History of Carbon Dioxide
First, let’s start with Figure SPM-1 from the Fourth Assessment of the IPCC, showing the history of carbon dioxide and some other greenhouse gases over the last 10,000 years. Ice-core data from multiple cores and labs cover most of the history shown, and overlap with the recent instrumental record, all with very close agreement. The recent rise is unprecedented in the 10,000 years shown. Based on additional ice-core records not shown, the greenhouse-gas levels are now above anything seen in the last 800,000 years. And, data from other sources indicate that carbon dioxide has not been this high for millions of years. (Note that much further back in history, nature did cause higher CO2 levels, a topic to which we will return later.)
IPCC Figure SPM.1 Atmospheric concentrations of carbon dioxide, methane and nitrous oxide over the last 10,000 years (large panels) and since 1750 (inset panels). Measurements are shown from ice cores (symbols with different colours for different studies) and atmospheric samples (red lines). The corresponding radiative forcings are shown on the right hand axes of the large panels.
Click for a text description of the IPCC Figure SPM.1.
This image is a set of three graphs illustrating the historical concentrations of greenhouse gases and their associated radiative forcing from the year 1000 to 2005. The data is sourced from the IPCC 2007 WGI-AR4 report. Each graph represents a different greenhouse gas: carbon dioxide, methane, and nitrous oxide.
- Top Graph (Carbon Dioxide):
The graph shows carbon dioxide concentration in parts per million (ppm) on the left y-axis, ranging from 250 to 400 ppm. The x-axis represents time from the year 1000 to 2005. From around 1000 to 1800, the concentration remains relatively stable, fluctuating slightly between 250 and 280 ppm, depicted with scattered colored dots (gray, purple, green). After 1800, there is a noticeable upward trend, with the concentration rising sharply to around 380 ppm by 2005, shown with a solid red line. On the right y-axis, radiative forcing is plotted, ranging from 0 to 1 W/m². A vertical bar on the right side shows the radiative forcing in 2005, with a colored gradient (blue to red) indicating a value slightly below 1 W/m².
- Middle Graph (Methane):
The graph displays methane concentration in parts per billion (ppb) on the left y-axis, ranging from 400 to 2000 ppb. The x-axis spans from 1000 to 2005. From 1000 to 1800, methane levels are relatively stable, fluctuating between 400 and 700 ppb, shown with scattered colored dots (gray, purple, yellow, green). After 1800, the concentration increases significantly, reaching around 1750 ppb by 2005, depicted with a solid red line. The right y-axis shows radiative forcing, ranging from 0 to 0.4 W/m². A vertical bar on the right side indicates the radiative forcing in 2005, with a colored gradient (blue to red) showing a value around 0.3 W/m².
- Bottom Graph (Nitrous Oxide):
The graph illustrates nitrous oxide concentration in parts per billion (ppb) on the left y-axis, ranging from 260 to 330 ppb. The x-axis covers the years 1000 to 2005. From 1000 to 1800, nitrous oxide levels remain steady, fluctuating slightly between 260 and 280 ppb, shown with scattered colored dots (gray, purple, green, blue). After 1800, there is a gradual increase, reaching around 320 ppb by 2005, depicted with a solid red line. The right y-axis shows radiative forcing, ranging from 0 to 0.1 W/m². A vertical bar on the right side indicates the radiative forcing in 2005, with a colored gradient (blue to red) showing a value slightly below 0.1 W/m².
- Additional Details:
The x-axis at the bottom of all graphs includes a secondary time scale labeled "Time (before 2005)" ranging from 1000 to 5000 years before 2005. The graphs use a combination of scattered dots in various colors (gray, purple, yellow, green, blue) to represent historical data from different sources, and a solid red line to show the aggregated trend over time. The source of the data, "IPCC 2007 WGI-AR4," is noted in the bottom right corner.
Credit: Intergovernmental Panel on Climate Change (IPCC), 2007: Summary for Policy Makers. In Climate Change 2007: The Physical Science Basis. Contribution of Working Group 1 to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning,and H.L. Manning (eds.].
The figure shows “radiative forcing” as well as atmospheric concentration. The Earth absorbs 240 W/m2 from the sun. The extra warming from rising CO2 is somewhat similar, although not identical, to the warming from a brighter sun, so the effect of the CO2 can be discussed in W/m2. CBy January of 2017, atmospheric CO2 was at a concentration of 405 ppm, up from 280 ppm before the industrial revolution, with the extra CO2 giving a radiative forcing of roughly 2 W/m2, equivalent to the sun getting almost 1% brighter. The contributions from methane (from rice paddies, cow guts, and other sources) and nitrous oxide (especially produced by processes in soil stimulated by nitrogen fertilizers and animal waste) are significant but smaller.
The amount of extra CO2 now in the air, and moving into the ocean to make it more acidic, closely matches the CO2 we know has been produced from fossil-fuel burning. The human source is roughly 100 times as large as the natural volcanic source, and volcanoes have not done anything bizarre recently, so cannot be blamed for the recent rise. CO2 is moving into the ocean rather than coming out, so oceans cannot be responsible for the rise.
Furthermore, the atmosphere confirms that humans are responsible, as discussed in the ETOM film clip below and the Enrichment linked below.
Want to learn more?
Read the Enrichment titles Humans are Primarily Responsible for the Rise in CO2..
Earth: The Operators' Manual
Watch the short video below on how we know that the rise in CO2 is primarily from our fossil-fuel burning, filmed at the Rotorua Thermal area of New Zealand.
Video: It's Us (2:41)
Click here for a video transcript of "It's Us".
DR. RICHARD ALLEY: So physics and chemistry tell us that adding carbon dioxide to the atmosphere warms things up. And Earth's climate history shows us there will be impacts, from melting ice sheets, to rising sea level. But how do we know, with equal certainty, that it's not just more natural variation, that humans are the source of the increasing CO2? When we look at a landscape like this one we know immediately that volcanoes put out all sorts of interesting things. And that includes CO2.
So how do we know that the rise of CO2 in the atmosphere that we see, comes from our burning of fossil fuels, and not from something that the volcanoes have done? Well, the first step in the problem is just bookkeeping. We measure how much CO2 comes out of the volcanoes. We measure how much CO2 comes out of our smokestacks and tail-pipes. The natural source is small. Humans are putting out 50 to a 100 times more CO2 than the natural volcanic source. We can then ask the air whether our bookkeeping is right, and the air says that it is. Volcanoes make CO2 by melting rocks to release the CO2. They don't burn and they don't use oxygen. But burning fossil fuels does use oxygen when it makes CO2. We see that the rise in CO2 goes with a fall of oxygen, which says that the rising CO2 comes from burning something.
We can then ask the carbon in the rising CO2 where it came from. Carbon comes in 3 flavors: the lightweight carbon 12, which is especially common in plants. The medium-weight carbon 13, which is a little more common in the gases coming out of volcanoes. And the heavyweight carbon 14. It's radioactive and decays almost entirely after about 50,000 years, which is why you won't find it in very old things like dinosaur bones or fossil fuels. We see a rise in carbon 12, which comes from plants. We don't see a rise of carbon 13, so the CO2 isn't coming from the volcanoes. And we don't see a rise in carbon 14, so the CO2 can't be coming from recently-living plants. And so the atmosphere says that the rising CO2 comes from burning of plants that have been dead a long time... That is fossil fuels. The CO2 is coming from our fossil fuels. It's us.
So, yes, humans are increasing the greenhouse effect, primarily by producing CO2 by burning fossil fuels, with very little uncertainty.
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