To expand on my statement in the last post that we know humans are warming the planet, here is an adaptation of part of a talk I gave at a local high school a couple years ago. It summarizes the scientific evidence not only for global warming, but for anthropogenic global warming. In other words, it’s not just getting hotter, we’re making it hotter. And we can measure the ratios of different carbon isotopes in CO2 to prove it.
Let’s start with the statement, “Rising temperatures today are caused by anthropogenic CO2.” This is something that has been said, in some form or other, by a lot of different people and in a lot of different places. Al Gore’s An Inconvenient Truth will be ten years old next year, and this statement been a part of the national discussion for some time now. Debates can go back and forth over what, exactly, we should do about global warming, but the statement that rising temperatures are caused by anthropogenic CO2 is a statement of fact. It is not a debate. With that, let’s break down that statement and ask, how do we know? What is the evidence?
Start with the first part: rising temperatures. How do we know temperatures are rising? Easy—we measure them. Scientists have been waving thermometers around since they were invented, and we have decades-long records of temperatures from the air, from the oceans, and from all around the world. We know that temperature can vary considerably from summer to winter, from day to night, and sometimes from hour to hour. But when you take a continuous record through all of those variations, and you average it out over long periods of time (for example, making one average value for a whole year and then comparing from year to year), you can see the long-term warming trend. The amount of warming is different for different parts of the globe, but on the whole, the planet has already gained about 1°C over the last century. And this warming trend is not showing any signs of stopping.
The IPCC, that international authority on climate change data, shows this global warming trend in the graph below. Here, the temperatures are presented as “surface temperature anomaly”–taking the annual average temperature of the whole surface of the earth, and subtracting from that the average temperature for the time period 1986-2005. That anomaly used to be negative, and now it’s positive.
If you want to see what’s happening in just the United States, NOAA, our national administration of experts on all things oceanic and atmospheric, has a webpage called Climate at a Glance, where you can make your own graphs of temperatures over time (and download the data for free, too). Here’s a graph I made of the annual temperature for the contiguous 48 states from 1895 to 2015:
The website can also graph hottest and coldest temperatures and precipitation, and you can look at states, regions, and cities, too. Want to see what happened to August temperatures in Houston over the last few decades? NOAA can do that for you:
There’s a lot of variability in those numbers from one year to the next, but the long-term trend is that Houston today is hotter than it was when I was growing up there.
So the surface of the Earth getting hotter. That part checks out. Now we can look at the bit about anthropogenic CO2. We know that we have more CO2 in our atmosphere than we used to, again because we can measure it. Below is the original version of the famous Keeling curve, first published in 1970 by Charles D. Keeling, who was a professor at Scripps Institution of Oceanography.
It shows the concentration of CO2 in the atmosphere, measured at the observatory situated near the top of the Mauna Loa volcano on Hawai’i, and reported as parts per million (meaning, the number of CO2 molecules in a million molecules of air). The up-and-down cycling apparent in each year of CO2 measurement is the result of photosynthesizing plants in the northern hemisphere taking up CO2 while they’re growing, drawing down the amount in the atmosphere each summer. When photosynthesis shuts off in the winter, respiration takes over, and atmospheric CO2 levels rise again. (Plants in the southern hemisphere also photosynthesize, of course, but with most of the land mass in the northern half of the planet, there just aren’t as many of them.) Just like temperatures cyclically rise and fall every summer and winter while at the same time the planet is getting hotter overall, this wave-like pattern in CO2 concentrations overlies an longer-term rise in concentrations, where each year there’s more CO2 than there was before.
Scripps continues to measure CO2 concentrations today, at the same Mauna Loa Observatory and at locations around the world. Here’s what the graph looks like now:
The 400 ppm threshold is an important one, which some experts think might be a critical tipping point for the global climate system. We’ve been steadily moving towards it for years, and pretty soon we’ll be seeing that number in the rearview mirror as CO2 concentrations keep climbing.
Along with measuring the overall amount of CO2 in the atmosphere, we can tally up the amount of CO2 emitted by human activities. Essentially, we can count up all the cars in the world, all the factories, measure the amount of cement made (cement production makes a lot of CO2), and we can estimate how much CO2 comes from all of those activities. That would give us a graph like this:
Gt stands for gigatons, “giga” representing a factor of a billion. 49 billion metric tons of CO2 were ejected into the atmosphere in 2010 because of human activity, according to the IPCC. That is a lot of CO2.
So, we’ve established that we can measure the following:
- The planet is warming.
- The concentration of CO2 in the atmosphere is increasing.
- We are adding a lot of CO2 to the atmosphere, and the amount that we’re add is increasing.
But! What about natural CO2 sources and sinks? Some of that CO2 goes into the ocean (which is causing ocean acidification, a discussion for later.) Some of it gets taken up by plants. How do we know that it’s the CO2 that we’re adding that’s increasing the overall concentration in the atmosphere? This is where the carbon isotopes come in. (This is the neat part for me, since carbon isotopes are part of what I study.)
