As graphs go, the Keeling Curve is simple, but it clearly illustrates the planet's vexing global warming challenge. In a decades-long upward zigzag it charts the unrelenting increase of heat-trapping carbon dioxide in the atmosphere.
An annual update of the curve done this week shows that CO2 once again spiked to a record high during the past year. The short-term throttling of emissions during the coronavirus pandemic didn't even show up as a blip, scientists said, adding that the readings are important because they help explain that fossil fuel pollution is changing the climate dangerously, and faster than expected.
The CO2 concentration during May averaged 417.2 parts per million, the highest monthly total ever recorded. It increased at about the same rate as throughout the 2010s, researchers with the National Oceanic and Atmospheric Administration and the Scripps Institution of Oceanography at the University of California San Diego said after analyzing thousands of samples taken over the past 12 months.
The Keeling Curve is named for climate scientist Charles David Keeling, who in 1958 started measuring atmospheric carbon dioxide concentrations from the Mauna Loa Observatory in Hawaii, when CO2 was at 313 parts per million.
"This is a measure of humanity overwhelming nature," said Ralph Keeling, who took over the measurements in 2005 after his father died. He now runs the Scripps Oceanography CO2 program. The graph's trajectory shows that atmospheric carbon dioxide concentrations will double from the pre-industrial level in 60-80 years, he said. That would warm the planet between 2.7 degrees and 8.1 degrees Fahrenheit above the pre-industrial average.
"It's going to be really hard to prevent that from happening, and it's not going to come about without a major investment in new energy," Keeling said.
When the Keeling Curve measurements started, scientists already suspected that greenhouse gases would heat the atmosphere, but data was sparse. Now the curve shows that, since humans started burning fossil fuels, atmospheric CO2 has increased 33 percent (138 ppm), to the highest concentration since at least the Pliocene era, 2.6 million years ago, when sea levels were 30 to 60 feet higher than today.
Other Gases Contribute to a Worrisome Milestone
Within the steep rise of the Keeling Curve is a smaller zigzag pattern that shows seasonal cycles of plant growth and decay. The CO2 level is highest each year in the late spring of the Northern Hemisphere, where most of the land and plants are. So the annual measurements are finalized and announced in early June each year.
This year's increase again shows that the clock is ticking faster in the race to limit global warming to less than 3.6 degrees Fahrenheit, the target of the Paris climate agreement, said Pieter Tans, who leads a carbon cycle research group at the National Oceanic and Atmospheric Administration's Global Monitoring Laboratory in Boulder, Colorado.
"In the 1990s, the CO2 concentration was increasing at 1.6 ppm each year. In the 2000s, it was 2 ppm per year, and the rate ticked up again in the current decade, to 2.4 ppm per year," Tans said. "CO2 is the main thing causing global warming. It's two-thirds of all climate forcing."
Climate forcing describes the heating power of all greenhouse gases combined, not just CO2. Tans works on NOAA's Annual Greenhouse Gas Index, which tracks the increase of that power over time, using measurements from the Mauna Loa lab and other sites in the worldwide monitoring network.
"We follow how much infrared radiation greenhouse gases absorb every year and how much that heats the surface of the Earth, measured in watts per square meter," Tans said.
Along with CO2, methane and nitrous oxide, the index also measures several less abundant but super-potent greenhouse gases. It shows that the heating power of all greenhouse gases combined is 45 percent higher than in 1990, a year chosen partly because the 1997 Kyoto Protocol set it as a baseline for international climate calculations. In the index released last week, combined greenhouse gases for the first time trapped the same amount of heat as an atmosphere with carbon dioxide at 500 ppm.
Using a slightly different formula, Tans compared the heating effect of all the greenhouse gases in the atmosphere today with Earth's pre-industrial climate, before 1800. The increase is "equal to adding 3.22 watts of heating power over every square meter of the Earth," he said.
"If that heat were focused over Greenland, it would melt 5.2 percent of the ice sheet and raise global sea level 15 inches in just one year," he said. "The heat retention caused by greenhouse gases in 2019 equals the electrical output of 1.64 million large power plants. That's staggering."
