Committee on Surface Temperature Reconstructions for the Last 2,000 Years
Dr. Gerald North, Chair
Good morning. On behalf of my colleagues, I would like to welcome those of you in the room as well as those of you listening on the Web. We are pleased to be here to discuss the findings of our new report, Surface Temperature Reconstructions for the Last 2,000 Years. This report was done in response to a request from Congress, and our goal was to assess the state of scientific efforts to reconstruct surface temperature records for the Earth over approximately the last 2,000 years and comment on the implications of these efforts for our understanding of global climate change. This report is not about projections of future climate change. It focuses on our knowledge of climate change in the relatively recent past.
Let me begin by explaining how scientists go about reconstructing surface temperatures for the past 2,000 years. Because scientists have been using thermometers to collect geographically widespread temperature records for only about 150 years, scientists estimate temperatures in the more distant past by analyzing what we call proxy evidence. Tree rings, corals, ocean and lake sediments, cave deposits, ice cores, boreholes, and glaciers are all proxies that give us information about the climate that was present when they formed. So are old paintings and diaries that document what the climate was like when they were created.
To give one example, the annual growth in trees can tell us much about the climate at the time the growth occurred -- a wider ring indicating better growing conditions than a narrow ring. Likewise, the advances and retreats of glaciers provide evidence of warming and cooling.
Starting in the late 1990s, scientists began using sophisticated methods to combine proxy evidence from many different locations in an effort to estimate surface temperature changes averaged over broad geographic regions during the last few hundred to few thousand years. These large-scale surface temperature reconstructions enabled researchers to estimate past temperature variations over the Northern Hemisphere or even the entire globe, often with time resolution as fine as decades or even individual years.
The debate that prompted our report began in 1998 when a paper by Michael Mann, Raymond Bradley and Malcolm Hughes was published in the journal Nature. The authors used a new methodology to combine data from a number of sources to estimate temperatures in the Northern Hemisphere for the last six centuries, and later for the last 1,000 years. This research received wide attention, in part because the authors concluded that the Northern Hemisphere was warmer during the late 20th century than at any other time during the past millennium, and also because it was illustrated with a simple graphic, the so-called hockey stick curve.
This controversy led to today’s study. Our committee, which was assembled by the National Research Council, is composed of 12 members with expertise in a range of fields including climate modeling, statistics, climate change and variability, and each of the types of proxies commonly used in reconstructions. The committee took multiple steps to accomplish its charge. First, we hosted a two-day workshop in March 2006 and invited numerous speakers from all perspectives in the debate to participate. We also examined the scientific literature in great depth, and considered written input from anyone wishing to provide their views. We looked at large-scale surface temperature reconstructions from six different research teams as well as at the instrumental record.
Let me summarize five key conclusions we reached after reviewing the evidence:
1. The instrumentally measured warming of about 0.6°C during the 20th century is also reflected in borehole temperature measurements, the retreat of glaciers, and other observational evidence, and can be simulated with climate models.
2. Large-scale surface temperature reconstructions yield a generally consistent picture of temperature trends during the preceding millennium, including relatively warm conditions centered around A.D. 1000 (identified by some as the "Medieval Warm Period") and a relatively cold period (or "Little Ice Age") centered around 1700.
3. It can be said with a high level of confidence that global mean surface temperature was higher during the last few decades of the 20th century than during any comparable period during the preceding four centuries. This statement is justified by the consistency of the evidence from a wide variety of geographically diverse proxies.
4. Less confidence can be placed in large-scale surface temperature reconstructions for the period A.D. 900 to 1600. Presently available proxy evidence indicates that temperatures at many, but not all, individual locations were higher during the past 25 years than during any period of comparable length since A.D. 900. The uncertainties increase substantially backward in time through this period and are not yet fully quantified.
5. Very little confidence can be assigned to statements concerning the hemispheric mean or global mean surface temperature prior to about A.D. 900.
