Waddle over, duck curve.
If you keep up with the renewable electricity sector, you’ve probably heard of the “duck curve.” Don’t let the cuteness fool you: the duck curve spells trouble for the power grid.
Solar energy peaks around mid-day, when the sun is highest, causing demand for other energy to drop. As capacity for solar energy increases over time, the belly of the duck takes shape. The graph below illustrates:
Here’s the problem: as the dip in demand becomes more dramatic, so does the neck. This slope represents the rate at which power generation must ramp up in the late afternoon to compensate for sunset. The height and steepness of the neck poses a serious technical and deployment challenge to utility managers.
Climate change mitigation, as it turns out, is swept up in its own curve-related drama. And if you ask me, it could use an unforgettable mascot like the duck curve.
We’re in luck: when you plot the forecasts for actual emissions, net emissions, and carbon removal in a “well-below 2C” world, an interesting picture emerges. And it looks an awful lot like an elephant.
As in the duck curve, the most critical aspect of the “elephant curve” is the slope of the line – in this case, the “trunk” of the elephant. The higher net emissions at the start, the faster and farther they must fall later on.
If the elephant’s trunk is outstretched, as it is in the chart, net emissions can fall gradually. If the trunk were drooping, however, it would mean net emissions have to fall much more rapidly, and probably in a shorter period of time, to meet agreed upon climate targets.
So, we want the elephant to be reaching for something. But what else does the elephant curve tell us?
1) Decarbonization and carbon removal are linked
The slope of the elephant’s trunk is important, but it’s not the whole story. The net emissions curve is drawn by taking what’s above the curve – actual emissions, or the total amount of carbon dioxide produced from sources like fossil fuels, cement production, and deforestation – and subtracting what’s below it – reductions in atmospheric carbon due to carbon removal. The resulting slope gives us the net trajectory of emissions over the next few decades if humans are to stay below 2 degrees of warming.
In our present world, where actual emissions are not dropping quickly enough, carbon removal must do the extra work to drag that curve down. But if actual emissions were to decrease faster, less of the slack would fall to “realized” negative emissions. In other words, the rate of carbon removal is tied to the pace of decarbonization.
2) We need carbon removal long before we reach net-negative emissions
Some critics have suggested that carbon removal will be deployed at the end of the century, or whenever we decide to stop emitting. But the elephant curve shows that we actually need carbon removal on the billions-of-tons scale within two decades.
Compare this to “net negative emissions,” which doesn’t show up until 2070. Net negative emissions will occur when net emissions fall below zero. At this point, actual emissions don’t have to be zero. But the higher their number, the more carbon removal needed to cancel them out – and then some.
Carbon removal, in other words, isn’t a replacement for or an excuse to avoid decarbonization. Even with decreasing actual emissions, it isn’t just important to the success of the elephant curve: it’s essential.
3) The elephant curve doesn’t tell us how
The area under the curve can come about in a number of ways, and the elephant curve leaves options open.
There are two possible flavors of carbon removal: land-based and engineered approaches. The land-based methods use photosynthesis to absorb carbon and store it in biomass. These include afforestation, reforestation, and “carbon farming” – agricultural practices that increase the amount of carbon held in vegetation and soils. Engineered solutions use artificial methods to remove carbon. These include direct air capture, bioenergy with carbon capture and sequestration (or BECCS), and enhanced weathering, which involves speeding up the natural process by which certain rocks react with atmospheric CO2 to form new minerals.
The more options, the better. A portfolio approach fills in the gaps, balancing the limitations of one method with the others. We should implement as many methods under the curve as possible, inventing new ones where necessary.
We already know we’ll need carbon removal. But let’s hope the elephant’s trunk falls as quickly as possible, or it’s burden may be too large.