To tell the story from the beginning, consider the carbon atom – 6 protons, 4 valence electrons, 4th most abundant element in the universe – basis of all life on earth. It’s locked up in rocks and plants, dissolved into our oceans, and mixed up with other gases in our atmosphere. As rocks it shows up as coal, limestone, or graphite for instance. In the rivers and oceans it’s mainly carbonic acid. In the atmosphere, carbon dioxide and methane gas.
While on the whole, restorative chemical processes  keep the relative distribution of carbon among these reservoirs fairly stable, each individual atom of carbon is in motion, traveling between the various phases, from gas to liquid to solid, between atmosphere and oceans and rocks and living matter. This is what’s known as the carbon cycle.
The carbon cycle has various loops, none of which is completely closed. For instance, in the fast carbon cycle, which is traversed on the time scale of a human life, carbon is taken up from the atmosphere by plants through photosynthesis, stored as sugars, then released back into the atmosphere when it is burned for energy, either by the plant itself, or by something that has consumed the plant, such as an animal, a microbe, or a fire.
But this loop is not closed. Dead plant matter which is buried before it has time to decompose do not release their carbon back into the atmosphere as a part of this fast cycle. Instead, it becomes coal, or oil, or natural gas, and is locked up for millions of years beneath the earth’s surface.
Before the industrial revolution, carbon stored in fossil fuels found its way into the atmosphere mainly through volcanic eruptions, as a part of the slow carbon cycle–called this, because a round-trip takes roughly 100 million years. In this leisurely cycle, rain dissolves atmospheric carbon, forms a weak acid – carbonic acid – which it then deposits into lakes and rivers and oceans. These ions are collected undersea by living organisms and built into shells. Carbon, now in solid form, settles to the sea floor when these organisms die, and builds up sedimentary rock layer by layer. Finally, the earth’s heat melts these rocks, and volcanoes and hot spots return carbon (including that which is contained in fossil fuels) to the atmosphere.
A key point about these natural processes is that they are roughly in balance. For instance, the rate of carbon release into the atmosphere, by respiration or volcanic activity, is matched by the rate of carbon absorption into plants and oceans. And this system is held in approximate equilibrium by various restoring forces. A sudden, small increase in the concentration of carbon in the atmosphere, absent other factors, leads to increased plant growth , more rain , and more direct absorption at the surfaces of oceans . In other words, the oceans acidify to deplete this extra carbon.
But how much carbon can our oceans take up? When, if ever, would the climate then return to its pre-perturbed state? What would the earth look like in the interim, in the far term?
By unearthing and burning fossil fuels, in our cars, factories, and electrical plants, we are harnessing energy by shortcutting a process which naturally occurs on geological time scales. About 30 billion tons of carbon dioxide are now added per year into the atmosphere directly by the burning of fossil fuels . A rate 100 times greater than that of volcanic emissions. As a result, atmospheric carbon, according to ice-core records which go back 800,000 years, is at its highest ever level .
We can use physical models to predict how the earth’s climate system might respond to different stimuli. To understand climate models, consider how a physical model can be used to predict the orbital motion of the planets. Given a set of parameters which describe the system (the position, mass, velocity of the planets and sun), the physical laws which govern the system (Newtonian physics or, more accurately, General Relativity), a certain set of simplifying assumptions (a planet’s interaction with another planet is insignificant compared to its interaction with the sun), and what emerges is the “future” of these original parameters. Some won’t change, such as the masses of the bodies, but others–their positions and velocities– will describe a trajectory.
Similarly, climate models aim to plot a trajectory for earth.
How well such a model performs depends crucially on the validity of its assumptions and completeness of its knowledge–our knowledge. Afterall, they know only what we know. We know, for instance, that earth exchanges energy with outer space through radiation, or light. We know that carbon dioxide and methane strongly absorb and re-emit certain IR frequencies of light while remaining largely transparent to visible frequencies. When incoming radiation is visible light (sunlight) and outgoing radiation is IR, we expect that an increase in greenhouse gases leads to an imbalance favoring energy influx over outflux. And, as Dr. Scott Denning stated in an earlier post: “When you add heat to things, they change their temperature.”
A deeper question is where the extra energy will go. To that end, we model the earth’s land, oceans, ice sheets, and atmosphere, allow them to absorb energy as a whole and exchange heat with each other through various thermodynamic processes. We track their temperatures, their compositions, and their relative extent. In this way, we can get a rough idea of the global response to a given amount of energy imbalance, called “forcing”.
But it gets more complicated.
The response itself may alter the amount of external forcing. The loss of ice sheets decreases the earth’s reflectivity, increasing the planet’s energy absorption . The thawing of permafrost and prevalence of hotter air are likely to elevate, respectively, levels of methane [9,10] and water vapor –two additional greenhouse gases–in the atmosphere. These are examples of known feedback mechanisms.
If the planet’s response to an energy flow imbalance is to increase this imbalance, the feedback is positive: climate change accelerates. On the other hand, negative feedback slows further climate change by re-balancing the earth’s energy flux. Changes in the carbon cycle, as in the ocean’s acidification by CO2 uptake, is one example of negative feedback . So far, about half of our CO2 emissions have found their way into our oceans .
It’s in this tug-of-war between positive and negative feedback mechanisms that the trajectory of earth’s future climate is drawn . Ultimately, thermodynamics guarantees that the earth’s climate will find stability . But we shouldn’t confuse a planet with a balanced energy budget with a necessarily healthy or habitable planet. Venus, for instance, has a balanced energy budget, and a composition very similar to earth. In other words, the question is not whether, but where.
The crucial role that climate models play in all this is that they help us catalogue and combine these separate pieces of knowledge. The more perfect the information, the more accurate its predictions. Right now, improving future accuracy of climate models depends heavily upon getting a good grasp of climate feedback mechanisms. As we slowly step toward a more complete understanding of our climate system, it’s important we continue to receive new science in context, reminding ourselves that each new study is a welcome refinement of our knowledge, that neither proves nor disproves global warming– simply moves us forward.