Thursday, March 9, 2017
My explanation of why we know it's CO2 causing Earth's current warming trend
I owe someone something.
Specifically, I owe an individual with the Twitter handle of "Mike Mangan" an explanation of why "we" (by which I mean the mainstream scientific community) knows that CO2 produced by human activities is the main cause of climate change -- which means, primarily, increasing global temperature.
Quick note: this assumes that it is accepted that the CO2 increase starting in the 1850s or so is anthropogenic. That's very easy to support in several ways, if necessary. But I don't think that's necessary here.
I promised him (Mangan) I'd do this awhile ago. It's been hard to motivate myself, because I had to convince myself it was useful. But I've reached a point where I think it might be. And I'm also going to write a second article (eventually) about the misconceptions and misstatements and distortions of science stated by Alex Epstein to promote his worldview that burning fossil fuels is a moral imperative or some such. But it's important to show that increasing atmospheric CO2 is the likeliest (by far) cause of the observed warming and related geo/eco/bio effects before taking on Epstein's flawed premise.
Now, before I continue, let me state that I'm not a member of the mainstream scientific community. As a nuclear power industry consultant, I'm on the periphery of actual science. But part of what I do is to justify and advocate nuclear power as part of the solution to climate change and the energy requirements of the human population, so I've striven mightily to understand the science and to contend with denialists, because they are getting in the way of real solutions to an increasingly difficult problem. Part of what must happen is that the criticality of dealing with climate change internationally and comprehensively has to be realized by a sufficient number of countries and their corresponding populations for it to become a major effort. There is momentum now. We can solve it -- in fact, it may be a trivial problem to solve, technologically. BUT It's not a trivial problem to deploy the solutions with enough global breadth to make them work.
Another part of what I do requires understanding ecology, so that impact statements can be authored authoritatively. So I've been in the water making measurements, I've taken measurements of air and emissions and radiation (from the Sun), and I've had to collect data. Maybe I am a scientist of a sort, but scientists don't like it when consultants call themselves scientists.
OK, enough about me. Let's lay out the facts of the case.
Number 1: CO2 absorbs longwave infrared radiation.
What happens is this (and believe me, this is an extremely simplistic summary of much more complicated processes). Incoming solar radiation is UV, IR, and visible. UV we don't have to worry about, except to avoid sunburn (but I'll be referring back to stratospheric ozone later). Visible -- can have an effect, especially in the oceans. Ice reflects visible light, dark water absorbs some of it, and that can enable heating as sea ice decreases.
But the important radiation here is IR - infrared. Incoming solar radiation is shortwave IR. It hits the land, gets absorbed, warms it up, and gets re-radiated (mostly upward) as longwave IR. This is what CO2 can absorb, and re-radiate, and absorb again. CO2 is therefore a major player in Earth's radiative balance. Let me illustrate. (There are many similar diagrams on the Internet.)
Now, I don't intend to go into how the greenhouse effect actually works in the atmosphere. It's quite complex. One of the errors of skeptics is the idea that the CO2 in the atmosphere is somehow getting saturated and can't absorb more IR. The reason that is wrong is explained quite nicely here: Is the CO2 effect saturated?
Let me also note that the planet Venus, with a thick atmosphere composed almost entirely of CO2 (with just some acids floating around to make it interesting) is hellaciously hot, because CO2 traps heat in the atmosphere.
Let's move on to the next point.
Number 2: If CO2 is absorbing longwave radiation, there must be a directly observable effect.
Yes, there must be -- and there is. It's called the cooling of the middle stratosphere.
Here's how this works, basically (and it too, in reality, is more complex, but fundamentally the result is the same). The stratosphere is so tenuous that it is heated and cooled radiatively; that is, the molecules absorb IR radiation rising upward from the surface, which makes them warmer, and radiate IR to space, which makes them cooler. Collectively, that determines the temperature of the stratosphere. Now, ozone depletion also has effects on the temperature of the stratosphere, to whit, less ozone means a cooler stratosphere, as there are less ozone molecules to interact with the IR from the surface.
If you're wondering, the troposphere warms and cools convectively, which is to say that the temperature is determined by the movement of air masses of different temperatures and densities.
So... if more and more longwave IR is being trapped and held in the lower atmosphere by GHGs, especially CO2, that means less of it is going to make it to the stratosphere. Less IR reaching the stratosphere means the stratosphere is going to cool. And that's what is happening. If the temperature of the entire stratosphere is examined, it has been cooling for a few decades due to this effect, but as ozone depletion lessened, the temperature of the entire stratosphere leveled out.
But not the middle stratosphere. That's because the effects of ozone depletion on the temperature of the stratosphere occur in the lower stratosphere, and the effects of increasing CO2 in the troposphere mainly occur in the middle stratosphere. And that has continued to cool, no matter what is happening with the temperature of the troposphere. So more heat is being trapped in the troposphere, near the surface of the Earth.
Here's a plot of the middle stratosphere temperature data, from Remote Sensing Systems:
Number 3: Extra heat being trapped in the troposphere has to go somewhere.
Skeptics like to point out that the warming at the Earth's surface hasn't been uniform, there have been cooling periods and flat periods (maybe), none of which I will give a name to, ever since the effects of increasing CO2 began to kick in. So, they ask, how can that happen if there is evermore heat being trapped.
