The greening Earth – Matthias Forkel interviewed for „VDI Nachrichten”
VDI Nachrichten, a German weekly newspaper on engineering and technology, published an article about the effect of climate change on vegetation (“Die Erde wird grüner” by Susanne Donner, 2017/03/03). Matthias Forkel from TU Wien’s EOWAVE project was interviewed for this article.
Unfortunately, some citations are incorrectly reported.
“Nadelwald ist intensiver grün als Laubwald und führt deshalb auch zu einem mehr an Grün”.
The “greenness” of vegetation can be measured from satellite observations. Therefore vegetation indices (e.g. NDVI) are calculated as metrics for vegetation greenness from the satellite measurements. Needle-leaved forests have usually lower NDVI values (i.e. less green) than broadleaved forests (i.e. more green). Satellite observations over the northern forests (i.e. boreal forests or Taiga) show that positive trends in vegetation greenness occur which is called “greening”. The greening in northern forests is largely caused by an increasing growth of broadleaved trees and shrubs.
“Pflanzen und Bäume nehmen zwar CO2 aus der Luft auf – etwa ein Viertel dem Treibhausgases, das in die Atmosphäre gelangt. Aber durch den Blätterboom wird es kaum mehr. ‚Der Wert stieg gerade einmal von 25 % auf 26 %‘, stellt Forkel klar.“
The burning of fossil fuels, cement production, and land use change (e.g. deforestation) are causing emissions of CO2 to the atmosphere. These emissions are approximately 8.9 Pg (peta grams, 1 Pg = 1015 g) of CO2 per year in the period 2000-2009 . The oceans take up 2.3 Pg per year (26 % of the emissions) and the land ecosystems take up 2.6 Pg (29 % of the emissions). The rest of the emissions (4 Pg = 45%) remains in the atmosphere and causes the increasing CO2 concentrations. Our own results show that the CO2 uptake by land ecosystems was increasing by around 6 % per year in the period 1980 to 2011 . This increasing uptake of CO2 by land ecosystems is largely caused by an increasing photosynthesis of the vegetation. The increase in photosynthetic carbon uptake is caused by several effects: The increase of CO2 in the atmosphere allows the plants to take up more CO2. The warming climate allows longer and more productive growing seasons especially in northern ecosystems. The increase in vegetation greenness (i.e. an increase in leaf area, “Blätterboom”) allows the vegetation to absorb more solar radiation for photosynthesis. The importance of these processes for changes in photosynthesis however depends on regional climate conditions and ecosystem types.
“Forkel stellt aber klar: ‘Der wichtigste Klimafaktor ist die Bodenfeuchte. Es sind vornehmlich bessere Bewässerungsmethoden und auch eine intensivere Düngung in den letzten Jahrzehnten, die in vielen Industrie- und Schwellenländern das Plus an Pflanzen erklären.‘“
Soil moisture is the most important factor for plant growth in regions that are limited by water (for examples Steppes, semi-deserts, the Mediterranean, Savannahs). The irrigation of agricultural areas in these regions contributes indeed to the observed increased plant growth. Fertilization of agricultural areas also contributes to the observed plant growth. However, temperature is a more important factor for plant growth than soil moisture in other regions such as in northern ecosystems (Taiga and Tundra) .
 Ciais, P., Sabine, C., Bala, G., Bopp, L., Brovkin, V., Canadell, J. G., Chhabra, A., DeFries, R. S., Galloway, J., Heimann, Martin, Jones, C., Le Quéré, C., Myneni, R., Piao, S. and Thornton, P. E.: Carbon and Other Biogeochemical Cycles, in Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, edited by T. F. Stocker, D. Qin, G.-K. Plattner, M. Tignor, S. K. Allen, J. Boschung, A. Nauels, J. Xia, Bex, V., and Midgley, P. M., pp. 465–570, Cambridge, United Kingdom and New York, USA. [online] Available from: www.climatechange2013.org/report/full-report/, 2013.
 Forkel, M., Carvalhais, N., Rödenbeck, C., Keeling, R., Heimann, M., Thonicke, K., Zaehle, S. and Reichstein, M.: Enhanced seasonal CO2 exchange caused by amplified plant productivity in northern ecosystems, Science, aac4971, doi:10.1126/science.aac4971, 2016.
 Forkel, M., Migliavacca, M., Thonicke, K., Reichstein, M., Schaphoff, S., Weber, U. and Carvalhais, N.: Codominant water control on global interannual variability and trends in land surface phenology and greenness, Global Change Biolology, 21(9), 3414–3435, doi:10.1111/gcb.12950, 2015.