|Brown, Patrick - Duke University|
|Li, Wenhong - Duke University|
|Cordero, Eugene - San Jose State University|
Submitted to: Scientific Reports
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/25/2015
Publication Date: 4/21/2015
Citation: Brown, P., Li, W., Cordero, E., Mauget, S.A. 2015. Comparing the model-simulated global warming signal to observations using empirical estimates of unforced noise. Scientific Reports. 5:9957 DOI:10.1038/srep09957.
Interpretive Summary: The increase in global mean surface air temperature (GMT) over the latter half of the 20th century is frequently cited as evidence of global climate change. However, since the late 1990’s this increasing trend in GMT has seen a leveling off, which some refer to as a global warming ‘hiatus’. Is this break in global warming a) just part of the noisy development of an overall long-term warming trend, or b), does it represent a real reduction of the trend consistent with a relatively weak response of the Earth’s climate system to increasing atmospheric CO2 levels? In this research we try to answer this question by estimating how much essentially noise-like climate variation should affect GMT, and then comparing GMT variation since the 1990’s with that range of variation. Although this range of GMT variation is usually estimated using climate models, these models do not reproduce the multi-decadal variation in GMT found in instrumental records. Instead, we estimate noise-like GMT variation based on instrumental and reconstructed global mean temperature records of the past. We used these estimates of global temperature ‘noisiness’ to compare observed GMT change to the forced GMT signal produced by climate models under possible increases in atmospheric CO2 . This analysis shows that the reduced GMT warming of the hiatus period is consistent with the noisy increase in GMT predicted by climate models forced by moderate increases in atmospheric CO2 levels, but likely inconsistent with projected changes in GMT associated with steeper increases.
Technical Abstract: The comparison of observed global mean surface air temperature (GMT) change to the mean change simulated by climate models has received much attention. For a given global warming signal produced by a climate model ensemble, there exists an envelope of GMT values representing the range of possible unforced states of the climate system (the Envelope of Unforced Noise; EUN). Typically, the EUN is derived from climate models themselves, but climate models might not accurately simulate the correct characteristics of unforced GMT variability. Here, we create a new empirical EUN that is based on instrumental and reconstructed temperature records of the past. We use this empirical EUN to compare observed GMT change to the forced signal produced by climate models. We find that the empirical EUN is wide enough so that the interdecadal variability in the rate of global warming over the 20th century does not necessarily require corresponding changes in the rate of increase of the forced signal. The empirical EUN also indicates that the GMT progression of the past decade or so is consistent with a forced signal corresponding to a middle emissions scenario, but likely inconsistent with a forced signal corresponding to the steepest emissions scenario.