Understanding GHG emissions: Stock vs. Flows
In discussing climate change and greenhouse gas (GHG) emissions, a key distinction must be made between the stock of GHGs in the atmosphere, and the flow of GHGs, primarily emissions. Understanding this difference is crucial for designing and implementing policies to effectively address the problem. Because a bathtub is something that most of us are familiar with, we will use it to illustrate the difference between stocks and flows.
Imagine our atmosphere is a giant bathtub, but instead of being filled with water, it is filled with a viscous fluid, something like maple syrup or ketchup. In this analogy, the fluid can be thought of as CO2 in our atmosphere, with the total volume of the fluid representing the total amount, or “stock,” of CO2 in the atmosphere at any given point in time (currently, CO2 is at 390 parts per million). Like most bathtubs, this one has a faucet that can pump more fluid into the tub and a drain that can remove fluid. The fluid coming out of the faucet represents the “flow” of GHG into the atmosphere, and the drain represents sinks (e.g. oceans, forests) that naturally remove CO2 from the atmosphere.
In order to better understand how stocks and flows of greenhouse gases affect the stability of our climate, we will add some nuances to our bathtub. Unlike conventional bathtubs, this one is equipped with a special feature: a floater sits on top of the bathtub fluid and is connected to a heating device that delivers heat into the tub as a function of the height of the fluid. As the level of the fluid and the floater rise, more heat is delivered to the tub. Furthermore, the viscous fluid in our tub has a unique property: as it heats up, its viscosity increases, and it flows more and more slowly out of the tub.
Although this scenario is simplified, this increase in the fluid’s viscosity as a function of temperature (making the sink less efficient) exists in our real-life bathtub: the ocean is the world’s largest sink of CO2, and although it currently absorbs roughly 22-35% of man made CO2 emissions, its ability to absorb CO2 decreases as it warms. Furthermore, because the solubility of CO2 decreases with increasing temperature, as the top layer of the ocean warms it could lead to degassing of dissolved CO2, which would lead to further decreasing the net oceanic sink.
Until the industrial revolution, our bathtub was in a state of equilibrium, where the rate at which the liquid entered the tub (the flow) and exited through the drain (the sinks) was such that the total volume of the liquid in the tub (the stock), and therefore its height and temperature, was kept nearly constant. After the Industrial Revolution, industrialized societies began burning fossil fuels, emitting large amounts of CO2 into the atmosphere.
This is analogous to increasing the amount and rate of delivery of viscous fluid into the bathtub. The level and viscosity of the fluid in our bathtub have been increasing, even though some of it is still going down the drain. The amount of liquid entering the tub is now greater than the amount draining out of it, so the tub is no longer in equilibrium.
Now, remember that this bathtub is equipped with a special heater that delivers heat as a function of the fluid’s height. It is widely accepted that the current level of CO2 in the atmosphere exceeds by far the natural range over the last 650,000 years. That is, the level of “fluid” is steadily climbing. As a consequence, the floater rises, forcing the heater to deliver more and more heat to the tub; the liquid in the tub is therefore warming at an increasing rate.
Imagine that nations of the world magically stop emitting CO2 (turning off the faucet). Because the tub is draining so slowly, and because the heating of the fluid depends on the total stock in the tub and not the flow, the temperature of the bath will continue to rise (above the temperature that was experienced when the tub was in equilibrium) until the level of the fluid falls back to its previous equilibrium level. This is what is meant by a “commitment to warming:” we have already added so much liquid to our tub that if we turn off the faucet, temperatures will continue to increase for a very long time. This is the guiding principle behind the goal to keep the concentration of CO2 in the atmosphere at or below 450 ppm: paleoclimate data and models simulating the response of the climate to CO2 indicate that increasing the stock of CO2 beyond this point is likely to lead to dangerous and irreversible anthropogenic warming.
What insight does our bathtub analogy hold for the real climate crisis we face? First, it helps illustrate the important difference between emissions of greenhouse gases and their amount in the atmosphere. Furthermore, it shows that no matter what we do going forward, we are already committed to further warming. This simple analogy also helps us visualize why delaying action will make it harder to solve the problem: as we increase the height of the fluid in the bathtub and as its temperature increases, it becomes more difficult to return it to a safe level.
With respect to policy decisions, our analogy illustrates a critical aspect of policies that cut emissions: the aggregate reduction of all policies needs to be on a scale large enough to keep the final concentration of CO2 within the accepted ranges. Policies that aim to curb the rate of emissions (the flow) need to be formulated with the overall concentration targets (the stock) in mind so that we end up with an amount that is within the “safe-zone” (around 450ppm). Multiple studies, including work published by Deutsche Bank with research from the Columbia Climate Center, highlight the fact that there is a significant “gap” between the overall impact of policies and the 450 ppm target.
Similar to our bathtub analogy, where draining the tub becomes harder as the stock builds up and the fluid warms, closing the “gap” between the impact of emission policies and overall concentration targets grows increasingly difficult the longer we wait. Although it is certainly feasible to achieve the goal through aggressive policies, the cumulative nature of GHGs in the atmosphere coupled with their long residence times means that we are running against the clock. Enough political will, at both domestic and international levels, must be mustered to construct policies that reduce flows consistent with the desired overall stock. To the misfortune of everyone on this planet, this is one particular case in which time is not on our side.