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WIREs Clim Change
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Observation‐based detection and attribution of 21st century climate change

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Climate change detection and attribution have proven unexpectedly challenging during the 21st century. Earth’s global surface temperature increased less rapidly from 2000 to 2015 than during the last half of the 20th century, even though greenhouse gas concentrations continued to increase. A probable explanation is the mitigation of anthropogenic warming by La Niña cooling and declining solar irradiance. Physical climate models overestimated recent global warming because they did not generate the observed phase of La Niña cooling and may also have underestimated cooling by declining solar irradiance. Ongoing scientific investigations continue to seek alternative explanations to account for the divergence of simulated and observed climate change in the early 21st century, which IPCC termed a “global warming hiatus.” Amplified by media commentary, the suggestions by these studies that “missing” mechanisms may be influencing climate exacerbates confusion among policy makers, the public and other stakeholders about the causes and reality of modern climate change.

Understanding and communicating the causes of climate change in the next 20 years may be equally challenging. Predictions of the modulation of projected anthropogenic warming by natural processes have limited skill. The rapid warming at the end of 2015, for example, is not a resumption of anthropogenic warming but rather an amplification of ongoing warming by El Niño. Furthermore, emerging feedbacks and tipping points precipitated by, for example, melting summer Arctic sea ice may alter Earth’s global temperature in ways that even the most sophisticated physical climate models do not yet replicate.

This article is categorized under:

