At http://phys.org/print326994700.html … we have an interesting story sent in by member William Thompson. He was fascinated by the fact there is actually a conflict of opinion regards global warming – between models and reality. An article in PNAS by Zhengyu Liu of the University of Wisconsin-Madison, and a variety of co-authors. They seem to have done the research ostensibly at the instigation of the IPCC. Why? It all revolves around an article in the journal Science published last year which claimed that for the last 7000 years global temperatures have been cooling. This research used proxy data – such as ice cores and oxygen isotopes from sea sediment cores. The new study directly contradicts the Science tale.
The interesting bit is what was thought to be relevant for inclusion in the models – used in the PNAS paper. For example, it is assumed the ice sheet from the Late Glacial Maximum took a long time to melt – but where is the evidence? People were living in that part of N America in early Holocene – and in the last stages of the Pleistocene. Secondly, they assume on dubious evidence that co2 has been gradually rising throughout the Holocene – and has only accelerated in the last 150 years. In other words, they know co2 causes warming (and say so) so it should have been warming – and their models show that it has been warming. In contrast, the Science article last year, which provoked the new study, a deliberate attempt to trash its findings, concluded that from about 5000BC, the height of the mid-Holocene Warm Period, the climate has been cooling (on an average graph). It doesn't say it has not warmed up in recent years – only that in the long term climate has cooled in comparison to what it was in 5000BC. They did in fact accept the idea of the hockey stick rise in the 1990s – presumably in order to get published in the first place. A total of 73 sites were used in the Science study, from different parts of the world, making use of various proxy climate data from ice cores and so on. This amounts to 'in field' research whereas the PNAS study is completely computer driven – and based on models (which effectively omit important information such as aerosols).
The line of Liu's thinking is also of interest – and then he comes out with a strange comment (which might be due to language differences). He said, 'the fundamental laws of physics say that as the temperature goes up it has to get warmer' in which he means as the temperature goes up according to the models it must get warmer in the real world – and the proxy data must be wrong. Thus, we have a new study that has omitted important impacts on climate, such as volcanic activity and aerosols in the upper atmosphere, saying that the models say it has warmed so it must have warmed. Volcanic activity causes an opaque sky via aerosols and therefore cools the weather. However, proxy data also shows some more significant periods of cooling, at 3200-3000BC, 2300-2000BC, and 1200-1000BC, as well as the ups and downs in climate between 1300 and 1850AD. Here you have the base line. Global warming since 1850 is used as a marker simply because temperatures are rising out of a particular cool period of weather – that is what the graphs CAGW types always like to show. Sometimes they choose to begin at around 1890, when again there was a cool phase dominated by La Nina (which is what is happening at the moment). The only thing demonstrated in the PNAS paper is that models don't conform to proxy data – neither do they correspond with the real world.