CO2 field experiment likely to cause "do-over" for climate models

I have two press releases here, one via EurekAlert, and one from Max Planck. Plus at the end a summary of points via WUWT regular Pierre Gosselin. First, a key point. You can see the CO2 respiration of the biosphere in the Mauna Loa CO2 record below:

Breath of the Earth: Cycling carbon through terrestrial ecosystems

New data on photosynthesis and respiration will improve models, researchers say

This release is available in Italian, German, Spanish, French, Chinese, and Japanese.

Two recent international studies are poised to change the way scientists view the crucial relationship between Earth’s climate and the carbon cycle. These reports explore the global photosynthesis and respiration rates—the planet’s deep “breaths” of carbon dioxide, in and out—and researchers say that the new findings will be used to update and improve upon traditional models that couple together climate and carbon.

The two reports will be published online by the journal Science at the Science Express Web site at 6 p.m., US ET, Monday, 5 July. Science is published by AAAS, the nonprofit science society.

Christian Beer from the Max Planck Institute for Biogeochemistry in Jena, Germany, along with colleagues from 10 other countries around the world, first take a look at Earth’s Gross Primary Production, or GPP, which represents the total amount of carbon dioxide that terrestrial plants breathe in through photosynthesis each year. With a novel combination of observations and modeling, they estimate the total amount of carbon that the world’s plant life inhales annually is 123 billion metric tons.

Then, Miguel Mahecha, also from the Max Planck Institute for Biogeochemistry, and another international team of researchers settle a long-standing debate over the effects of short-term variations in air temperature on ecosystem respiration, or the Earth’s exhalation of carbon dioxide back into the atmosphere. They show that the sensitivity of ecosystem respiration to short-term variations in temperature is similar around the world. The researchers also suggest that factors other than temperature, such as the slow, ongoing transformations of carbon in the soil and water availability, appear to play crucial roles in long-term ecosystem carbon balances.

Together, these findings shed more light on the global cycle of carbon into and out of the atmosphere and how those processes are coupled with Earth’s ever-changing climate. The researchers analyzed vast amounts of climate and carbon data from around the world, and they say their results should help to improve the validity of predictive models and help resolve how climate change might affect the carbon cycle—and our world—in the future.

“An understanding of the factors that control the GPP of various terrestrial ecosystems is important because we humans make use of many ecosystem services, such as wood, fiber, and food,” said Beer. “Additionally, such an understanding is important in the context of climate change as a consequence of carbon dioxide emissions from burning fossil fuels because vegetation greatly modulates the land-atmosphere exchanges of greenhouse gases, water, and carbon dioxide…”

In their report, Beer and his colleagues 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 calculate the spatial distribution of mean annual GPP between 1998 and 2006.

The researchers highlight the fact that uptake of carbon dioxide is most pronounced in the planet’s tropical forests, which are responsible for a full 34 percent of the inhalation of carbon dioxide from the atmosphere. Savannas then account for 26 percent of the global uptake, although the researchers note that savannas also occupy about twice as much surface area as tropical forests.

Precipitation also plays a significant role in determining the gross global carbon dioxide uptake, the researchers found. They suggest that rainfall has a significant influence on the amount of carbon that plants utilize for photosynthesis on more than 40 percent of vegetated lands, a discovery that stresses the importance of water availability for food security. According to the study, climate models often show great variation, and some of them overestimate the influence of rainfall on global carbon dioxide uptake.

“We reached a milestone with this paper by using plenty of data from FLUXNET in addition to remote sensing and climate reanalysis,” Beer said. “With our estimation of global GPP, we can do two things—compare our results with [Earth system] process models and further analyze the correlation between GPP and climate.”

In the second study, Mahecha and his team of researchers also relied on the global collaboration within the FLUXNET network during their investigation of ecosystems’ sensitivity to air temperature. Compiling and analyzing data from 60 different FLUXNET sites, these researchers found that the respiratory sensitivity to temperature of the world’s ecosystems, commonly referred to as Q10, is actually quite set in stone—and that the Q10 value is independent of the average local temperature and of the specific ecosystem conditions.

