Is There Consensus?

Part 2 of a series of 5 for NextGen Journal

We hear the phrase “climate change consensus” tossed around all the time. But what does that even mean? And does it actually exist?

In Part 1 we discussed the concept of a scientific consensus: overwhelming agreement (but rarely unanimity) among experts. Of course, such a consensus could be wrong, but it wouldn’t be very sensible for the public to ignore it or bet against it. If 19 out of 20 doctors said you needed surgery to save your life, would you sit in the hospital bed and argue about their motives?

When it comes to climate change, the consensus view can be summarized as follows:

  1. Human emissions of greenhouse gases, mainly from the burning of fossil fuels, are a significant force on the global climate.
  2. The expected warming from this force is beginning to show up.

Often, people will write these two points in opposite order: the Earth is warming, and it’s due to our actions. However, that’s not the order that scientists discovered them. The academic community realized the Earth was going to warm decades before that warming became clear. Flipping around these observations might imply “that the entirety of climate science is based upon a single correlation study”.

So, what do the scientists say? In fact, publishing climatologists – the most specialized and knowledgeable people there with regards to climate change – are almost unanimous in their position. 96.2% say the Earth is warming, and 97.4% say humans are causing climate change. It’s hard to know why the second figure is higher than the first – perhaps one scientist in the study thought the effects of our actions hadn’t shown up yet (i.e., point 1 but not point 2).

A year later, others built on this study. They had a larger sample of climate scientists, 97-98% of whom agreed with the consensus position. Additionally, those who agreed had higher academic credibility than those who disagreed: they had published more papers (“expertise”) and been cited more times (“prominence”).

However, it doesn’t really matter what a scientist says, as much as how they back it up. Having a Ph.D. doesn’t mean you get to stop supporting your claims. In the academic community, this is done in the peer reviewed literature.

In 2004, a random sample of almost 1000 scientific studies on climate change were examined. 75% of the studies explicitly supported the consensus position, while the remaining 25% didn’t mention it – for example, some papers wrote about climate change millions of years ago, so today’s climate wasn’t relevant. Incredibly, not a single one disagreed with the consensus.

This still doesn’t imply unanimity – remember, it was a random sample, not the entire literature. A very few dissenting studies do get published each year, but they are such a tiny fraction of the total papers that it’s not surprising that none showed up in a sample of one thousand. Additionally, these papers generally fail to stand up to further scrutiny – their methods are often heavily critiqued by the academic community. See, for example, Lindzen and Choi, 2009 and its response.

It’s clear that individual polls have limitations. They are restricted to a sample of scientists or papers, rather than the entire community. They don’t take into account which claims stood the test of time, and which were refuted. Luckily, the climate science community has another way to summarize the balance of evidence on global warming: the Intergovernmental Panel on Climate Change (IPCC). Since 1988, four assessment reports have been written by thousands of volunteer scientists worldwide. They examine the entire body of academic literature on climate change and create a summary, which is then painstakingly reviewed and scrutinized by others.

The latest report, published in 2007, is already quite out of date – due to the long review process, most of the data is from 2002 and earlier. However, it is still used by governments worldwide, so let’s look at some of its key findings:

  • “Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice and rising global average sea level.”
  • “Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic GHG concentrations.”
  • “Anthropogenic warming over the last three decades has likely had a discernible influence at the global scale on observed changes in many physical and biological systems.”
  • “Altered frequencies and intensities of extreme weather, together with sea level rise, are expected to have mostly adverse effects on natural and human systems.”
  • “Anthropogenic warming could lead to some impacts that are abrupt or irreversible, depending upon the rate and magnitude of the climate change.”

