The National Research Council (NRC) released a report to Congress today that says,
There is sufficient evidence from tree rings, retreating glaciers, and other ‘proxies’ to say with confidence that the last few decades of the 20th century were warmer than any comparable period in the last 400 years. Less confidence can be placed in proxy-based reconstructions of surface temperatures for A.D. 900 to 1600, said the committee that wrote the report, although the available proxy evidence does indicate that many locations were warmer during the past 25 years than during any other 25-year period since 900.
It was a report done in response to claims made by Michael Mann and his research group in the 1990’s. The NRC was less confident than Mann and his colleagues in their findings as data before the years 1600 becomes scarce, though the NRC did find the large-scale reconstructions helpful.
Also of note:
The committee pointed out that surface temperature reconstructions for periods before the Industrial Revolution—when levels of atmospheric greenhouse gases were much lower—are only one of multiple lines of evidence supporting the conclusion that current warming is occurring in response to human activities, and they are not the primary evidence.
On page 94, the report states:
- Greenhouse gasses and tropospheric aerosols varied little from 1 A.D. to around 1850. Volcanic eruptions and solar fluctuations were likely the most strongly varying external forces during this period, but it is currently estimated that the temperature variations caused by these forces were much less pronounced than the warming due to greenhouse gas forcing since the mid 19th century.
- Climate model simulations indicate that solar and volcanic forcings together could have produced periods of relative warmth and cold during the preindustrial portion of the last 1,000 years. However, anthropogenic greenhouse gas increases are needed to simulate late 20th century warmth.
An AP story reports that during the briefing to Congress:
The National Academy scientists concluded that the Mann-Bradley-Hughes research from the late 1990s was “likely” to be true, said John “Mike” Wallace, an atmospheric sciences professor at the University of Washington and a panel member. The conclusions from the ‘90s research “are very close to being right” and are supported by even more recent data, Wallace said.