Scientific consensus implies agreement and unanimity among an overwhelming majority of scientists, i.e. experts, about a particular scientific issue. Consensus is based on the evaluation of evidence from a scientific point of view and, therefore, it must be achieved by writing papers and through peer reviews aimed at investigating the veracity of a hypothesis or theory. Thus, the requirement – and the strength – of the scientific consensus is to reach such an agreement on certain theories by formulating evaluations as objectively as possible.

Scientific consensus is very important because it can influence the public opinion and, consequently, the socioeconomic development of countries. If there is scientific consensus about a theory, then this becomes a sort of public good, and everybody can start to believe its truthfulness and consequently act through an improved coordination and a reduced uncertainty. If there is no consensus among experts, then a debate can be raised between the different “parties”, i.e. the different views on the subject in question. It follows that the public opinion cannot take a sound shape, in the sense that each single person will believe what is more compliant with her own beliefs, which are formed by level and field of education, family values, and political and religious adherences. In summary, only experts can verify theories’ veracity in their area of competence and possibly reach an agreement. Only this kind of agreement can have a full scientific objective value and can be, therefore, broadcast among the non-experts.

What about the scientific consensus on climate change (CC) and, especially, on anthropogenic global warming (AGW)?  Naomi Oreskes, geophysicist and professor of history of science at Harvard, and John Cook, climate communication fellow for the Queensland’s Global Change Institute, found reliable methods to investigate this matter. The method used to collect data on scientific consensus on CC and AGW discussed by Naomi Oreskes (2004) involves the search for scientific papers written by experts in this field by typing the key word “climate change” in a database. Then, she checked whether and to what extent the papers endorsing the theory were supported, e.g. by the IPCC (International Panel on Climate Change), the NAS (National Academy of Sciences) etc. All papers agreed that the climate was changing, and in particular, the majority of them shared the opinion that it was human-caused, not natural. This result proves that there IS scientific consensus on the issue of global warming and, especially,  the anthropogenic one. Therefore, this should not be ignored both by the general public and by governments. In the other paper, John Cook (2013) collected data also by searching for scientific (climate-related) papers through key words. Then, he selected only their abstract. Afterward, these abstracts were randomly distributed to independent, anonymized raters via web, who evaluated the degree of endorsement.

The raters – who did not know anything about to whom, to which journal, etc. the papers belonged – followed Cook’s predetermined definitions of different endorsement-levels. It turned out that only 0.7% of papers rejected AGW, 0.3% were uncertain about its causes, the majority of them did not have a position on this topic, and 32.6% endorsed it. Then, the authors were asked to rate their own paper: 97.2% of the papers that had a clear position on the issue endorsed AGW. Thus, the conclusion reached by Cook was that the number of climate papers rejecting AGW is almost irrelevant and decreasing over time.

Therefore?  Is science on global warming “settled”? Of course not. Why? Because it is a complex system science. There is no way to reach a complete “settlement” of every single part of it. Even though there are well-established components about which nobody can argue – like “global temperatures are rising”– there are also competing explanations, like “ice melting is induced by humans” or “it is just a natural process.” Moreover, as in every complex science, there are speculative components, since people can also be very well paid to create confusion and misunderstanding.

What matters for either reaching or denying scientific consensus on CC and AGW is not falsification – Popper’s favorite way to “demarcate” science from non-science. In fact, ex ante, nobody’s aim is (or should be) to prove that there is absolutely no climate change. In this case the scientific method that needs to be applied is the preponderance of evidence. Nowadays, in fact, it is very easy for every polemical personality to claim that a theory is totally wrong and made up by conspirators who believe they live in a distopia. By exploiting  press coverage and people’s ignorance about climate and biological sciences, an understanding of the scientific side of the problem turns out to be impossible even by the bravest citizens and politicians without prejudices. Thus, without a vague understanding, emissions reduction policies involving expenditures (or even worse: certain costs today for uncertain benefits not tomorrow but maybe in a hundred years) will be – rationally – considered useless by the public opinion.

Actually, it shouldn’t really matter that there are speculative components, since climate science based on evidence cannot be easily falsifiable. IPCC, a scientific forum for climate change studies founded in 1988 by the World Meteorological Organization and the UN-Environmental Program, wrote that climate change is unequivocal and is happening now. Figures released by IPCC over time show clear trends of the relation between carbon dioxide emissions and rising temperatures. They do not provide a local, but a large scale level of confidence by taking into account many other factors which support this positive relation, like e.g. ice melting, sea levels rising, acid rains, oceans’ acidification and extreme weather events’ intensity. Thus, by following the scientific method known as “consilience of evidence” in which all the diverse lines of evidence come together in a consistent mutually reinforcing way, the IPCC (2007) was able to state that it is very likely (i.e. more than 90% sure) that most of the global warming in the last fifteen years is due to the increase in greenhouse gas concentration as a result of human activities. One single line of evidence, if taken alone, cannot give enough reason to believe AGW, but all these occurring factors jumping together give this theory a pretty persuasive character.