The final place to look for scientific consensus is statements from scientific organizations, such as the National Academy of Sciences. Not a single scientific organization worldwide disputes the consensus view, and many have published statements explicitly supporting it. A full list is available here, but here are some samples:

Climate change and sustainable energy supply are crucial challenges for the future of humanity. It is essential that world leaders agree on the emission reductions needed to combat negative consequences of anthropogenic climate change[.]
Thirteen national academies of science

It is certain that increased greenhouse gas emissions from the burning of fossil fuels and from land use change lead to a warming of climate, and it is very likely that these green house gases are the dominant cause of the global warming that has been taking place over the last 50 years.
Royal Society (UK)

The scientific evidence is clear: global climate change caused by human activities is occurring now, and it is a growing threat to society.
American Association for the Advancement of Science

[C]omprehensive scientific assessments of our current and potential future climates clearly indicate that climate change is real, largely attributable to emissions from human activities, and potentially a very serious problem.
American Chemical Society

Emissions of greenhouse gases from human activities are changing the atmosphere in ways that affect the Earth’s climate…The evidence is incontrovertible: Global warming is occurring. If no mitigating actions are taken, significant disruptions in the Earth’s physical and ecological systems, social systems, security and human health are likely to occur. We must reduce emissions of greenhouse gases beginning now.
American Physical Society

The Earth’s climate is now clearly out of balance and is warming. Many components of the climate system…are now changing at rates and in patterns that are not natural and are best explained by the increased atmospheric abundances of greenhouse gases and aerosols generated by human activity during the 20th century.
American Geophysical Union

It’s clear that a scientific consensus on climate change does exist. Since unanimity is virtually impossible in science, agreement over climate change can’t get much stronger than it is already.

Could all of these scientists, papers, reports, and organizations be wrong? Of course – nobody is infallible. Could that 3% of dissenting scientists triumph like Galileo? It’s possible.

But how much are you willing to risk on that chance?


10 thoughts on “Is There Consensus?

  1. Your fine summary concerns the capabilities of the science community to discover physical reality. If I may offer a preface:

    We may note the word consensus implies a voting or election – but this issue is not applied research – this is a discovery of a palpable reality which most all scientists have discovered it to be true.

    Take flat earth- believing the earth to be flat, did not make it flat. Humans did not invent the globe shape and reshape a flat earth. It was something discovered, not invented.

    Scientific consensus on CO2 warming just measures the capabilities of the scientists to discover AGW.

    Thanks for this wonderful summary of the science

  2. Guy Callender was pretty much a lone voice. Consensus was that man could never put in enough CO2 to adversely affect the climate. Eventually a few then many others came to accept the proposition than man could affect the climate. Now the consensus is; that we are already affecting the climate.

    While there is consensus that we are affecting the climate there is still argument on how much we are affecting climate, for how long and how fast future changes will occur.

    Human caused warming is not just the consensus position, but the consensus that has had to fight its way through, If you will a consensus that has been well tested and argued, not a consensus through being the resident argument.

    Good point. I am currently reading “The Warming Papers”, edited by David Archer and Ray Pierrehumbert, which reprints and discusses some of the relevant papers (including Callendar’s). I highly recommend it. -Kate

  3. I second Tony O’Brien’s point

    “Human caused warming is not just the consensus position, but the consensus that has had to fight its way through, If you will a consensus that has been well tested and argued, not a consensus through being the resident argument.”

    This illustrates the very important distinction between what a scientific consensus actually is and how the denialist crowd misrepresents it

  4. Tony O’Brien, one point to consider also is that back then, the number of people that had been actively studying the greenhouse effect was very small — I would guess countable on one hand. Is the notion of ‘consensus’ even meaningful then?

    I seem to remember that the reason ‘saturation’ fell down was, that the propagation of thermal IR in the atmosphere became interesting for remote sensing (also military) and missile guidance. It had to be understood correctly.

    Yes, a great deal of our knowledge of atmospheric science stemmed from the Cold War. Spencer Weart gives a great account of this in The Discovery of Global Warming. -Kate

  5. Perhaps some people (who aren’t liars) have this weird notion that the “consensus” was attained by having the scientists meet secretly in a smoke-filled room, or something close.

    Indeed, Lindzen, a denialist sort of climate scientist, had been promoting something similar to this idea, namely that the “consensus” was formed by some sort of secret campaign of corruption.

    I guess the actual process by which the “consensus” was achieved sounded a bit too mundane for some.

    — frank

  6. “Often, people will write these two points in opposite order: the Earth is warming, and it’s due to our actions. However, that’s not the order that scientists discovered them.”

    That’s a subtle but very well made point!

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