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The 'Big Lag' Index

How much additional pain or effort it will take to act tomorrow if we procrastinate today?

Big lag index


The pain of not acting decisively against climate change grows exponentially every day.

Yet, whether we begin today or tomorrow does not feel any differently to us. That is why we need a simple metric that unequivocally tells us how we are doing. Today, we don’t have that metric.

So far, reports and articles offer confusing uncertainties, such as the latest from the Intergovernmental Panel on Climate Change: "[There is] a two-in-three to one-in-two chance of not exceeding 1.5 degrees Celsius of global warming since pre-industrial times. […] There is also an approximately one-in-six chance that the remaining carbon budget for 1.5 degrees C has already been exceeded."

Wait, what?

When it comes to activating others, however, a directionally correct course of action is more effective.

The index looks to convey how much additional pain or effort it will take to act tomorrow if we procrastinate today.

As a global society, we can say with certainty that we are not reducing or removing greenhouse gas (GHG) emissions fast enough, therefore the climate continues to worsen, therefore our prospects of quality of life are declining even faster. Under these circumstances, the course of action is only one: avoid, reduce and remove as many GHGs as we can, within each of our own limited resources and spheres of influence. Because, as many have said, "Every ton matters."

That ought to be the metric we are looking for: How behind are we, how delayed? How big is the lag?

Introducing the 'Big Lag' Index

The "Big Lag" Index (BLI) looks to convey how much additional pain or effort it will take to act tomorrow if we procrastinate today. This is straightforward to estimate — if we agree on a few assumptions. The goal is not to arrive at an accurate, all-encompassing number but to communicate a sense of urgency from one period to the next.

First off, we assume our goal is to achieve a GHG emission rate of "net zero" by the end of 2050. At the same time, we assume that GHGs will continue to grow at 1.5 percent per year: beginning at 50 gigatons (Gt) in 2021, they will reach 77 Gt in 2050. Therefore, in order to achieve the net-zero goal, we will need to install enough GHG-removal capacity every year (be it trees, technology or otherwise) to counterbalance the emissions through 2050 (the same 77 Gt we will emit in 2050, we must eliminate in 2050).

Drawing a straight line between an assumed zero-removal rate at the beginning of 2021 and the end-goal rate of 77 Gt/year in 2050 yields an annual increase of 77/30 = 2.567 Gt/year.

At this rate, we can expect to remove a total of 1,193.48 Gt at the end of 2050.

Last, this exercise is solution-agnostic, therefore we do not explore advantages, feasibility or limitations.

The 'Big Lag' Index derivation

In a baseline scenario, every piece of capacity we install, we aim to use for actual removal as follows:

  • In 2021, the starting capacity is 2.57 Gt/year and allows the removal of 2.57 Gt by year end.
  • In 2022, we add 2.57 Gt/year of removal capacity, for a total of 5.14 Gt/year. The cumulative removed by year end is then 7.7 Gt (i.e., 2.57 Gt removed in 2021, plus 5.14 Gt removed in 2022).
  • In 2023, we add another 2.57 Gt/year of removal capacity, for a total removal capacity of 7.7 Gt/year. The total removed is then 15.4 Gt.

Eventually, the total capacity in 2050 will reach 77.0 Gt/year. At year end, the total removed will be 1,193.48 Gt. Fig. 1 shows this baseline of capacity growth from 0 to 77.0 Gt/year.



Fig. 1 — Starting 2021 with a removal capacity of 2.57 Gt/yr, at an annual increase rate of 2.57 Gt/yr, the EOY 2050 net removal capacity should reach 77 Gt/yr in order to meet the 1,193 Gt total cumulative removal goal (the area under the curve).

The 'Big Lag' Index at work

As stated above, whether we actually begin today or tomorrow, this year or next, seems irrelevant. So, what if we procrastinate a little?

Let’s say we do not begin yet, so that the 2021 capacity is 0 Gt/year. The cumulative removal in 2021 will stay at 0 Gt.

Because we still want to meet the removal goal by the end of 2050, in 2022 we still need to install the original capacity planned for 2022, plus the capacity to remove what we missed in 2021. Adding this capacity at a gradual rate over the remaining 29 years leads to the annual capacity step to increase slightly by 2(2.57/29) to 2.744 Gt/year.

This additional effort required is the "Big Lag."

The "Big Lag" equation thus goes as follows: BLIstart’22/start’21 = 2.744 [Gt/yr] / 2.567 [Gt/yr] = 1.069

Since the annual capacity install rate needs to grow in order to catch up with the overall removal goal plan, missing 2021 and 2022 would then send the annual capacity install rate for 2023 to 2.74 plus 2(2.74/28), equaling 2.94 Gt/year.

Missing two years then gives us BLI’23/’21 = 2.94/2.567 = 1.145

If we were to also miss 2023, the capacity in 2024 would need to be 3.157 Gt/year, for a BLI’24/’21 = 3.157/2.567 = 1.23. Fig. 2 depicts this dynamic.


Fig. 2 — Lines "Start ‘21" through "Start ‘24" show that the amount of effort to catch up in terms of total required removal capacity increases with every missed year. The "Big Lag" Index is the ratio between the slopes of each line to the baseline. Note: Each line’s origin (zero capacity) lies in the previous year to achieve the smooth start. This can be interpreted simply as "construction has to start the previous year to meet each year’s minimum start capacity."

Therefore, every year we miss or delay will result in additional effort required to catch up. This additional effort grows exponentially, which becomes apparent as we further delay or procrastinate, as Fig. 3 shows.


Fig. 3 — If we wait until the final year to do all the work not done in 29 years, we will have to install all required capacity to remove 1,193.48 Gt in one year. Compare this to the baseline scenario, where, for the same final year we would just need to install 2.57 Gt/yr (to achieve the final capacity of 77 Gt/yr).

We now need to put this in front of everyone.

To meet the 2021 goal of removing 2.567 Gt for the entire year, we should have installed that capacity in 2020, so we could have started removing GHGs immediately Jan. 1. If we did not install that capacity in 2020, then we are behind in removal and we are behind in capacity. And if we have not started yet, then we are eight months behind, so the effort to start now is: BLISEP/JAN = 7.8 (Fig. 4)


Fig. 4 – Looking at BLI annually is not very helpful. Instead, in order to impress upon everyone the sense of urgency, we need to look at BLI monthly. This graph shows what it looks like as of September.

Therefore, if we want to mitigate climate change and do so with as little pain as possible, we need to start now, in earnest.

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