Uncertain Climate Predictions and Certain Energy Progress – Part 1
The uncertainties of climate measurement.
There’s a lot of talk about “climate change,” with an emphasis on how humans’ use of energy is contributing to the increase in temperatures on Earth. There’s quite a bit of hyperbolic hysteria surrounding discussion of the issue, so much so that it’s become hard to separate the rhetoric of the most prominent spokesperson for radical reductions in energy use from the lyrics of apocalyptic “death metal” music.
What do we actually know about changes in the Earth’s climate, and their causes? What trends can we reliably discern about climate change, and are there any clear trends on energy use? If we took the word of those who ignore climate uncertainty and demand drastic action, who would be harmed, and how? And what are the prospects for a future world of both plentiful energy and the sensible mitigation of any damages an always-changing climate will inevitably bring? Those of the subjects of this series of essays.
The climate is warming a bit, and there is some human influence on that. But beyond those basic understandings, there are lots of misunderstandings about the issue.
Steven Koonin was President Obama’s former Under Secretary for Science at the Department of Energy. In his book Unsettled: What Climate Science Tells Us, What It Doesn’t, and Why It Matters, Koonin first sets out the uncertainties inherent in simply measuring climate change over time to begin with.
As Koonin writes:
Every measurement of the physical world has an associated uncertainty interval (usually denoted by the Greek letter sigma: σ). We can’t say what the measurement’s true value is precisely, only that it is likely to be within some range specified by σ. Thus, we might say the global mean surface temperature in 2016 was 14.85ºC with a σ of 0.07ºC. That is, there is a two-thirds chance that the true value is between 14.78 and 14.92ºC … For a scientist, knowing the uncertainty in a measurement is as important as knowing the measurement itself, because it allows you to judge the significance of differences between measurements. If the temperature in 2016 were 14.85 ± 0.07ºC (the first number is the value and the second its σ) and it was measured at 14.54 ± 0.07ºC in 2005, a scientist would declare the difference of 0.31ºC significant, since it’s more than four times the uncertainties in the measurements themselves. On the other hand, the measured annual increase of 0.04ºC between 2015 (14.81 ± 0.07ºC) and 2016 is insignificant since it is smaller than the uncertainties—about half as large, in fact.
With that in mind, Koonin goes on to set out our accumulated knowledge of the earth’s surface temperature over the past five hundred million years, as summarized in the following graph.
The graph shows global average surface temperature anomalies in degrees Celsius relative to the 1960–1990 baseline. There are five different panels, each spanning an interval between geologically significant events, and the timescale of each successive panel is about ten times shorter than the one before it in order to allow a huge swath of Earth’s history to be captured in a single chart.
As Koonin describes, the Earth in its history has experienced greater warming than it’s experiencing now:
Starting at the right-hand (most recent) panel, we see that the globe warmed about 5ºC (9ºF) starting some 20,000 years ago, when ice sheets last covered a large fraction of the earth. The relatively warm and stable temperature over the past 10,000 years supported the rapid development of civilization.
What causes the Earth to warm is a system of various feedback effects:
[T]he earth’s temperature results from a crucial balance between warming by sunlight and cooling by heat radiated back out into space … [The] greenhouse gas, carbon dioxide (CO2), is different from water vapor [water vapor is the most predominant gas relevant to warming] in that its concentration in the atmosphere is much the same all over the globe. CO2 currently accounts for about 7 percent of the atmosphere’s ability to intercept heat. It’s also different in that human activities have affected its concentration (that is, the fraction of air molecules that are CO2). Since 1750, the concentration has increased from 0.000280 (280 parts per million or ppm) to 0.000410 (410 ppm) in 2019, and it continues to go up 2.3 ppm every year. Although most of today’s CO2 is natural, there is no doubt that this rise is, and has been, due to human activities, primarily the burning of fossil fuels … The CO2 that humans have added to the atmosphere over the past 250 years increases the atmosphere’s ability to impede heat (it’s like making the insulation thicker), and is exerting a growing warming influence on the climate. Taking average clear-sky (no clouds) conditions as an example, the CO2 added from 1750 until today increases the fraction of heat intercepted from 82.1 percent to 82.7 percent.
