L4. Carbon and Climate
Introduction
One of the primary environmental issues resulting from our voracious consumption is Climate Change. This is because:
- At ever stage of a products life cycle there are energy inputs.
- Most of the energy is from fossil fuels
- Burning of these fossil fuels emits carbon dioxide
- These additional emissions of carbon dioxide change how much of the sun's energy is absorbed in our atmosphere and result in climate change.
NOTE: There is no video lecture for this lesson.
Carbon Cycle
So what is the carbon cycle and how does it relate to greenhouse gasses and climate change? Let's begin our discussion with a short animation showing the very basic concepts of the carbon cycle and climate change. If you have never studied the carbon cycle this is a good place to start. (If you think you know it, you can skip the video. There are no quiz questions on the video. It is all about learning : )
Understanding how carbon flows in the environment is important, however, a bit of quantification is also needed as we try to understand the human impact on the climate. In reality, human activity adds a very small amount of CO2 into this carbon cycle.
Carbon is the fourth most abundant chemical element in the universe (behind hydrogen, helium, and oxygen). There are about 65,500 billion metric tons of carbon in the earth (a metric ton is 1000 kg or 2200 lb) with the majority stored in rocks. The second biggest reservoir of carbon is the deep ocean (37,000 billion metric tons) and the third largest reservoir is fossil carbon (10,000 billion metric tons). Figure 1 shows these reserves of carbon and the emissions and absorption of this carbon in what is known as the carbon cycle. Carbon is moving or cycling all the time through these different reservoirs.
Figure 1. NASA. Carbon Cycle. Units in billion metric tons. Source (Links to an external site.)
The arrows (and yellow numbers) in Figure 1 indicate the movement or cycling of the carbon.
- Units are in Gt which is Gigaton which is the same as billion metric tons.
- There are about 210 Gigatons or 210 Gt) of carbon cycling in and out of the atmosphere naturally each year.
- The atmosphere stores about 800 Gt of carbon.
- Arrows pointing down indicate that carbon is being taken out of the atmosphere. Plants and oceans are referred to as carbon “sinks” as they take in this carbon from the atmosphere.
- Arrows moving up (plant respiration, microbial respiration, and oceans) indicate movement of carbon to the atmosphere and these are referred to as sources of carbon emissions. Yes, plants and oceans are both carbon sources and sinks.
- The red numbers indicate human influence in the cycle. Human additions are 9 Gt, with about 2 Gt being absorbed by the ocean and 3 Gt being taken up by plants through photosynthesis. This means that annually there is an additional 4 Gt of carbon going into the atmosphere – thus increasing the atmospheric concentration of Carbon. The 4 Gt to the atmosphere is important (as we will see) but so are the total of 5 Gt being absorbed the ocean and plant material. This additional carbon creates some changes there also.
Fast Carbon Cycle
Many of us are familiar with photosynthesis and the carbon cycle of plants. During photosynthesis plants take in carbon dioxide (one carbon and 2 oxygen molecules) and give off oxygen. Using energy from the sun, the carbon dioxide taken in by the plant combines with other elements (primarily hydrogen, oxygen, and nitrogen) to form cellulose, lignin, proteins, sugars, etc in the plant. When the plant dies, microbes eat this plant material (called biomass) and give off carbon dioxide through a process called respiration. Animals and humans also respire – eating plants and animals, breathing in oxygen, and giving off carbon dioxide. This plant carbon cycle is part of what is known as the Fast Carbon Cycle. For more information see NASA: The Carbon Cycle (Links to an external site.). This fast cycle refers to cycles of carbon in the biosphere – exchanges between plants, animals, soils, oceans, and the atmosphere. CO2 concentrations fluctuate on a seasonal basis due to this faster carbon cycle. You can see the impact of this fast cycle on an annual basis in the atmospheric CO2 concentrations (animation Figure 2). (There are some other great visualizations of carbon and climate change here Links to an external site.. Not required.)
Figure 2. Real time atmospheric carbon dioxide concentrations over one year. Graphic shows changes by latitude and over a one year period.
Human Influence on Atmospheric Carbon
There is also a “Slow” Carbon Cycle where carbon slowly cycles between rocks, oceans, and soils over periods of 100-200 million years. Historically fossil fuels (petroleum, coal, and natural gas) have been locked in the slow carbon cycle. Fossil fuels are made of principally of carbon and hydrogen and were formed long ago from plant material (biomass) at high temperature and pressure. The carbon in this fossil fuel was essentially locked away in the slow carbon cycle until about 100 years ago when we started using it for an energy source. As we burn this fossil fuel (old plant material) we release the carbon and put it into the atmosphere – moving it from the slow carbon cycle into the fast carbon cycle. Figure 3 shows that the various contributions of anthropogenic Links to an external site. emissions (human caused). Of this, about 40% was from from coal, 34% from petroleum, and 19% from natural gas. The "other" in the graphic includes emissions from cement production and the flaring of natural gas at extraction sites.
Figure 3 also includes land use changes. Land use change reflects any changes to the number or types of vegetation on the earth. Land use changes include such things as the conversion of forests to agricultural land or the building of roads, houses, or cities on areas that were agriculture or forest. Since plants take up carbon - any conversion from forest or agriculture to buildings and roads results in more carbon emitted to the atmosphere.
Figure 2. CO2 contribution by type of fuel (worldwide) plus land use change. (Source: Global Carbon Budget 2014). (Note: This is carbon dioxide (CO2) not elemental carbon (C) hence the emissions are reported as about 40 Gt CO2 rather than 9 Gt C that we were using before.)
