Pinpointing the timing of sudden climate change

The researchers looked at changes in the sediment of a lake in Germany and compared it to lake sediment records of a Norwegian lake. They were able to pinpoint exactly when the local climate in each country began to recover from glacial conditions by identifying where microscopic volcanic ash from a major Icelandic eruption around 12,100 years ago occurred in the annually layered sediments of the German lake. According to the study published in Geology, the ash anchors in time the climatic events before and after the eruption. The study says this information gives new insights into the mechanism and pace of past climate change, as well as the variable effects of future climate change.

Lake sediment acts like a time capsule, containing both inorganic and organic material (such as algae, soil material and plant matter) collected at the bottom of the lake where the layers lie undisturbed for thousands of years. In some lakes, sediments are annually laminated due to seasonal algal blooms and washing in of material from the lake's borders. These types of lake sediment offer some of the most precisely dated records of environmental change, allowing conditions to be compared from one year to the next. However, even these very precise records have uncertainties when the timing is scaled down to decades when you are looking back over many thousands of years. This presents problems if scientists are trying to establish whether changes in different parts of the world were simultaneous or not, which is why the marker provided by the volcanic ash within the lake sediments is so important.

The researchers from the University of Oxford, GFZ Potsdam and Royal Holloway University of London were able to compare the environmental changes displayed in sediment of lake Meerfelder Maar in the Eifel region in western Germany with Lake Kråkenes in southern Norway. Iceland's Katla volcano caused ash to be blown over much of the north Atlantic, with some of it settling across sites in central and northern Europe. These microscopic particles, which today appear as glass shards in the lake sediment, could be identified in both lake basins. The researchers used this to line up the timescales of the two records in order to measure the apparent time lag.

By counting the annual layers in Meerfelder Maar's sediments, the researchers have shown that the first signs of a more stable climate in Germany occurred exactly 100 years before the volcanic ash was deposited. In the lake in Norway, however, the same ash layer is found 20 years before the sediment record shows a response to the same climatic event. The researchers believe that the apparent 120-year lag in the climate signal was due to the time taken for the polar front to gradually retreat northwards from the latitude of Meerfelder Maar (50 degrees north) to its present day position (60 degrees north). Interestingly, there is no signal for this transition within the Greenland ice core records, which remained within the polar air masses at all times. The authors suggest that the Greenland ice cores are therefore not the best record of past climate change everywhere else.

Lead author Dr Christine Lane, from the University of Oxford's School of Archaeology, said: 'These findings have important implications for our treatment of past climate records, showing that we must not assume that climate change occurred everywhere at the same time. Instead, we need to recognise the dynamic and variable nature of climatic change, even within the same continent. The findings should also help us understand the workings of climate in the future. Models need to be able to incorporate relatively short-term processes as well as changes over the long term in order to accurately predict the patterns and rapidity of climate change around the globe.'