The Carbon Cycle (Week 4) - Post 1

The Carbon Cycle - Notes

4.1 Notes

The Unperturbed Carbon Cycle

Here we’ll start to get familiar with the key stocks and flows of carbon in Earth’s climate system. We’ll focus on carbon flowing to and from the atmosphere, and the timescales on which different processes involving carbon exchange occur. For practice thinking about the carbon cycle from a systems dynamics perspective, we’ll examine the processes driving seasonal cycles of atmospheric CO₂ in some detail.

Learning Goals:

By the end of this section, you will be able to:

-List the main reservoirs/stocks of carbon and their relative sizes.

-Describe the processes by which carbon exchanges among the atmosphere, hydrosphere, biosphere, and geosphere, and their relative time scales of operation.

-Explain the timing and processes involved in annual cycles of atmospheric CO₂ concentration.

Video

-Carbon resides in various places on Earth, including the atmosphere, biosphere, hydrosphere, and geosphere.

-Atmospheric carbon exchanges with all the other major reservoirs. (Oceans, Plants, etc)

-Exchanges with biology (all the natural greenery) and the surface ocean are fast.

-Mixing carbon into and out of the deep ocean is very slow. (1,000 t0 10,000 years)

-Exchanging carbon between rock and the atmosphere is really, really slow.

-The concentration of CO2 in the atmosphere at any time depends on the balance of inflows and outflows up until that point.

-On a seasonal basis, atmospheric CO2 in the northern hemisphere cycles up and down in response to the balance of photosynthesis (an outflow away from the atmosphere) and respiration (an inflow into the atmosphere) through the year.

Readings

https://earthobservatory.nasa.gov/Features/CarbonCycle/page1.php

The Carbon Cycle:

-Carbon is the backbone of life on Earth. We are made of carbon, we eat carbon, and our civilizations—our economies, our homes, our means of transport—are built on carbon. We need carbon, but that need is also entwined with one of the most serious problems facing us today: global climate change.

-Most of Earth’s carbon—about 65,500 billion metric tons—is stored in rocks. The rest is in the ocean, atmosphere, plants, soil, and fossil fuels.

-Carbon flows between each reservoir in an exchange called the carbon cycle, which has slow and fast components. Any change in the cycle that shifts carbon out of one reservoir puts more carbon in the other reservoirs. Changes that put carbon gases into the atmosphere result in warmer temperatures on Earth.

-Over the long term, the carbon cycle seems to maintain a balance that prevents all of Earth’s carbon from entering the atmosphere. This balance helps keep Earth’s temperature relatively stable. This stabilizing system works over a few hundred thousand years, as part of the slow carbon cycle. However, that means that for shorter time periods—tens to a hundred thousand years—the temperature of Earth can vary.

Slow Carbon Cycle

-Chemistry regulates this dance between ocean, land, and atmosphere. If carbon dioxide rises in the atmosphere because of an increase in volcanic activity, for example, temperatures rise, leading to more rain, which dissolves more rock, creating more ions that will eventually deposit more carbon on the ocean floor. It takes a few hundred thousand years to rebalance the slow carbon cycle through chemical weathering.

-Before the industrial age, the ocean vented carbon dioxide to the atmosphere in balance with the carbon the ocean received during rock weathering. However, since carbon concentrations in the atmosphere have increased, the ocean now takes more carbon from the atmosphere than it releases. Over millennia, the ocean will absorb up to 85 percent of the extra carbon people have put into the atmosphere by burning fossil fuels, but the process is slow because it is tied to the movement of water from the ocean’s surface to its depths.

Fast Carbon Cycle

-In all four processes, the carbon dioxide released in the reaction usually ends up in the atmosphere. The fast carbon cycle is so tightly tied to plant life that the growing season can be seen by the way carbon dioxide fluctuates in the atmosphere. In the Northern Hemisphere winter, when few land plants are growing, and many are decaying, atmospheric carbon dioxide concentrations climb. During the spring, when plants begin growing again, concentrations drop. It is as if the Earth is breathing.

Changes in The Carbon Cycle

-Left unperturbed, the fast and slow carbon cycles maintain a relatively steady concentration of carbon in the atmosphere, land, plants, and ocean. But when anything changes the amount of carbon in one reservoir, the effect ripples through the others.

