Despite these limitations, the large, diverse, and coherent collection of evidence represented by the samples shown in Figure O-5 indicates that global surface temperatures were relatively cool between 1500 and 1850 (the Little Ice Age) and have risen substantially from about 1900 to present. The tree-ring-based and multiproxy-based surface temperature reconstructions shown in panel C also suggest that the Northern Hemisphere was relatively warm around A.D. 1000, with at least one reconstruction showing surface temperatures comparable in warmth to the first half of the 20th century. The timing, duration, and amplitude of warm and cold episodes vary from curve to curve, and none of the large-scale surface temperature reconstructions show medieval temperatures as warm as the last few decades of the 20th century.
What conclusions can be drawn from large-scale surface temperature reconstructions?
Based on its deliberations, the plots shown in Figure O-5, and the evidence described in the chapters that follow and elsewhere, the committee draws the following conclusions:
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.
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. The existence of a Little Ice Age from roughly 1500 to 1850 is supported by a wide variety of evidence including ice cores, tree rings, borehole temperatures, glacier length records, and historical documents. Evidence for regional warmth during medieval times can be found in a diverse but more limited set of records including ice cores, tree rings, marine sediments, and historical sources from Europe and Asia, but the exact timing and duration of warm periods may have varied from region to region, and the magnitude and geographic extent of the warmth are uncertain.
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.
Less confidence can be placed in large-scale surface temperature reconstructions for the period from 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 associated with reconstructing hemispheric mean or global mean temperatures from these data increase substantially backward in time through this period and are not yet fully quantified.
Very little confidence can be assigned to statements concerning the hemispheric mean or global mean surface temperature prior to about A.D. 900 because of sparse data coverage and because the uncertainties associated with proxy data and the methods used to analyze and combine them are larger than during more recent time periods.