Recent data related to our global emissions of heat-trapping gases suggest that humanity may have reached a turning point, or even several. We may be moving from increasing emissions, to peaking and starting to decline. We could be close to such peaks, or even have passed it, for several of the main sources of greenhouse gases, including coal and deforestation—perhaps even for humanity’s total emissions.
If so, this would be a momentous occasion, reversing centuries of growing global warming pollution. But before we start celebrating, we should realize that peaking is not nearly enough.
Before looking at the data, it’s useful to remember that even if you have reached a turning point, things could turn again. “Peak oil,” an animated subject of discussion a decade or so ago, provides an excellent example. The argument was based on the peak in U.S. oil production in 1970, which was predicted by geologist M. King Hubbert in 1956. If the same thing was imminent for the planet as a whole, not just for the U.S., it was thought to imply the End of Civilization as We Know It.
Well, we didn’t find out whether it would or not, because oil production dropped but then began to increase again, including for the U.S. In fact, it’s now essentially at the same level as that 1970 peak. Here’s the U.S. data from 1900 to 2015:
U.S. oil production, 1900-2015, thousands of barrels per day. Source: Energy Information Administration
I actually remember having heard M. King Hubbert give a lecture on this trend, when I was an undergraduate at Yale, and wondering how he could predict the future of oil using the normal distribution. (I had just taken a mathematical statistics course, and knew that the normal distribution is generally fit to frequency distributions, not to time series as Hubbert was doing.) But I was a mere undergraduate and figured he was the expert and that I didn’t understand the issue nearly as well as he did. Looking back, maybe I did.)
With this as a warning about the dangers of predicting peaks, let’s press on nonetheless. In March the International Energy Agency (IEA) has released their estimate of global energy-related CO2 emissions—which make up the large majority of global warming pollution—and they seem to indicate a levelling-off in the past two years:
Energy-related carbon dioxide emissions — global total in Gt CO2/year. Source: International Energy Agency
Such a “pause” (to use a loaded word) is not by any means unprecedented—see the arrows for the late 70s, early 90s and the Great Recession of 2008—but the IEA pointed out that this time, it’s happening without an economic shock, at a time of continued growth. Thus, we may be seeing the beginning of a “decoupling” of global warming pollution from the global economy.
This trend is related to the indication that such decoupling is happening with respect to coal in China, which is the world’s biggest emitter. Qi and colleagues have compared the relation between GDP and coal consumption in China to that in the U.K. (where it peaked in 1956) and in the U.S. (an apparent peak in 2007):
Coal consumption vs. per-capita GDP for China, the UK and the US (log-log scale). Source: Qi et al. 2016, Nature Climate Change, Figure 3.
The energy sector, and within it, coal, are the biggest sources of heat-trapping emissions, but the land sector (basically forests and agriculture) accounts for nearly ¼ of the total. Here too, there’s new data concerning peaking, published by the University of Minnesota’s Institute on the Environment and the Food and Agriculture Organization (FAO):
Greenhouse gas emissions from agricultural and deforestation, 1990-2014 (Gt CO2eq/year). Source: University of Minnesota, Institute on the Environment and FAO
With respect to deforestation, it’s increasingly clear that we’ve passed a peak (although its exact date—somewhere between the late 1990s and the early 2010s—varies a lot depending on which data set you use). As with energy, there’s good evidence for decoupling, with rapid reductions in deforestation emissions while the economy was growing in Brazil and other tropical countries.
On the other hand, for direct emissions from agriculture (“agricultural management” in the graph above—particularly from cattle, some from nitrogen fertilization, and to a lesser extent from paddy rice), there’s no sign at all of peaking, with emissions continuing to rise slowly but steadily. The same goes for fossil fuels other than coal (oil, natural gas). So there are important sectors where there’s relatively little indication of progress so far, and these could easily make the overall emissions total reverse course again.
Nonetheless, the pieces of evidence for peaking, and more importantly for the decoupling emissions from economic growth, are encouraging signs. But it’s worth remembering that reaching a peak in global warming emissions is not nearly enough to solve our climate problem. To stop global warming—to make the temperature stop increasing —we need to peak and then reduce emissions rapidly, to a level below the absorption of carbon dioxide by sequestration of the biosphere. And to stabilize the temperature at a reasonable level—1.5 or 2 degrees—this has to happen within just a few decades.
So, the signs of peaking, and particularly the indications that they are due to the decoupling of emissions from the economy, are welcome signs of a potentially historic change. But what we actually need to do is a whole lot more.