What Is—and Is Not—Considered Settled with Climate Science?

, senior climate scientist | October 8, 2015, 10:55 am EDT
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I was reminded this week, by an exchange of words between Senator Ted Cruz and Sierra Club President Aaron Mair, that at hearings on policy, the discussion can go off on a tangent toward climate science and what is or is not settled. Spoiler alert: settled is as close as scientists get to knowing that a scientific finding has been a widely accepted explanation or law for which no credible alternative exists. For some concepts, like gravity or the fact that carbon dioxide traps heat in Earth’s atmosphere, the science has been widely accepted for over a century or longer.

Senator Ted Cruz questions Sierra Club President Aaron Mair at an October 7, 2015 subcommittee hearing. Photo: Video screenshot from senate.gov.

Screen capture of Senator Ted Cruz during the U.S. Senate Committee on the Judiciary hearing “Opportunity Denied: How Overregulation Harms Minorities.” The hearing in the Subcommittee on Oversight, Agency Action, Federal Rights and Federal Courts was on October 6, 2015 with Chairman Cruz presiding.

What are the top 3 signs that a topic in science is settled?

  1. No study on the topic has stood the test of time as having offered a compelling alternative.
  2. Topic is common knowledge
  3. It is declared a scientific law or theory (a technical term for a widely accepted explanation).

Before we jump to the top three widely accepted climate science developments below, let’s quickly cover how we got to these.  The operative phrase in sign number one above is that the topic has “stood the test of time.” Many studies are conducted after a finding is presented in order to test if it can be disproved.  During this process some studies may temporarily look like they have disproven the original concept. Upon closer inspection, flaws in the approach, human error, or other factors may emerge that, once discovered, return the scientific community back to the fundamental explanation.

We have seen this with climate science. For example, at one point a study of satellite measurements of the atmosphere suggested it was not warming at the expected rate, which was soon overturned.  The original study had flaws mainly due to factors not accounted for such as the decay in the satellite orbit over time. Once these were properly accounted for, it was shown the satellite data did measure warming of Earth’s atmosphere in the expected way from excess heat-trapping gases. Politicians may continue to mention overturned studies, but fact checkers in the media and the scientific community are here to set the record straight.

Three widely accepted scientific understandings in climate science

  1. Carbon dioxide traps heat and exerts major influence on Earth’s temperature when its concentration increases or decreases: upheld since the late 19th century.
  2. Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia.
  3. Human influence has been the dominant cause of the observed warming since the mid-20th century.

The second point includes global average surface temperature which increased over the first 15 years of this century at a rate of warming at least as great as the rate over the last half of the 20th century. With regard to the third point, my favorite piece of evidence is the fingerprint of fossil fuel carbon atoms representing more and more of the CO2 molecules of the atmosphere over time since isotopic measurements began around 1980.  Take a look at the second box within this IPCC figure from the fifth climate assessment report where more negative values equal a higher proportion of fossil fuel carbon in the atmospheric CO2 molecules (see figure 1).

With these three widely accepted scientific understandings, we have the basic points to confidently tell friends, relatives and colleagues, “Climate change is occurring now, we are the primary cause, and scientists agree.”  This has sparked many conversations about what to do in light of this knowledge. Personally, with these three scientific understandings, I do not use the word “belief.”

IPCC AR5 Figure 6-3

Figure 1. Image and caption source: Intergovernmental Panel on Climate Change Fifth Assessment Report Working Group 1 Figure 6.3 | Atmospheric concentration of CO2, oxygen, 13C/12C stable isotope ratio in CO2, CH4 and N2O recorded over the last decades at representative stations (a) CO2 from Mauna Loa (MLO) Northern Hemisphere and South Pole Southern Hemisphere (SPO) atmospheric stations (Keeling et al., 2005). (b) O2 from Alert Northern Hemisphere (ALT) and Cape Grim Southern Hemisphere (CGO) stations (http://scrippso2.ucsd.edu/ right axes, expressed relative to a reference standard value). (c) 13C/12C: Mauna Loa, South Pole (Keeling et al., 2005). (d) CH4 from Mauna Loa and South Pole stations (Dlugokencky et al., 2012). (e) N2O from Mace-Head Northern Hemisphere (MHD) and Cape Grim stations (Prinn et al., 2000).

What is currently not settled in climate science (as of October 8, 2015)

The date is important as this topic is evolving and new findings are appearing with each successive research publication. Major gains have occurred in our understanding of how climate change is influencing global scale changes on average. Significant advances on the scale of what matters most to people (e.g. my local river floodplain, my farm field, the town where my children live, the coastal road along my commute to work) are still evolving. This is especially true when it comes to our understanding of the pace and magnitude of change, which involves the interplay of energy choices we make around the world and the current state of scientific research.

For most scientists, it’s not enough to just say that the science is settled; to be more precise, we know that the science is settled to the point of knowing that we do have a choice about the future we can inherit. We know human energy choices and land use decisions influence the climate and thereby the world within which we live and work. With every choice we make we will continue to have a need to monitor, measure, and calculate the likely consequences, measure the economic decisions with various thresholds crossed, and evaluate the successful and unsuccessful adaptation decisions. The reaction to scientific uncertainty is not to throw our hands up and walk away from the risks scientists have identified; the answer is to do what we can now, based on what we know, and to keep learning together.

Correction: The original version of this post contained an error in the paragraph beginning “The second point includes global average surface temperature”: “the last half of the 20th century” was misstated as “the first half of the 20th century”.

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  • Dano2

    IMHO you should also address the Detection and Attribution problem settled in the 1980s.



  • alex7070

    Discussing these existential matters with these blind clerics is a complete waste of time. Only when the money funding their blindness dries up will they come out of their coma.