Given that, how can we proceed next? Even if there are large unknowns in the risk assessment, we can do risk management, which indeed involves risk that depends on what can happen and the probability it will happen, and management which is not as objective and scientific as the former. It is a value judgment on whether society fears more the risk of investing resources that may turn out to be “wasted” and could  be used to solve other issues today, or doing nothing and bearing the risk of accelerating the time in which we reach the moment of a possible – but, if true, irreversible – climatic tipping point. And that is the value trade-off that we face right now. For example, talking again about competing explanations, there is the issue of meltwater rivers occurring from melt ice bars in Greenland and Antarctica: is this water going to the bottom, being dispersed and slowly reaching oceans, causing higher sea levels, future submerged peopled land and the redistribution of coast lines, or it is just refreezing along its way? We are not 100% sure but again, we need to consider evidence: satellite data have shown that ice melting in 2007 in Greenland was occurring more quickly than IPCC estimations (60% faster) which have widely underestimated the actual sea level rise. In fact, IPCC models of the fourth assessment report (AR4) have not included Greenland’s and Antarctica’s ice sheets contributions because, at that time, there was no evident scientific reason to think they will melt so soon. Thus, natural scientists need to do their job and calculate the risk, a matter which economists do not have the competence to question. Economists play a role after the “consensus process” is done, in particular when the society needs policy advice to understand what to do once acquainted with that objective risk. Even though we are not going to know if the climatic tipping point will occur in centuries or millennia, it is highly  irresponsible not to warn policy makers and the general public about the worst climatic and biological scenarios possible: we (general public and policy makers) need to be fully aware of the risk – scientifically and consensually determined – that something inevitable may happen if we don’t do anything to avoid it. At the same time, we have full political liberty to decide how to manage this risk, given the degree of our risk aversion. However, economists need to know about a cascade of uncertainties: unsure carbon cycle response, climate sensitivity (which depends on the internal dynamics of sensitivity (which  depends on the internal dynamics of the ecosystems, oceans, ice, clouds etc.), and the inevitable unpredictability associated to future happenings (IPCC produced three possible future scenarios corresponding to a low, medium, high temperature rise). A first conclusion may be that global warming issues do not need a multidisciplinary approach – because this allows naysayers much room to maneuver speculative methods – but a proper interdisciplinary action. The latter integrates (not just puts side by side) different sets of studies based on evidence, not deduction. In contrast to the “normal science”, attempt to force nature within a certain paradigm (Kuhn, 1978), we need a “post-normal” epistemological framework that analyses the limitations of “normal” science when facts are uncertain, values are in dispute, stakes are high and decisions are urgent (Funtowicz&Ravetz, 1991, etc.).

Thanks to an “extended peer community”, i.e. an “expert community” rather than a “community of experts” (D’Alisa, ICTA, 2010), not just quantitative, but also qualitative evaluations are possible by taking into account multiple perspectives under irreducible uncertainty. The public opinion needs to know that climate scientists have to research on AGW even if they do not have absolutely certain proof of it. Actually, nobody in science has ever possessed the indisputable truth. Also the Ptolemaic system had been working for over 1500 years (even if at that time some theories tried to show its gaps), and it allowed to us make several correct astronomical predictions and calculations, e.g. on the position of some planets. However, after about two millennia we found out that geocentrism was not the right explanation of how things work up in space. Therefore the scientific consensus of that time had changed and so have our beliefs. But again, how can we be sure we are not wrong? Probably because of the preponderance of evidence: this offers many observations that support this theory through consilience. By following this reasoning, if climate scientists would not research what their statistics suggest, if the public opinion would not be informed about that, and if policy makers would just ignore scientific consensus on AGW, then all this would certainly be considered irresponsible not just by our grandchildren – in the case where the worst scenarios occur – but already now by ourselves. Of course, looking at some plotted data and statistics is not enough to say we are following the method of evidence. Scientists need a theory that involves detection, i.e. the acknowledgment of rising temperatures and increasing CO2 levels in the atmosphere, and attribution, i.e. the determination of the inferred causality after having tested it. Indeed, it was not just an upwardsloping curve showing the correlation between rising temperatures and CO2 emissions that had IPCC using the words “unequivocal” and “very likely” in its reports (as we all know, correlation does not imply causation). A theory – not a dangerous experiment – has been set up, and this is not to scare people, not to make the government lose its control over people who could start to act against past and current policies, but to drive a computer model for a better understanding of the repercussions of human activity on nature among time.

Nature has to be protected and defended for itself and not because of the services it provides to humans or as natural capital or as a means of production. By fighting now for the changes we would like to see in the future, we can pave the way for stronger and more unified efforts, populations and nations. In this way, Europe will soon get a wake-up call on global warming issues which should not wear any political color or regional pin.