The evidence for Anthropogenic Climate Change

Yes, it's changing. Yes, we're causing it.

From Claire—Dr. X is an old friend and a distinguished climate scientist. Why, you may ask, does he prefer to be anonymous? Because his wife finds the American political climate—and the things people say on social media, in particular—utterly repellent and vicious. She wants no part of it near their family. She married a scientist, not a public figure.

Can you blame her?

Congratulations to America for screwing up freedom of expression so badly that earnest scientists now act like Soviet dissidents.

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By Dr. X

Thanks to Claire for inviting me to contribute to this conversation. She suggested I offer a broad overview: How do we know climate scientists are right—or at least, so overwhelmingly likely to be right that the scientific consensus should be the foundation of our energy strategy? Here, I’ll sketch some arguments for the consensus view.

A few years ago, I was invited to join my aunt and uncle, along with another couple, for dinner. The other couple, Dr. and Mrs. Paterson, as I’ll call them, had many questions about climate science. Dr. Paterson remarked that the Earth’s climate has undergone natural swings for millions of years, so he didn’t see how we could be sure that humans are changing the climate today.

I spent the next few minutes talking about climate and orbital cycles. The Earth’s tilt and precession cycles line up so that Arctic summers are cold, allowing snow to survive the summer and build up ice sheets in North America and Eurasia. The ice sheets grow large enough to change the climate in ways that favor more growth—for example, by storing more carbon in the ocean, so that the global atmosphere cools and the entire Earth enters a glacial period. This continues for tens of millennia, until orbital cycles favor summer Arctic melting. Ice sheets collapse, atmospheric CO2 rises, and we enter a warm interglacial period.

I explained that the timing and triggers are different for global warming caused by greenhouse gas emissions, but many of the underlying physical processes are the same. When I finished, Mrs. Paterson said, “That makes sense. Why didn’t someone explain that to me before?” It was one of the nicest things anyone ever said to me. The Patersons came to dinner as climate skeptics with open minds; they went home a bit less skeptical.

My uncle, however, was unmoved. He’s convinced that climate science is nothing but a jumble of hoaxes, hyperbole, and delusions. I no longer talk with him about climate science, since our arguments distress my aunt and are pointless.

I assume any subscriber to The Cosmopolitan Globalist who is not already convinced of the scientific consensus on climate change would have an open mind, so it’s worth some effort to outline the main arguments. I will begin with this essay, but will subsequently address points I’ve seen raised in the comments section.

How do we know that climate scientists are right? Obviously, scientists don’t know everything about the climate, so they keep measuring, modeling, and debating. But climate scientists are very confident of this statement:

The Earth’s climate is warming, and most of the warming is caused by humans.

When I say “very confident,” I mean confident in the way that medical researchers are confident that smoking raises the risk of cancer. This claim rests on a web of well-established theories, backed by millions of observations and model simulations, documented by thousands of peer-reviewed papers and painstakingly scrutinized reports.

Why so confident? Let’s assess the statement in two parts: the observation of warming and the attribution to humans. The evidence for warming since the mid 20th century is overwhelming. Nearly every location on the Earth’s surface has warmed—by an average of about 1.2 degrees C (around 2 degrees F) since the start of the instrumental record in the late 1800s. Every month since February 1985 has been warmer than the 20th-century average. Record daily high temperatures are about twice as common as record lows. The oceans are warming, at and beneath the sea surface. The troposphere (the lower part of the atmosphere, up to 10 or 20 km) is warming. The Arctic sea ice cover is thinning and shrinking. The Greenland and Antarctic ice sheets, and nearly all the world’s glaciers, are losing mass.

There are some complications, such as the relative sparseness of temperature measurements in places like the South Pacific and Antarctica, and biases such as the urban heat-island effect. Climate scientists have reliable methods to address these problems. About ten years ago, a distinguished physicist, Richard Muller, questioned the statistics and spent many months reviewing temperature data. He wrote a piece in The New York Times confirming that to his surprise, prior estimates of warming rates were correct (I found this extraordinary. Not many of us have written editorials saying that after a close look at the evidence, we changed our minds.)

