While correct in the context of glacial/interglacial cycles, annually resolved ice core records from central Greenland demonstrate increased complexity in climate through the Holocene—as, for example, cold climate feedbacks produced by the presence of ice sheets dissipated during the early Holocene. Moreover, detailed examination of these and other paleoclimate records suggests that subdued versions of glacial-age rapid climate change events regularly punctuated the Holocene and that major atmospheric phenomena such as ENSO and monsoons varied markedly in frequency and magnitude throughout the Holocene. In fact, closer examination of Holocene climate records reveals more variability than that typically observed in the instrumental records covering the past century. Paleoclimate reconstructions, while not generally as accurate as the instrumental record, can uncover information from times and regions not covered by the instrumental record, thus supplementing it.
The past 2,000 years of Holocene climate offer examples of climates both warmer than modern (the MWP) and colder (LIA). While these climate events do not serve as strict analogs for future warmer or colder climates because they predate the industrial era, they do offer important “climate opportunities.” Furthermore, despite the fact that instrumental records are most commonly less than a century in length, abundant and relatively untapped records in the form of historical journals and natural archives (e.g., tree rings, ice cores, corals) are relatively abundant for this period. Embedded in these records is evidence of both climate response (e.g., changes in temperature, precipitation, circulation strength) and climate forcing (e.g., solar variability, biogenic emissions, volcanism).
The most recent major climate event of the 0Holocene—the Little Ice Age—was a period of regionally lowered temperatures, increased atmospheric circulation intensity, and significant decadal-scale variability. While Holocene climatic shifts larger than those of the LIA are recorded in several proxy records, the LIA appears to have started more abruptly (within several decades) than other Holocene climate change events. The significance of this climatic “surprise” is not fully understood.
Despite general agreement that temperatures characteristic of the LIA have not characterized the twentieth century, there is considerable debate over the timing and geographic extent of this event and little discussion of the other climate parameters (e.g., precipitation, atmospheric circulation intensity) that characterized it. While climate forcings such as solar variability and volcanism explain some degree of LIA climate, more complex multiple forcings and nonlinear responses to these forcings must be investigated. Therefore, while the LIA is the most recent example of a naturally forced cooler climate, our knowledge of this event is not sufficient to understand its significance or explain its causes. Less is known about the MWP. Understanding of modern climate is, therefore, hampered not only by questions concerning the influence of humanly induced forcing through emissions of radiatively important trace gases and aerosols but as