For years, experts have debated the effect that air temperature has on global respiration, or the collective metabolic processes of organisms that return carbon dioxide to the atmosphere from Earth’s surface. Most empirical studies suggest that such ecosystem respiration around the world is highly sensitive to increasing temperatures, while the majority of predictive models suggest otherwise. Scientists say that global air temperatures may rise due to the presence of heat-trapping carbon dioxide from the burning of fossil fuels. But, this new result suggests that the temperature sensitivity of the natural exhalation of carbon dioxide from ecosystems has been overestimated and should be reevaluated.

This latest study, in settling the controversy, suggests that previous field studies failed to disentangle processes acting on different time-scales. Mahecha and his team considered the processes of the 60 different ecosystems on the exact same time-scale in order to nail the global mean Q10 down to a value of 1.4. Their new, standard value for various ecosystems’ sensitivity to air temperature suggests a less pronounced short-term climate-carbon feedback compared to previous estimates.

“Our key finding is that the short-term temperature sensitivity of ecosystem respiration to air temperature is converging to a single, global value,” Mahecha said. “Contrary to previous studies, we show that the sensitivity of ecosystem respiration to temperature variations seems to be independent from external factors and constant across ecosystems. In other words, we found a general relationship between variation in temperature and ecosystem respiration… Our findings reconcile the apparent contradictions of modeling and field studies.”

In the future, these two separate studies should allow for more precise predictions of how Earth’s warming climate will affect the exchange of carbon between our ecosystems and the atmosphere—and vice versa. They provide scientists with important tools for better understanding the world’s ecosystems and how the human race continues to influence and alter them.

The report by Beer et al. was funded by CarboEuropeIP, FAO-GTOS-TCO, iLEAPS, Max Planck Institute for Biogeochemistry, National Science Foundation, University of Tuscia, Université Laval, Environment Canada, and the U.S. Department of Energy.

The report by Mahecha et al. was funded by CarboEuropeIP, FAO-GTOS-TCO, iLEAPS, Max Planck Institute for Biogeochemistry, National Science Foundation, University of Tuscia, the U.S. Department of Energy, and grants from the European Research Council, the European Commission project CARBO-Extreme, and the Max Planck Society.

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Here’s the Press Release from the Max Planck Institute:

A new balance for the global carbon balance

Researchers determine how rates of photosynthesis and ecosystem respiration depend on the climate in order to obtain a better assessment of the consequences of climate change

Climate predictions could become more accurate and more reliable in the future – thanks to new findings on the role of terrestrial ecosystems in the global carbon cycle. International teams of researchers headed by the Max Planck Institute for Biogeochemistry in Jena now present comprehensive data analyses in two related studies. The analyses also lead to more precise estimates of how the ecosystems could react to climate change. In most ecosystems, the photosynthesis rate at which plants fix carbon dioxide from the atmosphere changes relatively little as temperature varies. Over 40 percent of the Earth’s vegetated surface reacts very sensitively to changes in the amount of precipitation, however. The respiration of the ecosystems, when flora and fauna release carbon dioxide, also increases to a lesser extent than has recently often been assumed when the temperature rises. Moreover, this temperature dependence is the same all over the world – even in ecosystems as different as the tropical savannah and the Finnish needleleaf forest, for example. (Science Express, July 5, 2010)

Fig. 1: Countless measurement stations (red) around the globe record the exchange of carbon dioxide and water in different ecosystems.

Image: Ulrich Weber, MPI for Biogeochemistry

The climate is quite temperamental: countless factors are involved and many feedback mechanisms enhance effects such as the anthropogenic greenhouse effect. This makes it difficult to make predictions, especially as many processes in the Earth system are still not completely understood. More light is now being shed on the part played by terrestrial ecosystems in the global carbon cycle. This applies to the role of photosynthesis, whereby plants fix carbon dioxide, as well as the process of respiration, during which plants release carbon dioxide once again. The scientists are thus making an important contribution to understanding how the global carbon cycle reacts to global warming and climate change. “Our results suggest that the availability of water, in particular, plays a decisive role for the carbon cycle in ecosystems. It is often more important than temperature,” says Markus Reichstein, a scientist at the Max Planck Institute for Biogeochemistry who has investigated these issues together with his colleagues and two international teams.

