Carbon-dioxide levels in the atmosphere rose at a record pace in 2013

The amount of carbon-dioxide in the Earth's atmosphere increased at a record pace in 2013, setting the stage for "potentially devastating" climate change in the decades ahead, the World Meteorological Organization warned Tuesday.

The Oceans and forests may be absorbing less CO2 than before

There are two possible reasons why the amount of carbon-dioxide in the atmosphere is growing so rapidly. One is obvious: Humans continue to emit more and more carbon-dioxide from power plants, cars, and factories each year.

But the other possibility is a bit more surprising: According to the WMO, early data suggests that the world's oceans and forests are now absorbing a bit less of our extra carbon-dioxide than they used to — which means that more CO2 ends up in the atmosphere, where it traps heat and warms up the planet.

Traditionally, oceans and forests have acted as giant sponges, soaking up roughly half of our carbon-dioxide emissions each year. (Indeed, that's why oceans are now acidifying at the fastest rate in 300 million years, with dire consequences for marine life.) But if they're now starting to absorb less carbon-dioxide overall — and, again, it's still too early to say for certain — that could mean even more global warming going forward.

The WMO report (pdf) found that atmospheric levels of three planet-warming greenhouse gases — carbon-dioxide (CO2), methane (CH4), and nitrous oxide (N2O) — are all at record highs. That's what the top row in the chart below shows. But the year-over-year increase for carbon-dioxide was the fastest since records began in 1984:

(World Meteorological Organization)

More precisely: In 2013, CO2 concentrations were 142 percent of what they were in the pre-industrial era, methane concentrations were 253 percent, and nitrous oxide concentrations were 121 percent.

How we're disrupting the carbon cycle

Of all the greenhouse gases above, climate scientists tend to agree that carbon dioxide is the most important — it can linger in the atmosphere for hundreds of years and traps heat on the planet, increasing temperatures on the Earth's land and oceans.

Of all the greenhouse gases, CO2 is the most important

Before the industrial revolution, the Earth's atmosphere already contained some carbon-dioxide, roughly 270 parts per million. (That's why the planet wasn't a frigid wasteland.) Overall CO2 levels were regulated by the natural carbon cycle: Animals would breathe out carbon-dioxide, plants would absorb it to convert sunlight to energy, and the oceans and soils would both absorb and emit CO2.

But human activities have significantly altered that cycle. As we began burning oil, gas, and coal for energy, we released additional carbon-dioxide into the atmosphere that was previously buried deep underground.

Traditionally, about one-fourth of that additional carbon-dioxide has been absorbed by the oceans (which in turn became more acidic), one-fourth by soils and plants (which are bolstered by the extra CO2). The other half made it into the atmosphere, contributing to global warming:

The carbon cycle

(NASA Earth Observatory)

The WMO notes that levels of carbon-dioxide in the atmosphere are now hovering near 400 parts per million — and will likely cross that threshold in 2015 or 2016. Levels have never been that high since human civilization has been around.

Meanwhile, the WMO's preliminary data suggests that the oceans and forests together are no longer absorbing quite as much carbon-dioxide as they used to. Scientists have long predicted this would happen as the planet got warmer and man-made emissions increased — largely because the oceans' capacity for soaking up carbon dioxide is steadily diminishing.

This is essentially a feedback mechanism, in which global warming leads to further global warming. That said, the precise timing of this feedback has always been a bit uncertain — it's complicated by things like fluctuations in plant growth — and it's not yet clear whether it's already begun.

The oceans are now acidifying at the fastest pace in 300 million years

There's another twist here: As the oceans absorb carbon-dioxide, they become more acidic — the carbon dissolves to form carbonic acid. The WMO said "the current rate of ocean acidification appears unprecedented at least over the last 300 million years."

More acidic seawater can chew away at coral reefs and kill oysters

More acidic seawater can chew away at coral reefs and kill oysters by making it harder for them to form protective shells. Acidification can also interfere with the food supply for key species like Alaska's salmon.

One study in the journal Climatic Change estimated that the loss of mollusks alone could cost the world as much as $100 billion per year by the end of the century.

Scientists are still trying to understand exactly how acidification will affect different species and the marine food chain, through both lab experiments and by looking at past acidification events. About 55 million years ago, during the Paleocene-Eocene Thermal Maximum, the oceans became warmer and more acidic. As a result, coral reefs became scarcer and the food chain had difficulty supporting larger predators.

Further reading: These five charts show why the world is still failing at climate change.

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