Studying Climate Before the Modern Era
Understanding the past is always useful in predicting the future. In this case, how climate fluctuates over time due to natural effects before and and his industry affected it. Tree-rings, ice-cores, and speleothems can all be used to reconstruct climate of the past millennia. But these records may be of local effects and not global as well as being impacted by other events not clear in the mists of time. Researchers of the Swiss Federal Research Institute WSL, the University of Bern’s Oeschger Center, and the University of Mainz have found out that yearly temperature and precipitation variation and extremes have previously been underestimated in comparison to longer-term climatic trends. These findings were just published in an early release issue from the renowned journal Nature Climate Change.
In the study of past climates, known as paleoclimatology, climate proxies are preserved physical characteristics of the past that stand in for direct measurements (as statistical proxies), to enable scientists to reconstruct the climatic conditions that prevailed in the past. As reliable modern records of climate only began in the 1880s, proxies provide a means for scientists to determine climatic patterns before record-keeping began. Examples of proxies include ice cores, tree rings, sub-fossil pollen, boreholes, corals, and lake and ocean sediments.
The character of deposition or rate of growth of the proxies' material would have been influenced by the climatic conditions of the time in which they were laid down or grew. Chemical traces produced by climatic changes, such as quantities of particular isotopes, can be recovered from proxies. Some proxies, such as gas bubbles trapped in ice, enable traces of the ancient atmosphere to be recovered and measured directly to provide a history of fluctuations in the composition of the Earth's atmosphere.
Climate extremes not always recognized in proxy archives. The scientists learned that these proxy archives provide an incomplete record of climate variation. The annual width or density of tree-rings is not only influenced by temperature while the ring is developing, but also from the climate of the past years and other factors like tree age. This makes it difficult to extract pure temperature signals from these natural archives. Importantly, the researchers found out that proxy data underestimate climate fluctuations of, for example, air temperature over the land surface where large year-to-year variability is common. In contrast, long-term trends in precipitation tend to be exaggerated by the proxy records. These findings indicate that the proxy data often result in a "blurry picture" of climate variation.
The researchers were able to conclude from their work that short-term extreme climate events, such as individual years with hot summers, are not well captured by the proxy reconstructions.
Temperature trends can’t be used to understand rainfall. Investigations on the individual factors and processes fingerprinted in tree-ring, ice-core and similar records are needed to develop a more accurate history and understanding of the climate system. The authors explicitly warn that proxy records that predominately reflect temperature variation should not be used to make conclusions about precipitation change and vice-versa.
"Our results point to uncertainties in the global climate system that were previously not recognized," says David Frank, co-author of this study. He continues "This might be surprising because we know more about the Earth’s climate now than say 20-years ago. Part of the scientific process is to confront and uncover these unknowns while developing climate reconstructions." There is still a lot of basic research needed to reduce uncertainties about how the Earth’s climate system operated prior to the industrial era and how it may operate in the future.
For further information see Ancient Climate.
Section of annual tree-rings image via WSL.