User:William M. Connolley/Att

Attribution_of_recent_climate_change says Attribution of recent change to anthropogenic forcing is based on the following facts:

• The observed change is not consistent with natural variability.
• Known natural forcings would, if anything, be negative over this period.
• Known anthropogenic forcings are consistent with the observed response.
• The pattern of the observed change is consistent with the anthropogenic forcing.

but doesn't really back this up. So...

The observed change is not consistent with natural variability

The climate system, unforcd by humans, will vary due to a combination of internal and external climate forcings. The size of these forcings can be estimated by a combination of reconstruction of past termpature records, reconstruction of past forcings, and estimates of the internal variability of the climate system. The latter is hard to do from direct or indirect observations, so is usually deduced from the behaviour of multi-millenial climate model integrations. The forcings - mostly solar or volcanic - can be estimated by proxy records such as volcanic ash layers in ice cores, and translated into likely temperature changes by theory, models, or analogy with more recently observed episodes. Past temperature changes - see Temperature record of the past 1000 years - can be reconstructed from a variety of sources including ice cores, tree rings and historical records.

The combination of these sources indicate that the 20th-century warming is larger than can be explained by natural causes. The late 20th century has been unusually warm; the second half of the 20th century was likely the warmest 50-year period in the Northern Hemisphere in the last 1300 years [1]. This level of variability, whilst possible, would not be expected due to natural causes.