Climate scientists first laughed at a ‘bizarre’ campaign against the BoM - then came the harassment

For more than a decade, climate science deniers, rightwing politicians and sections of the Murdoch media have waged a campaign to undermine the legitimacy of the Bureau of Meteorology's temperature records.

Those records say Australia has warmed by 1.4C since 1910, the year when the bureau's main quality-controlled climate dataset starts.

Extremely hot days come along more often than they used to, and the warming trends are happening everywhere, at all times of the year.

As a target for those with an often visceral distrust of the established science of human-caused global heating, the bureau's temperature record might be seen as ground zero.

"This has frankly been a concerted campaign," says climate scientist Dr Ailie Gallant, of Monash University. "But this is not about genuine scepticism. It is harassment and blatant misinformation that has been perpetuated."

Despite multiple reviews, reports, advisory panels and peer-reviewed studies rejecting claims that its temperature record was biased or flawed, Gallant says the "harassment" of the bureau has continued.

In the latest challenge, the Australian newspaper - which over the years has elevated claims from sceptics and bloggers on to its pages - is calling for a "proper, expert appraisal" of temperature records after running claims that mercury thermometers occasionally used by the bureau were recording lower temperatures at one location than newer automatic probes in the same place.

One former executive, who for eight years was responsible for the bureau's main climate record, says the constant criticism has affected the health of scientists over many years, who were diverted from real research to repeatedly answer the same questions.

One of the earliest formal attacks came in 2010, when the then Liberal senator Cory Bernardi joined sceptics to demand the National Audit Office investigate the internationally-recognised technique known as homogenisation, which aims to remove artificial changes in temperature readings such as if a weather station is moved or buildings are erected that could alter readings.

A small band of sceptics have long claimed a conspiracy, without evidence, that the bureau deliberately used homogenisation in its centrepiece dataset of 112 weather stations - known as the Australian Climate Observations Reference Network - Surface Air Temperature (Acorn-Sat) - to make the heating of the continent's climate appear worse.

That effort to engage the audit office failed, but the bureau established its own review with a panel of independent international experts.

That panel reported that "as the world's first national-scale homogenised data set of daily temperatures" it had "a high level of confidence in the national temperature trends" from Acorn-Sat.

Another independent expert panel reported for three years from 2015 and again backed the bureau's methods, despite "unsolicited submissions" which they said "do not provide evidence or offer a justification for contesting the overall need for homogenisation and the scientific integrity of the bureau's climate records."

Undeterred, sceptics have continued, sending a barrage of questions to the bureau while publishing blogs that were echoed by multiple stories and opinion pieces in the Murdoch media.

Former bureau boss Rob Vertessy, who left the agency in 2016, has said the attacks were pushed by an organised "fever swamp" of climate science denial.

"Every minute a BoM executive spends on this nonsense is a minute lost to managing risk and protecting the community. It is a real problem," he told Guardian Australia.

Neil Plummer worked at the bureau for 33 years, leaving in 2019, and between 2010 and 2018 was the senior executive responsible for Acorn-Sat.

"The answers [from the reviews] should have been very clear," he says. "There was always a close to zero chance that the bureau had deliberately introduced a warming trend."

The warming trend in Australia is visible in the adjusted and adjusted data, and Plummer says it also aligns with the warming seen in the ocean around the continent, and with "18 other independent data sets around the world, including from satellites looking at the lower atmosphere".

Plummer says from 2010 onwards, a constant stream of "distractions coming from critics" meant leading climate scientists under his charge were having to respond repeatedly to the same questions.

"The cost was high because we were diverting scientists to addressing these very, very specific questions which overall had very little public benefit."

Plummer says when the bureau was accused of conspiring to make warming seem worse, he thought it was initially "bizarre and laughable".

"But then we heard climate scientists in Australia and overseas were receiving death threats or getting hate messages and there were personal attacks on our own people. Then it gets serious and raised a big concern about where this was all going. Would we end up in some politicised inquiry that was potentially career-ending?

