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ONE MORNING EACH WEEK a scientist takes a stroll on the barren upper slopes of Hawaii's Mauna Loa volcano, with a basketball-sized glass sphere in hand. At some point the researcher faces the wind, takes a deep breath and strides forward while twisting open a stopcock in the sphere.
With a whoosh lasting no more than a few seconds, five litres of the most pristine air on the planet replaces the vacuum inside the thick-walled orb.
Every couple of weeks, a more thoroughly wrapped-up researcher at the South Pole conducts the same ritual. At these remote sites and dozens of others around the world, instruments also sniff the air, adding measurements of atmospheric chemistry to a dataset that stretches back for more than 50 years.
This nearly continuous record results from one of the most comprehensive and longest-running Earth science experiments in history, says Ralph F. Keeling, a climate scientist at Scripps Institution of Oceanography in La Jolla, California. He carries on the effort his father, Charles Keeling, began as a graduate student back in the 1950s.
Several trends emerge from the data, says Keeling. First, in the northern hemisphere, the atmospheric concentration of carbon dioxide (CO2) rises and falls by about seven parts per million (ppm) over the course of a year (around a current average of approximately 380 ppm).
The concentration typically reaches a peak each May, then starts to drop as the hemisphere's flush of new plant growth converts the gas into sprouts, vegetation and wood. In October, the decomposition of newly fallen leaves again boosts CO2 levels. Populations of algae at the base of the ocean's food chain follow the same trend, waxing each spring and waning each autumn.
A second trend is that each year's seven ppm saw-tooth variation in CO2 is superimposed on an average concentration that's rising steadily. Today's average is more than 380 ppm, compared with 315 ppm 50 years ago, and 280 ppm prior to the Industrial Revolution. And it's still rising by about 2 ppm each year, mainly due to combustion of fossil fuels.
Largely because CO2 traps heat, Earth's average temperature has climbed by about 0.74° C over the past century, a trend that scientists expect will accelerate. In the next 20 years, the average global temperature is projected to rise at least another 0.4° C.
Squelching additional temperature increases depends on limiting - if not eliminating - the rise in CO2 levels. And as Keeling says, "It's clear that if we want to stabilise CO2 concentrations in the atmosphere, we need to stop the rise in fossil fuel emissions."
But halting the increase in atmospheric CO2 doesn't necessarily mean doing away with fossil fuels. Many experts think that capturing CO2 emissions, rather than simply reducing them, could ultimately provide climate relief.
Possible solutions range from boosting natural forms of carbon capture and storage - fertilising the oceans to enhance algal blooms, say, or somehow augmenting the soil's ability to hold organic matter - to schemes for snatching CO2 from smoke stacks and disposing of it deep underground or in seafloor sediments.
Success in sequestering carbon depends on meeting two major challenges: how to remove CO2 from the air (or prevent it from getting there in the first place) and what to do with it once it has been collected.
