The location of the tree line is largely controlled by summer temperatures and growing season length. However, conifer trees have 'not yet recolonised many areas where trees were present during the Medieval Warm Period and the Holocene Thermal Maximum'. We are then told, back in 2007, that global warming will allow trees to colonise these areas – eventually. The research paper was published by the Royal Society and clearly involved a lot of research into the past tree line in northern Russia. What was surprising in this paper is that the tree line was further north in the Medieval Warm Period – and all this in spite of the hockey stick and the claim it did not exist. The Early Holocene Thermal Period is lumped in this paper as 10,000 to 3000BC (which is a fair assessment as temperatures were warm in the early Holocene and the differential between early and mid Holocene has been blurred by recent redating methodologies). In some of the older literature the advance of the tree line is confined to the period between 6000 and 3000BC and therefore we have an ambiguity – to what extent did the tree line advance during the Boreal period, for example. The paper is at rstb20072200.pdf and is foucussed on modern warming and how long it will take trees to reach the older tree line – and they assume this will happen. This assumption is based on their statement, 'climate model experiments suggest temperature increases due to global warming would be particularly pronounced in the northern higher latitudes'. Increased temperatures will allow a NW extension of boreal forest into tundra and this in turn would decrease high altitude albedo and provide a positive feedback for further enhancing global warming. Studies by Overpeck et al in 1997, Hansen et al in 1999, Briffa et al in 2001 and Lugina et al in 2006 claim increased temperatures over the last 100 years – yet so far no change in the tree line. The authors outline the expected changes from modern global warming and then add, these changes have not yet generated an extension of conifer species limits, to, or beyond, the former position occupied during the Medieval Warm Period. Pause. They have NOT yet reached the limit of the MWP. In other words, the models have been projected but Nature has yet to respond.
The authors go on to infer that projected tree line zones will be greater than before 3000BC (even though they are currently static). What the authors fail to connect with is the evidence they have found that temperatures were much higher in the past and the world did not come to an end and humans went on to thrive and increase in numbers. It is also obvious that it was colder in the past – especially in the 16th and 17th centuries – and still it did not stop human advancement. It is just as likely it will be colder in the future as it will be warmer. Climate models currently do not factor in the possibility of that or even of temperatures standing still. Projection of climate models is always upwards – ever and onwards. That has never happened in the past and one wonders why they should think it will happen nowadays. The idea co2 in the atmosphere will increase temperatures relies on the assumption that co2 remains in the atmosphere for hundreds of years when in fact it has a much shorter life span in the air. If there was any solid evidence of the tree line expanding in the north we would have had the environmentalists frothing at the mouth – but they have ignored the issue. It is almost certain that no movement in the tree line has occurred since the paper was published in 2007. Another anomaly is the fact AGW is mostly confined to the western world (and driven by western forces). We don't hear a lot about Russian research on global warming issues – which is somewhat strange as they would be affected more than any other nation by warming (except perhaps Canada which also has all to gain and little to lose). Is there a western black out of Russian research – and come to that, Chinese research published in China.