The critical question is whether soils release more CO2 because faster-growing plants pump more in, or if soils release CO2 that would have stayed in the ground at lower temperatures.
Carbon dioxide enters the soil through the roots of living plants and from the decaying bodies of dead plants, and is processed by microbes, fungi and insects. Over time, some of that CO2 releases back into the atmosphere.
At any given time, there’s about twice as much CO2 in Earth’s soils as in its atmosphere.
Because more heat means more energy and faster chemical reactions, Earth scientists have suspected that rising global temperatures would increase the rate of soil respiration.
The last review of soil respiration studies (.pdf) took place in 1992, however, and though it found a link between temperature and respiration rates, data was relatively sparse.
Whatever the anomaly’s explanation, that data was still included when global soil-respiration rates were calculated and the rise identified.
What’s not clear from the analysis is whether soil-respiration rates have increased without actually affecting atmospheric balances of CO2, or if CO2 that would have remained earthbound is now being released.