Persistent episodes of extreme weather in the Northern Hemisphere summer have been shown to be associated with presence high-amplitude quasi-stationary atmospheric Rossby waves within a particular wavelength range (zonal wavenumber 6–8). The underlying mechanistic relationship involves phenomenon quasi-resonant amplification (QRA) synoptic-scale that becoming trapped an effective mid-latitude waveguide. Recent work suggests increase recent decades occurrence QRA-favorable conditions and...

The recent slowdown in global warming has brought into question the reliability of climate model projections future temperature change and led to a vigorous debate over whether this is result naturally occurring, internal variability or forcing external Earth's system. To address these issues, we applied semi-empirical approach that combines observations simulations estimate Atlantic- Pacific-based multidecadal (termed "AMO" "PMO," respectively). Using method, AMO PMO are found explain large...

A volcanic source of variation The Atlantic Multidecadal Oscillation (AMO), a 50- to 70-year quasiperiodic climate centered in the North region, was long thought be an internal oscillation system. Mann et al. now show that this is forced externally by episodes high-amplitude explosive volcanism. They used ensemble models evaluate causes AMO, finding volcanos are most important influence, and there no evidence it has been internally generated during last millennium. Science , issue p. 1014

Persistent episodes of extreme weather in the Northern Hemisphere summer have been associated with high-amplitude quasi-stationary atmospheric Rossby waves, zonal wave numbers 6 to 8 resulting from phenomenon quasi-resonant amplification (QRA). A fingerprint for occurrence QRA can be defined terms zonally averaged surface temperature field. Examining state-of-the-art [Coupled Model Intercomparison Project Phase 5 (CMIP5)] climate model projections, we find that events are likely increase by...

Abstract The level of agreement between climate model simulations and observed surface temperature change is a topic scientific policy concern. While the Earth system continues to accumulate energy due anthropogenic other radiative forcings, estimates recent evolution fall at lower end projections. Global mean temperatures from are typically calculated using air temperatures, while corresponding observations based on blend sea temperatures. This work quantifies systematic bias in...

We estimate the low-frequency internal variability of Northern Hemisphere (NH) mean temperature using observed variations, which include both forced and components, several alternative model simulations (natural + anthropogenic) component alone. then generate an ensemble historical histories based on statistics estimated variability. Using this ensemble, we show, first, that recent NH temperatures fall within range expected multidecadal synthetic histories, also show certain procedures used...

In the Northern Hemisphere, springtime events are frequently reported as advancing more rapidly at higher latitudes, presumably due to an acceleration of warming with latitude. However, this assumption has not been investigated in analytical framework that simultaneously examines latitude while accounting for variation phenological time series characteristics might also co-vary We analyzed 743 trend estimates spanning 86 years and 42.6 degrees well rates Hemisphere over same period...

Abstract For several decades the existence of interdecadal and multidecadal internal climate oscillations has been asserted by numerous studies based on analyses historical observations, paleoclimatic data model simulations. Here we use a combination observational state-of-the-art forced control simulations to demonstrate absence consistent evidence for decadal or longer-term oscillatory signals that are distinguishable from climatic noise. Only variability in interannual range associated...

Abstract Separating low-frequency internal variability of the climate system from forced signal is essential to better understand anthropogenic change as well variability. Here both synthetic time series and historical simulations phase 5 CMIP (CMIP5) are used examine several methods performing this separation. Linear detrending, commonly in studies variability, found introduce large biases amplitude estimated Using estimates obtained ensembles can reduce these biases, particularly when...

Multiple paleoclimate proxies are required for robust assessment of past hydroclimatic conditions. Currently, estimates drought variability over the several thousand years based largely on tree-ring records. We produced a 1,500-y record winter precipitation in Pacific Northwest using physical model-based analysis lake sediment oxygen isotope data. Our results indicate that during Medieval Climate Anomaly (MCA) (900–1300 AD) experienced exceptional wetness and Little Ice Age (LIA) (1450–1850...

The mining and use of copper by prehistoric people on Michigan's Keweenaw Peninsula is one the oldest examples metalworking. We analyzed concentration lead, titanium, magnesium, iron, organic matter in sediment cores recovered from three lakes located near mine pits to investigate timing, location, magnitude ancient pollution. Lead concentrations were normalized lithogenic metals account for processes that can influence natural (or background) lead delivery. Nearly simultaneous enrichments...

Low-frequency internal climate variability (ICV) plays an important role in modulating global surface temperature, regional climate, and extremes. However, it has not been completely characterized the instrumental record Coupled Model Intercomparison Project phase 5 (CMIP5) model ensemble. In this study, temperature ICV of North Pacific (NP), Atlantic (NA), Northern Hemisphere (NH) historical CMIP5 all-forcing simulations is isolated using a semiempirical method wherein ensemble mean applied...

Abstract Climate’s influence on late Pre-Columbian (pre-1492 CE), maize-dependent Native American populations in the midcontinental United States (US) is poorly understood as regional paleoclimate records are sparse and/or provide conflicting perspectives. Here, we reconstruct changes precipitation source and seasonality local warm-season duration rainstorm events related to Pacific North pattern (PNA) using a 2100-year-long multi-proxy lake-sediment record from US. Wet climate reflecting...

Abstract In this paper we examine various options for the calculation of forced signal in climate model simulations, and impact these choices have on estimates internal variability. We find that an ensemble mean runs from a single [a (SMEM)] provides good estimate true even models with very few members. cases where only member is available given model, however, SMEM other general out-performed by scaled all simulations [the multimodel (MMEM)]. The MMEM may therefore be used as observations....

We present a 6,000-yr record of changing water balance in the Pacific Northwest inferred from measurements carbonate δ(18)O and grayscale on sediment core collected Castor Lake, Washington. This subdecadally resolved drought tracks 1,500-yr tree-ring-based Palmer Drought Severity Index reconstructions Cook et al. [Cook ER, Woodhouse CA, Eakin CM, Meko DM, Stahle DW (2004) Science 306:1015-1018] extends our knowledge back to 6,000 yr B.P. The results demonstrate that low-frequency...

2014 was nominally the warmest year on record for both globe and northern hemisphere based historical records spanning past one a half centuries. It latest in recent run of temperatures decade half. Press accounts reported odds as low one-in-650 million that observed global temperature would be expected to occur absence human-caused warming. reports notwithstanding, question how likely may have been with without human influence is interesting its own right. Here we attempt address using...

Simulations conducted using a coupled lake‐catchment, hydrologic and isotope mass‐balance model indicate that small, closed‐basin lakes in north‐central Washington are isotopically sensitive to changes precipitation, relative humidity, temperature. Most notably, simulations predicted inconsistent lake responses precipitation due differences outseepage rates surface area volume (SA :V) ratios. Greater within experiments resulted increased sensitivity mean precipitation. Moreover, suggest...

Abstract We use a previously developed semiempirical approach to assess the likelihood of sequence consecutive record‐breaking temperatures in 2014–2016. This combines information from historical temperature data and state‐of‐the‐art climate model simulations Coupled Model Intercomparison Project Phase 5 (CMIP5). find that this had negligible (<0.03%) occurrence absence anthropogenic warming. It was still rare but not implausible event (roughly 1–3% likelihood) taking warming into effect....