In one of the current studies, the researchers involved in the Fluxnet initiative measured how the respiration of ecosystems reacts to short-term variations in temperature at 60 stations spread across the globe. They found that the rate at which plants and microorganisms convert sugar into carbon dioxide does not even double when the temperature increases by ten degrees from one week to the next, for example. “With the aid of suitable models it is then possible to calculate how climate change could affect the respiration of the ecosystems and the global carbon cycle,” says Markus Reichstein.

Some earlier investigations at the ecosystem level resulted in threefold to fourfold accelerations, which would enhance the greenhouse effect. It was not possible to reconcile these data with global models and atmospheric measurements of carbon dioxide concentrations and their seasonal variations, however. “We can now settle obvious contradictions between experimental and theoretical studies,” says Miguel Mahecha, who played a crucial role in coordinating and evaluating the new measurements on ecosystem respiration. His colleague Markus Reichstein adds: “Particularly alarmist scenarios for the feedback between global warming and ecosystem respiration thus prove to be unrealistic.”

These measurements also contradict a further assumption which earlier investigations seemed to suggest: that the respiration of the ecosystems in the tropics and temperate latitudes is influenced to a lesser degree by temperature than at higher latitudes. As the Jena scientists have now discovered, the respiration of very different ecosystems intensifies to the same extent when it becomes hotter. The factor which determines the acceleration of the respiration thus obviously does not depend on the local temperature conditions and the specific characteristics of an ecosystem. “We were very surprised that different ecosystems react relatively uniformly to temperature variations,” says Miguel Mahecha. “After all, for example, we analysed savannahs, tropical rain forests, and also central European broadleaf and needleleaf forests and agricultural ecosystems.”

Fig. 2: Anemometers measure the wind speed ten times per second over leaf canopies and grasslands. With the movement of the air, gases such as carbon dioxide are exchanged between ecosystems and the atmosphere.

Image: MPI for Biogeochemistry

Climate researchers must now examine how the new findings affect the predictions for the carbon dioxide balance and climate change. “It is still not possible to predict whether this attenuates the positive feedback between carbon dioxide concentration and temperature,” says Markus Reichstein. “The study shows very clearly that we do not yet have a good understanding of the global biogeochemichal cycles and their importance for long-term developments.”

The researchers want to change this with their investigation of the rate of photosynthesis. According to this, the terrestrial ecosystems store 123 billion tonnes of carbon dioxide annually. “Although a similar value had been assumed before, it was only an hypothetical estimate,” says Christian Beer, who was in charge of the study. Sixty percent of the carbon dioxide which plants globally take up from the atmosphere is swallowed up by the tropical rain forests and savannahs. The savannahs owe their comparatively important role to the huge area which they cover. The rain forests, in contrast, take up particularly large amounts of carbon dioxide over relatively small areas in order to produce biomass.

The global analysis has also enabled the researchers to establish that the amount of carbon dioxide which is fixed by photosynthesis in leaves is influenced by different climatic factors in different vegetation zones. Sometimes the temperature plays a more important role, sometimes the intensity of the solar radiation, and sometimes the amount of water which the plants can take up from the ground.

From a global point of view, however, water is the factor which has the strongest effect: over 40 percent of Earth’s vegetated surface plants photosynthesize more when the supply of water increases, and less during droughts. In temperate grasslands and shrublands, the amount of carbon dioxide which plants fix as sugar depends to 69 percent on their water supply, in the tropical rain forest this figure is only 29 percent. The researchers call the amount of carbon dioxide which ecosystems annually take up primary production. “We were surprised to find that the primary production in the tropics is not so strongly dependent on the amount of rain,” says Markus 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.”

