Observed global mean surface temperature (GMST) combines a forced response with internal variability, and quantifying internal variability is important in assessing the reaching of key thresholds, such as the 1.5 °C warming threshold in the Paris Agreement. This paper uses observational data to estimate internal variability. Since the current period of warming began in the 1970s, the 10-year mean of GMST has been very close to the 30-year mean for the period it is centred in and can therefore be considered as a robust indicator of the recent state of the climate. The range between the 5th and 95th percentile of annual residuals of observed GMST is 0.319 °C, substantially less than the corresponding range in large model ensembles, implying that the first individual year above 1.5 °C may occur later than indicated by climate models. The largest annual residuals are mostly associated with large-amplitude El Niño-Southern Oscillation (ENSO) events or major volcanic eruptions, with the relationship between cool years and La Niña more consistent than that between warm years and El Niño. The relationship between multi-year GMST means for differing periods indicates that the probability that the 1.5 °C threshold has been crossed (using the IPCC definition of the midpoint of the first 20-year period above the threshold) exceeds 50% once the most recent observed 11-year mean reaches 1.43 °C.

Quantifying warming of the Earth’s climate system since the late 19th century and the ratio of regional to global warming are required when examining implications of Paris Agreement global warming targets. To estimate these terms reliably, the limitations in quality and length of observed records, differences between climate models and observations, and different results dependent on temporal and forcing contexts must be taken into account. Here we use observational datasets currently available back to 1860 and the latest set of global climate model simulations from CMIP5/6 to examine the warming of Australia in the past and projected future. We find that Australia has warmed by 1.5 °C (1.3–1.8 °C) during 1850 - 2019, at a ratio of ~1.4 times the global warming of ~1.1 °C. Models generally produce a lower ratio of Australian to global warming than in observations, which may be related to model biases or internal climate variability.