New Carbon Cycle Data Casts Doubt On Climate Models

Many factors and feedback mechanisms are involved in the climatic response to increasing carbon dioxide levels. The uncertainties in these effects, and our general ignorance, make it almost impossible to predict what will happen to the Earth’s ecosystems as temperature increases. This has not stopped some predicting on the basis of climate models that the world will get drier as the temperature increases and for example, as a consequence, the Amazon rain forest will die and be replaced by savannah or bare earth.For years, there has been debate about the effect of air temperature on global respiration - or the accumulated metabolic processes of organisms that return carbon dioxide to the atmosphere from Earth's surface. It’s vital to understand this process if we are to have any ability to predict what may happen if the global temperature rises in the future. But how does the global carbon cycle react to global warming?Most studies suggest that ecosystem respiration around the world is highly sensitive to increasing temperatures, while the majority of computer climate models suggest otherwise. Studies just published may throw some light on this problem as they suggest that the temperature sensitivity of the natural exhalation of carbon dioxide from ecosystems has been overestimated and needs to be reevaluated. Scientists from the Max Planck Institute for Biogeochemistry pooled large amounts of data from FLUXNET - an international initiative established more than 10 years ago to monitor exchanges of carbon dioxide between Earth's ecosystems and the atmosphere - with remote sensing and climate data from around the world to investigate the spatial distribution of mean annual Earth's Gross Primary Production (GPP) - which is the total amount of carbon dioxide that terrestrial plants produce each year via photosynthesis - between 1998 and 2006.FLUXNET involves data from 250 sites around the world. Scientists have strung sensors on high towers above grasslands and forests to record water and carbon dioxide concentrations in the air as well as meteorological data. This enables them to calculate how much carbon dioxide is taken up and released by a certain ecosystem. Satellites provide data about how much light vegetated surfaces absorb. Until such data was available scientists had to use estimates.The researchers found that the rate at which plants and microorganisms release carbon dioxide changes little with temperature variations. This is in contrast to earlier investigations that suggested a three or fourfold increase in carbon dioxide production at quite modest temperature changes. According to one of the researchers, Markus Reichstein, “Particularly alarmist scenarios for the feedback between global warming and ecosystem respiration thus prove to be unrealistic.”The measurements also contradict another assumption used in climate models: that the respiration of the ecosystems in the tropics and temperate latitudes is not as temperature sensitive as higher latitudes. This does not seem to be borne out by these FLUXNET observations. “We were very surprised that different ecosystems react relatively uniformly to temperature variations."These findings will have implications for predicting the relationship between carbon dioxide balance and global warming. It is currently not possible to predict whether the response between these two factors. According to Reichstein, “The study shows very clearly that we do not yet have a good understanding of the global biogeochemical cycles and their importance for long-term developments.”A parallel study by Christian Beer from the Max Planck Institute for Biogeochemistry in Jena, Germany, and colleagues looked at the GPP. They estimate that the world's plant life inhales 123 billion metric tons of carbon dioxide every year.The study showed that uptake of carbon dioxide is most pronounced in tropical forests, which are responsible for 34 percent of the inhalation of carbon dioxide from the atmosphere. Savannas account for 26 percent of the global uptake, although they occupy about twice as much surface area as tropical forests.The study suggests that water is the most important factor that influences photosynthesis. Over 40 per cent of the Earth’s vegetated surface plants photosynthesise more when the supply of water increases. This is a potentially important finding as some climate models appear to overestimate the influence of rainfall on global carbon dioxide uptake.A curious finding is that it is the temperate grasslands and shrublands that are most affected by water variation and not tropical rain forests. According to Reichstein, “Here too, we need to therefore critically scrutinize the forecasts of some climate models which predict the Amazon will die as the world gets drier.”Feedback: david.whitehouse@netzerowatch.com