Medieval diaries aid scientists ascertain increase in hot spots due to global warming

Tree rings and ice cores also help scientists build up picture of global warming

The temperature of the northern hemisphere has increased over a larger area in the last century than at any time in the past millennium a report published in Science reveals this week.

The study finds that the number of ’hot spots’ has increased dramatically in the Northern Hemisphere in the last century compared to the past 1200 years ¨C adding to the growing evidence of wide-scale global warming.

Dr Tim Osborn and Prof Keith Briffa, of the Climatic Research Unit team at the University of East Anglia, analyzed thermometer measurements of temperature from 1856 onwards to establish the spatial extent of recent warming, and compared it with evidence from as far back as AD 800 (from tree rings, ice cores and shells).

The study found evidence for periods of significant warmth (890 ¨C 1170) in the Northern Hemisphere during medieval times and for clearly colder periods (1580 ¨C 1850) during the so-called “Little Ice Age”.

Their key conclusion was that the 20th century stands out as having unusually widespread warmth, compared to all of the natural warming and cooling episodes during the past 1,200 years.

The research team gathered climate change data from a number of regions in the Northern Hemisphere especially:

Long life evergreen trees growing in Scandinavia, Siberia and the Rockies, which had been cored to reveal the patterns of wide and narrow tree rings over time ¨C wider rings relating to warmer temperatures.

Ice from cores drilled in the Greenland ice sheets revealed which years were warmer than others by the chemical composition of the ice.

They also used a record developed from diaries of people living in the Netherlands and Belgium over the past 750 years that revealed for example the years when the canals froze.