While Antarctic ice cores indicate that atmospheric carbon-dioxide concentrations shifted by a maximum of 12 parts per million (ppm) by volume in pre-industrial times, a recent study of stomata in ancient leaves indicates the variability could have been much higher, at up to 34 ppm.
“We [have] proved that carbon dioxide did play an important role in climate variability during the pre-industrial part of the past 1000 years,” Tom van Hoof of the Netherlands Organisation of Applied Scientific Research told environmentalresearchweb. “Furthermore, the level of natural carbon-dioxide variation detected is comparable to about 30% of the man-induced carbon-dioxide increase of the past ˜150 years. The flexibility of the short-term carbon cycle is therefore bigger then previously assumed.”
The IPCC fourth assessment report assumed that carbon dioxide did not have a significant role as a climate-forcing factor before industry caused atmospheric levels of the gas to rise.
van Hoof and colleagues from the Netherlands Organisation of Applied Scientific Research and Utrecht University examined leaf remains from English oak (Quercus robur) trees found in the Netherlands. The numbers of stomata – gas exchange pores – on the leaves acted as a proxy for carbon-dioxide concentrations. Generally, the smaller the number of stomata on a leaf, the higher the carbon-dioxide concentration when it grew.
“We knew from earlier studies that stomata-frequency analysis of fossil leaves shows a higher rate of carbon-dioxide variability during the earlier parts of the Holocene and we wanted to test whether such variations did occur during the past thousand years, how they correlate to the ice-core data and if natural carbon-dioxide variability could have been a significant climate-forcing factor during this period,” said van Hoof.
The team found that carbon-dioxide concentrations varied between 319 and 292 ppmv from 1000 to 1500 AD. Between AD1000 and AD1200 this amounted to a decrease in climate forcing of 0.5 W/m2, interrupted by a temporary increase of 0.2 W/m2 around AD1100, and followed by an increase of 0.7 W/m2 between AD1200 and 1300 as a result of a 34 ppmv rise in carbon dioxide. After AD1300 carbon-dioxide forcing decreased by 0.4 W/m2.
However, the team beleives any direct coupling between trends in atmospheric carbon dioxide and air temperature during the period are likely to be masked by changes in solar forcing and the volcanic event in AD1258.
“There is still a debate how big natural climate variability was during the past millennium,” added van Hoof. “Climate model studies showed that a proposed higher natural climate variability during the past millennium would need a higher flexibility of the carbon dioxide signal than known from the ice core records. Furthermore, the low variability of natural atmospheric carbon dioxide as reconstructed by the ice-core studies made the IPCC suggest that carbon dioxide was an insignificant climate forcer during the past millennium.”
According to van Hoof, the research shows that we do not know exactly how the short-term carbon cycle naturally works. “This might have severe implications for predictions about future global-warming scenarios,” he added. “We hope that the model community takes up this study and starts running climate models under a more flexible carbon dioxide regime.”
Now the team plans to cover a longer period at higher resolution and to develop several stomata-based carbon dioxide curves from around the world. They will also focus on the possible sources and sinks of the detected carbon dioxide variability – they believe that North Atlantic Ocean sea-surface temperatures are involved but need to carry out more research.
The researchers reported their work in PNAS.