Global Temperature Hitting New Heights
Some of the clearest evidence we have of climate change is the dramatic rise in global atmospheric temperature in recent decades. Reliable temperature records go back to around 1850, but to assess whether this temperature rise is due to natural variations or human activity we need data from further back. Climate scientists have been able to reconstruct a temperature record using various "proxies" for temperature, such as tree rings, ice cores, corals and stalactites. Combining many such records, researchers in 1998 drew up the famous "hockey stick" graph which shows average temperature declined gradually through most of the past 1000 years but then shot up during the last200 years.
But 1000 years is just a blip in geologic time. To put the current rise in a longer historical context a team of researchers from Oregon State University and Harvard have done a similar analysis for the whole of the Holocene epoch, the 11,300 years since the last glaciation. The study, published this week in Science, analysed sediments from the ocean floors from 73 locations around the globe.
By looking at fossils in the sediment--their species and chemical and isotopic make-ups--the researchers were able to reconstruct a global temperature record for the period. It showed that temperatures initially rose for 2000 years and then levelled off for another 4000 years followed by a slow decline from 5500 years ago until the early 19th century. Over the course of the past century, the temperature climbed from close to the coldest level in the whole Holocene to nearly its warmest level. If global warming continues as climate models currently predict, the researchers say, even if the strictest limits on greenhouse gas emissions are applied, by 2100 the global temperature will be the highest it has been at any time since the last ice age.