One of the oft-quoted arguments of the climate change deniers is that the weather/climate has changed in the past and/or is always changing, so the rise in temperature over the last few decades is nothing unusual. It thus has nothing to do with man's activities, specifically the increasing emissions of carbon dioxide from the burning of fossil fuels - assuming of course that they accept that the temperature has risen at all recently! One of the periods that they often refer to as "proof" is the so-called Medieval Warm Period, dating from around the 10th to the 14th centuries.

Given that it was not until the beginning of the 18th century that the first instrument capable of measuring temperature was devised by Gabriel Fahrenheit, how can we possibly know what the temperature was many centuries earlier? There is considerable anecdotal evidence which supports the idea of warmmer climes: vine growing in northern England, increased crop yields in parts of Europe, earlier plant flowering and animal breeding, Vikings breeding cattle in Greenland. But there is also scientific evidence in proxies for actual temperature measurements: tree ring growth in timbers of known age, pollen in lake sediments, phytoplankton in marine sediments and oxygen isotope ratios in ice cores. All of these can yield reliable measures of temperature at known periods in the past. But were these global increases or lust local ones?

The evidence is that they were not global phenomena but rather that these higher than average temperatures were neither widespread across the globe nor did they occur at the same time in those different regions. In particular, they were almost entirely restricted to the Northern Hemisphere, those in the Tropics and Southern Hemisphere being lower than the long-term average.

The most recent studies have shown that over that timescale (11th to 14th centuries) there were no globally-synchronous decade-long warm intervals that would define a worldwide Medieval Warm Period. If anything, the mean global temperature was slightly lower than that which obtains today. Nor, for that matter, is there any evidence which would support the idea of a global Little Ice Age: it was a purely Northern European event. What is clear is that for the past 1400 years or so, the Earth temperature has been on a slow but steady decline, until the late 20th century when it has been on the rise. 

As for the Neolithic Period, yes it was warmer than now but again mainly in the Northern Hemisphere, and that warming can be attributed to known natural phenomena such as subtle changes is the Earth’s orbit round the Sun. Such a process, along with reduced volcanic activity and changes in ocean currents, also explain the local temperature changes in the MWP. Crucially, none of these can explain the recent global temperature rise: only the huge increase in man-made carbon dioxide emissions from the burning of fossil fuels can do that.

Barry Colam