But, as Koonin makes clear, carbon dioxide isn’t the only gas affecting warming:
The problem with human-caused carbon dioxide and the climate is that … all else isn’t necessarily equal, as there are other influences (forcings) on the climate, both human and natural, that can confuse the picture. Among the other human influences on the climate are methane emissions into the atmosphere (from fossil fuels, but more importantly from agriculture) and other minor gases that together exert a warming influence almost as great as that of human-caused CO2 … Methane, the second most important human-caused greenhouse gas, has also been increasing over the past century and so also exerts a growing warming influence on the climate … But there are several important differences between methane and carbon dioxide. One is that methane concentrations are much lower (2,000 parts per billion, which is about 1/200th that of CO2’s 400 parts per million). Another difference is that a methane molecule lasts in the atmosphere for only about twelve years—though after that, chemical reactions covert it to CO2. And a third difference is that, because of the peculiarities of how molecules interact with the different colors of infrared radiation, every additional methane molecule in the atmosphere is thirty times more potent in warming than a molecule of carbon dioxide. These differences—lower concentration and shorter lifetime, but greater warming potency—must be taken into account when comparing CH4 and CO2 emissions. For instance, the 300 million tons of methane humans emit each year is only 0.8 percent of the 36 gigatons of CO2 emitted by burning fossil fuels. But as shown in Figure 3.2, that methane has a disproportionate warming influence, equivalent to ten gigatons of CO2.
As Koonin writes:
One additional point about methane that surprises many people is that fossil fuels account for only about one quarter of global human-caused methane emissions … Rather, most methane emissions arise from enteric fermentation (digestion in cattle—mostly emitted from the front of the animal, not the back) and other agricultural activities, particularly rice cultivation …
(Interestingly, the burping and flatulence of cows constitute around 6 percent of all greenhouse gas emissions. And human vegetarians, since they share the same sort of plant-based herbivore diet as cattle, also produce more methane than non-vegetarians.)
At the same time certain gases tend to increase warming, there are other factors that tend to cool, rather than warm, the Earth, such as certain particles in the air (called aerosols):
Aerosols make the globe more reflective both by directly reflecting sunlight and by inducing the formation of reflective clouds. Human-caused aerosols, together with changes in land use like deforestation (pasture is more reflective than forest), increase the albedo [the ability of the Earth to reflect heat back into space] and so exert a net cooling influence that cancels about half of the warming influence of human-caused greenhouse gases.
And there are other counter-warming influences as well:
Then there are natural forcings: erupting volcanoes loft aerosols high into the stratosphere, where they remain for several years reflecting a bit more sunlight than usual and so exerting a cooling influence. Such eruptions are unpredictable, but they’re sometimes significant enough to negate human influences completely for a few months and therefore have to be taken into account. (For example, the earth was about 0.6ºC cooler during the fifteen months that followed the eruption of Mt. Pinatubo in June 1991.) And changes in the sun’s intensity of even a fraction of a percent over decades (due to its own internal variability) can change the amount of sunlight reaching the earth, further complicating our attempts to account for all the human and natural forcings affecting the planet’s delicate energy balance. But if we’re to understand the climate’s response to growing CO2 levels, it is important to know what those other influences are, how big they are, and how and when they come into play.
Koonin then describes the details of this climatic give-and-take:
The energy that flows in and out of the climate system is measured in watts per square meter (W/m2). The sunlight energy absorbed by the earth (and hence the heat energy radiated by the earth) amounts to an average of 239 W/m2. Since a 100-watt incandescent light bulb gives off, well, one hundred watts (almost all as heat), this means the planet radiates heat as if there were a bit more than two light bulbs in every square meter (eleven square feet) of its surface. Human influences today amount to just over 2 W/m2, or slightly less than 1 percent of that natural flow … The totality of human and natural influences on the climate is shown in Figure 2.4. It illustrates much of what we’ve discussed already. We can see the growth of greenhouse gas warming (predominantly from rising CO2 and methane concentrations, but also other human-emitted greenhouse gases), and that this has been partially offset by growing aerosol cooling. The episodic cooling by large volcanic eruptions is also evident. We can also see that, before 1950, total human influences (the sum of “CO2, “Other GHG,” and “Human cooling”) were less than one-fifth of what they are today. Also shown in Figure 2.4 is how uncertain we are about these various forcings. While the warming effects of CO2 and other greenhouse gases are known to within 20 percent, the uncertainty in the cooling influence of human-caused aerosols is much larger, making the total human-caused forcing uncertain by 50 percent—that is, the best we can say is that the net human influence today is very likely to be between 1.1 and 3.3 W/m2.
Koonin writes that, all things considered:
The fact that human influences currently amount to only 1 percent of the energy that flows through the climate system has important implications, and means there’s a lot to understand. To usefully measure them and their effects, we have to observe and understand the larger parts of the climate system (the other 99 percent) with a precision better than 1 percent. Small natural influences must also be understood to that same precision, and we’ve got to be sure they’re all accounted for. This is an enormous challenge in a system for which we have limited observations for a limited time, and about which our uncertainties are still large.
The greater uncertainties involved in climate prediction will be the subject of the next essay.
Links to all essays in this series: Part 1; Part 2; Part 3; Part 4; Part 5; Part 6; Part 7; Part 8; Part 9.