Nonglacial rapid climate events: Past and future

Abstract

The paleoclimate record makes it clear that rapid climate shifts of the 20th century are only a subset of possible climate system behavior that might occur in the absence of glacial conditions, and that climatic surprises could be a challenge for society even in the absence of significant greenhouse warming. P aleoclimatologists have been aware for decades that the climate system is capable of behavior quite unlike that of the present day. At the same time, however, paleoclimatology has tended to follow the lead of the climate dynamics community, and to pursue research primarily under the paradigm that the Earth's climate tends to change gradually in response to slowly changing climate forcing. For example, the first major revolution of modern paleoclimatology was the identification of a slowly oscillating Earth-Sun relationship as the pacemaker of the Ice Ages (i.e., ''Milankovitch Theory''). On the other end of the climate variance spectrum, early successes with seasonal to interannual climate prediction benefited from a well described stable mode of the El Niño͞Southern Oscillation (ENSO) variability in the late 1980s.However, a new paradigm of climate variability is emerging. Rapid step-like shifts in climate variability that occur over decades or less, as well as climatic extremes (e.g., drought) that persist for decades, occurred repeatedly in recent earth histor y. For example, a substantial amount of work now focuses on abrupt ice-age climate shifts (e.g., other papers in the Rapid Climate Change special feature of this issue of PNAS), and significant research has also focused on the abrupt climate shift that took place over the Pacific basin in the mid-1970s (1). This mid-1970s shift represents a transition into a mode of variability characterized by frequent and strong El Niño conditions relative to earlier decades.Here we review a representative subset of the growing body of paleoclimatic evidence regarding rapid climatic change since the last deglaciation and highlight the conclusion that climate variability in the past has changed substantially in response to altered climate forcing. Building on a recent review (2), we focus on rapid climate change of ''warm climates'' like those of today and the future, rather than on the well described rapid changes of glacial or deglacial climatesperiods of time when the Earth's climate system may have been heavily inf luenced by large Northern Hemisphere ice sheets. Equally important, we focus on climate phenomena (e.g., El Niño, monsoons, and drought) of primary interest to society. The growing body of paleoclimatic evidence indicates that the climate system may have many potential surprises in store for scientists and society alike.Case Study 1: ENSO͞Pacific Variability. Recent well known shifts in Pacific climate variability have heightened awareness that seasonal to interannual variability is modulated by longer-period modes of climate variation. Moreover, shifts between different ''modes,'' or ''states,'' of variability have taken place without warning and ...

Lake sediments record large-scale shifts in moisture regimes across the northern prairies of North America during the past two millennia

“…Regardless of the exact timing of these shifts or the direction of these changes, the persistent and abrupt nature of these events represents a scale of variation that is not well understood. Such shifts today would prove a major challenge for society regardless of global warming (42), particularly given that persistent periods of drought have been shown in numerous cases to coincide with societal stress and collapse (2).…”

Stabilizing the Earth’s climate is not a losing game: Supporting evidence from public goods experiments

“…This release will lead to adverse effects on many societies, especially if it involves threshold crossing and abrupt climate change (1)(2)(3)(4)(5)(6)(7)(8). To avoid major long-term climate change, average per capita greenhouse gas emissions must be reduced to a small fraction of the present levels of developed countries on the time scale of a century (1,9).…”

Middle Holocene climate change and human population dispersal in western North America

“…This, however, is inconsistent with indications of widespread Middle Holocene aridity in western North America. Instead, evidence for relatively stable decadal-scale climate variability in Site 893A/B appears to be more consistent with weaker ENSO activity during the Middle Holocene (Sandweiss et al, 1996(Sandweiss et al, , 1997(Sandweiss et al, , 2001Overpeck and Webb, 2000;Tudhope et al, 2001;Koutavas et al, 2006). If so, it follows that generally warmer SSTs at the millennial-scale are not necessarily accompanied by more intense or frequent ENSO activity.…”