Neue Studie fördert überraschendes Ergebnis zutage: Erderwärmung hat keinen Einfluss auf Kohlendioxidabgabe aus Mooren

Auf dem Höhepunkt des Klimaalarms erschien 2008 in Nature Geoscience eine Arbeit von Takeshi Ise und Kollegen, in der den Mooren der kühlen Klimazonen ein böses Schicksal prognostizert wurde. Im Zuge der Klimaerwämung würden sie in den kommenden Jahren große Mengen an Kohlenstoff bzw. CO2 in die Atmosphäre abgeben. In der Kurzfassung hieß es:

High sensitivity of peat decomposition to climate change through water-table feedback
[...] In our long-term simulation, an experimental warming of 4 °C causes a 40% loss of soil organic carbon from the shallow peat and 86% from the deep peat. We conclude that peatlands will quickly respond to the expected warming in this century by losing labile soil organic carbon during dry periods.

Wie man so schön sagt: Die Klimaaalarmgeschichte passte gut ins Bild und zum Zeitgeist. Spektrum der Wissenschaft berichtete damals besorgt:

Erderwärmung steigert Kohlendioxidabgabe aus Mooren
Sinkt in Mooren aufgrund wärmerer Temperaturen der Grundwasserspiegel, werden erhebliche Mengen Kohlendioxid freigesetzt. Dies zeigen Ergebnisse einer Langzeitsimulation von Takeshi Ise von der Japan Agency for Marine-Earth Science and Technology in Yokohama.

Aber stimmt das auch? Sieben Jahre Forschung später wissen wir es endlich: Nein, die Horrorgeschichte hat sich zum Glück nicht bewahrheitet. Die Klimaerwärmung hat nahezu keinen Einfluss auf die nördlichen Moorgebiete. Im Gegenteil, es scheint sogar so zu sein, dass sich die Ablagerungsrate des Torfs unter warmen klimatischen Bedingungen erhöht. Herausgefunden hat dies ein US-amerikanisches Forscherteam um Michael Philben. In einer Pressemitteilung der University of South Carolina vom 8. Juni 2015 fassen sie ihre Ergebnisse wie folgt zusammen:

Rising global temperatures will have little effect on boreal peatlands

To some scientists studying climate change, boreal peatlands are considered a potential ticking time bomb. With huge stores of carbon in peat, the fear is that rising global temperatures could cause the release of massive amounts of CO2 from the peatlands into the atmosphere—essentially creating a greenhouse gas feedback loop.

A new study by researchers at the University of South Carolina and University of California Los Angeles challenges that notion, and demonstrates that the effect of temperature increases on peat storage could be minor. Funded by the National Science Foundation (NSF) and published in “Global Biogeochemical Cycles,” the study instead points to the length of time peat is exposed to oxygen as a much more important factor in how it releases carbon into the atmosphere.

The researchers used the biochemical composition of a peat core collected from the James Bay Lowland in Canada to assess the historical relationship between climate and the extent of peat decomposition. The core is a record of peat accumulation over the last 7,500 years and contains two intervals (the Medieval Climate Anomaly and the Holocene Thermal Maximum) when temperatures were about 2°C warmer than normal, providing a natural analogue for modern warming.

However, peat formed during these warm intervals was not extensively decomposed compared to peat formed during cooler periods. Instead, the most extensive decomposition coincided with drier conditions and longer oxygen exposure time during peat formation. This indicates oxygen exposure time was the primary control on peat decomposition, while temperature was of secondary importance. This was supported by comparing the extent of decomposition along a climate transect in the West Siberian Lowland, Russia. Cores from the northern end of the transect, which experienced longer oxygen exposure times, were more decomposed than cores from the south, which formed under warmer temperatures.

The low apparent sensitivity of peat decomposition to warming has important implications for the future of the peatlands, as warming is unlikely to result in widespread carbon loss. Instead, the lengthening growing season is expected to stimulate plant growth, which combined with unchanging decomposition could increase the rate of carbon sequestration.

Ron Benner, director of the Marine Science Program at the University of South Carolina and one of the study’s authors, says the findings are important in understanding how the earth’s changing climate will affect peatlands.

“It is too early to declare peatlands and their massive carbon stocks are secure. Changing precipitation patterns could cause drier conditions, increasing oxygen exposure time and promoting decomposition,” Benner said. “Thawing permafrost in arctic peatlands could also trigger the loss of previously inaccessible carbon. In addition, increasing atmospheric nitrogen pollution can allow rapidly decomposing vascular plants to outcompete the more recalcitrant Sphagnum (peat moss). However, the results of the study indicate the direct effect of increasing temperatures on decomposition will be relatively minor.”

Das entsprechende Paper von Philben et al. erschien in den Global Biogeochemical Cycles. Hier die Kurzfassung der Arbeit:

Temperature, oxygen, and vegetation controls on decomposition in a James Bay peatland
The biochemical composition of a peat core from James Bay Lowland, Canada, was used to assess the extent of peat decomposition and diagenetic alteration. Our goal was to identify environmental controls on peat decomposition, particularly its sensitivity to naturally occurring changes in temperature, oxygen exposure time, and vegetation. All three varied substantially during the last 7000 years, providing a natural experiment for evaluating their effects on decomposition. The bottom 50 cm of the core formed during the Holocene Climatic Optimum (~7000–4000 years B.P.), when mean annual air temperature was likely 1–2°C warmer than present. A reconstruction of the water table level using testate amoebae indicated oxygen exposure time was highest in the subsequent upper portion of the core between 150 and 225 cm depth (from ~2560 to 4210 years B.P.) and the plant community shifted from mostly Sphagnum to vascular plant dominance. Several independent biochemical indices indicated that decomposition was greatest in this interval. Hydrolysable amino acid yields, hydroxyproline yields, and acid:aldehyde ratios of syringyl lignin phenols were higher, while hydrolysable neutral sugar yields and carbon:nitrogen ratios were lower in this zone of both vascular plant vegetation and elevated oxygen exposure time. Thus, peat formed during the Holocene Climatic Optimum did not appear to be more extensively decomposed than peat formed during subsequent cooler periods. Comparison with a core from the West Siberian Lowland, Russia, indicates that oxygen exposure time and vegetation are both important controls on decomposition, while temperature appears to be of secondary importance. The low apparent sensitivity of decomposition to temperature is consistent with recent observations of a positive correlation between peat accumulation rates and mean annual temperature, suggesting that contemporary warming could enhance peatland carbon sequestration, although this could be offset by an increasing contribution of vascular plants to the vegetation.

Sind Sie überrascht, dass bislang keine einzige deutsche Zeitung über die wichtige neue Studie berichtet hat? Man könnte fast auf die Idee kommen, dass in Vorbereitung auf die diesjährige Klimakonferenz in Paris unbequeme Themen aus der Öffentlichkeit herausgehalten werden sollen.