One of my colleagues here in the earth science department at the University of Southern California is teaching a general education course on climate science this semester.  He recently surveyed the class, and asked the students to submit their “favorite” myths about climate change–the things they hear repeated most often, the objections they find the most compelling, or whatever.  After analyzing all the submissions, he found that almost all of the myths fell into one of eleven different categories.  He was good enough to forward me the list, and I spent some time compiling a response to each of them.  For those of you who run into climate change skeptics (either online or at Thanksgiving dinner) but don’t know what to say to them, this one’s for you.

I’ve lightly edited some of the myths for spelling or grammar, but otherwise these are verbatim responses from USC undergraduates to the question “what’s your favorite climate myth?”

Myth# 1: Some Places Are Cold

“The strongest argument that I have heard about not doing something about climate change is three statistics that read on ice and in 2014. According to “Global Climate Scam,” in 2014 there was a record sea ice in Antarctica, with only 3 months of the year with no ice. Also, in 2014 there was a record snowfall in many places of the year and a record of cold weather in many places of the world. Of course I believe in all of these statistics, but what lead them as misleading is because they forgot to add that we’ve also had the hottest years at some places of the world. They are also forgetting that carbon emissions in the air that have exceeded anything that one could have imagined many years ago. Overall, no one should believe in these statements and  wise person shouldn’t allow strong arguments like these to create opinions for them on climate change. Climate change is real and we need to take action as soon as possible!”

Answer: There are a couple of different points that are worth making here.  First, as you rightly point out, at the same time that there have been record-breaking snowfalls or record-breaking low temperatures in some places, we’ve been setting record after record for hottest temperatures at a variety of scales.  Both January and February of 2016 set the record for the warmest January and February on record (with global temperature anomalies of+1.14°C/2.05°F and +1.35°C /2.43°F), and the February record now stands as the warmest month on record overall (that is, the month with the greatest temperature anomaly since we’ve been tracking this stuff).  2014 was the warmest year on record, until that record was broken by 2015 (temperature anomaly of + 0.69°C/1.24°F and 0.90°C/1.62°F, respectively).  It’s important to remember that the climate is a global system, so these overall trends are much more indicative of the actual state of the climate.  

As NOAA notes, “2015 marks the fourth time in the 21st century a new record high annual temperature has been set (along with 2005, 2010, and 2014) and also marks the 39th consecutive year (since 1977) that the annual temperature has been above the 20th century average. To date, including 2015, 15 of the 16 warmest years on record have occurred during the 21st century. 1998 is currently tied with 2009 as the sixth warmest year on record.  Overall, the global annual temperature has increased at an average rate of 0.07°C (0.13°F) per decade since 1880 and at an average rate of 0.17°C (0.31°F) per decade since 1970.”

There’s a well-known saying from Aristotle’s Nicomachean Ethics, “one swallow does not a summer make,” which alludes to the fact that a single data point is not indicative of a trend (swallows were the harbingers of summer for the Ancient Greeks, much as Robins are seen as the harbingers of spring in the Western world today).  Exactly the same thing is true here: the fact that some places see extremely cold temperatures doesn’t show that there isn’t an overall trend toward warming; the calculations of average surface temperature anomalies referenced above include such data points, and their existence is greatly outweighed by the trends toward warming in other locations.  In just the same way that a single warm day in January does not mean that winter is over, a record low in a single location does not mean that the warming trend has stopped globally.  The climate, remember, is an extremely “noisy” system, with lots of variation at short time-scales.  That’s why we pay attention to long-term trends, and not day-by-day (or even year-by-year) data.  

The Antarctic sea ice extent is a more interesting case.  It’s true that the sea ice surrounding Antarctica reached a record maximum during 2014, but this too is not by itself indicative of a global trend.  The accumulation of sea ice around Antarctica is due to a number of factors, but generally has more to do with slightly elevated global temperatures being more conducive to local ice accumulation in Antarctica. This sounds just as paradoxical, but it isn’t really. The water vapor capacity of air is partially a function of temperature, and (all other things being equal) warmer air is better at holding moisture. The polar regions are cold–really, really cold. In the polar winters, they’re generally so cold that they don’t hold much moisture at all. This is particularly true in Antarctica, which is significantly colder than the Arctic. Slightly warmer temperatures around the southern pole have slightly increased, which has allowed the air to take up far more water vapor than it could before, and winter snowfall has increased. This isn’t paradoxical because the Antarctic is still far, far, far below the freezing point of water during the winter months, but the slight increase in temperature has allowed more snow (rather than just ice) to fall and accumulate down there.  This snowfall on the ice can push thinner parts of the sea ice (i.e. at the edges) underwater, allowing the extremely cold water to mix with the surface snow, reducing the salinity of the local water.  Since fresher water freezes at higher temperatures than saltier water, this results in more ice formation.

Moreover, the increase in surface ice is precisely what we’d expect to see if deep water ocean currents were warming, slowly melting ice underwater and bringing very cold water to the surface. The Earth’s oceans are very dynamically active, with currents moving not just horizontally but vertically as well. The structure of these currents is driven by many factors, including water temperature, salinity, and prevailing winds.  As all of these factors change on and around Antarctica, the structure of the ocean currents changes as well.  This has led to some increased melting below the surface of the ocean, with the super-cold fresh water then rising to the surface, where it refreezes as sea ice.

It’s also worth pointing out that the amount by which the Antarctic sea ice extent has grown recently is much smaller than the amount by which the Arctic ice sheet has shrunk.  We are, on the balance, losing much more ice than we are gaining every year.

