A former Liberal Democrat Senate candidate has attempted to expose global warming as a “doomsday cult”, claiming that global temperatures have gone sideways in the past 25 years.
However, the best indicators of global temperatures show a consistent warming trend over the past 25 years with the last eight years being the hottest eight on record.
Topher Field made the claim in an August 7 Facebook post. He makes several claims about the environment, including: “The global temperature has been going sideways for 25 years and has now dipped below historic baselines.”
He adds: “Please, someone, help me understand, WHERE IS THIS GLOBAL APOCALYPSE??? I WANT TO SEE IT!!!”
When contacted for the source of his claim, Mr Field pointed to the University of Alabama in Huntsville’s (UAH) global temperature report, which uses satellite data to determine temperatures in the lower troposphere.
Mr Field claimed in an email that satellite records are the only reliable source of global temperature data available because terrestrial instrumental datasets, including land-based thermometer readings, are influenced by external factors such as the urban heat island effect, land use changes, poor siting and patchy coverage.
However, climate experts told AAP FactCheck that the reality was the opposite and land-based readings are far more reliable than potentially problematic satellite records.
University of Melbourne climate scientist Dr Linden Ashcroft said Mr Field’s claims “have been disproven many, many times”.
“Studies that use long-term temperature records are VERY CAREFUL to identify and reduce the types of impacts mentioned,” Dr Ashcroft said in an email.
The IPCC Sixth Assessment Report (AR6) states multiple groups of researchers independently calculate global surface temperature data, accounting for external factors that may impact the datasets, a process known as homogenisation.
“Each group aggregates the raw measurement data, applies various adjustments for non-climatic biases such as urban heat-island effects, and addresses unevenness in geospatial and temporal sampling with various techniques,” the report says (page 175).
Associate Professor Malte Meinshausen, a lead author of the IPCC report, told AAP FactCheck that Mr Field’s claims are not new and researchers put great effort into addressing the impacts of urbanisation.
“The urban heat island effect is well-known and the scientists working on the temperature data series are analysing and correcting for that issue. It is not something that is just swept under the carpet,” he said.
Dr Meinshausen pointed to a review of global temperature records, which observed no significant differences in the various datasets used to construct the records after correcting for these biases.
To correct for the heat island effect, researchers will leave urban stations out of global temperature analysis or where that’s not possible will calculate the impact of urban heat by “comparing the observations with data from nearby stations in less built up areas,” Dr Ashcroft said.
According to NASA’s global land-ocean temperature index, the five-year moving average (Lowess smoothing) has risen from 0.44C above the long-term average in 1998 to 0.93C in 2021. This is backed up by regional data from Australia, which addresses any impact of urban heat islands.
A 2010 study on the reliability of surface temperature records in the US found no evidence that temperature trends were inflated as a result of poor siting of measuring stations.
“There are many datasets developed around the world, using different approaches, and they have all found the same trend (of global warming),” Dr Ashcroft said.
“There have even been studies that have derived global temperatures from pressure observations and sea surface temperature data only i.e. all land-based thermometer data have been ignored. Unsurprisingly, it also showed that temperatures are increasing around the globe.”
Dr Kevin Trenberth, from the National Center of Atmospheric Research, has written extensively on the reliability of satellite temperature data.
He told AAP FactCheck that researchers perform several alterations to cross-calibrate the various individual satellites which input data to create a global temperature dataset. But he said there are limitations as to how accurate the data can be, even with corrections.
“The instruments on satellites systematically degrade over time and the orbits of each satellite also degrade and so the geometry of what is seen changes, and it changes the time of day of the observations,” he said via email.
“If sequential satellite instruments overlap then corrections are possible but in the 1990s there was not always an adequate overlap.”
Satellite data goes through significantly more adjustment than surface temperature data. The two organisations collecting the data – UAH and Remote Sensing Systems (RSS) – use different adjustment methods, resulting in different warming trends across the two datasets.
The RSS dataset exhibits a stronger warming trend than UAH, which still shows consistent long term warming.
Mr Field also raises cycles of solar activity as being behind the increase in temperatures evident in satellite data.
Solar activity does have an impact on global temperatures and is included in IPCC figures to determine the influence of natural factors on temperature variation. However, it has been found to have a negligible impact on global warming compared to greenhouse gas accumulation, as shown in figure SPM.2 of the IPCC Summary for Policymakers.
University of Birmingham researchers also found no correlation between sunspot activity and global temperature variation.
AAP FactCheck previously debunked a similar claim based on UAH satellite data.
The Verdict
The claim that global temperatures have gone sideways in the last 25 years is false. Surface temperature data reliably shows that global temperatures have increased significantly over the past quarter of a century.
False – The claim is inaccurate.
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