"My biggest concern was for the health of the climate scientists. They did that work admirably and stuck with it but for a significant number it affected their health and wellbeing and their professional standing. They were concerned it was going to damage their careers."

Dr Greg Ayers, a former director of the bureau and leading CSIRO atmospheric scientist, has written four peer-reviewed papers testing claims made by sceptics.

"There's a lot of assertion [from sceptics] but I haven't seen much science," said Ayers. "If you are going to make claims then we need to do peer-reviewed science, not just assertion."

In one paper, Ayers, who left the bureau 13 years ago, compared the Acorn-Sat warming trend with four other international data sets that use weather balloons, satellites and raw data from the bureau. In all cases, Ayers found a comparable warming trend.

One longstanding bureau critic is Dr Jennifer Marohasy of the rightwing Institute of Public Affairs - the Melbourne-based group that has been a central cog in attempts to deny or undermine human-caused climate change in the eyes of the public since the 1990s.

For several years, Marohasy has claimed the bureau's practice of taking automatic measurements from the final second of each minute breached guidelines from the UN's World Meteorological Organization, which recommends temperatures should be averaged over a minute.

But Ayers and the bureau say the response time of its automatic probes means the recorded measurement is effectively an average of the temperature over the previous 40 seconds to 80s.

In another study, Ayers examined if the bureau's recording method could generate a bias towards higher temperatures.

Ayers took all the data recorded at two locations to see if taking extra readings across a minute made any difference to the temperatures recorded. While tiny differences were found, the study concluded the bureau's method was "not at risk of bias".

Marohasy, and the Australian, have claimed the bureau's methods contradict guidance from the WMO, despite previous studies arguing the contrary.

Dr Anthony Rea, a director at the WMO, says the organisation does not audit members but does produce guidelines "and members can implement these in different ways depending on their specific requirements".

He says: "No measurement is perfect, the bureau's temperature measurements included, but we all know there are multiple lines of evidence proving that global temperatures are rising, from satellites to ocean buoys to deep sea profilers. All evidence points in the same direction."

Marohasy says her own analysis of three years of Brisbane airport temperature data shows automatic thermometers record warmer temperatures than parallel mercury probes 41% of the time and are cooler 26% of the time. This, she argues, means "future new record hot days could be a consequence of the probe rather than global warming".

But scientists and the bureau say it was never possible - and it was never claimed by them - that mercury probes recorded the exact same temperatures as automatic probes.

The bureau has analysed the same Brisbane data, finding the maximum daily temperatures recorded manually by the "liquid-in-glass" mercury probes were on average only 0.02C cooler than automatic probes across three years. For minimum daily temperatures, the manual probes were 0.02C warmer than the automatic probes.

A bureau statement said: "There is no significant systematic difference in maximum and minimum temperatures, or the [daily] temperature range, or the effective response time of measurement, observed using mercury thermometers compared with platinum resistance probes."

Gallant says: "I don't even know what [the sceptics] are trying to highlight. That there are differences in measurements? We know that already and we take that into account.

"Some people might ask why can't the temperature record be scrutinised? Scrutiny is part of the scientific process, but [critics] never put their findings into reputable peer-reviewed journals. The bureau has done this. Scientists have done this, and they all show the same thing.

"It's just someone's opinion until it's published. That's why I would argue this is harassment. They need to put up or shut up."

The Guardian sent questions to The Australian, but did not receive a response.

In an 11-page response to questions, Marohasy defended her claims and said she intended to publish responses to the work of Ayers and to publish details of her current work on mercury probes with her IPA colleague, Dr John Abbot.

She said it was true that she had "pursued this issue with the bureau and some members of its staff for close to a decade", adding: "But if they had acknowledged the genuine issue and the public interest in sharing the data, in say, 2015, we could have moved on."

"There has been no harassment on our part," she said. Because the bureau had initially refused to release the Brisbane data from mercury probes, leaving her and Abbot to resort to the Administrative Appeals Tribunal to get the information, "the harassment, obstruction and misinformation has been by the bureau".