Anthropogenic and natural warming inferred from changes in Earth’s energy balance

Journal name:
Nature Geoscience
Volume:
5,
Pages:
31–36
Year published:
DOI:
doi:10.1038/ngeo1327
Received
Accepted
Published online

The Earth’s energy balance is key to understanding climate and climate variations that are caused by natural and anthropogenic changes in the atmospheric composition. Despite abundant observational evidence for changes in the energy balance over the past decades1, 2, 3, the formal detection of climate warming and its attribution to human influence has so far relied mostly on the difference between spatio-temporal warming patterns of natural and anthropogenic origin4, 5, 6. Here we present an alternative attribution method that relies on the principle of conservation of energy, without assumptions about spatial warming patterns. Based on a massive ensemble of simulations with an intermediate-complexity climate model we demonstrate that known changes in the global energy balance and in radiative forcing tightly constrain the magnitude of anthropogenic warming. We find that since the mid-twentieth century, greenhouse gases contributed 0.85°C of warming (5–95% uncertainty: 0.6–1.1°C), about half of which was offset by the cooling effects of aerosols, with a total observed change in global temperature of about 0.56°C. The observed trends are extremely unlikely (<5%) to be caused by internal variability, even if current models were found to strongly underestimate it. Our method is complementary to optimal fingerprinting attribution and produces fully consistent results, thus suggesting an even higher confidence that human-induced causes dominate the observed warming.

At a glance

Figures

  1. Radiative forcings and observed and simulated warming using observationally constrained model parameters.
    Figure 1: Radiative forcings and observed and simulated warming using observationally constrained model parameters.

    a, Radiative forcings from historical reconstructions and the SRES A2 scenario for different forcing agents. b,c, Emulation of observed global-mean temperature (b) and observed ocean heat uptake to 700m (c; ref. 27) with the Bern2.5D climate model. The grey shading denotes the 5–95% uncertainty range.

  2. Probabilistic estimates of the historical and future cumulative radiative forcing and the Earth/'s energy budget.
    Figure 2: Probabilistic estimates of the historical and future cumulative radiative forcing and the Earth’s energy budget.

    a,b, Composition of the cumulative forcing energy (a) and the Earth’s energy budget (b), given by eq. (1), during the years 1850–2100 for historical forcing and the SRES A2 scenario. c,d, Comparison of the simulated results with observations (circles2) for 1950–2004. The error bars denote the 5–95% uncertainty range of the probabilistic estimates.

  3. Contributions of different forcing agents to the total observed temperature change.
    Figure 3: Contributions of different forcing agents to the total observed temperature change.

    a, Time series of anthropogenic and natural forcings contributions to total simulated and observed global temperature change. The coloured shadings denote the 5-95% uncertainty range. bd, Contributions of individual forcing agents to the total decadal temperature change for three time periods. Error bars denote the 5–95% uncertainty range. The grey shading shows the estimated 5–95% range for internal variability based on the CMIP3 climate models. Observations are shown as dashed lines.

  4. Comparison of observed changes in global mean temperature and energy content with model estimates of internal variability.
    Figure 4: Comparison of observed changes in global mean temperature and energy content with model estimates of internal variability.

    a,b, Distribution of linear trends for surface temperature and total energy content from unforced control simulations (grey bars) and observations (red lines) during 1956–2007. c,d, Upper 95% percentile as in a,b estimated for each CMIP3 model (grey lines). Observations are shown in red. b,d, Observations of the ocean heat uptake to 700m are compared with the net radiation imbalance of the CMIP3 models. The total energy content of the Earth is difficult to measure but is about 40% higher than the 700m heat uptake, which is indicated in b,d as dashed lines.

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Affiliations

  1. Institute for Atmospheric and Climate Science, ETH Zurich, Universitätstrasse 16, 8092 Zurich, Switzerland

    • Markus Huber &
    • Reto Knutti

Contributions

M.H. performed the climate model computations and statistical analysis. Both authors designed the study and wrote the paper.

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The authors declare no competing financial interests.

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