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Anthropogenic forcing

In this video Dr. Nathan B. English teaches us about anthropogenic forcing
<v ->Hello again.</v> And we’ve just covered in the previous steps a lot of the causes of what we call, greenhouse warming, greenhouse gas warming, or global climate change or global warming, call it what you want. The previous steps where we talk about radiation and how it interacts with the atmosphere is important in showing how solar radiation is converted to terrestrial radiation and that in turn causes the atmosphere to warm. And I also wanted to show you in a really sort of tangible, quantitative way, how our own actions and how burning fossil fuels can narrow and shrink those atmospheric windows that terrestrial radiation can escape through.
And thus by narrowing those windows we’re essentially closing the, thickening the glass on the atmospheric greenhouse. So just, we’re still in the causes here and in the following, following on from this step, we’ll talk about some of the effects of these greenhouse gas emissions into our atmosphere. And those are the effects on the temperature in the atmosphere and on the oceans. So I think, just to sort of recap, I hope everybody had fun calculating their carbon emissions from their mode of transportation. When I went through and did this for my own vehicle a while ago, by the end of its lifetime, it was up to about 188,000 kilometers.
And in that time I burned through, because it was a V8, I burned through about 28,000 liters of petrol. Which is the equivalent of 16 elephants. And I think one of the things I wanted to ask you guys to do that I didn’t get a chance to do in the previous step is when you’re driving around town or riding the bus around town, I want you to try visualizing every car with at least three, four or five elephants on it. Because by the end of that car’s lifetime, or by usually, by the average middle of that car’s lifetime, that’s what they’re, you’re looking at elephant-wise.
And just a matter, and now remember also how thin and fragile our atmosphere really is. It’s only 12 kilometers thick, about the 90% of it.
And so now you can begin to see how individuals and societies, can have a large impact and effect on our atmosphere.
These impacts and effects are measurable. Now this is a chart. This is a rather famous chart from atmosphere science. It’s the atmospheric concentration of carbon dioxide at Mauna Loa Observatory in Hawaii, United States. Now, the Mauna Loa Observatory is on a, I think it’s a 4,000 meter tall mountain in the middle of the ocean in Hawaii. So it’s relatively free from any kind of human influence. It’s got a really well-mixed atmosphere. So it’s a great place to sort of look at concentrations of different molecules in the atmosphere. And one of these is CO2, and back in the 1950s,
climate scientists and environmental scientists began to actually collect samples every month from Mauna Loa and measure the concentration of carbon dioxide in the atmosphere. And now remember pre-1850, atmospheric CO2 was about 285PPMV or parts per million by volume. And that’s what’s shown on the Y-axis of this graphic here. On the X-axis is the year and he started in 1958. He’s name was Keeling.
He’s got a couple of cool nifty things about atmospheric gases named after him. I think it was Paul Keeling, and he measured monthly. And one of the things he noticed was this increasing trend in CO2. It was visible even back in 1960. Remember, it had been rising from 285 in late 1850. So he’s still on the tail end. Not only was CO2 increasing, it was increasing faster and faster to higher and higher rate. And then he also noticed these cool little month seasonal variations and it as well. And those seasonal variations are actually the temperate force in the Northern hemisphere, breathing in the summertime and assimilating carbon dioxide in the, in the summertime and putting it into the plant matter.
And then in the winter time, lots of those dead leaves and needles decay, and they release CO2. So this is essentially the breathing planet, the breathing Northern hemisphere forces and plants in the Northern hemisphere. But overall, this has been increasing and you can see here that we’ve passed 400 PPMV a couple of years ago, and it’s not likely we’ll ever get back below 400 parts per million by volume. So this is an effect that we can see. Now in the latest International Panel on Climate Change or IPCC report, this is the AR6, Assessment Report 6. This is the sixth assessment report. They’ve been doing these since I believe the early 1990s.
They find that it’s based on a lot of, calculations, very similar to what you did for your vehicle, but on just imagine a grander scale. It’s unequivocal that human influence has warmed the ocean atmosphere and land. Widespread and rapid changes in the atmosphere, ocean cryosphere and biosphere have occurred. And unequivocal is a, is a pretty broad statement about the probability that climate change is human-caused. Now, I want to spend some time talking about attribution, and it’s, what is it in our atmosphere, that’s actually increasing the temperature or decreasing the temperature of our atmosphere. And these attribution studies are done in a couple of ways or forcing studies are done in a couple of ways.
Attribution studies look at trying to measure the, impact of these particular items, in a real world sense. So whether they add energy to the Earth’s surface or subtract energy from the earth surface through reflecting at the, at the atmosphere’s top. So before that radiation even reaches earth, it can be reflected off into space. And then there’s these radiative forcing studies that look at the physics, behind the greenhouse gas warming, or aerosol reflection, reflectance or warming as well. Though, let me just go from left to right here. This is a figure from the summary for policymakers, which is a part of the IPCC AR6 report.