The Evidence in the Isotopes
To establish some background, we need to understand what isotopes are, and why they matter for carbon. An element like carbon is defined by how many protons it has in its nucleus. Carbon has six protons. Add a proton, and you get nitrogen. Subtract a proton, and you get boron. But you can add or subtract neutrons, which also reside in the nucleus, and the element stays the same. You just get different isotopes. Most carbon in the universe (98.9%) has 6 protons and 6 neutrons, giving it a mass number of 12 and the name carbon-12. This nice balance of nuclear particles makes for a stable isotope—one that does not undergo radioactive decay.
Add a neutron to carbon-12, and you get carbon-13, which is also stable. Add one more neutron, and you get carbon-14, which is not.
These are the three naturally occurring isotopes of carbon. They can all be found mixed in as all of the carbon of the atmosphere, in the tissues of living plants and animals, and in our own bodies.
Carbon-14 is generated naturally in the upper atmosphere when a cosmic ray slams into a nitrogen atom. Plants take up CO2, some of which has carbon-14, and turn it into sugars and carbs. Animals, including humans, eat those plants (or other animals) and take up that same carbon-14 and incorporate it into their tissues. But when something dies, it stops assimilating carbon, and that’s when the carbon-14 clock really starts, with the isotope decaying at a steady rate back into nitrogen. Carbon-14 has a half-life of 5715 years, so it what’s used to date Egyptian mummies and the Shroud of Turin. But after 10 half-lives, only about 0.1% of the original carbon-14 remains, and it gets really difficult to measure accurately. So radiocarbon dating is only really useful for stuff that’s younger than 50 to 60,000 years old. Fossil fuels come from plants and algae that lived hundreds of millions of years ago. Their carbon-14 is all gone.
So then that means that if we’re adding CO2 from fossil fuels to the atmosphere, we should see the ratio of carbon-14 to carbon-12 decrease, because we’re adding CO2 that has no carbon-14 but still plenty of carbon-12. We have a prediction to test!
And not surprising, that is indeed what scientists observe. Here is a graph I made in Excel using publicly available data from Scripps, from the same Mauna Loa Observatory where we saw increasing CO2 concentrations in the Keeling curve. It shows the ratio of carbon-14 to carbon-12, which scientists report as Δ14C (which is a slightly more complicated ratio that involves comparing the sample value to a standard value. Here’s the equation explained.) As predicted, the ratio of carbon-14 to carbon-12 decreases, all while CO2 concentrations are increasing.
Now for that other stable isotope, carbon-13. It lasts just as long as carbon-12, but it’s heavier by one neutron. Being heavier, it makes a slightly less bouncy and less reactive CO2 molecule than carbon-12 does. And that means that when a plant is taking in CO2 and converting it to sugar during photosynthesis, that plant is a little bit more likely to use a CO2 molecule that has carbon-12 than a molecule that has carbon-13. So the plant’s tissues, all of which come from that photosynthetic process, end up with a skewed ratio of carbon-13 to carbon-12. They have a lot less carbon-13 than we would otherwise expect.
Fossil fuels are the pressure-cooked remains of ancient plants and algae, and because carbon-13 and carbon-12 are both stable isotopes and don’t decay like carbon-14 does, fossil fuels keep that same skewed isotope ratio that was the result of photosynthesis.
So our second prediction is this: if atmospheric CO2 concentrations are increasing because we’re burning fossil fuels, we should expect to see the ratio of carbon-13 to carbon-12 decrease. Just like we expected to see with carbon-14, but for different reasons.
And again, that is indeed what scientists observe. Here’s a graph of the carbon-13 to carbon-12 ratio in CO2 from Mauna Loa, again generously made publicly available by Scripps, and again plotted by myself on Excel. This time the ratio is reported as δ13C, with a lower-case delta instead of upper-case, and again using a slightly more complex comparison of samples with a known standard (Wikipedia has the equation). For the purposes here, the lower the δ13C value, the less carbon-13.
The amount of carbon-13 in atmospheric CO2 is also decreasing, just as the amount of carbon-14 is decreasing. And all while CO2 concentrations are increasing. Those big annual wiggles in the carbon-13 ratio are caused by the same seasonal cycling of photosynthesis that drives the wiggles we saw in the Keeling curve.
So there it is! Temperatures are rising. CO2 concentrations are rising. We’re generating billions of tons of CO2, and the isotopes of CO2 in the atmosphere are responding in exactly the way we’d expect if the CO2 we’re making is what’s causing the increase. The last piece is the connection between CO2 and temperature known as the greenhouse effect, and that’s been well-understood since the Swedish scientist Svante Arrhenius described it way back in 1896.
So having validated the statement that we humans are warming the planet, what exactly does that mean for us and the planet? Has the planet ever experienced anything like this before? Those are questions for another post.