Myth #2: It Would Cost Too Much To Fix

“The strongest argument I have heard on the side of not taking action to curb climate change is that changing our methods and habits would put too much economic strain on the middle and lower class. While alternative practices such as moving to renewable resources would help to curb the impact we are currently having on the environment, these alternatives are almost always more expensive. There is an initial cost of building the physical pieces necessary to start using these other forms of power, and many require much more research until they can produce energy at a cost closer to that of using gas or coal. These extra costs would have to be passed on to the consumer in one way or another, and many people simply would not be able to afford it.”

Answer: It’s worth emphasizing that this point is an argument in favor of a particular response to climate change (i.e. adaptation rather than mitigation), rather than an argument against either the reality of climate change or the fact that human behavior is a major contributing factor.  It is possible for reasonable people to disagree about policy approaches, but we should not mistake that disagreement for disagreement about the science behind climate change.

With that said, this argument rests on a number of extremely questionable assumptions: (1) that economic success/GDP is the most reasonable measure of human welfare to use in evaluating these policy decisions, (2) that adaptation is significantly cheaper than mitigation, and (3) that the costs associated with mitigation would necessarily be borne disproportionately by developing or underprivileged populations.  Let’s take these in order.

(1) is a favorite of economists, but strikes me as extremely suspect.  The assumption is grounded in a belief that quality of life (in the most general possible sense of the term) can be quantified in terms of a concept called “utility,” which in turn is “cashed out” (so to speak) in terms of purchasing power (i.e. wealth).  This belief is foundational to a certain way of doing macroeconomics, but it’s far from obvious that it’s defensible as a general principle.  We might think that some things that people genuinely value resist a straightforward translation into talk of monetary worth: how much is the health of your significant other or your parents worth to you?  What’s the monetary value of your freedom of expression?  What’s the cash-value difference between a year of healthy life and a year of chronic illness?  Even if these questions are sensible in the first place–that is, even if they have answers–it’s certainly not clear what those answers are, or how to scale-up individual answers to evaluate the overall utility of global public policy.  Yet this is precisely what this line of argumentation assumes: that the host of potential problems associated with climate change–the spread of disease, drought, massive refugee crises, flooding, starvation–can all be neatly represented under the auspices of purchasing power, or economic success.  The argument assumes, that is, that all human values are commensurable with the economist’s notion of utility, and that a careful calculation of the economic costs associated with each policy proposal will tell you everything that’s worth knowing about which proposal is better.  At the very least, this is an extremely controversial suggestion in need of exploration and argument.

(2) is similarly unclear.  The most comprehensive analysis of the economic costs of climate change–the 2006 report by Nicholas Stern from the London School of Economics–argues that the global cost of unmitigated climate change will be equivalent to permanently losing at least 5% of global GDP each year, and that climate change could be effective mitigated through an investment of 1-2% of global GDP annually.  The longer the problem is allowed to proceed without mitigation, though, the greater both those costs are expected to rise; this suggests that the most economically sound policy approach is early and effective mitigation.  This conclusion is (of course) controversial, but that’s a big part of my point here: there’s good reason to think that there’s no approach that’s uncontroversially all-things-considered preferable, even if we accept the assumptions in (1).

(3)  It isn’t clear to me why the costs of climate change mitigation would necessarily be borne disproportionately by developing nations and/or the middle/working class population.  Exactly how to deal with the market failure that is anthropogenic climate change, after all, is precisely what’s at issue here: it’s perfectly possible to design a policy in which the majority of the costs associated with mitigation are borne by actors (both state and non-state) who are both most able to bear those costs and who are most responsible for the state of the climate.  The economist and philosopher Dale Jamieson has argued for a concept he calls “intervention responsibility,” in which the responsibility for mitigation “….should be ascribed to international regimes and organizations, states and other jurisdictions, individuals, and firms. Each has different capacities and thus different intervention-responsibilities responsibilities, but these differences are not always mirrored in public discussion. In particular, the moral responsibility of firms has been greatly neglected.”  This is one model of ascribing economic (and ethical) liability that wouldn’t result in developing nations being disproportionately burdened by the costs of mitigation, but there are others.  The costs will only be passed on to already vulnerable populations if that is the policy we choose to pursue.  This is, if anything, an argument against pursuing such a policy, not against mitigation itself.

Myth #3:  It’s Just a Natural Cycle

“The strongest argument I have heard is that climate change does not actually exist. The slight warming that has been observed is just a part of a warming/cooling cycle that the Earth has been experiencing since the beginning. The warming is a naturally occurring phenomenon that will reverse in time just as it always has done.”

“The strongest climate denial argument I’ve heard is that the climate has always been changing, and that the climate has changed like this without humans.”

“I believe that the strongest argument I have heard about not doing something regarding climate change is that climate change is natural, and it is something that comes and goes and needs to occur. It is said that climate change has happened before. It was warmer during the Holocene Climatic Optimum than it is today, and it had no human influence. It was just as warm in the Medieval Warm Period as it is today. Global warming has been going on for the past 20,000 years. These are all examples that help the claim that climate change has happened before and it is okay to happen again.”

“I think that the strongest argument I’ve heard about not doing something about climate change is that it is inevitable anyways. The Earth goes through different warm and cold periods and it could just be another one of those times that is just being sped up a couple years by anthropogenic forcings.”

Answer:  It’s certainly true that the global climate has changed in the past, and has done so drastically!  The climate is a highly complex dynamical system, and is constantly changing in response to everything from small variations in the Earth’s orbit (i.e. Milankovitch cycles) to changes in the relative concentration of organisms in the biosphere.  Whether or not the climate has changed in the past is not under debate.  The relevant question here is what’s responsible for the trends we see happening now.  The fact that the climate has warmed without anthropogenic influence in the past is no more evidence against anthropogenic climate change today than the fact that some houses burn down in the absence of arson is evidence for the claim that arson never happens.

