A recent report on the warming suggested that the generally held view that the 20th century rise in temperature was unprecedented may be false. Instead of the ‘hockey-stick’ graph produced by many scientists, which shows small oscillations in global temperatures since over the last 2000 years with a sharp rise at the turn of the 20th century, the author Dr Craig Loehle suggested that in fact temperatures had been higher during the Medieval Warm Period around 1000AD and that wild fluctuations in global temperature were not uncommon.
If his calculations are correct, it would cast doubt on the proponents on global warming as a purely manmade phenomenon. If it’s happened before, long before the industrial revolution, then why should we assume that current ideas surrounding global warming are anything other than a myth?
But were his calculations correct? And why is there such a discrepancy?
The problem stems from the difficulty of getting data about past climate change. We know for a fact that global temperatures have been on the rise during the 20th century. Scientists are no longer disputing this fact. What is disputed is whether this is a natural rise or something that has been influenced by humans.
In order to discern this, we need to know what has happened in the past. But while we have adequate temperature data from scientific instruments for the past 100 years, getting data from the past few millennia is far harder – nobody was taking daily temperature readings back in medieval times.
In order to get temperature data, we need to do analysis on tree cores, glaciers, sea floor sediment layers – anything biological or geological that was around at the time. By analyzing the age of a substance and the way it was formed we can tell something about the average temperature that prevailed at the time.
For example, dendrochronology involves the study of tree rings. By looking at how thick a tree ring is at the time of formation we can ascertain something about the climate at the time. By looking at the concentration of different isotopes in glaciers we can gain further knowledge about the average annual temperature when the ice was formed.
But none of these measures are exact – they all involve interpretation and all have a margin for error. Furthermore, we can only get temperature data from certain locations where such sources are found. So with a relatively small amount of potentially inaccurate data, it is inevitable that there will be discrepancies between different studies of climate history.
And this is why global warming may yet be a myth – because do not yet have full understanding of the way that climate works. We know that the temperature has been rising over the last few decades but we cannot say for sure whether or not this has happened before and we do not know if it is the result of a natural fluctuations or human intervention.