstructions for the last 2,000 years are not the primary evidence for the widely accepted views that global warming is occurring, that human activities are contributing, at least in part, to this warming, and that the Earth will continue to warm over the next century. The primary evidence for these conclusions (see, e.g., NRC 2001) includes: