MANY lines of evidence confirm that humans are causing global warming. Scientists measure more heat-trapping greenhouse gases in the atmosphere, a result of burning fossil fuels such as coal and oil. Satellites measure less heat escaping to space at the exact wavelengths that greenhouse gases trap heat. Human fingerprints are being observed all over our climate.
As the evidence piled up, overwhelming agreement developed among climate scientists. A 2009 survey of Earth scientists found that among actively researching climate scientists, more than 97 per cent agreed that humans were changing global temperatures. A 2010 study found that among climate scientists who had published peer-reviewed climate research, there was 97-98 per cent consensus.
I was part of a team that examined 21 years of climate research, identifying all papers stating a position on human-caused global warming. Some papers explicitly said humans were causing global warming. Others were more specific, quantifying just how much of global warming was caused by humans. Among papers giving a position on the topic, 97.1 per cent agreed humans were causing global warming.
Replication is the heart of scientific research. We checked our results by asking the actual scientists who authored the climate papers to rate their own research. As a result 1200 scientists rated their own papers. Among papers self-rated as stating a position on human-caused global warming, 97.2 per cent endorsed the consensus.
Just as many independent observations confirm human-caused global warming, there are many independent indicators of overwhelming agreement among climate scientists.
Consensus matters. When people correctly perceive that scientists agree about climate change, they're more likely to support climate action. Consequently, those who oppose policy to mitigate climate change have sought to cast doubt on the consensus for over two decades.
This is done with the same techniques of the tobacco industry and right-wing ideologues who denied smoking causes cancer.
Fake experts are invoked to portray the impression of ongoing scientific debate. Evidence is cherry-picked and any inconvenient data is ignored. Just as the tobacco industry demanded unreasonable levels of proof that smoking caused cancer, opponents of climate action employ the same technique of unrealistic expectations.
The same tactics are on offer in Anthony Cox's opinion piece "Politics muddy global warming debate" (Herald 27/11). Mr Cox magics away the overwhelming scientific consensus via "unrealistic expectations". He ignores any paper that doesn't specify the percentage of global warming humans have caused.
Thousands of papers endorsing the consensus conveniently disappear. Statements such as "global warming caused by greenhouse gases emitted into the air is a result of the human activities" or "accumulating evidence points to an anthropogenic 'fingerprint' on the global climate change that has occurred in the last century" do not endorse the consensus, according to Mr Cox.
Our understanding of climate change is based on many lines of evidence. The result is an overwhelming scientific consensus, observed in a number of independent studies.
However, those who reject climate science persistently muddy the waters in the same manner right-wing ideologues and the tobacco industry denied the science linking smoking with cancer.
John Cook is research fellow in climate communication at the Global Change Institute, University of Queensland. His paper is at iopscience.iop.org/1748-9326/8/2/024024
See his video at http://bcove.me/c1li8rcl