  • Paleoclimates and Current Trends > Climate Forcing
(a, e) Observed variations in the recent past and plausible variations in near‐future anthropogenic forcing (based on extrapolation of the average of the RCP4.5 and GISS anthropogenic forcings shown in Figure c and c) and solar irradiance (based on repetition of an 11.2‐year cycle with amplitude 0.07%). Also shown are plausible but different scenarios for future El Niño and La Niña fluctuations and volcanic aerosol optical depth. The scenario of future variations in panel (a) is such that from 2017 to 2041 cooling by El Niño and La Niña, solar irradiance and volcanic aerosols mitigates much of anthropogenic warming over this specific period in global temperatures in (b) the middle troposphere (~5 km), (c) the lower troposphere (~2 km), and (d) at the surface, analogous to the recent “global warming pause” of the early 21st century. In contrast, in the alternative scenario of future variations shown in panel (e) there is net warming by El Niño and La Niña, solar irradiance, and volcanic activity from 2021 to 2039 that “advances” global warming by exacerbating (rather than offsetting) anthropogenic warming in (f) the middle troposphere (~5 km), (g) the lower troposphere (~2 km), and (h) at the surface
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Compared in the upper panel are the average global surface temperature anomalies (black lines) in (a) observations and (b) simulations made by the CCSM4 physical climate model (Meehl et al., ). Statistical models of the observed and modeled global surface temperature anomalies are also shown (green lines). The statistical models were constructed using monthly values of the ENSO, volcanic, solar, and anthropogenic predictors from 1979 to 2005, which is the end of the CCSM4 simulations. The lower panels compare the individual El Niño and La Niña, solar irradiance, volcanic, and anthropogenic components according to the statistical models
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Shown are the spatial patterns of surface temperature rates of change (in oC per decade) that the space‐era statistical model attributes to (a) El Niño and La Niña, (b) volcanic aerosols, (c) solar irradiance, and (d) the anthropogenic influence. On the left are the rates of change from January 2001 to December 2011 and on the right are the rates of change from January 1997 to December 2013. The integrals of these spatial patterns correspond to the global values in Table 1
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Global surface temperature observations (average of three, black line) and two statistical models of the (average) observations are compared in (a) for the period 1995 to 2012 which encompasses the “global warming hiatus.” The two statistical models are constructed from the monthly observations over two different time periods, specifically the space era (orange line), from 1979 to 2017, and the historical era (blue line), from 1882 to 2017. Compared in panel (b) are the rates of change (i.e., slopes in 12‐year intervals) and their 1σ uncertainties (in successive 12‐year intervals) in the observations and in each of the two statistical models. Compared in panel (c) are the rates of change and the 1σ uncertainties of the slopes in temperature attributable to each of the individual El Niño and La Niña, volcanic, solar, and anthropogenic components, according to both the space‐era (orange) and historical‐era (blue) statistical models
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Compared in the top panels from 1979 to 2017 are observations of global temperature anomalies (black lines) with statistical models (blue lines) constructed from the observations by linearly combining time series of natural and anthropogenic components. (a) The average of the three observational records of global surface temperature anomalies (shown in Figures a and a), (b) the average of the two observational records of lower troposphere (shown in Figure a), and (c) the average of two observational records of middle troposphere (shown in Figure a). The individual El Niño and La Niña (ENSO) and volcanic aerosol components of the global temperature anomalies determined by the statistical models are shown in the middle panel, and the solar and anthropogenic components are shown in the bottom panel, for (a) global surface temperature, (b) global lower tropospheric temperature, and (c) global middle tropospheric temperature. Table 1 lists the values of the slopes of the observed and modeled average temperatures and their respective anthropogenic, El Niño and La Niña, solar irradiance, and volcanic aerosol components for the two time periods from January 2001 to December 2011 and from January 1997 to December 2013
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The range of IPCC’s (2013) 42 CMIP5 physical model simulations of global surface temperature anomaly changes in response to the RCP4.5 anthropogenic forcing scenario (Figure a and a) is shown from 2005 to 2050 relative to a baseline of average global temperature from 1986 to 2005. For comparison, also shown from 1985 to 2012 are the three observational records of global surface temperature anomalies (Figures c and c), adjusted to the same baseline. The time series in this figure are the same as those in fig. 11.9a of IPCC AR5
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(a) Observations of global temperature anomalies in the middle troposphere, lower troposphere, and at the surface, from 1995 to 2017. The tropospheric temperature anomalies are those determined independently by the University of Alabama Huntsville, UAH (Christy, Norris, Spencer, & Hnilo, ) and Remote Sensing Systems (RSS; Mears, Wentz, & Thorne, ). The surface temperature anomalies are the most recent values of the three independent records shown in Figure 1a. The slopes in the observations and their 1σ uncertainties are compared for the period from January 2001 to December 2011, indicated by shading. This specific interval is chosen for illustration because the magnitude of the slope of five of the seven individual global temperature records is less than the 1σ uncertainty of the slope. Compared in panel (b) are the rates of change in the averages of the two middle troposphere, two lower troposphere, and three surface temperature records in successive 12‐year intervals. Compared in panel (c) are three estimates of anthropogenic climate forcing, specifically that due to CO2 concentrations, alone, determined as 3.71/log(2) × log(CO2/278), RCP4.5 (Moss et al., ), the total anthropogenic forcing used in Intergovernmental Panel on Climate Change (IPCC, ) physical climate model simulations, and an alternative estimate of the total anthropogenic forcing by GISS (Miller et al., ), from Figure c
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(a) Observations of monthly‐averaged global surface temperature anomalies from 1880 to 2017 reported separately by the UK Meteorological Office Hadley Center (Morice, Kennedy, Rayner, & Jones, ), NASA GISS (Hansen, Ruedy, Sato, & Lo, ), and NOAA (Vose et al., ). The anomalies are determined relative to the average of the absolute temperature from 1971 to 2000, with the annual cycle removed. The thick black lines are the slopes of the anomalies; their values and 1σ uncertainties are given for three separate intervals, 1900–1950, 1950–2000, and 2001–2011. The thin black lines are estimates of the temperature anomaly variations determined by statistical regression of the observations with anthropogenic and natural influences. (b) The rates of change in the observed temperature anomalies, determined as the slopes of the monthly values in successive 12‐year intervals for each of the three separate records. Indicated are the uncertainties in the slope coefficients of the linear trends in each 12‐year interval, taking into account autocorrelation. Compared in panel (c) are three estimates of anthropogenic climate forcing, specifically that due to CO2 concentrations, alone, determined as 3.71/log(2) × log(CO2/278), RCP4.5 (Moss et al., ), the total anthropogenic forcing used in Intergovernmental Panel on Climate Change (IPCC, ) physical climate model simulations, and a recent alternative estimate of the total anthropogenic forcing by GISS (Miller et al., )
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