Spread out over the entire planet, the warming is rapidly melting ice sheets and glaciers, thawing permafrost, raising sea level, intensifying storms and shifting weather patterns. Tans said that more global warming will lead to even more such climate extremes in the future.
Steve Montzka, who coordinates the analysis of the global sampling at NOAA's Earth Systems Research Laboratory in Boulder, focuses on the warming effect of 40 lesser-known gases that also affect the climate, as well as the ozone layer. Some of them have 10,000 times the heat-trapping power of CO2, so it's important to track them because even a small increase can compound global warming. The warming influence of the sum of these related gases continues to increase, he said.
Flattening the Curve
But carbon dioxide is the greenhouse gas causing most of the warming, and the accelerating rise of the Keeling Curve shows its impacts on the climate continuing to increase.
"The Keeling Curve is the most important graph for humankind because that's where we show our success, or so far, our failure to address climate change," said Dave Reay, who teaches carbon management at the University of Edinburgh and snagged the @KeelingCurve handle on Twitter in 2010.
He says he decided to use that name because, as a climate educator, he knows that the curve represents vital climate information that can help people understand what's at stake with global warming. The official Scripps account is @Keeling_Curve.
"It's the big curve for humanity," he said. "If we can stabilize CO2 below 500 parts per million, we can meet the goal of staying below 2 degrees Celsius (3.6 Fahrenheit) warming. The sooner we stabilize the better," he noted, because recent research suggests the climate is more sensitive than estimated by major global climate assessments.
Reay said that incoming students at the university "have a degree of anger about the fact that we've known about the risks of rising emissions for a long time without doing anything about it."
But, he added, they also see opportunities in the transformation to a zero-carbon, sustainable economy, as well as in studying ways to make communities more resilient to the impacts of global warming.
"Damage limitation, mitigation, resilience, a lot of them get a huge passion for that," he said. "There are a lot of jobs in this, too."
So, while the economic slowdown brought by the pandemic hasn't significantly lowered carbon dioxide concentrations, the recovery could. Reay noted that scientists around the world see that as a chance to take a significant step toward building a carbon-neutral economy.
How and When Will it End?
Stanford University civil and environmental engineer Mark Z. Jacobsen has researched the ongoing global energy transition extensively and says there is little to be lost and much to be gained by ending the planet's fossil fuel addiction.
"Transitioning the U.S. and world to 100 percent clean, renewable energy will reduce energy, health and climate costs substantially while creating jobs and increasing energy security," he said. "To avoid the most severe consequences of global warming, we need an 80 percent worldwide transition by 2030 and 100 percent no later than 2050, and ideally, earlier."
Colorado State University climate scientist Scott Denning argues that such a transition could be achieved with less disruption and cost to society than previous transformations, like rural electrification or the internet revolution. The only thing hindering that is the lack of political and social will, he said, as well as lobbying by the fossil fuel industry.
"The consequences of failure are staggering," said Denning, who studies greenhouse gases.
Unchecked, global warming impacts will spiral out of control, wiping out critical ecosystems, threatening global food supplies, displacing millions of people and raising the likelihood of conflicts over resources like water, he said.
"It would be worse than anything anybody alive has ever experienced," he said. "Worse than the Great Depression. Worse than the World Wars. Worse than 9/11 and coronavirus and the USSR and everything. Maybe the Black Death of the 14th Century is a good analog in terms of socio-economic and political and cultural impacts."
It's also important to remember that the climate won't start cooling when the Keeling Curve shows global emissions dropping to zero. The heating power of greenhouse gas pollution will increase for several more centuries after that.
"The Keeling Curve will not go back down when total emissions stop, except over many, many generations," he said. "The good news is that neither will it go up once we stop burning carbon ... the Keeling 'Curve' will become the 'Keeling Flat Line' with seasonal wiggles."
Denning believes this is guaranteed to happen "as the terrible consequences of climate change become more clear and stopping them becomes a matter of survival for the world economy and civilization."
"The only question is how bad we make it on ourselves before we stop making it worse," he said.