The variability of sea surface temperatures (SSTs) at multidecadal and longer timescales is poorly constrained, primarily because instrumental records are short and proxy records are noisy. Through applying a new noise filtering technique to a global network of late Holocene SST proxies, we estimate SST variability between annual and millennial timescales. Filtered estimates of SST variability obtained from coral, foraminifer, and alkenone records are shown to be consistent with one another and with instrumental records in the frequency bands at which they overlap. General circulation models, however, simulate SST variability that is systematically smaller than instrumental and proxy-based estimates. Discrepancies in variability are largest at low latitudes and increase with timescale, reaching two orders of magnitude for tropical variability at millennial timescales. This result implies major deficiencies in observational estimates or model simulations, or both, and has implications for the attribution of past variations and prediction of future change.sea surface temperature | climate variability | multiproxy synthesis | proxy data reconstruction V ariations in sea surface temperature (SSTs) have widespread implications for society and are the basis of most regional decadal prediction efforts (1). Magnitudes of variability in regional SSTs are inferred either using observations or simulations from general circulation models (GCMs). At synoptic and interannual timescales, there is overall agreement between observational and GCM estimates of SST variability (2-4). At decadal timescales, however, instrumental records generally show greater regional SST variability than found in the Coupled Model Intercomparison Project Phase 5 (CMIP5) ensemble of GCM simulations (4) and in earlier simulations (5-7). Discrepancies are greatest at low latitudes where model−data mismatches in variance reach a factor of 2 (Fig. 1).Estimating regional SST variability at multidecadal and longer timescales presents a quandary. Discrepancies with instrumentally observed SST variability at decadal timescales calls into question the credibility of GCM estimates at longer timescales. At the same time, instrumental observations covering more than 100 y of SST variability are sparse. For example, whereas 68% of ocean grid boxes in the Climate Research Unit's instrumental compilation of SSTs have a 30-y interval with an observational density of at least 100 observations per year, only 19% of grid boxes have such coverage over a 100-y interval (8). Furthermore, SST variability observed during the last century represents contributions from natural and anthropogenic sources that are difficult to disentangle and are not necessarily representative of any other interval (9).One is inevitably led to using paleoclimate proxies to constrain multidecadal and longer-term variability. Numerous paleoclimate reconstructions document low-frequency SST variability (10-15), but the degree to which these reconstructions afford quantitative constraints of annual...