This addition of fossil carbon to the atmosphere (and changes in land use) has increased the concentration of carbon in the atmosphere. In the last several hundred thousand years atmospheric concentrations were in the range of 200-300 ppm. Over the last 100 or so years this concentration has risen by about 30% or to over 400 ppm (ppm is parts per million). This is important because of the role carbon dioxide (and other gases) play in the chemistry of the earth’s atmosphere and the energy balance of the planet (more on this later). In the graph below (Figure 3) you can see the seasonal fluctuations because of plant growth (part of fast carbon cycle) and the general long term increasing trend because of the addition of fossil fuels.
Figure 3 shows the variation in concentration seasonally, by latitude and over time. Note the relatively constant CO2 concentrations in the historic past compared to the recent and abrupt change. Please watch the whole 4 minutes!! Speed it up if necessary. What was the concentration in 1979? The year you were born? In the ice ages? Preindustrial times? Did any concentrations before 1000 AD go above 300 ppm? (BCE is Before Common Era and KY is 1000 years so 800 KYBCE is 800,000 BCE.)
Figure 3. Atmospheric carbon dioxide concentration due over time and by latitude.
Greenhouse Gasses
Our atmosphere is comprised of about 78% nitrogen, 21% oxygen, 0.93% argon and 0.04% carbon dioxide (400 ppm) plus a few other gasses at even lower concentrations (e.g. methane at 0.00017%). Although the concentrations of carbon dioxide and methane are very small, they are extremely important gasses. as they regulate the earths temperature. These gases are known as greenhouse gases (GHGs) because, like a greenhouse, they trap heat by absorbing some of the energy from the sun. Increasing GHG's in the atmosphere results in a warming planet. In the last 100 or so years the CO2 in the atmosphere has risen bout 30% from about 300 ppm to 400 ppm (0.04%).
Climate Change Science
The climate is always changing due to many factors that are well understood by scientists. What we are interested in is how human activity is changing the climate.
There are just a few simple things to know:
- Climate change is all about Earth's energy balance. Energy arriving from the sun must equal the energy returned to space or the earth gets warmer (or colder).
- Incoming energy from the sun: The amount of sun entering the earths atmosphere is a function of the energy output of the sun and the distance the sun is away from the earth. On average the sun's energy reaches our outer atmosphere at an intensity of about 340 Watts/meter2. Natural cycles of the earth's orbit and the solar output will impact this number slightly and have been the cause of past climate change. Humans have no influence on this incoming solar energy.
- Outgoing energy: The energy coming from the sun is reflected or radiated from the earth back into space. This outgoing energy must be equal to the incoming energy from the sun or the planet will warm or cool. There are many human influences to this outgoing energy.
The term "radiation" is is used when something absorbs energy and then re-emits the energy. Think about a brick building. As the sun shines on the bricks they will warm up. They are reflecting some of the energy, but also absorbing some energy. When the sun stops shining, the bricks release that energy (*radiate that energy) back into the atmosphere. Note: If the bricks did not release as much energy as they got when the sun was shining over time the bricks would keep getting hotter and hotter.
If we look at the reflection, absorbtion, and radiation of energy (the Earth's Energy Balance) we have a complex graphic that looks like Figure 4. The important thing to note in the graph is that the atmosphere reflects, absorbs, and radiates energy as does the clouds and the earth's surface. This absorption, reflection, and radiation of energy is from carbon dioxide and other greenhouse gasses (e.g. water vapor, methane and nitrous oxide). Changing the concentration of GHG's in the atmosphere, changes this energy balance. This is not theory, it can be tested in the laboratory and has been known for over 100 years. Estimates are that we are currently receiving about 340 Watts/m2 from the sun and returning slightly less. This difference is about 0.8 Watts/m2 (NASA Links to an external site.) but it has the consequence of slightly warming the planet.
Figure 4. Earths Energy Budget from NASA. (Source Links to an external site.)
How Much Warming?
In the past the average temperature of the earth was 15 °C. Because of the additional energy being absorbed, earths temperature is now about 0.8°C (1.4°F) hotter than it was in 1880 and is rising about 0.2°C (0.36°F) per decade.
Figure 5 shows the global temperature measurements (black line) since 1880. The graph shows the data as a temperature offset from the average earth temperature of 15°C (59°F). The graph also shows the range of predicted temperatures based on computer models. The blue shaded area are the results of computer models that include the human factors of land use change and increases in atmospheric GHG's. These results are similar to the observed data (black line). The green shaded area represents the computer model outputs if only natural factors are included then the computer models. This is a bit of verification that the human influences are real and can be predicted based on atmospheric science.
Figure 5. Modeling and measurement of global temperatures (Source Links to an external site.)
You might ask "So what it the big deal in just a few degrees? Daily temperature changes are way more than a couple of degrees."
The problem is that just slight changes in global temperatures have a tremendous impact on the planet. During the last ice age (about 13,000 years ago) the average global temperature was only about 4°C cooler than normal and yet all of Canada and most of the US was covered in ice.
Figure 6. Global average temperatures over time.
The additional energy in the atmosphere changes rainfall patterns and shifts ocean currents. This changes the length of the growing season in northern climates. It impacts the strength of hurricanes the the frequency of droughts and flooding. As a result of our current global warming, the Arctic Ocean is expected to become essentially ice free in the summer by 2050. These climate changes are not hypothetical, but actually being documented around the world. If you are interested in exploring these changes in detail visit the NASA Climate Change Page Links to an external site..
Have these changes been witnessed before? Indeed. Sea levels have changed, global temperatures have been warmer and colder causing a variety of climate changes over time. These changes dramatically impacted the environment and the populations of plants and animals. The problem with the current climate change is how quickly it is changing and how it will impact the human population which is vulnerable to sea level rise, heat waves, hurricanes, consistent food production and a variety of other climate sensitive things.