And I strongly recommend you, if you have a look at this report and to take a look, and I do provide a summary after these following steps and ask you to think about which impact, which effect from climate change will impact you most. On the left-hand side is the observed warming. And this is just the warming we’ve observed, from an average temperature taken from 1850 to 1900. And the average temperature between 2010 and 2019 was about a degree Celsius warmer than it was a century ago. Okay. So how do we, you know, how do we break this up? What part of this is just maybe natural variability? What part of this is anthropogenic, meaning human-caused, climate change?
What part of this is due to nitrous oxide, methane and carbon dioxide, or what part of this is due to aviation contrails or black carbon aerosols, right. In the attribution studies, it’s really clear that internal variability and solar and volcanic drivers just aren’t a player. We, we can measure these pretty well in the last century. They, they have very little to do with current warming. Over geologic timescales, absolutely, they have an effect, but when I say geologic timescales, I’m talking about millions of years. Okay. Hundreds of thousands to millions of years, not, not decades and centuries. Okay, well-mixed greenhouse gases have a large forcing effect.
So adding energy to the Earth’s surface through back radiation, from the atmosphere of terrestrial radiation and then the total human influence, you can find here. Other human drivers, these are things like aerosols, which are small particles that drift in the atmosphere. And if they’re light colored aerosols, like sulfur dioxide
or nitrogen oxides, or sorry, not nitrogen oxides, organic carbon or ammonia, if they’re light colored aerosols, they can reflect that high-frequency solar radiation at the top of the atmosphere before it even reaches earth, cooling the planet. But if they’re dark particles like black carbon, they can actually absorb that solar radiation and add that heat energy to the Earth’s atmosphere. So when we add up the human drivers, greenhouse gases and other human drivers, we find that the total human influence equals about one degree celsius. Okay. Which matches with what we’ve seen. So there’s some, some nice attribution studies that are consistent with the warming we’ve seen today.
When we look at the radiative forcing studies, these are the physical models that people create. We find some similar patterns. Carbon dioxide, is responsible for about two thirds of the observed warming, methane about one third. And then a lot of these other ones sort of mucking about in the, in the small numbers here, I will point out that sulfur dioxide, which is a common pollutant from coal fired or fossil fuel fired power plants is a anthropogenic aerosol that cools the planet because it’s a light color and it reflects solar radiation back out into space. Sulfur dioxide can also be found in volcanic emissions as well. Black carbon like soot adds, to the radiative forcing of the earth surface.
And then aviation contrail’s have an interesting impact. They’re both reflective and absorptive. And so they have sort of just a kind of, they null each other out and nulls itself out. Also land-use reflectance and irrigation have changed the reflectance of the surface of the earth. So that high frequency solar radiation is actually reflected back into space through the same wind, atmospheric windows that it entered through. And you can think of things like changing, changing a dark forest, from a dark forest, you cut down all the trees and you plant a wheat field that is relatively light in color and reflect solar radiation better than a dark forest.
So when we look at all of these things, it’s, it’s relatively clear that anthropogenic forcing through atmospheric additions is really a good, and consistent explanation, for the effect we see in the atmosphere of a one degree warming. So cause and effect are, are right there.

We know that human influence has an impact on the level of energy in the earth’s atmosphere. Let’s look more closely at what in our atmosphere increases and decreases that energy level.

Watch Dr. Nathan B. English examine how our own actions are impacting the windows that radiation can pass through and contributing to the warming of the Earth.

Video guide

Since this video is a little longer than most videos in this course, here is a guide to the topics it covers.

Topic Timestamp
How our actions narrow atmospheric windows 0:40
Atmospheric CO2 at Mauna Loa 2:45
Attribution studies 6:00
Where do greenhouse emissions come from? 12:00

Now it’s your turn

Having examined the cause and effect of some of the major contributors to global climate change, what actions can we as individuals take to reduce our greenhouse gas emissions? Share your thoughts in the comments.


Figures SPM.1, SPM.2, SPM.5 and SPM.8 from IPCC, 2021: Summary for Policymakers. In: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson- Delmotte, V., P. Zhai, A. Pirani, S. L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M. I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J. B. R. Matthews, T. K. Maycock, T. Waterfield, O. Yelekçi, R. Yu and B. Zhou (eds.)]. Cambridge University Press. In Press.
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