Statistical proof of 'the pause' – Overestimated global warming over the past 20 years

Commentary from Nature Climate Change, by John C. Fyfe, Nathan P. Gillett, & Francis W. Zwiers

Recent observed global warming is significantly less than that simulated by climate models. This difference might be explained by some combination of errors in external forcing, model response and internal climate variability.

Global mean surface temperature over the past 20 years (1993–2012) rose at a rate of 0.14 ± 0.06 °C per decade (95% confidence interval)1. This rate of warming is significantly slower than that simulated by the climate models participating in Phase 5 of the Coupled Model Intercomparison Project (CMIP5). To illustrate this, we considered trends in global mean surface temperature computed from 117 simulations of the climate by 37 CMIP5

models (see Supplementary Information).

These models generally simulate natural variability — including that associated

with the El Niño–Southern Oscillation and explosive volcanic eruptions — as

well as estimate the combined response of climate to changes in greenhouse gas

concentrations, aerosol abundance (of sulphate, black carbon and organic carbon,

for example), ozone concentrations (tropospheric and stratospheric), land

use (for example, deforestation) and solar variability. By averaging simulated

temperatures only at locations where corresponding observations exist, we find

an average simulated rise in global mean surface temperature of 0.30 ± 0.02 °C

per decade (using 95% confidence intervals on the model average). The

observed rate of warming given above is less than half of this simulated rate, and

only a few simulations provide warming trends within the range of observational

uncertainty (Fig. 1a).

Figure 1 | Trends in global mean surface temperature. a, 1993–2012. b, 1998–2012. Histograms of observed trends (red hatching) are from 100 reconstructions of the HadCRUT4 dataset1. Histograms of model trends (grey bars) are based on 117 simulations of the models, and black curves are smoothed versions of the model trends. The ranges of observed trends reflect observational uncertainty, whereas the ranges of model trends reflect forcing uncertainty, as well as differences in individual model responses to external forcings and uncertainty arising from internal climate variability.

The inconsistency between observed and simulated global warming is even more

striking for temperature trends computed over the past fifteen years (1998–2012).

For this period, the observed trend of 0.05 ± 0.08 °C per decade is more than four

times smaller than the average simulated trend of 0.21 ± 0.03 °C per decade (Fig. 1b).

It is worth noting that the observed trend over this period — not significantly

different from zero — suggests a temporary ‘hiatus’ in global warming. The

divergence between observed and CMIP5-simulated global warming begins in the

early 1990s, as can be seen when comparing observed and simulated running trends

from 1970–2012 (Fig. 2a and 2b for 20-year and 15-year running trends, respectively).

The evidence, therefore, indicates that the current generation of climate models

(when run as a group, with the CMIP5 prescribed forcings) do not reproduce

the observed global warming over the past 20 years, or the slowdown in global

warming over the past fifteen years.

This interpretation is supported by statistical tests of the null hypothesis that the

observed and model mean trends are equal, assuming that either: (1) the models are

exchangeable with each other (that is, the ‘truth plus error’ view); or (2) the models

are exchangeable with each other and with the observations (see Supplementary

Information).

Brief: http://www.pacificclimate.org/sites/default/files/publications/pcic_science_brief_FGZ.pdf

Paper at NCC: http://www.nature.com/nclimate/journal/v3/n9/full/nclimate1972.html?WT.ec_id=NCLIMATE-201309

Supplementary Information (241 KB) CMIP5 Models