So, then, what’s responsible for the trends we see today?  We have a broad spectrum of observations and models, virtually all of which agree with one another to an almost perfect degree. All the evidence points to the fact that current trends can’t be explained without reference to anthropogenic contributions, and that those contributions are currently among the most significant factors influencing the evolution of the global climate. The global climate is a monstrously complex system, and attributions of single causes are notoriously tricky in cases where there are a lot of interlinking feedbacks operating on multiple different scales. However, we’ve come up with a lot of clever ways to check our results. The degree of intermodal agreement between observations and different models / model methods strongly suggests we have this right.

Jim Hansen pioneered this back in the mid-90s, but our models have improved a lot since then, so let’s stick to more recent stuff. One of the more prominent recent pieces looking at this is “Combinations of natural and anthropogenic forcings in twentieth-century climate” by Gerald Meehl et. al. in the Journal of Climate in 2004. They used an ensemble of general circulation models to try to hindcast the observed temperature trends of the 20th centuries using four different scenarios. The first three included the various natural forcings only (volcanic aerosols, varying insolation, &c.), while the last included both the natural variations and anthropogenic greenhouse gas (GHG) emissions. They were unable to reproduce the observed trends without accounting for anthropogenic emissions, and including those emissions resulted in ensemble predictions that matched the observed data almost perfectly. Here’s the graph:

Climate change forcings compared

We’re able to discern anthropogenic GHG emissions from natural emissions of the same compounds by tracking the relative prevalence of different isotopes of carbon in CO2 molecules in the atmosphere. Because fossil fuels are composed of decomposed organic matter, they have a distinctive ratio of carbon-13 to carbon-12 that’s not found in other sources. By tracking changes in the observed ratio in the atmosphere, we can get a fairly good sense of what proportion of CO2 is coming from us and what proportion is coming from other sources, giving the picture that Meehl found in his study.

Climate models also predict that if a large portion of the shifts in temperature ranges at the regional level are due to anthropogenic GHG emissions, we should also see a reduction in the difference between maximum and minimum daily temperatures, a value called the “diurnal temperature range” (DTR). We wouldn’t expect to see a significant shift in the DTR if warming trends were the result of natural forcings alone, as natural forcings would alter temperatures more-or-less uniformly or (depending on the forcing) would have a much larger positive effect on the maximum temperature. Multiple data sets have shown that observed DTR in the last 50 years has narrowed significantly, and that there’s been a relatively larger increase in minimum temperatures than in maximum temperatures, a fact which again can’t be explained without reference to the impact of anthropogenic GHG. This is a good roundup of those studies can be found here.

One of the most important thing about the DTR as a fingerprint is that variations of DTR are more-or-less independent of variations in global mean temperature, as increases in the global mean could be accounted for by a lot of different combinations of changes in the DTR in different regions (a large increase in daily maximums in an isolated region will artificially inflate the global mean, for instance). However, we see a fairly constant increase in daily minimum temperatures in geographically disparate regions, and a consequent shrinking of the average DTR. This can’t be accounted for without including anthropogenic GHGs, and is an important independent confirmation of humanity’s impact.

Yet another independent measure is the variation in the observed wavelengths of incoming radiation. Because the physical basis of the greenhouse effect is quantum mechanical, different molecules absorb and reradiate energy at distinctive frequencies because of differences in their structure. By using a spectrometer, we can figure out how much of the incoming radiative forcing is due to insolation itself and how much is due to the reradiative effect of different GHGs. If anthropogenic emissions were a major factor in planetary warming, we’d expect to see a significant contribution to overall radiative forcing coming from the molecular constituents of anthropogenic GHGs. This is exactly the observed result. In “Measurements of the radiative surface forcing of climate,” (2006), Evans and Puckrin observed that anthropogenic GHGs were responsible for a radiative flux increase of 3.52 W/m^2, which is hugely significant (and in line with model predictions). 

Those are three independent measures for estimating the human emission impact on observed temperature trends, and all three agree in their attribution. There are other independent measures as well that we could talk about (atmospheric temperature gradient, wavelength flux in upward IR radiation) that also agree, but I think I’ve said enough here already. In each of these cases, the models not only agree with one another, but agree with observation: human behavior is the most significant cause of recent climate change.

It’s also true that global average temperatures have been different during various times during the Holocene (i.e. since humans have been kicking around).  It’s important to remember, though, that human civilization today and human civilization during the early Holocene (say, 10,000 years ago) or even during the Medieval period (say, 1,000 years ago) are markedly different.  During the early Holocene, most humans still lived in small, scattered bands and were living nomadic hunter-gatherer lifestyles: this was just at the dawn of early agriculture and animal husbandry.  A “human civilization” (insofar as what existed during the early Holocene can even be called that) which consists of small, scattered nomadic tribes is vastly different from a human civilization that consists of 7 billion individuals, most of whom live in relatively densely populated (and, incidentally, coastal) cities, and who are linked together via a globalized economy based on large-scale agriculture and international trade.  The former is in a much better position to adapt to a changing climate, as it is widely distributed, diverse, mobile, and accustomed to relocating regularly–traits that contemporary human civilization most certainly does not possess.  

