Long-term natural variability and 20th century climate change

Abstract

Global mean temperature at the Earth's surface responds both to externally imposed forcings, such as those arising from anthropogenic greenhouse gases, as well as to natural modes of variability internal to the climate system. Variability associated with these latter processes, generally referred to as natural long-term climate variability, arises primarily from changes in oceanic circulation. Here we present a technique that objectively identifies the component of inter-decadal global mean surface temperature attributable to natural long-term climate variability. Removal of that hidden variability from the actual observed global mean surface temperature record delineates the externally forced climate signal, which is monotonic, accelerating warming during the 20th century.climate modeling ͉ ocean variability D elineating the relative role of anthropogenic forcing, natural forcing, and long-term natural variability in 20th century climate change presents a significant challenge to our understanding of the climate system (1-7). Observations suggest the warming of the 20th century global mean surface temperature has not been monotonic, even when smoothed by a 10-20 year low-pass filter. Temperatures reached a relative maximum around 1940, cooled until the mid 1970s, and have warmed from that point to the present. Radiative forcings due to solar variations, volcanoes, and aerosols have often been invoked as explanations for this non-monotonic variation (4). However, it is possible that long-term natural variability, rooted in changes in the ocean circulation, underlies much of this variability over multiple decades (8)(9)(10)(11)(12). Quantifying whether there is a large role for long-term natural variability in the climate system is important, as such variability could exacerbate or ameliorate the impact of climate change in the near future. Further, large magnitude variability may require revisiting the types and magnitudes of imposed forcings thought to be responsible for the observed 20th century climate trajectory (12). More ominously, a climate with large magnitude natural long-term variability in general is a climate very sensitive to imposed forcings, raising concerns about extreme impacts due to future climate change (13).Due to its large heat capacity, the ocean is the likely source of natural long-term climate variability on interdecadal time scales. The oceans can impact global mean surface temperature in several ways; directly, through surface fluxes of heat, or indirectly, by altering the atmospheric circulation and impacting the distribution of clouds and water vapor. However, our understanding of how the ocean impacts the global mean surface temperature is strongly limited by available observations, which historically have consisted primarily of sea surface temperature (SST) measurements.The desire to optimally use these SST observations suggests a two-stage approach to objectively quantify the role of internal variability in the 20th century climate trajectory. The first step requires linking SST a...

Detecting an external influence on recent changes in oceanic oxygen using an optimal fingerprinting method

“…Moreover, coupling frequency between the ocean and atmosphere (once a day in MPI-ESM-LR and HadGEM2-ES simulations) might also be insufficient to fully capture climate variability in state-of-theart ESMs. This could support the assertion that the response of models to secular climate change may also be underestimated, since a more naturally variable ocean-climate system will likely be more sensitive to imposed external forcings (von Storch and Zwiers, 1999;Swanson et al, 2009). However, the piControl derived estimates of natural internal variability used in this analysis are shown to be consistent with observed variance using a standard residual consistency test and comparison of simulated noise with two observational estimates (Sect.…”

Global air temperature variability independent of sea-surface temperature influences

“…Removing both interannual and multidecadal SST variability from the observed global mean SAT record results in a climate signal that is monotonic (Swanson et al, 2009;Thompson et al, 2009). Swanson et al (2009) used a suite of climate model simulations to examine resultant global mean temperature variability after removing internal natural climate variability driven by SSTs.…”

“…Swanson et al (2009) used a suite of climate model simulations to examine resultant global mean temperature variability after removing internal natural climate variability driven by SSTs. They showed that removing the natural variability of SSTs from global mean temperature resulted in a monotonic warming trend during the 20th century.…”

“…Previous studies of the relations between SSTs and SAT have used combinations of measured data and climate-model simulated data (Kravtsov and Spannagle, 2008;Parker et al, 2007;Swanson et al, 2009;Zhang et al, 2007), and have often used filtering to remove high-frequency variability from the SSTs and SAT data (Kravtsov and Spannagle, 2008;Swanson et al, 2009;Zhang et al, 2007). No study to date has explicitly identified the trend in SAT with SST influence removed, based simply on unfiltered instrumental data for both series.…”

“…In addition to a long-term positive trend in SAT, another major feature of the SAT time series is multi-decadal variability: a warming in the early 20th century, a cooling in roughly mid-century, and an increase again since the mid-1970s. The multi-decadal SAT variability may be controlled by multi-decadal variability of the world's oceans (Baines and Folland, 2007;Knight et al, 2005;Kravtsov and Spannagle, 2008;Parker et al, 2007;Swanson et al, 2009;Zhang et al, 2007).…”