The data forming the basis of the two reports now published originate from an international network of over 250 measuring stations which undertake long-term observations in different ecosystems across the world. Teams of researchers have high measurement towers above grasslands or forests to record the water and carbon dioxide concentrations in the air and the wind turbulences all over the world. This enables them to calculate how much carbon dioxide is taken up and released by a certain ecosystem. They relate these values to the local climate and divide the Earth’s surface into quadrants measuring 50 by 50 kilometres. The researchers derive the global primary production by using satellite data to extrapolate the photosynthesis rates of the individual ecosystems. The satellites measure how much light energy the vegetated surface of the Earth absorbs on a global basis. Until now, climate researchers have extrapolated the interplay of climate and photosynthesis or ecosystem respiration on the basis of hypotheses.

The new Fluxnet initiative, which Markus Reichstein and colleagues from Italy and North America initiated, brings together the results of the global measurements for the first time. “We are gaining new insights into Earth’s climate system because various research groups are prepared to disclose their data,” says Reichstein. “The current studies, in particular, prove the added value of long-term observations of the Earth system for research. They help to reduce the uncertainty of current climate predictions and exclude some model predictions.”

[JM / PH]

Related links:

[1] Fluxnet Initiative

Original work:

Miguel D. Mahecha, Markus Reichstein, Nuno Carvalhais, Gitta Lasslop, Holger Lange, Sonia I. Seneviratne, Rodrigo Vargas, Christof Ammann, M. Altaf Arain, Alessandro Cescatti, Ivan A. Janssens, Mirco Migliavacca, Leonardo Montagnani, Andrew D. Richardson

Global Convergence in the Temperature Sensitivity of Respiration at Ecosystem Level

Science Express online, July 5, 2010 | http://dx.doi.org/10.1126/science.1189587

Christian Beer, Markus Reichstein, Enrico Tomelleri, Philippe Ciais, Martin Jung, Nuno Carvalhais, Christian Rödenbeck, M. Altaf Arain, Dennis Baldocchi, Gordon B. Bonan, Alberte Bondeau, Alessandro Cescatti, Gitta Lasslop, Anders Lindroth, Mark Lomas, Sebastiaan Luyssaert, Hank Margolis, Keith W. Oleson, Olivier Roupsard, Elmar Veenendaal, Nicolas Viovy, Christopher Williams, F. Ian Woodward, Dario Papale

Terrestrial Gross Carbon Dioxide Uptake: Global Distribution and Covariation with Climate

Science Express online, July 5, 2010 | http://dx.doi.org/10.1126/science.1184984

Max Planck Society

for the Advancement of Science

Press and Public Relations Department

PDF (300 KB)

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Here is a summary of key points via Pierre Gosselin:

1. In most ecosystems, the photosynthesis rate at which plants fix carbon dioxide from the atmosphere changes relatively little as the temperature varies.

2. The respiration of the ecosystems, when flora and fauna release carbon dioxide again, also increases to a lesser extent than has recently often been assumed when the temperature rises.

3. Moreover, this temperature dependence is the same all over the world – even in ecosystems as different as the tropical savannah and the Finnish needleleaf forest.

4. The climate is quite temperamental: countless factors are involved and many feedback mechanisms enhance effects such as the anthropogenic greenhouse effect. This makes it difficult to make predictions, especially as many processes in the Earth system are still not completely understood.

5. Results suggest that the availability of water, in particular, plays a decisive role for the carbon cycle in ecosystems. It is often more important than temperature.

6. Particularly alarmist scenarios for the feedback between global warming and ecosystem respiration thus prove to be unrealistic.”

7. The factor which determines the acceleration of the respiration thus obviously does not depend on the local temperature conditions and the specific characteristics of an ecosystem. “We were very surprised that different ecosystems react relatively uniformly to temperature variations.”

8. “It is still not possible to predict whether this attenuates the positive feedback between carbon dioxide concentration and temperature,” says Markus Reichstein. “The study shows very clearly that we do not yet have a good understanding of the global material cycles and their importance for long-term developments.”

9. “We were surprised to find that the primary production in the tropics is not so strongly dependent on the amount of rain,” says Markus Reichstein. “Here, too, we therefore need to critically scrutinize the forecasts of some climate models which predict the Amazon will die as the world gets drier.”