The ease with which a small tribe of a few dozen individuals can relocate following a climate shift has virtually no bearing on the ease with which a globalized civilization can accommodate a similar change.  Most of the people who stand to be most impacted by climate change (i.e. those in developing nations, especially in sub-Saharan Africa, Asia, and parts of South America) are those who are least capable of adapting to changes on their own: they have relatively few resources, are often tied to local subsistence farming, and lack the means to relocate to less impacted areas.  Moreover, the interconnectedness of the global economy–particularly with respect to agriculture–means that climate-related damage done to one region is likely ripple across the globe.  Globalization is a double-edged sword here: it provides both the means for humanity to combat climate change, but makes us more vulnerable if we fail to rise to the challenge.

Myth #4:  Climate Change Is a Liberal Conspiracy

“Right-wing climate deniers also argue that climate change is a liberal conscription to take down the oil companies. Although this is quite a ridiculous argument, it is hard to come up with a response to it sometimes.”

Answer: This is a version of what Alan Turing described as the “Head in the Sand” objection in his 1950 paper “Computing Machinery and Intelligence.”  Though Turing was talking about artificial intelligence, the spirit of the objection is the same in both cases: this can’t be true because I don’t want it to be true.  This is, as you point out, a ridiculous argument.  Still, it’s worth addressing in more detail because of its prevalence.  One of the most important pieces of evidence against this is the fact that fossil fuel companies like Exxon-Mobile very clearly knew about the link between fossil fuels, greenhouse gas emissions, and climate change very early.  A whole pile of leaked internal memos from the fossil fuel industry shows that their own scientists had, by the 1980s, reached the same conclusion as the mainstream scientific establishment: CO2 emissions caused by burning fossil fuels leads to significant (and potentially dangerous) global climate change.  Those same memos show that these findings were ignored or actively suppressed by the executives of those corporations, and that a deliberate campaign of misinformation was devised in order to discredit the science behind anthropogenic climate change.  Naomi Oreskes and Erik Conway document this campaign  meticulously in their excellent 2011 book Merchants of Doubt (now a major motion picture!), showing in detail how the same small cadre of science lobbyists and public relations firms were recruited to obfuscate the science behind the tobacco-cancer link, acid rain, ozone depletion, and global climate change.  In each of these cases, there is clear evidence that the industry knew quite well that the story they were pushing in public was false, but that they chose to hold the line as long as possible in order to safeguard their profits.  It’s hard to overstate how damning the evidence in the leaked memos is: these corporations quite clearly knew (and still know) that their businesses were contributing to a warming world, but actively chose to run a disinformation campaign against the public while simultaneously lobbying elected officials to prevent the passage of restrictive regulations.

In addition to this, the notion that such a broad scientific consensus could be maintained via a “liberal conspiracy” is almost unspeakably far-fetched.  The number of distinct individuals who would have to be “in on it” for such a conspiracy to persist for more than half a century boggles the mind.  Earlier in 2016, Oxford mathematician David R. Grimes published a paper called “On the Viability of Conspiratorial Beliefs” which used network theory and empirical data to analyze the relationship between the number of people who know a secret and how long that secret can plausibly be kept.  The paper establishes “...a simple mathematical model for conspiracies involving multiple actors with time, which yields failure probability for any given conspiracy. Parameters for the model are estimated from literature examples of known scandals, and the factors influencing conspiracy success and failure are explored.”  Dr. Grimes found that even assuming very good individual secret-keeping behavior, large conspiracies–those with 1000+ conspirators–are extremely unstable, and unlikely to persist for long.  He estimates that in order for a climate change conspiracy to function, at least 405,000 individuals would need to have been “in on it” over the course of the last few decades.  Even under his most optimistic model parameterization, he found that such a large conspiracy would almost certainly fall apart within 3.7 years.  The idea that it has persisted for decades longer than this defies belief.

Finally, it is worth asking the question of what scientists would stand to gain by maintaining such a conspiracy, versus what fossil fuel corporations would stand to gain by promulgating the idea that climate change is a conspiracy.  It’s not clear what the answer is in the former case: climate science is neither particularly lucrative, nor is it tied to the truth of anthropogenic climate change directly.  If we were wrong about climate change, the need to study the climate would not disappear–it’s no more predicated on the truth of this specific scenario than any other branch of science is predicated on a particular theory.  

Moreover,  science as a social institution is constructed in such a way that novel insights–especially those which overturn widely held orthodoxies–are highly rewarded, not suppressed.  A scientist with strong, comprehensive evidence against anthropogenic climate change would become a celebrity overnight, just as a scientist with strong, comprehensive evidence against (say) general relativity would.  The scientific method was deliberately constructed so as to be self-correcting in this way; scientists make mistakes all the time, but the overall system of science has proven very effective at catching those mistakes and correcting them.  By contrast, fossil fuel companies quite transparently stand to gain quite a lot by obfuscating the truth about climate change; their whole business model is predicated on a process that emits many tons of GHG annually.  Which scenario seems more likely here?

Myth #5: We’ll Be Dead Before It Affects Us

“We have heard many strong arguments regarding not doing something about climate change. Regardless of the speculations that climate change would cause snow to disappear, in 2014 we saw record snowfall. Similarly, people state it is not their problem because they will be dead before climate change affects us. They also think it is expensive to combat.”

Answer: See answers to #1 and #2 for the point about snowfall.  With respect to the argument that we’ll all be dead before climate change affects us, there are two points worth making.  First, it’s just manifestly not true that this is a future problem only.  Climate change is making a real, discernable impact around the world already (again, see answers to the other myths for more on this).  Second, we generally believe that we have at least some moral obligation to future generations, and that it is wrong to effect short-term gains at the expense of long-term damage.  This principle has guided many public policy decisions for hundreds of years; it’s not clear why climate change presents a unique case here.

