Was there a definitive moment when the scientific community reached a consensus about climate change?More in our defining moments in climate change series

Climate scientists are canaries in the coalmine - highly attuned to sense danger before we blunder into it. For decades, various researchers have issued calamitous warnings about climate change. But was there a moment when science collectively, definitively, dropped off the perch?

During the 1990s, scientists were still debating the most basic assertions of climate change science. Was the world indeed warming? Consensus was growing, but slowly and many scientists remained undecided.

Two bold scientific statements bookended the decade – James Hansen's statement to the US senate in 1988 and the 1999 hockey stick graph. Maligned and celebrated, the two were influential in bringing climate change into the public consciousness. Yet both were accused of using unproven methods to reach their conclusions, damaging the credibility of climate science and paving an easy road for denialism.

Hansen, head of Nasa's Goddard Institute for Space Studies, told the energy and natural resources committee of the United States Senate in 1988 that his research on human-induced global warming was unequivocal. "The greenhouse effect has been detected, and it is changing our climate now."

The next day, the New York Times ran the headline "Global Warming Has Begun". It was a galvanising moment for public opinion and Hansen became a poster boy for the environment movement, but not all were convinced.

"Hansen was a scientist that bumped right up against the edge of activism and a lot of scientists have been very uncomfortable going over into that. And rightly so ... if you're seen too much as an activist then people won't trust your science," said Marshall Shepherd, 2013 president of American Meteorological Society.

The US National Climate Assessment (NCA), released last week, echoed Hansen's words 26 years later: "Climate change is already affecting the American people".

But even the NCA, built as it was on the work of 800 scientists, has been criticised for a lack of nuance.To maintain credibility, climate science must walk the narrow ledge between conservatism and activism.

Some scientists feel the hockey stick graph, published in 1999, dangled both feet over this edge. Michael Mann, Raymond Bradley and Malcolm Hughes used data gathered from tree rings, lake sediments, ice cores and corals to recreate the global temperature over the past 1,000 years. The image they produced was a startling visual communiqué of the world's post-industrial warming trend. It was featured prominently in the Intergovernmental Panel on Climate Change's (IPCC) 2001 report.

But Mann et al's willingness to use unproven methodology irked some scientists, including Hulme: "I don't think it was seminal for scientists. To me that was never a decisive interventional piece of evidence. The data was absolutely scanty."

Shepherd disagrees. "I think it would be characterised as a watershed moment in climate science," he says, although he recognises it as "one of the singular most polarising graphs or scientific pieces of data that exist".

The problem for Mann and Hansen is the world wants to see all the canaries keeling over together, a clear public moment of unequivocal proof. Shepherd says the public wrongly see science like a court case, in which reasonable doubt can outweigh a larger body of evidence. But science has a natural indifference to the desire for certainty. Each time a scientist gets too far ahead of the curve it makes the scientific community deeply uncomfortable. Disagreements of this kind can be latched onto as evidence that the scientific process is flawed, fuelling the denial movement.

Consensus on climate change built incrementally through the 1990s until, by the time the 2001 IPCC report came out (with the hockey stick graph in it), there were very few scientists who felt uncomfortable attributing some climate change to human activity.

But Hulme says there was no collective eureka moment and there will always be doubt and questions. "Science doesn't really do that. It is always an unending process of confirmation, correction, refutation … It is the collective social practice of science that in the end gives science its particular credibility and status. But it's a rather harder thing to get to the bottom of because you can't just focus on one charismatic individual."

Notably absent from the consensus building of the 1990s were the voices of climate scientists from developing countries, says Chandra Bhushan, deputy director of the Centre for Science and Environment in Delhi. For the most part, this was because the research was simply not happening. But increased interest from political leaders during this time led some countries (mainly in the Indian subcontinent, China and southeast Asia) to implement climate science programmes. Even so, the imbalance perpetuates today.

Bhushan says climate scientists from the south "still play a very little role in developing consensus on climate change negotiations." The latest IPCC report drew more than 90% of its research material from developed countries.

Scientists participate in the compiling of IPCC reports with funding from their governments, meaning wealthy countries can afford to participate more in the process. This has the effect, Bushan argues, of politicising the reports, which he says have focussed unduly on the impacts of climate change on the developed world.

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