(In Earth’s past, the carbon cycle has changed in response to climate change. Variations in Earth’s orbit alter the amount of energy Earth receives from the Sun and leads to a cycle of ice ages and warm periods like Earth’s current climate.)

-Today, changes in the carbon cycle are happening because of people. We perturb the carbon cycle by burning fossil fuels and clearing land.

When we clear forests, we remove a dense growth of plants that had stored carbon in wood, stems, and leaves—biomass. By removing a forest, we eliminate plants that would otherwise take carbon out of the atmosphere as they grow. We tend to replace the dense growth with crops or pasture, which store less carbon. We also expose soil that vents carbon from decayed plant matter into the atmosphere

-Emissions of carbon dioxide by humanity (primarily from the burning of fossil fuels, with a contribution from cement production) have been growing steadily since the onset of the industrial revolution. About half of these emissions are removed by the fast carbon cycle each year, the rest remain in the atmosphere

Atmosphere

-It is significant that so much carbon dioxide stays in the atmosphere because CO₂ is the most important gas for controlling Earth’s temperature. Carbon dioxide, methane, and halocarbons are greenhouse gases that absorb a wide range of energy—including infrared energy (heat) emitted by the Earth—and then re-emit it. The re-emitted energy travels out in all directions, but some returns to Earth, where it heats the surface. Without greenhouse gases, Earth would be a frozen -18 degrees Celsius (0 degrees Fahrenheit). With too many greenhouse gases, Earth would be like Venus, where the greenhouse atmosphere keeps temperatures around 400 degrees Celsius (750 Fahrenheit).

(while carbon dioxide contributes less to the overall greenhouse effect than water vapor, scientists have found that carbon dioxide is the gas that sets the temperature. Carbon dioxide controls the amount of water vapor in the atmosphere and thus the size of the greenhouse effect.)

-Greenhouse warming doesn’t happen right away because the ocean soaks up heat. This means that Earth’s temperature will increase at least another 0.6 degrees Celsius (1 degree Fahrenheit) because of carbon dioxide already in the atmosphere. The degree to which temperatures go up beyond that depends in part on how much more carbon humans release into the atmosphere in the future.

Ocean

-Dissolving carbon dioxide in the ocean creates carbonic acid, which increases the acidity of the water. Or rather, a slightly alkaline ocean becomes a little less alkaline. Since 1750, the pH of the ocean’s surface has dropped by 0.1, a 30 percent change in acidity. (more acidic water will dissolve the carbonate shells of marine organisms, making them pitted and weak.)

 Land

-Plants on land have taken up approximately 25 percent of the carbon dioxide that humans have put into the atmosphere. The amount of carbon that plants take up varies greatly from year to year, but in general, the world’s plants have increased the amount of carbon dioxide they absorb since 1960.

-So far, it appears that carbon dioxide fertilization increases plant growth until the plant reaches a limit in the amount of water or nitrogen available.

-All of these land use decisions—better farming techniques, planting forests on old farmland, and putting out forest fires—are helping plants absorb human-released carbon in the Northern Hemisphere…In the tropics, however, forests are being removed, often through fire, and this releases carbon dioxide. As of 2008, deforestation accounted for about 12 percent of all human carbon dioxide emissions.

-Dry, water-stressed plants are also more susceptible to fire and insects when growing seasons become longer. In the far north, where an increase in temperature has the greatest impact, the forests have already started to burn more, releasing carbon from the plants and the soil into the atmosphere.

Studying the Carbon Cycle

-Many of the questions scientists still need to answer about the carbon cycle revolve around how it is changing. The atmosphere now contains more carbon than at any time in at least two million years. Each reservoir of the cycle will change as this carbon makes its way through the cycle.

4.2 Notes

Human Perturbations of the Carbon Cycle

Learning Goals:

-Quantify carbon flow imbalances due to anthropogenic activities. Compare to natural sources and sinks

-Evaluate chemical and mass balance evidence linking human activities to the atmospheric carbon increase in the recent past.

-Evaluate hypothesis regarding when human activities began to measurably alter atmospheric greenhouse gas concentrations.

Video Lesson

-It takes about 2.1 billion tons of carbon to increase atmospheric carbon by about 1 part per million.