Myth #6: The Warming Pause

“I believe that the problem with proving that global warming really exists is that it is very global and broad topic: therefore there are hundreds of ways to measure it. Many people take advantage of this and try to convince us that global warming has stopped. I am sure they don’t even believe what they say and the only reason they try to prove 99% of  climate scientists wrong is that they want to be in the centre of the media. Only looking at the fact that sea level is rising is enough to understand that global warming exists because there are two ways that it could happen.First due to ice (not the sea ice) melting in the poles and second water is expending. Both cases are result of overall temperature rise. The next obvious fact is the increasing rate of increasing CO2 in the atmosphere. I think it would be much better if these people, who try to deny the global warming, think of the ways that what effects can the global warming have on our planet and how to stop it.”

Answer: The relevant point here is that there are many different ways to measure the climate, and that trends may appear or disappear depending on how you measure things, what data set you use, and so on.  The objection being alluded to here is the so-called “pause” in temperature increase that has supposedly persisted for the last 10-20 years.  This is what I will address here.  I also have a much more detailed separate post focusing on this topic.

One of the most important general figures in climate modeling is also one of the most contentious: the equilibrium climate sensitivity (ECS).  The ECS represents the expected increase in global average surface temperature as a result of doubling the atmospheric concentration of CO2 (or CO2-equivalent greenhouse gasses).  That is, the ECS represents the answer to the question “If we go from 400 parts per million of CO2 to 800 parts per million of CO2, how much would things warm up before the climate reached a new equilibrium?”  Like almost all aspects of climate modeling, the value of the ECS differs from model to model.  Some models are extremely sensitive, and predict a lot of warming as a result of greenhouse gas (GHG) doubling.  Some models aren’t very sensitive at all, and predict very little warming from the same forcing.  The models that were included in the most recent IPCC report–which used a collection of models from the Coupled Model Intercomparison Project (CMIP)–vary in their estimation of ECS.  The least sensitive included model gives the ECS value as just over 2°C, while the most sensitive model gives an ECS of almost 5°C.  That’s a big spread, but part of the point of CMIP (as well as the IPCC itself) is to combine the disparate outputs of very different models into an ensemble, which helps us take the best parts of many different models to generate a more reliable picture than any model alone could give us.  On the basis of this ensemble, the IPCC’s AR5 states that the ECS is  “likely in the range 1.5°C to 4.5°C (high confidence), extremely unlikely less than 1°C (high confidence), and very unlikely greater than 6°C (medium confidence).”

It’s interesting to see that this is actually a slight revision down from the Fourth Assessment Report (AR4), which said that the ECS is “likely to be in the range 2°C to 4.5°C with a most likely value of about 3°C, based upon multiple observational and modelling constraints. It is very unlikely to be less than 1.5°C.”  That’s good news!  Lower ECS is unequivocally better for humanity, because it means that we have a little more space in which we can safely operate, and a little more time to decarbonize our economy before we begin to risk really catastrophic consequences.  To a significant extent, this revision down was based on observations in the period from the year 2000 until about 2011, which in general experienced much slower warming than most models predicted, as shown in the figure below:

Yearly climate change trend 1880

The flattened curve that often gets passed around in these discussions is usually a depiction not of temperature, but rather of the first derivative of temperature: the last few years have showed a slowdown in the rate of warming, but average temperatures have continued to increase. However,, remember that climate science is the science of long-term changes, and that we should be attending to trends that take place on the scale of decades (or more) to get a good picture of what’s going on; we expect to see some fluctuation on a year-by-year basis, even with the very strong change in radiative forcing caused by anthropogenic greenhouse gas emissions. In the same way that a freak snowstorm in July (or an unseasonably warm day in February) doesn’t signal the onset of a season change, a few years with a warming rate slowdown doesn’t signal the end of a decades-long trend. Rather than attending to small fluctuations, we look at the overall trajectory of the system across a relatively long time. There are a lot of neat mathematical tricks that we can use to extract signals like this from the data that don’t rely on “well, this graph looks a little flatter to me” style evidence, and the data are unequivocal that warming is not pausing or decreasing.

In many cases, people who are pushing the “pause” argument will be very careful about where they place their start and end dates for their statistics.  As you can see on the graph above, there’s quite a lot of yearly variation in temperatures.  By very carefully selecting an unusually warm starting year and an unusually cool ending year, you can create the illusion that the global temperature is declining over time.  This is nothing more than sleight-of-hand, though, and represents an egregious abuse of the data we have.  An examination of the trend as a whole is quite clear: the Earth has warmed significantly over the last century and a half.

The second major point that needs to be made in this discussion is that air surface temperature data only give us a small slice of the entire picture of what’s going on with our climate. Part of why climate science is difficult to a degree that (I would argue) is unrivaled in either contemporary or historical science–with the possible exception of cognitive neuroscience–is that a complete understanding of the state of the global climate requires collaboration between experts in what would have previously been considered disparate fields. Oceanography and atmospheric physics, for instance, both require highly specialized knowledge and very different training; they are sciences in their own right. Understanding the climate as whole, though, means that we have to understand what’s going on deep underwater, as well as what’s going on in the upper atmosphere (and those are just two examples). Water makes a significantly better reservoir for heat than does air, and there’s a lot of water in the oceans[citation needed]. One of the ocean’s biggest influences on the global climate comes from its ability to act as a heat reservoir for the atmosphere. This also explains why coastal regions tend to be more temperate than regions of the same latitude, but further inland. When the air is cold, the ocean radiates heat to keep it warm. When the air is warm, the ocean absorbs heat to cool it down. This is why, for instance, Santa Monica and Irvine, CA can have such wildly different temperatures on the same day, despite being only a few dozen miles apart: the ocean acts to moderate the temperatures in Santa Monica. On a global scale, this is important for a few different reasons. First, since the vast majority of the Earth’s surface is covered by the oceans, and because ocean water has a relatively low albedo–it absorbs far more sunlight than it reflects–much of the incoming solar energy gets absorbed by the oceans. As the air has warmed up, the oceans have absorbed more of this heat energy, where it is sequestered in the deeper waters via the thermohaline (again, see 1). The reason why warmer temperatures make the oceans absorb more heat is complicated, but it’s not terribly important for our purposes here: it’s enough to understand that we don’t have a complete picture of warming just by considering atmospheric temperatures. All of the evidence indicates that deep water temperatures are rising, and this helps explain the nature of the atmospheric temperature slowdown; the Earth isn’t warming less, it’s simply warming in different places. As I said before, there’s reason to think that this is even more worrying than mere atmospheric warming, since the oceans are in some ways far more delicate than the atmosphere.

