I believe our '98 Nature article is highly cited because it establishes a new line of evidence, independent of the predictions of theoretical climate models, for the assertion that human beings are responsible in large part for 20th century global warming.

  Does it describe a new discovery or new methodology that's useful to others?

Our paper presented a novel multivariate statistical approach for assimilating the information contained in disparate "proxy" indicators of past climate change (natural archives such as tree rings, corals, and ice cores, which by their very nature record past climate changes at the annual timescale). Our methodology allowed this information to be synthesized and compared to modern instrumental climate records, allowing us to reconstruct large-scale patterns of surface temperature changes in the past, and, importantly, providing an estimate of the uncertainties inherent in the reconstructions.

  Could you summarize the significance of your paper in layman's terms?

Our paper established that the large-scale global warming which took place in the latter part of the 20th century is unprecedented over a fairly long period of geological time. The now oft-cited assertion that "the 1990s are the warmest decade of at least the past 1000 years" is attributable to our '98 Nature article (which established the result for the last 600 years) along with an extension of our work (which extended this conclusion to the past 1000 years) which we published in the journal "Geophysical Research Letters" in 1999. Our Nature article established that the warmth of the 1990s was outside the range of variability as indicated in our reconstruction of past Northern Hemisphere temperature variations, taking the uncertainties in the reconstruction into account. The paper also showed, from a statistical point of view, that the recent warming could not be explained in terms of "natural" influences (such as changes in solar output or explosive volcanic activity), but could only be explained in terms of anthropogenic factors (specifically, the increase of greenhouse gas concentrations due to modern industrial activity).

  How did you become involved in this research?