Since the warming of the past few decades is unequivocal, what is the proximate cause? Scientists are highly confident that most of the warming is caused by increasing amounts of greenhouse gases in the atmosphere.¹ By definition, a greenhouse gas is mostly transparent to visible light from the sun, but relatively opaque to the infrared radiation emitted back to space by the Earth. By trapping infrared radiation near the surface, greenhouse gases warm the Earth’s surface and its lower atmosphere. This effect has been understood since the 19th century and is easily shown in a laboratory.

CO2 is the most important greenhouse gas, but methane, nitrous oxide, and other trace gases add to the warming. Humans do not add another greenhouse gas, water vapor, to the atmosphere, but it amplifies the effects of the other gases since a warmer atmosphere holds more water. We know that greenhouse gas concentrations are increasing because we can measure them with great precision. We began monitoring CO2 at Mauna Loa in the 1950s. With somewhat less precision, we can track greenhouse gas concentrations about a million years into the past by analyzing air bubbles trapped in ice cores. Ice cores also allow us to estimate variations in temperature, which are well correlated with greenhouse gas changes. (The causation is mutual: Greenhouse gases increase temperature; and for more complex reasons, warming raises atmospheric concentrations of greenhouse gases.) Between glacial and interglacial periods, CO2 has varied in concentration between about 200 and 280 parts per million, or ppm. The value was about 315 ppm when the Mauna Loa measurements began in the 1950s and is 420 ppm today.

Is the observed warming commensurate with the increase in greenhouse gases? It is. Theoretical calculations, simple radiation models, and complex global climate models tell us how much warming goes with a given increase in CO2. There are uncertainties of about a factor of two. If CO2 doubles compared to to the pre-industrial value of 280 ppm, we expect the climate eventually to warm by 2 degrees to 4 degrees C, maybe 5 degrees C if we’re unlucky.

Much of the uncertainty comes from clouds, which are infernally complex. We know that low clouds reflect sunlight and cool the climate, while high clouds absorb infrared radiation and warm it, but it’s hard to predict where low and high clouds will become more or and less prevalent.

Another complication is the variability in natural and human emissions of aerosol particles, which generally cool the climate by reflecting sunlight. Clouds and aerosols largely explain why some climate models predict more warming than others. But if we assume, for example, a 50 percent increase in CO2 and similar increases in other greenhouse gases such as methane, no model consistent with the laws of physics will simulate an unchanging or decreasing temperature.

Carbon is exchanged between the surface and the atmosphere by many processes, both natural and human. How do we know the extra CO2 is coming from humans? The strongest evidence comes from carbon isotopes. There are three isotopes of carbon in the atmosphere: carbon-12 and carbon-13, which are stable, and the rarer carbon-14, which is formed naturally in the atmosphere and decays over several thousand years. Fossil carbon—that is, the carbon in coal, oil, and natural gas—is depleted in carbon-13 and carbon-14. Increasingly, carbon in the atmosphere is also depleted in these isotopes, showing that most of the added carbon has a fossil origin.

In addition to measuring atmospheric CO2, we can independently estimate the total carbon added to the atmosphere by human activities. We find that only about half the emitted carbon remains in the atmosphere; the land and ocean take up the rest. This uptake is fortunate, since it has reduced the warming to date and gives us some ways cheaply to remove carbon, for instance by planting trees.

Could other factors be contributing significantly to the recent warming? Orbital changes are too slow to be relevant. Besides greenhouse gases, the main external drivers of short-term climate changes are volcanic eruptions and variations in solar intensity. The sun has not become more active over the past few decades, and there have been no major changes in volcanic activity. Could the recent warming be a natural fluctuation, caused by changes in El Niño or other circulation patterns? This is extremely unlikely. Over the past 1,000 years, the Earth’s average temperature—deduced from tree rings, ice cores, and other natural records—has varied within a much smaller range than the recent warming. Temperatures today are the highest they’ve been in at least the past millennium, and probably higher than at any time since the last interglacial period, about 125,000 years ago.