Finally, let me again emphasize a point from Myth #1: As NOAA notes, “2015 marks the fourth time in the 21st century a new record high annual temperature has been set (along with 2005, 2010, and 2014) and also marks the 39th consecutive year (since 1977) that the annual temperature has been above the 20th century average. To date, including 2015, 15 of the 16 warmest years on record have occurred during the 21st century. 1998 is currently tied with 2009 as the sixth warmest year on record.  Overall, the global annual temperature has increased at an average rate of 0.07°C (0.13°F) per decade since 1880 and at an average rate of 0.17°C (0.31°F) per decade since 1970.”

This is not a pause.

Myth #7: Individual Action is Futile

“The strongest argument I’ve heard about not doing something about climate change is that a person’s action is not good enough to change the whole problem. It argues that only big steps would change the course of the phenomenon like getting rid of companies that contribute to climate change and so on.”

“That one person cannot change anything about it. For example, I cannot stop climate change or do anything about it. So, why worry about it?”

Answer: “First, is the danger of futility: the belief there is nothing one man or one woman can do against the enormous array of the world’s ills – against misery and ignorance, injustice and violence. Yet many of the world’s greatest movements, of thought and action, have flowed from the work of a single man. A young monk began the Protestant Reformation, a young general extended an empire from Macedonia to the borders of the earth, and a young woman reclaimed the territory of France. It was a young Italian explorer who discovered the New World, and the thirty-two-year-old Thomas Jefferson who proclaimed that all men are created equal.”

Robert F Kennedy, Cape Town 1966

Climate change is what’s known as a “collective action problem.”  Collective action problems are particularly tricky to solve because they involve the coordination of a large number of individuals working toward a common goal in a situation where any one of those individuals would be better off breaking away from the coalition to pursue their own interests, while letting the rest of the group continue working toward the goal.  This is called the “free rider problem,” and it’s well-known in economics.  The most familiar illustration is the parable of “the tragedy of the commons:”

Imagine a small village of people who make their living through cattle farming.  In the middle of the village is a large plot of grass that’s owned by noone and shared by all–a public park.  This public land is prime cattle grazing land, and it would be to the benefit of all the villagers to share the land equally, rotating their cattle through so that each villager gets an equal share of grazing time.  All the villagers agree to this.  However, the public park stands unguarded at night, and any villager could secretly graze his own cattle on the land at night, depleting the resource but boosting his own herd’s fitness.  One or two individuals doing this would make little difference, and so such behavior would go unnoticed.  However, since each villager knows this fact, each decides to graze his cattle secretly at night, boosting his own profit at marginal expense to the others.  Since every villager makes this (apparently rational) decision, the public park is quickly ruined from overgrazing, and all the villagers suffer from the loss.

There’s a clear parallel between this parable and the case of climate change: the global atmosphere is a shared resource just like a public park, and so the temptation exists for individuals to exploit it for their own gain, assuming other individuals will act in a way that’s conducive to the public good.  However, if everyone exploits the resource, we all suffer (as we’re seeing now).  Figuring out how to coordinate the actions of everyone so that we all benefit and avoiding this free-rider problem is the big challenge of global climate policy.  This challenge is extremely daunting, and eclipses any previous challenge in human history in both urgency and scope.  This, however, is not the same as saying that it is insurmountable: humans have solved many hard problems in the past, and there is no good reason to think that this challenge is uniquely insoluble.  Our success in banning CFCs–the agents responsible for ozone damage–is perhaps the best existence proof for the claim that we can in fact solve problems like this.

We know, in general, what sorts of things we would need to do to solve the challenge posed by climate change.  The difficulty is in implementing those changes–in collectively organizing our behavior and avoiding the free-rider problem.  The fact that we have not yet solved the problem is not itself a reason to believe that it cannot be solved.  Indeed, it should only serve as motivation for us to work harder toward a solution.  It’s true that you as an individual lack the power to single-handedly stop climate change, but you are no more an island than is anyone in contemporary global society.  You’re part of a huge array of interlocking social systems, operating at a variety of scales.  You have some degree of influence over how those social systems operate, as well as some degree of influence over other individuals in your social circle; you participate in many sorts of collective actions that would be impossible for an individual to achieve, and your role here is no different.  The kind of cooperation needed to solve this problem first requires us to recognize that there is a problem, and then to commit ourselves to solving it.  That starts with individual behavior, but it also involves the recognition that your behavior can shape the behavior of those around you, and of the larger social system in which you’re embedded.

