Carbon Dioxide Hits a New Peak this Spring: 404 ppm

Weekly carbon dioxide measurements from the pristine air atop Hawaii’s Mauna Loa have just topped another predictable yet worrisome milestone: 404 parts per million. The actual preliminary value reported by NOAA for last week (April 12–18) was 404.02 ppm. By all evidence, we now have the largest amount of CO2 present in Earth’s atmosphere for at least the last 800,000 years, and probably several million. The most prevalent of the human-produced greenhouse gases, carbon dioxide has been measured regularly by scientists at Mauna Loa since 1958. The gas is also measured at other sites around the world, but the Mauna Loa dataset is the most widely tracked index of global trends because of its uninterrupted 57-year length.

The weekly CO2 readings at Mauna Loa will crest over the next couple of months, making a run at 405 ppm before the annual seasonal decline begins (see below). Eyeballing the multiyear trend shown in Figures 1 and 2, it’s a fair guess that the final time we see a weekly value below 400 ppm will be somewhere toward the end of 2017, perhaps a year sooner or later. From that point on, we’re unlikely to again see a week below 400 ppm for many years—probably centuries, if not millennia—because of the ever-increasing accumulation of atmospheric CO2 produced by burning fossil fuels.


Figure 1. The last two years of daily, weekly, and monthly averages for carbon dioxide concentration measured by the Scripps Institution of Oceanography atop Mauna Loa, Hawaii. NOAA operates a parallel measurement program at Mauna Loa. Image credit: Scripps/The Keeling Curve.

What’s in a curve?
One of the most renowned images in climate science is the Keeling curve (see Figure 2), generated from the Mauna Loa data. This trace is famous for its inexorable year-to-year increase in CO2, as well as the seasonal rise and fall embedded in the graph’s sawtoothed pattern, a trait that became evident as early as 1960.


Figure 2. The Keeling Curve, 1958-present. Image credit: Scripps/The Keeling Curve.

Because the Northern Hemisphere has far more plant-friendly land mass than the Southern Hemisphere, it has an oversized impact on the global CO2 pattern. The result is a net global addition of carbon dioxide to the air as northern plants decompose, from around October till May, then a net removal as northern vegetation surges from roughly June through September. These natural seasonal spikes are about twice as large as the amount added each year by fossil-fuel burning, which has recently averaged just over 2 ppm per year. Unlike the human contribution, though, the seasonal spikes cancel each other out over time. After removing the seasonal cycle from the long-term record, we end up with a steady increase that topped 400 ppm for the first time in March, according to NOAA.

Close inspection of the the Keeling curve reveals some embedded nuance apart from the obvious seasonal cycle and the long-term rise. Figure 3 (below) shows how the percentage increase in carbon dioxide concentration at Mauna Loa varies from year to year. These bumps and dips arise from both natural and human factors.


Figure 3. The annually averaged growth rate of carbon dioxide, in parts per million, as measured at in the atmosphere at Mauna Loa. Horizontal black lines show the growth rate for each decade from the 1960s to 2000s. Image credit: NOAA Earth System Laboratory.

In a typical year, about 57% of the CO2 emissions put into the atmosphere by human activity remain in the air, showing up in the long-term measurements at Mauna Loa and elsewhere. The other 43% is removed by plants, soil, and oceans. These percentages have held remarkably steady over the long haul, but they can also vary markedly from year to year. El Niño, for example, tends to pinch off the cold equatorial upwelling that normally sends large amounts of CO2 into the air, thus causing a temporary drop in the overall global rate of increase.

The human contribution from fossil fuel also varies from year to year. Global emissions of carbon dioxide actually dropped slightly during the recession years of 1992 and 2009. Likewise, CO2 emissions tend to increase at a faster clip when the global economy is especially robust. Policymakers have long taken this connection between emissions and economic activity for granted. Many were surprised, then, when global CO2 emissions in 2013 came in essentially flat even though the world’s gross domestic product had risen by about 3%. This could be a one-year fluke--scientists and policy experts have been debating this point--but it’s also a hopeful sign that our global economic engine just might be able to run on less coal, oil, and gas while still performing well.

The long view
How high the concentrations get in this century and beyond will depend in large part on what measures the global community takes to restrict carbon emissions, including any agreements hammered out at the crucial UN climate meeting in Paris this December. Technology is a huge player, of course: wind and solar power, hydropower, and nuclear power are all close to carbon-neutral when compared to fossil fuels. But unless a price is set on carbon through some globally accepted process, there will be powerful market incentives for a growing world to use as much of our existing reserves of oil, coal, and natural gas as possible. And a key insight vividly highlighted by author and activist Bill McKibben remains: Earth holds several times more fossil fuel than needed to push global warming above the 2°C benchmark widely accepted as a target to minimize the odds of major climatic disruption.

Bob Henson


Figure 4. Atmospheric carbon dioxide concentrations derived from ice cores (prior to 1958) and Mauna Loa data (from 1958 onward) show the rises and falls associated with several ice ages and the dramatic spike of the last 100 years. Image credit: Scripps/The Keeling Curve.