LIA appears to play an important role in understanding modern climate. Based on the GISP2 atmospheric circulation record (see Figure 6.8), the LIA had the most abrupt onset (AD 1400 to 1420) of any of the Holocene rapid climate change events.50 This extends findings from a 1,500-year-long ice core record in the Andes which suggests that entrance into and out of the LIA was abrupt.51
Previous research summarized by Lamb (1995) demonstrates changes in climate such as increased severity of winter storms and sea ice extent, plus accompanying changes in food harvests during the LIA and contrasting milder conditions during the MWP. Recently developed marine sediment records from the Sargasso Sea52 suggest that sea surface temperatures in the Bermuda Rise region were ~1 degree cooler than today ~400 years ago (during the LIA) and ~1,700 years ago, and ~1 degree warmer than today 1,000 years ago (during the MWP). On the basis of this work,53 it is suggested that part of the general climate warming of the past few decades54 could be natural. During the MWP extreme and persistent drought characterized such regions as California and Patagonia,55 implying potential “surprises” during warmer-than-present climates.
Composite time series for El Niño recurrence (see Figure 6.10) suggest that fewer such events occurred during the MWP than during colder intervals prior to and following this time.56 Studies conducted over only the past 500 years, which do not include the LIA/MWP transition, suggest that El Niño recurrence rate is stationary over the long term but that strong El Niño events are nonstationary over centennial scales.57
Analysis of records covering the past 500 years suggests the presence of persistent natural interdecadal and century-scale climate oscillations. A compilation of paleoclimate records representative of the past 400 years of circum-Arctic climate variability indicates that the highest temperatures over this period have occurred since 1840, demonstrating the role of natural climate variability and, as of 1920, the added climate influence of atmospheric trace gases.58 Multidecadal modes and step-function changes in precipitation, temperature, and wind regimes have been identified in a number of regions, ranging from the Intertropical Convergence Zone (ITCZ) to both northern and southern midlatitudes. Recent attempts to match decadal-scale climate change events from region to region do not, however, necessarily reveal synchronous behavior over the past few centuries.59
Although their relative importance is still debated, several mechanisms have been proposed for the natural changes in climate of the past millennium, including changes in solar output, an increase in volcanic aerosols during specific periods, an increase in long-term average atmospheric aerosol loading, variations in thermohaline circulation, and changes in greenhouse gases.60
The paleoclimate record offers the potential to deconvolve the region-to-region climate variability that characterizes the Holocene on millennial to decadal plus finer timescales. Although few such records are available at present, these records offer immense potential (see Box 6.1).