The main reason that our confidence in large-scale surface temperature reconstructions is lower before A.D. 1600 and especially before A.D. 900 is the relative scarcity of precisely dated proxy evidence. Other factors limiting our confidence in surface temperature reconstructions include the relatively short length of the instrumental record, the fact that all proxies are influenced by many climate variables, and the possibility that the relationship between proxy data and local surface temperatures may have varied over time. All of these considerations introduce uncertainties that are difficult to quantify.
Overall, the committee finds that efforts to reconstruct temperature histories for broad geographic regions using multiproxy methods are an important contribution to climate research and that these large-scale surface temperature reconstructions contain meaningful climatic signals. The individual proxy series used to create these reconstructions generally exhibit strong correlations with local environmental conditions, and in most cases there is a physical, chemical, or physiological reason why the proxy reflects local temperature variations. Our confidence in the results of these reconstructions becomes stronger when multiple independent lines of evidence point to the same general result, as in the case of the Little Ice Age cooling and the 20th century warming.
The basic conclusion of Mann et al. (1998, 1999) was that the late 20th century warmth in the Northern Hemisphere was unprecedented during at least the last 1,000 years. This conclusion has subsequently been supported by an array of evidence that includes both additional large-scale surface temperature reconstructions and pronounced changes in a variety of local proxy indicators, such as melting on icecaps and the retreat of glaciers around the world, which in many cases appear to be unprecedented during at least the last 2,000 years.
Based on the analyses presented in the original papers by Mann et al. and this newer supporting evidence, the committee finds it plausible that the Northern Hemisphere was warmer during the last few decades of the 20th century than during any comparable period over the preceding millennium. However, the substantial uncertainties currently present in the quantitative assessment of large-scale surface temperature changes prior to about A.D. 1600 lower our confidence in this conclusion compared to the high level of confidence we place in the Little Ice Age cooling and 20th century warming. Even less confidence can be placed in the original conclusions by Mann et al. (1999) that "the 1990s are likely the warmest decade, and 1998 the warmest year, in at least a millennium" because the uncertainties inherent in temperature reconstructions for individual years and decades are larger than those for longer time periods, and because not all of the available proxies record temperature information on such short timescales.
One significant part of the controversy on this issue is related to data access. The collection, compilation, and calibration of paleoclimatic proxy data represent a substantial investment of time and resources, often by large teams of researchers. The committee recognizes that access to research data is a complicated, discipline-dependent issue, and that access to computer models and methods is especially challenging because intellectual property rights must be considered.
Our view is that all research benefits from full and open access to published datasets and that a clear explanation of analytical methods is mandatory. Peers should have access to the information needed to reproduce published results, so that increased confidence in the outcome of the study can be generated inside and outside the scientific community. Paleoclimate research would benefit if individual researchers, professional societies, journal editors, and funding agencies continued their efforts to ensure that existing open access practices are followed.
Where do we go from here? It is worth it to pursue large-scale surface temperature reconstructions. They have the potential to further improve our knowledge of temperature variations over the last 2,000 years, particularly if additional proxy evidence can be identified and obtained from areas where the coverage is relatively sparse and for time periods before A.D. 1600 and especially before A.D. 900. It would be helpful to update proxy records that were collected decades ago, in order to develop more reliable calibrations with the instrumental record.
Improving access to data used in publications would also increase confidence in the results of large-scale surface temperature reconstructions both inside and outside the scientific community. New analytical methods, or more careful use of existing ones, may also help circumvent existing limitations. Finally, because some of the most important potential consequences of climate change are linked to changes in regional circulation patterns, hurricane activity, and the frequency and intensity of droughts and floods, regional and large-scale reconstructions of changes in other climatic variables, such as precipitation, over the last 2,000 years would provide a valuable complement to those made for temperature.
In summary, as science has made progress over the past few years, we have learned that large-scale surface temperature reconstructions are important tools in our understanding of global climate change. They contribute evidence that allows us to say, with a high level of confidence, that global mean surface temperature was higher during the last few decades of the 20th century than during any comparable period during the preceding four centuries.
Thank you for your attention. My colleagues and I are now open to answering questions.