Earth's Fidgeting Climate

Greater insight about the role of natural variability may come from the field of paleoclimatology -- a specialized branch of climatology that uses scientific sleuthing to summon the ghost of climates past.

Trees show the fingerprints of Earth's Climate

The "fingerprints" of Earth's climate hundreds or even thousands of years ago remain imprinted in the rings of temperature-sensitive trees, the chemicals trapped in ancient ice, and the layers of sediment on the ocean floor.

Several studies by paleoclimatologists have suggested that natural variability can't fully explain the warming of the last century.

For example, Dr. Thomas J. Crowley, a geologist at Texas A&M University, used similar techniques to reconstruct basic climate data -- such as average global temperature -- back to 1000 A.D.

Crowley examined natural climate variations in a simple computer climate model caused by two external influences: fluctuations in the sun's intensity and aerosols injected into the atmosphere by volcanoes.

He deduced the history of solar flux from concentrations of carbon-14 in tree rings and of beryllium-10 in ice cores. Then, he deciphered past volcanic activity from sulphate aerosol deposits in ice cores.

Crowley ran the computer climate model with the solar and volcanic forcing terms, then compared the average temperatures it produced with a temperature record constructed from tree-ring data.

Despite the relative simplicity of his model, Crowley found good agreement between the temperature fluctuations it calculated for the years 1000 AD to 1850 AD and the fluctuations actually measured from tree rings during that interval. Over that 850-year period, fluctuations in solar intensity along with volcanic eruptions could account for roughly 50 percent of the variation seen in the tree-ring record -- give or take 10 percent.

Something happened, however, after 1850. Crowley's model could only account for about 25 percent of the observed temperature changes. Something else was needed -- volcanic eruptions and solar variability were not enough.

Crowley then introduced a human-triggered greenhouse effect to the model and it produced a much better match.

"It all comes out as indicating that you can't resort to (natural variability) to explain the recent warming," Crowley said. "The (recent) warming is consistent with a greenhouse effect but inconsistent with any explanation from natural variability."

So with the weight of a 1,000-year climate record on human shoulders, can scientists finally say that they've proven humanity is causing an unnatural warming of the globe?

Not necessarily.

"The time series we developed is statistically significant -- highly significant," Crowley said. "That doesn't prove something is right, but it still makes a good case that we're on the right track."

The problem with the "P" word --Proof!