Uncertainties in Assessing Global Warming during the 20th Century: Disagreement between Key Data Sources

Abstract

We have reanalyzed the available data on the terrestrial temperature in the Northern Hemisphere over the past 1000 years using both direct observations and various proxy records. It is shown that the only plausible conclusion, which can be drawn from these data is that global temperature has been increasing since the end of the 19th century. However, due to discrepancies between different large-scale temperature proxies, it is impossible to determine whether this warming is anomalous or a result of natural climatic cycles. Furthermore, the amplitude of the temperature rise cannot be estimated correctly because of considerable disagreement between thermometer records, proxy data and satellite measurements over the last 20–30 years. Thus, the origin of the rise of global temperature should be considered to be unknown due to a lack of adequate knowledge about any of the factors that may be responsible for this phenomenon (the greenhouse effect, the rise of solar activity, natural climatic variability, regional anthropogenic impact etc.). A few probable scenarios of climatic changes in the first part of the 21st century are discussed.