Is Australia warming more rapidly than the rest of the world?
In 1979-1992, ACORN 2.1 showed 0.17C more anomaly warming than 10 global, southern hemisphere and Australian datasets that are considered accurate international monitors of temperature trends (see source data).
In 1993-2020, ACORN 2.1 anomalies showed 0.32C more anomaly warming, nearly twice the previous difference, with the adjusted Australian temperature anomalies almost 0.5C warmer since 2013 than the average of the main datasets measuring climate change around the world.
An ACORN 2.1 anomaly shift occurred in 2013.
The comparison starts in 1979 because that was the first full year of observations calculated from lower troposphere satellite measurements by the University of Alabama in Huntsville (UAH).
The chart below shows the difference between adjusted ACORN 2.1 mean temperatures and 10 other datasets including global land and ocean, land only, southern hemisphere, satellite (including Australia) and sea surface temperatures.
The anomaly differences between ACORN 2.1 and each of the 10 other datasets are charted below.
The Bureau of Meteorology notes on its ACORN FAQ that "Globally, recent studies have shown that the satellite data are warming at a similar rate to both the surface observations, and radiosonde records. This is also the case over Australia, where warming trends since 1979 are very similar for satellite and surface data."
The BoM FAQ links to On the sensitivity of Australian temperature trends and variability to analysis methods and observation networks which provides the chart below comparing ACORN with other global temperature measurements from 1911 to 2010, including UAH satellite estimates since 1979 (top lines). These differences can be compared with trends shown above since 2010.
The BoM FAQ also notes that "The trends in the Bureau’s temperature data are in close agreement with trends derived independently by other agencies. Warming in Australian surface temperature closely matches warming seen in the oceans surrounding Australia and in the Pacific Islands."
The bureau below updates the chart above from 1910-2010 to 1910-2018, showing anomalies in a range of global datasets based on data provided by the BoM, as well as Australian regional sea surface temperatures. The charts below again illustrate the divergence from UAH satellite lower troposphere measurements and sea surface temperatures, mostly since around 2013, that are at odds with the claim they are "are in close agreement with trends derived independently by other agencies" :
The Bureau of Meteorology also provides annual sea surface temperature anomalies for Australia (sourced to America's National Oceanic and Atmospheric Administration ERSSTv5 dataset) which can be compared against ACORN 2.1 anomalies from 1979 to 2020 :
A possible explanation is low rainfall in Australia since 2013 compared to other parts of the world.
2019 was Australia's driest year on record with a rainfall anomaly of -190.32mm. However, the shift in ACORN mean temperature anomalies began in 2013.
For example, below are the annual average differences between ACORN 2.1 and the 10 other datasets since 2010.
2010 : 0.22C
The anomaly difference in 2019 was due to Australia's low rainfall and cloud cover which caused a natural 0.12C jump in maximum temperatures compared to the anomaly difference in 2018.
However, the timing and magnitude of the increasing difference between ACORN 2.1 anomalies and other global temperature datasets is too significant to be entirely attributable to rainfall.
A possible contributor is the BoM's use of automatic weather stations and small rather than large thermometer screens since the mid 1990s, with research suggesting these can result in electronic observations being biased high on warm to hot days (see BomWatch).
Such days were more frequent in 2013-2020 due to below average rainfall and cloud cover.
Ken Stewart's How Accurate is Australia's Temperature Record? shows similar anomaly divergence when ACORN 2.1 is compared to rainfall, sea surface temperatures, the Southern Annular Mode, cloudiness and evaporation.
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