A Short Note On How We Know About Anthropomorphic Climate Change

The purpose of this post is to provide a simple explanation for how we know with a high degree of confidence that climate change is due to humans.

The plot shows the land-ocean temperatures for the years that saw a huge increase of the quantity of carbon that humanity has added to the atmosphere.  The points are yearly values and the red curve a five-year average, which smooths out the fluctuations to highlight the general trend (source: NASA with data at https://data.giss.nasa.gov/gistemp/graphs_v3/Fig.A2.txt).  The shaded area shows the total carbon emission each year from human activities (source: Department of Energy with data at http://cdiac.ornl.gov/ftp/ndp030/global.1751_2011.ems).

I won't go into details of how heating of the oceans and air affect climate because this phenomenon is well known and not a topic of debate.  The point of contention is proof that humans are responsible.  The plot shows excellent correlation between the mass of greenhouse gasses that are emitted by human activities and the temperature.  But correlation is not proof of causation, so how do we know that we are to blame?

The science is too complex to argue in a public forum.  The reason for the high degree of certainty is based on the many different streams of evidence that converge on the same conclusion.  It's like all compasses pointing to magnetic north; those that don't are found to be broken and point randomly, refuting the assertion that the vast number of north-pointing compasses must be wrong.

This convergence of many independent lines of evidence in support of a conclusion is called consilience, a topic that is well explained by Michael Shermer in Scientific American, and can be found at https://www.scientificamerican.com/article/why-climate-skeptics-are-wrong/.  This is a must read for all non-scientists who want to understand how science works and why it is so reliable.

---

Some people may blame scientists, who base professional judgements on extensive knowledge and consilience, of being arrogant.  A proclamation of a "truth" that goes against the accumulated evidence of a scientific field is true arrogance.  Basing policy that goes against consilience is sheer folly and muzzling the fruits of consilience is the ultimate crime.

---

It would be a waste of my time to fully analyze the literature on my own.  I trust the scientists working in the field who build high-tech apparatuses, take data over decades, and meticulously analyze it.  After all, the consilience they have reached is the result of extensive professional debate.  It is arrogant for citizens without expertise in a field to deny anthropomorphic climate change based on the fact that Moscow Idaho has been exceptionally cold this winter.  Policy should be based on the best evidence that science has to offer, and the scientists -- as a group -- with expertise in areas related to climate science are the ones who should be trusted by the public.


NOTE:

Global warming is a scientifically imprecise term because it focuses on the temperature increase, which is a consequence of extra trapped energy.  When light from the sun is absorbed by air, the earth's surface, and oceans, that energy is converted to heat.  Heat is simply the flow of energy in the form of motions of atoms and molecules.  Some of the energy of the warm earth is radiated through infrared light, but greenhouse gasses absorb that light, preventing it from escaping the earth's atmosphere, thus  converting it to heat.  Greenhouse gasses pass sunlight but block the infrared light that is needed for cooling, leading to a buildup of heat and thus an increase of the temperature.

The temperature increase due to a unit of heat energy depends on the mass of the object.  Since the oceans are so much more massive than the earth's atmosphere, the average ocean temperature will increase less than the atmosphere for a given amount of heating.  Since the energy imbalance driven by greenhouse gases is the key process at work, ocean and atmosphere temperatures must be combined if the effects of greenhouse gasses are to be accurately assessed.  That is why the land-ocean temperature composite is a good index of global heating.

Cherry Pickers Deny Global Warming

Global warming deniers are like a broken record, repeating the same old climate change myths.  It is always possible to find supposed counterexamples to any well-established and impeccably-tested scientific theory such as evolution, the big bang theory, the germ theory of disease, etc. by cherry picking the data.  This despicable practice is forbidden by the scientific community, and those who practice it are censured and shunned.  Unfortunately, cherry picking is the favorite tool of politicians, lobbyists, and purveyors of ideology, who use it unashamedly with impunity.

Take the claim that global warming has stalled.  A plot of the mean global land-ocean temperature shows that the 2013 temperature is indeed below the 1998 value.  However, such variations are well within the expected range of statistical fluctuations.  Taken out of context and ignoring the rest of the dataset leads to the erroneous conclusion that warming has stopped.  The long-term trend is clear and agreement between the data and computer models (within computational and measurement uncertainties, of course) increases our confidence in the results.

What about the claim that scientists are changing terminology in an effort to hide the perception that global warming has stopped?  To be precise, the earth is experiencing global heating.  It is a fact that CO2 is a greenhouse gas that traps heat, which warms the atmosphere, land and oceans.  Manifestations of heating include temperature increase, evaporation of water, melting of ice, volatility, and changes in climate patterns.  Deniers see a conspiracy in what they believe is an intentional obfuscation on the part of scientists.  The fact that the media uses different words or has changed its terminology is irrelevant to the underlying facts.  The terms  global warming and climate change have been used by scientists for decades, and it has always been understood that these are consequences of heating.

Deniers like to downplay the scientific consensus.  According to several studies that analyze the vast literature on climate change, more than 97% of active climate scientists conclude that the data supports anthropogenic climate change.  The most meticulous study of this sort assembled a list of researchers who are both active climate scientists (judged by numbers of publications in refereed journals), and who have been signatories of public statements that either support or denounce anthropogenic climate change.  This study confirms the 97% figure.  Global warming deniers point to one specific report that suffers from a documented flaw in its methodology as evidence that the 97% number is incorrect.  They ignore all the other studies that confirm the sentiments of 97% of the climate researchers who concur that global warming is real.

Deniers call for productive discussions on climate change.  These are already taking place. Vigorous debate is common at scientific conferences, new and updated studies are inundating the journals with data, and interpretations of the data are continually being fine-tuned.  Scientists are not zombies who tow the party line.  Only after years of debate is a scientific consensus reached; and even then, scientists doggedly try to find holes in the evidence.  In contrast, the climate change denying machine cherry picks the data in a deliberate effort to cast doubt and plant misinformation in the popular press.

Demanding 100% certainty before taking action on global warming is a stalling tactic.  Prudence demands that steps be taken in proportion to the degree of confidence in the information.  Public debate should focus on finding the most economical ways with the greatest impact on reducing greenhouse gas emissions.  The scientists who have the deepest and broadest understanding of the literature have reached a consensus based on the preponderance of evidence.  We are fools for not heeding their warnings.