A Limited Role for Unforced Internal Variability in Twentieth-Century Warming

Abstract

The early twentieth-century warming (EW; 1910–45) and the mid-twentieth-century cooling (MC; 1950–80) have been linked to both internal variability of the climate system and changes in external radiative forcing. The degree to which either of the two factors contributed to EW and MC, or both, is still debated. Using a two-box impulse response model, we demonstrate that multidecadal ocean variability was unlikely to be the driver of observed changes in global mean surface temperature (GMST) after AD 1850. Instead, virtually all (97%–98%) of the global low-frequency variability (>30 years) can be explained by external forcing. We find similarly high percentages of explained variance for interhemispheric and land–ocean temperature evolution. Three key aspects are identified that underpin the conclusion of this new study: inhomogeneous anthropogenic aerosol forcing (AER), biases in the instrumental sea surface temperature (SST) datasets, and inadequate representation of the response to varying forcing factors. Once the spatially heterogeneous nature of AER is accounted for, the MC period is reconcilable with external drivers. SST biases and imprecise forcing responses explain the putative disagreement between models and observations during the EW period. As a consequence, Atlantic multidecadal variability (AMV) is found to be primarily controlled by external forcing too. Future attribution studies should account for these important factors when discriminating between externally forced and internally generated influences on climate. We argue that AMV must not be used as a regressor and suggest a revised AMV index instead [the North Atlantic Variability Index (NAVI)]. Our associated best estimate for the transient climate response (TCR) is 1.57 K (±0.70 at the 5%–95% confidence level).

Moving beyond the catchment scale: Value and opportunities in large‐scale hydrology to understand our changing world

“…As well as requiring a system‐wide approach to studying the cascade of processes linking large‐scale variation to catchment outputs, a temporally‐holistic perspective is necessary. By investigating large‐scale atmospheric processes dynamics according to time‐scale (e.g., Lovejoy, ), it becomes more possible to clarify the relative importance of different contributions to large‐scale hydrological variation and apparent trends (e.g., Haustein et al, ). Similarly, characterising long‐term hydrological persistence can also be insightful for understanding extreme events and observed hydrological variations under climate change (Markonis & Koutsoyiannis, ).…”

Past warming trend constrains future warming in CMIP6 models

“…Estimates of the internal component of Atlantic multidecadal variability depend largely on how the forced signal is estimated and removed (25). It has been argued that decadal Atlantic sea surface temperature (SST) variability might largely reflect a forced signal in the period after 1980 [e.g., in (26)]. We estimate a Pacific internal variability contribution to GSAT of about −0.02°C per decade and an Atlantic contribution of about +0.01°C per decade from 1981 to 2017 (see Materials and Methods; and fig.…”

A Satellite Era Warming Hole in the Equatorial Atlantic Ocean

“…The satellite era warming hole in the equatorial Atlantic is associated with increased NASST anomalies and a cross‐equatorial SST gradient. The low‐frequency behavior of the NASST has been linked to the variability in ocean circulation (Gulev et al, 2013; McCarthy et al, 2015), surface fluxes (Clement et al, 2015 ; Keenlyside et al, 2015), and external forcing (Bellomo et al, 2017; Booth et al, 2012; Haustein et al, 2019). It is also strongly related to low‐frequency variability over the Pacific (Kucharski et al, 2016; Sun et al, 2017; Zhang & Delworth, 2007).…”

“…The role of aerosols in Atlantic climate variability is a contentious subject of ongoing research (Bellomo et al, 2017; Booth et al, 2012; Evan et al, 2009; Haustein et al, 2019; Yan et al, 2019; Zhang et al, 2013). Previous studies discussed interhemispheric SST gradient over tropical Atlantic (with warmer south and colder north) as robust response to aerosol forcing in climate models during the twentieth century (Biasutti & Giannini, 2006; Booth et al, 2012; Chang et al, 2011; Held et al, 2005).…”