Myth #8: All Polar Bears Should be Dead

“There are many facts that can contest to people who argue about the fact that climate change is a hoax. Many people said that 2014 was going to be the hottest year. One can conclude, that as a result of this, many animals that live in cold environments would die off. Many people will start to assume that Polar Bears are in trouble, when in reality, that year they were thriving. There are many cases just like this that will constantly put those who oppose climate change to shame. Another case, people thought that Global Warming was going to cause snow to disappear. Again, 2014 saw record snowfall.”

Answer: We are, in fact, in the middle of one of the worst mass-extinction events in the history of the planet, with species going extinct at about 1,000 times the rate that they were during the previous 60 million years.  It’s never been claimed by anyone that I’ve seen that climate change would “cause snowfall to disappear” or that all animals living in cold environments would die off: these are both excellent examples of what philosophers call “straw-man arguments,” and don’t represent genuine positions held by anyone.  Polar bears are currently classified as “vulnerable” by the IUCN, with worldwide populations declining: they are most certainly not thriving.  The majority of the damage to polar bears is attributable to sea ice melting in their habitats, which limits their hunting grounds.  Given this, we would expect to see the most significant population impacts in areas where sea ice has declined most significantly–i.e. at the southern extreme of polar bear habitats.  This is in fact exactly what has been observed.  Continued warming will only magnify this effect, and increase the negative impact on polar bear population.

See the response to #1 and #2 for a discussion of the rest of this objection.

Myth #9: There’s No Such Thing as the Greenhouse Effect!

“Temperature has not gone up any faster compared to the rise in carbon dioxide levels throughout the history. If temperature was up in parallel with the carbon dioxide, the planet would be uninhibitedly hot.”

Answer: The relationship between temperature and CO2 levels is not in dispute: the greenhouse effect is grounded in extremely well-understood physics.  Molecules of different gases have different molecular structures, which (among other things) affects their size and chemical properties. As incoming radiation passes through the atmosphere, it strikes a (quite large) number of different molecules. In some cases, the molecule will absorb a few of the photons (quanta of energy for electromagnetic radiation) as the radiation passes through, which can push some of the electrons in the molecule into an “excited” state. This can be thought of as the electron moving into an orbit at a greater distance from the nucleus, though it is more accurate to simply say that the electron is more energetic. This new excited state is unstable, though, which means that the electron will (eventually) “calm down,” returning to its previous ground state. Because energy is conserved throughout this process, the molecule must re-emit the energy it absorbed during the excitation, which it does in the form of more E/M radiation, which might be of different wavelengths than the energy originally absorbed. Effectively, the gas molecule has “stored” some of the radiation’s incoming energy for a time, only to re-radiate it later.

More technically, the relationship between E/M radiation wavelength and molecular absorption depends on quantum mechanical facts about the structure of the gas molecules populating the atmosphere.  The “excited” and “ground” states correspond to electrons transitioning between discrete energy levels, so the wavelengths that molecules are able to absorb and emit depend on facts about which energy levels are available for electrons to transition between in particular molecules. The relationship between the energy change of a given molecule and an electromagnetic wave with wavelength λ is:

ΔE = ħ/λ

where ħ is the reduced Planck constant (h/2π), so larger energy transitions correspond to shorter wavelengths. When ΔE is positive, a photon is absorbed by the molecule; when ΔE is negative, a photon is emitted by the molecule. Possible transitions are limited by open energy levels of the atoms composing a given atom, so in general triatomic molecules (e.g. water, with its two hydrogen and single oxygen atoms) are capable of interesting interactions with a larger spectrum of wavelengths than are diatomic molecules (e.g. carbon monoxide, with its single carbon and single oxygen atoms), since the presence of three atomic nuclei generally means more open energy orbital states.

Because the incoming solar radiation and the outgoing radiation leaving the Earth are of very different wavelengths, they interact with the gasses in the atmosphere very differently. Most saliently, the atmosphere is nearly transparent with respect to the peak wavelengths of incoming radiation, and nearly opaque (with some exceptions) with respect to the peak wavelengths of outgoing radiation. Specifically, incoming solar radiation is not absorbed efficiently by any molecule, whereas outgoing radiation is efficiently absorbed by a number of molecules, particularly carbon dioxide, nitrous oxide, water vapor, and ozone. This is the source of the greenhouse effect.

This image depicts the absorption spectrum for the constituents of the atmosphere:
Climate change GHG spectrum absorption rates

The E/M frequency spectrum is represented on the x-axis, and the absorption efficiency (i.e. the probability that a molecule of the gas will absorb a photon when it encounters an E/M wave of the given wavelength) of various molecules in Earth’s atmosphere is represented on the y-axis. The peak emission range of incoming solar radiation is colored yellow, and the peak emission range of outgoing radiation is colored blue (though of course some emission occurs from both sources outside those ranges). Note the fact that incoming solar radiation is not absorbed efficiently by any molecule, whereas outgoing radiation is efficiently absorbed by a number of molecules, particularly carbon dioxide, nitrous oxide, water vapor, and ozone. The absorbed and reradiated photons increase the amount of energy hitting a particular area of the ground–they increase radiative forcing. This is not controversial in any way.

What you’re disputing here, it seems to me, is the climate sensitivity. Sensitivity is expressed in °C/(W/m^2), and corresponds to the amount of the expected mean temperature increase resulting from a given increase in radiative forcing. The radiative forcing value most people work with is 3.7 W/m^2, which corresponds (uncontroversially) to a doubling of CO2-equivalent GHG in the atmosphere. It’s true that calculating the temperature response to this change in forcing is non-trivial because of the interplay of various feedbacks and other non-linear processes, but the difficulty lies in calculating the sensitivity, not the forcing itself (and we’ve gotten pretty confident in our estimates of the lower bound on the sensitivity).