-Our own records of emissions align well with measurements of carbon dioxide concentrations over time and with chemical evidence from carbon isotopes and oxygen. These records link human activities to the recent increase in greenhouse gas concentrations.

Key Points:

-Human activities, in particular fossil fuel burning, clearing forested land for agriculture, and cement making, have increased the inflows of carbon to the atmosphere.

-Some of the excess inflow is taken back out by plants and soils on land, and by the ocean, but about 45% of our emissions stay in the atmosphere each year.

-Chemical data from carbon isotopes and oxygen align well with the explanation that the recent rise in atmospheric carbon dioxide is due to human activities.

-Farther back in time, it’s plausible that we began altering the composition of the atmosphere as far back as 8,000 years ago, with the expansion of agriculture. (Tearing down forests and replacing the land with agricultural land; killing forests releases the carbon that all the vegetation has stored up.)

4.3 Notes

Learning Goals:

-Describe potential amplifying and stabilizing feedbacks in the climate system involving carbon.

-Explain the process that influences the long-term fate of anthropogenic CO2 and the timescales on which they operate.

Video Notes

 -Paleocene Eocene Thermal Maximum: About 55 million years ago something happened that perturbed the carbon cycle, and atmospheric CO2 increased fast. Global temperatures went up, the oceans got acidic, lots of shells of marine organisms dissolved, and a bunch of things went extinct…(In response to the perturbation the Earth worked its way back to equilibrium) The oceans took up a lot of the excess CO2. Carbonates slowly dissolved to neutralize the acid. And eventually the really slow process of silicate weathering brought things back to the previous baseline. If you look at the data, you will see that the recovery back to something we could consider baseline took a couple hundred thousand years. Our current perturbation of the carbon cycle may have similarly lengthy recovery time.  

Key Points:

-Carbon cycle responses to perturbations include various feedbacks involving carbon on land. Many of these are amplifying feedbacks.

-The carbon we’re emitting to the atmosphere today will take a long time to work its way back into other carbon stocks.

            The oceans take up some of the carbon

Carbonate rocks and sediments help neutralize the CO2 in the oceans, on time scales of thousands of years.

Silicate weathering will ultimately draw the excess carbon out of the atmosphere, but that will take much longer. (100,000s-millions of years)

Reading

The term “tipping point” commonly refers to a critical threshold at which a tiny perturbation can qualitatively alter the state or development of a system. Here we introduce the term “tipping element” to describe large-scale components of the Earth system that may pass a tipping point.

Many of the systems we consider do not yet have convincingly established tipping points. Nevertheless, increasing political demand to define and justify binding temperature targets, as well as wider societal interest in nonlinear climate changes, makes it timely to review potential tipping elements in the climate system under anthropogenic forcing

Policy-Relevant Tipping Elements in the Climate System:

     Arctic Sea Ice-We conclude that a critical threshold for summer Arctic sea-ice loss may exist, whereas a further threshold for year-round ice loss is more uncertain and less accessible this century. Given that the IPCC models significantly underestimate the observed rate of Arctic sea-ice decline, a summer ice-loss threshold, if not already passed, may be very close and a transition could occur well within this century.

     Greenland Ice Sheet (GIS)- In some simulations with the GIS removed, summer melting prevents its reestablishment, indicating bistability, although others disagree. Regardless of whether there is bistability, in deglaciation, warming at the periphery lowers ice altitude, increasing surface temperature and causing a positive feedback that is expected to exhibit a critical threshold beyond which there is ongoing net mass loss and the GIS shrinks radically or eventually disappears…If a threshold is passed, the IPCC gives a >1,000-year timescale for GIS collapse. However, given the acknowledged lack of processes that could accelerate collapse in current models, and their inability to simulate the rapid disappearance of continental ice at the end of the last ice age, a lower limit of 300 years is conceivable.

     Amazon Rainforest- A regional climate model predicts Amazon dieback due to widespread reductions in precipitation and lengthening of the dry season. Changes in fire frequency probably contribute to bistability and will be amplified by forest fragmentation due to human activity. Indeed land-use change alone could potentially bring forest cover to a critical threshold. Thus, the fate of the Amazon may be determined by a complex interplay between direct land-use change and the response of regional precipitation and ENSO to global forcing.