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FWIW: How is the U.S. actually doing on reducing Greenhouse Gas emissions?

In honor of the COP28 conference (28th UN Conference on Climate Change) going on throughout this week and next, and of the uniformly woebegone and denunciatory press coverage about it…

Per the EPA (emphasis added):

Greenhouse gas emissions in 2021 (after accounting for sequestration from the land sector) were 17 percent below 2005 levels

from “Inventory of U.S. Greenhouse Gas Emissions and Sinks” (data through 2021, published in 2023)

That includes “sequestration” by land-use policies — meaning, we give ourselves credit for things we are are doing to reduce effects on the back end by pulling carbon back out of the atmosphere (like, say, reforesting previously deforested areas).

But, note, also that

In 2021, CO2 emissions from fossil fuel combustion increased by 7 percent relative to the previous year. This increase in fossil fuel consumption emissions was due primarily to economic activity rebounding after the height of the COVID-19 pandemic.

So, this represents a real and long-term reduction, not a short-term artifact of the pandemic-induced economic downturn.

The report doesn’t provide an equivalently neat and easily accessible summary of the effect of emissions themselves, in isolation, but does provide this graph of emissions by sector of the energy economy over time:

U.S. Greenhouse Gas Emissions by Economic Sector, 1990-2021

The graph shows a clear pattern of reduction in actual emissions since about 2005: by eye (that is, trying to read off the graph axes without benefit of the detailed numbers), I estimate that gross emissions (not including the effect of sequestration) declined by something like 15% to 20% — about in line with net emissions.  That implies our sequestration efforts held about even and that the bulk of the net reduction was, in fact, the result of reductions in actual emissions.

Over the course of the 16 years between 2005 and 2021, and assuming a constant proportional rate of decrease (that is, so many percent/year, which implies an exponential decay), that overall 17% decrease in net emissions would imply an average annual rate of decrease of a bit more than 1%/year.  If we were to assume we could maintain the same average rate of decrease (no improvement but, also, no backsliding) over the next few decades, that would lead  to reductions (below 2005 levels) of ~41% by 2050 and ~67% by 2100.

Further, that 17% decrease in emissions happened while the American population increased by ~12.3% (from 295.5M to 331.9M per the U.S. Census Bureau).  So, under the same assumption, the rate of greenhouse gas emissions per capita has been decreasing at an even faster average rate: more like 2%/year, nearly twice as fast as the rate of decrease in the actual emissions.  Demographic projections by the U.S. Census Bureau predict a declining rate of population growth (and even the beginning of a decline in actual population) over the next century (to something like 350M by 2050 and to 366M (and decreasing) by 2100).  Under the same assumption about per capita emissions that we made about overall emissions — that the average percentage rate of change will remain consistent —  we might then expect the net effect, taking slowing population growth into account, to be an overall reduction in emissions (again, below 2005 levels) of more like ~57% by 2050 and ~83% by 2100.

Note:  There is enough ‘noise’ in the data (bumps in the graph) that observing the shape of the reduction is difficult.  Is the amount of annual improvement constant over time (i.e, is it a straight line)?  Is the amount of improvement increasing over time as we put more effort into it (is it a downward curve)?  Is it decreasing over time as we run out of low-hanging fruit to pick (is it an upward curve)?  It’s hard to say.

My eye tells me that there is a slight downward curve, implying that the amount of improvement is getting larger — that, each year, things get better a bit more rapidly.  That could, however, be a trick of the eye prompted by wishful thinking.

Assuming a relatively constant proportional rate of change is a reasonably safe bet because it matches the way the real world often works: the effort required to improve things gets bigger over time as you run out of easy changes and start working on the harder ones; and, so, the absolute amount of change you get for that effort goes down over time but stays roughly proportionately constant.  In the absence of better information, it’s a fairly common and plausible a priori model for how such things will evolve.  It is also the most conservative (meaning the most pessimistic — the worst-case) of the three assumptions…

It is, moreover, particularly pessimistic in light of the immense attention and effort — cultural, political, economic, and technological — being focused on ameliorating climate change at the moment.  That attention and effort, combined with demonstrable technical progress in the practical applications of various renewable energy sources and in the greening (if not outright electrification) of transportation (not to mention a few small but genuine breakthroughs in methods for carbon sequestration), all but assures that the rate at which things improve will, indeed, accelerate (rather than slow down or stay constant) over the next few years.  That is, it is highly likely that I have under-estimated the amount of reduction in greenhouse gases that we will see in the coming decades.

Those numbers are not, perhaps, what climate activists want or what climate scientists would say we should target.  They are certainly not “net zero”.  But, they represent real, measurable, and steady progress.

So they are also not, perhaps, what the most alarmist headlines would lead you to believe about how we are doing.

It may well be that we are not doing quite enough.  But, we are far from doing nothing…

© Copyright 2023, Augustus P. Lowell

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