The simple answer to that is: most of the heat is going into the ocean. It's been measured, documented, and quantified. Yet still, the ocean's uptake of the heat can vary, depending on a few factors, and just a slight variation can have a big effect on the atmospheric temperature. That might be part of the explanation for what happened in the early years of the 21st century (if in fact it happened). But more heat kept going into the oceans -- until it came back out decisively, in a massive El Nino event that drove atmospheric temperatures to all time highs, and also lower tropospheric temperatures measured by satellites.
Thus, in modern times, we have these basic, measurable indications of the greenhouse effect of increasing atmospheric CO2. The heat is being trapped in the troposphere, so less is reaching the stratosphere. Most of the heat trapped in the troposphere ends up going into the oceans, which are measurably warming. ALL of the other effects of global warming (I'm especially fond of phenology, like spring thaw/autumn freeze trends for lake ice, migration timing, flower blooms, etc.) are connected to these demonstrable observations.
But what about the past? Two additional examples will be made here; the warming of the Earth at the end of the last glacial period, and the Paleocene-Eocene Thermal Maximum (PETM).
Point 4: Add a lot of CO2 to the atmosphere, and the Earth gets a lot warmer.
The first is the PETM. What happened then is that a massive amount of carbon went into the atmosphere. I haven't checked to see what the most popular theorized mechanism for that is, but there's no doubt that it happened. And the carbon entering the atmosphere, likely as methane, oxidized rapidly to CO2. And Earth's temperature shot up (on geological timescales). It was a perfect natural experiment to demonstrate that if you add CO2 to the atmosphere, a lot of it, Earth's temperature is going to go up, up, up. See what it looks like:
Back in my early days of blogging, I wrote a whole article on this, with lots and lots of references. Just checked (February 2017) -- most of them still work, enough for this to still be useful.
5. CO2 leads, not lags.
That brings us to my final point -- the warming at the end of the last glacial period, which ushered in the Holocene (though the Younger Dryas forestalled that at the end of the warming). Now, if you have been paying some attention to this issue, particularly as a skeptic of anthropogenic climate change, one of the arguments about CO2 and the Ice Ages was that the temperature changes appeared to occur ahead of the forcing factor; i.e., temperature appeared to go down before CO2 decreased in an interglacial-glacial ' transition, and temperature appeared to go up before CO2 increased in a glacial-interglacial transition. Very briefly, this meme was called "CO2 lags, not leads".
First of all, the triggering factor of a transition is a superposition of all three of the Milankovitch cycles: eccentricity, axial tilt, and precession. The superpositions cause the biggest swings in insolation. To trigger a glacial-interglacial transition, there has to be sufficient Northern Hemisphere insolation to induce major melting of the ice sheet, which causes a loss of albedo, allowing more solar insolation to begin the warming process. (Positive feedbacks.)
Once the warming has begun, the combination of warmer temperatures and ice melting triggers ocean circulation changes. This leads to what is called "ventilation" of the deep ocean. "Ventilation" simply means increased upwelling of deep waters, which have higher amounts of dissolved inorganic carbon. Bringing these waters to the surface releases CO2, which ' causes more temperature increase, furthering ice melt, albedo decrease, and increased ocean circulation. Classic positive feedback.
The "CO2 lags, not leads" conundrum was based on examination of the Vostok ice core, which has CO2 concentrations trapped in bubbles, and which uses stable oxygen isotope ratios as a paleothermometer. And in the Vostok core, the temperature rise precedes the CO2 increase. That was perplexing, until it is simply realized that the Vostok ice core is in Antarctica, and Antarctica doesn't act like the rest of the world. It is, in fact, very isolated, primarily due to the atmospheric circulation induced by the frigid continental ice. (It was also perplexing because the only major factor that changes enough to induce the temperature change of a glacial-interglacial transition is CO2, but it didn't make intuitive sense then, that CO2 would lag behind the temperature increase, since nothing else was nearly as potent a forcing factor.)
This leads to Shakun et al. 2012. What was done in this paper was to use a large set of temperature proxies to represent the global temperature (not just the Antarctic temperature). From https://www2.bc.edu/jeremy-shakun/FAQ.html:
"A key issue here is that while the ice cores record global atmospheric CO2 concentration (since it is well-mixed by the winds), they only reflect local temperatures in Antarctica. Just as no place today can be reliably expected to reflect the global average temperature, it is questionable if Antarctica does this over the ice ages. One basic reason for this is simple: heat moves around with winds and ocean currents. So, one place might get warmer at the expense of another place getting relatively colder, and looking at either location individually would give a skewed view of the larger temperature patterns. Therefore, the only way to reliably track changes in the total amount of heat at the surface of the planet is to average temperature data from as many locations as possible. This approach should help to cancel out these heat redistributions, removing a complicating variable, and make it easier to interpret what drove global temperature in the past."
So when you do that, as the researchers did, what's the result?
"The pattern of global temperature rise over the end of the last Ice Age as reconstructed from 80 proxy records around the world is strongly correlated with the increase in atmospheric CO2 concentrations recorded in ice core air bubbles. Furthermore, global warming generally appears to have lagged a few centuries behind the rise in CO2. These two points are consistent with the idea that CO2 was a major driver of global warming at the end of the ice age."
Exactly! This paper was important because it resolved the conundrum with real data. Occam's Razor was satisfied, and the simplest explanation fit both the underlying geophysical foundation (physics) and the data.
So that's why "we" know that the increasing CO2 caused by human activities is causing the observed warming of Earth's climate. No matter what the head of the EPA says (I had to add that).
Any questions? Especially from you, Mr. Pruitt?
Hope you like this, Mike.
at 7:55 PM