Reconstructing Past Climate Using Tree Ring Data from Ancient Bristlecone Pine

[@petersteenkamp]

I have a question about tree rings, the CO2 fertilization effect, and calibrating tree ring size to temperature. I understand that tree growth speed depends on a number of factors, like nitrogen in the soil, precipitation, and temperature. However, nowadays CO2 in the atmosphere is above 400 ppm while in the pre-industrial era, CO2 was below 300 ppm. Higher CO2 leads to faster tree growth and thus thicker tree rings. Assuming nitrogen in the soil and precipitation are equal, if a tree ring today is XX millimeters thick, and you find a medieval tree ring that is exactly the same XX millimeters thick, then you can't simply assume the temperatures now and then were equal, because the medieval tree ring thickness is based on warmth alone while the present-day tree ring thickness is based on warmth plus the extra CO2 fertilization effect. So if you equalize the two and use a present-day thermometer to assign a temperature to that thickness, then the medieval temperature will be artificially low compared to today, which leads to an artificial hockey stick that misrepresents the temperature. So how do you calibrate tree ring size to temperature?

[@Rucuz]

Does more CO2 increase drought resistance thus increasing the tree line?

[@EmilNicolaiePerhinschi]

ring widths did anyone try to reproduce the 1970s studies on oxygen and hidrogen isotopes using tree rings ?