So what is the scientific consensus about the answers to the key questions concerning global warming?
The British magazine New Scientist gives a review of the state of affairs concerning climate change, along with a handy summary sheet of the main points, and the Intergovernmental Panel on Climate Change (IPCC) report (thanks to Brian Gray of the Kelvin Smith Library who runs the blog e3 Information Overload for the link) provides more detailed information. Here are some tentative answers to the five key questions I raised in a previous post.
1. Is warming occurring? In other words, are average temperatures rising with time?
Here we have to distinguish between the more recent period (starting in 1861) when we have direct measurements of temperature and the prior periods, for which we have to infer temperatures using proxy measures such as using tree rings or bubbles trapped in ice cores that date back 750,000 years.
For the recent past, the IPCC report says that “The global average surface temperature has increased by 0.6 ± 0.2°C since the late 19th century”.
For the period prior to that, the report says “It is likely that the rate and duration of the warming of the 20th century is larger than any other time during the last 1,000 years. The 1990s are likely to have been the warmest decade of the millennium in the Northern Hemisphere, and 1998 is likely to have been the warmest year.”