Another climate milestone soared by last weekend when scientists announced that atmospheric carbon dioxide levels hit 415 parts per million for the first time ever (Climatewire, May 7).

It’s the latest in a long list of broken records, and like the others, it promises to hold the title temporarily. Atmospheric CO2 is rising at accelerating rates—currently climbing at close to 3 ppm each year, and getting faster. Every year, the world sees new levels that were previously unrecorded in modern human history. The last time CO2 concentrations hit 415 ppm was likely close to 3 million years ago.

Atmospheric CO2 levels are directly correlated with rising global temperatures. But it’s the warming, itself, that often captures the most international attention. World nations participating in the Paris climate agreement have chosen to set their goals in terms of global temperatures, aiming to keep the climate from warming more than 2 degrees Celsius, or 1.5 C if possible, above its preindustrial condition.

And, indeed, global temperature milestones may make even bigger headlines when they occur. It’s major news when yearly or monthly temperature records are broken. Same when nations experience their hottest heat waves or when winter temperatures are the highest ever recorded.

So why does it matter when we hit a new CO2 milestone? And why monitor carbon dioxide concentrations instead of just paying attention to global temperatures?

There are a variety of reasons, some more obvious than others.

Measuring progress

The primary value in keeping tabs on CO2 concentrations is to monitor global progress on addressing climate change—and to keep tabs on how quickly global climate targets are approaching.

There’s still some scientific uncertainty regarding exactly how much warming is associated with a given increase in atmospheric CO2. But scientists are able to calculate a broad range of likely outcomes. This has given rise to a concept known as the “carbon budget”—a scientific estimate of how much additional CO2 would cause the world to blow past its temperature targets.

A special report from the Intergovernmental Panel on Climate Change, released last fall, estimated that emitting no more than 420 billion to 570 billion tons of carbon dioxide would give the world approximately a 66% chance of meeting the 1.5 C temperature goal. That’s only around 12 years’ worth of current global greenhouse gas emissions.

With this budget in mind, the report also included a set of recommendations on global policies consistent with the target—namely, that nations will need to collectively reduce their carbon emissions to zero within the next 30 years.

Monitoring global CO2 concentrations allows scientists to keep tabs on the progress they’re making in real time, and the speed at which the temperature targets are approaching, under the estimated carbon budget.

“And at this point, we are doing a terrible job,” said Pieter Tans, head of NOAA’s Carbon Cycle Greenhouse Gases group.

Not only are atmospheric concentrations still climbing, but the rate seems to be accelerating. Ralph Keeling, director of the Scripps Institute of Oceanography’s CO2 Program, which monitors CO2 concentrations at the Mauna Loa Observatory in Hawaii, noted that this year’s total increase will probably be around 3 ppm. The recent annual average has been hovering around 2.5 ppm.

The emissions cuts needed to meet the Paris climate targets, on the other hand, would manifest as a slowing, and ultimately a halt, to the rise of atmospheric CO2 levels.

Warnings from the past

In addition to measuring progress on global climate action, atmospheric CO2 may be able to provide some clues about the climate consequences the world should expect in the coming years.

Samples taken from ancient sediments at the bottom of the ocean, fossilized corals, or ancient ice from Greenland or Antarctica can provide researchers with chemical information about the Earth’s climate millions of years in the past. This kind of data can tell scientists what the planet was like the last time global carbon dioxide levels were as high as they are today—or as high as they’re expected to get in the coming decades.

During the last geological era that saw CO2 concentrations over 400 ppm, global temperatures were about 3 C higher, according to paleoclimate expert Gavin Foster of the University of Southampton in the United Kingdom.

“There was very little ice on Greenland, there was no ice on West Antarctica, and the East Antarctic ice sheet was a little bit reduced,” he said. “So sea levels as a result were a lot higher.”

Many of the consequences that climate models project for the future are similar to the kinds of effects that paleoclimate studies suggest have happened in the distant past. In that way, previous ages may provide some useful hints about the Earth’s response to major climate change.

