With the mood music suggesting a global deal on climate change is still far away, commentators are becoming fearful of a future world with runaway global warming. On the other side of the coin the sceptics claim that the situation is over-played, and point to the complexities of the science and a decade of stable or falling temperatures as a call for caution. There are two core aspects to the debate: 1. The Science – How significant is CO2 amongst other greenhouse gases? How significant is the human component? How do positive and negative feedbacks influence climate? 2. The Statistics – Is there an upward trend in temperatures, or are we simply seeing natural variability The first of these aspects is even too difficult for Figure it Out. However we can dip our toe into the debate and take a look at the second.
There are a number of sources of temperature data that are used to study long term temperature trends. We take a UK perspective by using the Central England Temperature records (http://hadobs.metoffice.com/hadcet/), an excellent source of past data. This data is representative of a triangular area of the United Kingdom enclosed by Lancashire, London and Bristol, and is the world’s longest set of temperature records, dating back to 1659. So how do we go about analysing the data? Firstly plot the raw daw and apply a simple trend line (by minimising overall errors between the data and the line). The chart below shows an overall upward trend in average temperatures of just under 1 degree between 1659 and 2009. However the second chart below shows a similar trend of temperatures increasing by almost 1 degree over the last one hundred years alone. So the statistics seem conclusive, although temperatures have dipped slightly this decade, in the UK there has been an underlying increase of just under 1 degree over the last 100 years, following 250 years with little overall trend. Similar analysis to this on other global data sources, with similar findings, is regarded by commentators as fairly conclusive proof of a continuing upward trend in temperatures... ...However take a step back and look at the first chart again and you will see that the upward temperature trend is not because recent temperatures have been historically much hotter than previous hot years, but rather because recently there has been a cluster of years with annual average temperatures in line with the hottest years of the last 300 years. An annual average of around 10.5 degrees is a hot year for the UK and has occurred roughly every 40 years or so. With the exception of 2006 which at 10.83 was a record year average, recent hot years have been in line with this figure. So does this mean we should question whether the upward temperature trend is real or whether it can be explained by natural variability? Well take another look at the first chart again and you may see something else interesting in the early years. The chart below highlights the temperature trend in the 80 years up to 1738, well before the days of budget air travel and Chelsea tractors. It shows an equally dramatic temperature trend increase of 1 degree within 80 years. So what is the conclusion? Are we seeing a long term increase in temperatures or is it simply natural variability? To be sure of an underlying trend of 1 degree of warming over the last 100 years we would expect to be seeing a year with an average Central England annual temperature of 11.5 degrees, which we are a long way away from, so the statistics cannot be deemed conclusive. Interestingly if we take a look at the extremes of temperature we see similar trends with a little more variability –The three warmest Julys were 1783, 1983 and 2006 – The three warmest Januarys were 1796, 1916 and 1921. Whatever the situation, Figure it Out recommends switching of those lights and getting on that bike... even if only for the sake of your wallet and waistline rather than the planet!