The Bayesian probabilities for climate sensitivity to a doubling of CO2-e (that is, an increase in radiative forcing of 3.7 W/m^2) range from 1.5°C to 4.5°C, with 90% confidence. It is extremely unlikely that the sensitivity is less than 1°C (less than a 10% probability), and very unlikely greater than 6°C (less than a 25% probability). It’s possible to point to model runs in which extreme outlier values like 0.5°C/(3.7 W/m^2) or 10°C/(3.7 W/m^2) have been found, but we tend to pay more attention to the statistical ensemble of model predictions than we do to the outliers in either direction. Even still, there is significantly more of a chance that the sensitivity value is much higher than the ensemble estimate than that it is much lower.  These estimates are all consistent with observed temperature and atmospheric CO2 concentration trends over the course of the planet’s lifetime.

Myth #10: Warming Isn’t Caused by CO2

“There are a couple studies suggesting that CO2 may not necessarily cause global warming even if there is correlation. A look at the past 4 climatic cycles suggests that CO2 levels and temperature do not have a direct relationship. That is, at some point of extraordinarily high CO2, there was actually periods of cooling. Since this is one of the main arguments of global warming, I am a little confused how to respond to this.”

Answer: Atmospheric GHG concentrations play a strong role in regulating the planet’s temperature (see #9), though there are other factors.  It’s true that there have been periods in which the global average temperature has increased before CO2 concentrations increased.  The physical mechanism for this is well understood.  The oceans are the globe’s largest carbon reservoir, storing many orders of magnitude more CO2 than the atmosphere.  Our best estimates indicate that the oceans as a whole contain something on the order of 40,000 gigatons of carbon, mostly as dissolved CO2, but also as organic detritus and other compounds.  In contrast, the entire Earth’s atmosphere only contains about 700 gigatons–that’s a huge difference, and it means that even small changes in the storage capacity of the oceans can make a tremendous impact on atmospheric CO2 concentration.

Warm water is much worse at holding dissolved CO2 than cool water is.  If you’ve ever left a bottle of soda in a parked car on a hot day, only to have it go flat (or fizz all over you when you open it), you’ve experienced this effect.  When you heat a liquid up, you force some of the dissolved gasses out of it and into the surrounding atmosphere.  What’s true of soda is also true of the oceans: when they get warmer, some of the CO2 that’s usually trapped in their waters gets released into the atmosphere, and given the size of the CO2 reservoir in the oceans, even tiny changes in temperature can result in enormous CO2 releases, potentially altering the composition of the atmosphere significantly.

The global climate is a complex system, with many interconnected processes and forcings coming together to regulate things like the global temperature.  Small variations in axial tilt and orbital patterns–some of which vary on timescales of 100,000 years or more–called Milankovitch cycles are among the slowest processes to make a significant impact on the climate.  Some features of the Milankovitch cycles are thought to be drivers of the glaciation/interglaciation cycles that create the ebb and flow of ice ages on Earth.  When the planet heats up as a result of things like Milankovitch cycles (or other exogenous or endogenous factors), the oceans warm along with the atmosphere.  This results in more CO2 being released into the atmosphere.  On top of this, many other greenhouse gases (especially methane-based compounds) are trapped underground by permafrosts, and can be released into the air as a result of warming temperatures.  The end result of this is that when the planet warms due to outside forcings, atmospheric greenhouse gas concentrations tend to go up significantly.

This can create a positive feedback in which warmer temperatures give rise to conditions that create even warmer temperatures, amplifying the small and slow processes like Milankovitch cycles into major climate change events that shape the history of the planet.  Left unchecked, these positive feedbacks could result in runaway warming, which is one of our best guesses as to how Venus ended up the way that it is now.  Luckily, the Earth’s climate also has a number of negative feedbacks that, given time, can break this cycle and drive temperatures back down again: the explosion of plant life that results from warmer temperatures and increased CO2 during an interglacial period is one of these negative feedbacks.

The problem is that such temperature “braking” processes take time to work.  Historical climate changes took place on timescales of thousands or even tens of thousands of years–plenty of time for negative feedback processes to prevent runaway warming.  Anthropogenic climate change, however, is moving much faster than any natural historical process ever has.  Our greenhouse gas contributions are causing the kinds of changes that used to take millennia in just a few decades.  This means that positive feedback processes like ocean outgassing and albedo decreases due to melting ice (which makes the planet reflect less solar energy) may not have time to be checked by negative feedbacks as they have been historically, and thus will only magnify the effects of anthropogenic climate change.  

This is one of the reasons that it’s important to act to mitigate our climate damage immediately.  Even if we were to stop burning fossil fuels today, processes like these would result in continued temperature increases for many more decades, potentially doing a lot of damage to human society before the climate turned back toward equilibrium.  The longer we wait, the more intense this lag will be, leading to greater changes for us to cope with.

Myth #11: Can’t Fight the System

“For the better of human being, I believe most people would support  the decision about preventing our planet from the continuous climate change. However, in reality, it would be difficult to take immediate actions towards the achievement due to the significant involvements of politics.  Because of their mutual interests between a country’s politics and CO2-generator industries. For example, to win an election, a politician needs a huge financial support from business people including fossil fuel industry, which would prevent them from discharging  or eliminating those industries from a nation. Thus, as a common citizen, regardless how much you want to do about protecting the Earth from the  drastic climate change, it would turn to be tough to accomplish without the support from the governments and institutions because they are the ones who have the ultimate power to do so.”

Answer: The fact that this is a difficult problem to solve does not imply that it is an impossible problem to solve, or that we should not attempt to solve it.  It just means we need to try harder.  

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