Still, that’s not to say these kinds of effects are likely to materialize tomorrow. As Foster notes, there’s a time lag between the emission of greenhouse gases and the total effects they have on the global climate system.

Even if humans stopped emitting all carbon today, it would still take decades or even centuries for global temperatures to stabilize, and perhaps between 1,000 and 2,000 years for the climate system as a whole—including the response of the world’s ice sheets, sea levels, ecosystems and so on—to reach a point of equilibrium.

It’s also important to note that today’s conditions aren’t perfectly comparable with those in the past. Even if today’s CO2 concentrations are similar to the levels seen millions of years ago, the rate at which they’re currently climbing probably “outstrips anything we’ve seen in the geological record for at least 65 million years,” Foster said.

Although the past may provide some important clues to the future, it’s impossible to say for sure if the planet will respond to such rapid increases in the exact same way it responded to the slower changes in CO2 millions of years ago.

And that knowledge can be valuable for scientists, as well, Foster points out. Just as it’s helpful to use the past for clues about what the future may hold, it can also be useful to note the ways the present-day climate response is different from what’s believed to have happened in the past. That information can help scientists sharpen their understanding of the Earth’s response to extremely rapid changes in global carbon levels and, thus, their predictions for the future.

“We’ve sort of switched from pointing out the analogous behavior to expressing the nonanalogy,” Foster said.

Clues to the carbon cycle

Atmospheric CO2 can also help scientists keep tabs on the global carbon cycle—not just how much carbon humans are emitting into the atmosphere, but how much carbon is being soaked up or released from the Earth’s forests, wetlands, oceans and other natural ecosystems.

For one thing, scientists can compare their observations of total carbon dioxide concentrations in the atmosphere to their estimates of how much carbon dioxide is being emitted by humans. That allows them to calculate how much extra carbon is being absorbed by these natural sinks.

“We see that carbon dioxide is increasing less rapidly than we would expect as a result of all the emissions of CO2 from fossil fuel burning and land use,” Keeling pointed out. “And that extra carbon that’s not ending up in the air must be ending up in the ocean and the land.”

They can also monitor carbon dioxide levels—and concentrations of other greenhouse gases, like methane—at specific sites around the world to determine whether there are any significant changes occurring in the world’s natural carbon sinks.

Researchers have suggested that continued climate change, or other human disturbances, could cause some natural ecosystems to begin storing up less carbon than they used to, or even to begin releasing more greenhouse gases into the atmosphere. It’s important to keep tabs on these kinds of changes, which can affect scientists’ estimates of how quickly global greenhouse gas concentrations are rising.

Thawing permafrost in the Arctic, for instance, is one of the biggest uncertainties about future greenhouse gas emissions from natural sources. As the landscape warms, the thawing soil is known to release large quantities of methane and carbon dioxide.

A significant increase in Arctic emissions would, theoretically, cause scientists to notice a growing divergence between the emissions measured in the Arctic versus the midlatitudes farther south, according to Tans, the NOAA scientist. So far, monitoring suggests that the gradient between the two sites is fairly constant, he said. But he added that “we are keeping tabs on that” because of the high potential risks should thawing permafrost get out of hand.

It’s also worth noting that rising CO2 concentrations can pose extra risks to the Earth system that are unrelated to rising temperatures, such as ocean acidification, the chemical process that occurs as carbon dioxide dissolves into the sea.

So just as monitoring milestones in global temperatures is useful to researchers, keeping tabs on global greenhouse gas concentrations also provides a variety of valuable scientific insights.

Still, one of its greatest values may be the simplicity with which it presents the problem to the public. In the same way each new “hottest year on record” provides a real-time reminder of the physical progression of climate change, each carbon dioxide milestone also creates a marker of the human actions behind that warming.

“The CO2 build-up is an example of a milestone that we want to keep track of,” Keeling said. “It’s important that people keep actual numbers in their head so they get an appreciation for how this is playing out.”

Reprinted from Climatewire with permission from E&E News. E&E provides daily coverage of essential energy and environmental news