In 1990, when I started learning about climate science, a majority of climate scientists thought it was caused by humans, but a sizable minority weren’t sure. By 2000, based on the evidence I’ve described, working scientists arrived at a near-universal consensus that humans are causing climate change. The evidence has grown ever stronger, but public opinion, for various reasons, has lagged the science by a decade or two.

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I should like to correct an assertion offered by Adam Garfinkle in an essay that is otherwise scientifically unobjectionable. He writes, “[O]ur models cannot yet firmly capture the dynamic relationship between temperature and the mix of gasses in the atmosphere.” This statement is untrue, unless by “firmly capture” one means an impossibly high standard, like “capture without any significant uncertainty.” NOAA’s overview of climate models is correct:

Climate models are based on well-documented physical processes to simulate the transfer of energy and materials through the climate system. Climate models, also known as general circulation models or GCMs, use mathematical equations to characterize how energy and matter interact in different parts of the ocean, atmosphere, land. Building and running a climate model is a complex process of identifying and quantifying Earth system processes, representing them with mathematical equations, setting variables to represent initial conditions and subsequent changes in climate forcing, and repeatedly solving the equations using powerful supercomputers.

When someone says that models don’t do a good job of representing (or capturing) the dynamic relationship between temperature and GHGs, I understand them to be arguing that models fail adequately to represent the transfer of heat and other forms of energy between different parts of the Earth system.

But they do a very good job. For example, if GHGs are added to the tropical atmosphere, the added heat absorption will lead to greater evaporation and a stronger Hadley cell (i.e., rising of air in the tropics and subsidence at higher latitudes). The model might then predict more intense rainfall events in the tropics and drought in the subtropics, consistent with what’s observed. This is the kind of thing models do well, because the dynamics are based on fundamental principles such as Newton’s laws of motion and the conservation of energy. Translating these laws to computer algorithms is tricky, but climate scientists have worked on this for decades, and the methods are now very good.

Cloud processes are harder, in part, because the processes take place on small physical scales that can’t be resolved by global models. You would need very fine grid resolution—say, 1 km or less—and the resulting model would be too expensive for decade-to-century simulations. So scientists have to find other ways to get accurate answers without resolving all the relevant scales of motion. This is known as parameterization.

To use the term of art, the dynamical core is sound, but the parameterization of physical processes needs more work—and will always need more work, because climate models can’t have infinitely fine resolution. If Adam Garfinkle questions the dynamical core, he has misunderstood the problem.²

In a subsequent essay, I hope to add some general thoughts on the epistemology of climate science.

Dr. X is a climate scientist who has spent more than two decades developing global climate and Earth system models.

1. Claire—careful readers will note that The Cosmopolitan Globalist has suddenly begun referring to greenhouse gases, as opposed to gasses. This is because “greenhouse gasses,” it seems, is wrong. Gasses is the third-person singular form of the verb gas. The plural form of the noun gas—one of the four states of matter—is gases. So, for example, “If Hannibal Lecter gets too close, the warden gasses him.” We bitterly regret the error.

2. Claire—again, my error. Adam’s original words were these: “the dynamic relationship between temperature and the mix of gases in the atmosphere is not yet firmly in the grasp of current models.” I did not like this phrase—strictly on prose merits—because he used the passive voice, which I abominate. What’s more, models don’t have hands to grasp, not even metaphorically; if they did, they could hardly grasp a dynamic relationship, for grasping is something one does to a baseball, say, or a boyfriend. I decided he meant we don’t grasp this relationship, or perhaps that models don’t capture it—I opted for the latter. I was wrong to do so. I should instead have asked him what he meant. I apologize, Adam. (But let that be a terrifying lesson to anyone thinking of sending me a sentence written in the passive voice.)