Greening of the Earth: High Latitudes
From ClimateWiki
In the introduction to their report on the response of High Arctic tundra vegetation to the warming experienced in that part of the world over the past quarter-century, Hudson and Henry (2009) note the Arctic warmed by about 1.6°C over the past four decades, citing McBean et al. (2005). They state this temperature increase “led the Arctic Climate Impact Assessment (ACIA) and Intergovernmental Panel on Climate Change (IPCC) to predict that tundra ecosystems will be particularly threatened by climate change [i.e., warming] over the next century.” To test this prediction, the authors set out to find if plants of the High Arctic tundra have been growing more or less vigorously or abundantly during the recent warming period.
At an 8-km2 coastal lowland adjacent to Alexandra Fiord on the east-central coast of Ellesmere Island, Nunavut, Canada, Hudson and Henry measured biomass and composition changes in a heath community dominated by several vascular plants and bryophytes. They did this over a period of 13 years (1995–2007), using a point-intercept method in permanent plots, and over a period of 27 years (1981–2008) using a biomass harvest comparison. “Results from both methods,” in the words of the Canadian scientists, “indicate that the community became more productive over time.” The note “bryophyte and evergreen shrub abundances increased,” while “deciduous shrub, forb, graminoid, and lichen cover did not change,” so that “species diversity also remained unchanged.” All of these changes—and non-changes—are a far cry from the “particularly threatened” view of the ACIA and IPCC.
In further support of their findings, Hudson and Henry report “satellite-based remote sensing models, such as green trends derived from the normalized difference vegetation index (NDVI; e.g., Myneni et al., 1997; Zhou et al., 2001; Stow et al., 2004; Verbyla, 2008), and global vegetation and ecosystem process simulations of the terrestrial carbon cycle (e.g., Kimball et al., 2006; Zhang et al., 2008), indicate increasing trends in vegetation photosynthetic activity and net primary production in the Arctic over the past several decades.” As for what drove this welcome transformation of the tundra, Hudson and Henry say “it is likely that warming directly increased plant growth and reproduction and indirectly increased resource supply,” while “increased temperatures also lengthened the growing season, increased soil temperature, deepened the active [soil] layer, and consequently may have influenced nutrient uptake in the plant community.”
Nevertheless, some people continue to claim anthropogenic global warming will have widespread adverse effects on ecosystems, and one of the regions they claim to be most vulnerable is the Arctic. Jia et al. (2009) utilized 25 years of satellite data covering the period of most rapid recent warming (1982 to 1996) to evaluate this contention via NDVI data obtained from the GIMMS dataset, which consists of 64-km2 cells minus those cells with too much open water or bare ground within them that are known to inject significant bias into NDVI data analyses. This study showed tundra ecosystems exhibited an average increase in greenness of 5.6 percent per decade over 96 percent of the pixels evaluated, which was proportional to the rate of summer warming as measured by growing degree days. The three researchers state the decadal increases of vegetation greenness over the tundra biome in summer “reflect increasing vegetation productivity during the growing season.”
Working at a site just three kilometers from the Abisko Scientific Research Station (68°21’N, 18°49’E) in the Northern Swedish Scandes, Hallinger et al. (2010) studied male plants of the medium-sized Juniperus nana shrub, collecting the main stems of five to eight shrubs every hundred meters of elevation until the shrub zone ended. They then performed ring-width measurements on these stems, “to measure radial and vertical growth, to track growth changes over time, to age the shrub individuals and to correlate annual shrub growth with climate.” Data for the latter factor were derived from records of the nearby Abisko Station. By these means the three researchers identified “a distinct increase in radial and vertical growth rates of J. nana shrubs during recent decades in the subalpine zone of North Sweden,” and they state “the age structure of shrubs along the elevational gradient provides evidence that an upslope advance of the altitudinal shrubline is underway.” In addition, they state they observed “significant, strong and stable correlations between annual ring width and summer temperatures (June, July, August),” and “the acceleration of radial and vertical growth since 1970 also coincides with the recent three decades of rising arctic air temperatures and the warming trend of 0.2°C per decade for the average temperature since 1956 at Abisko.” Thus, the German scientists’ study adds to what they call the “mounting evidence that shrubs are expanding into alpine and arctic areas because of climate warming,” and they note “this expansion occurs in both evergreen and deciduous shrub types,” citing Forbes et al. (2010).
Other remote-sensing data suggest tundra vegetation in North America may be responding to recent warming via increased photosynthetic activity (Goetz et al., 2005; Verbyla , 2008). Forbes et al. (2010) write, “at a circumpolar scale, the highest photosynthetic activity and strongest growth trends are reported in locations characterized by erect shrub tundra (Reynolds et al., 2006),” noting “live leaf phytomass from deciduous shrubs, shown to have increased in northern Alaska during the second half of the last century (Sturm et al., 2001; Tape et al., 2006), is believed to be a key driver of the observed trends (Jia et al., 2003; Goetz et al., 2005; Verbyla, 2008).” Therefore, working with Salix lanata L. (sensu latu)—an abundant deciduous dioecious willow with nearly circumpolar geographic distribution from the northern boreal forest to the northern limits of the Low Arctic—Forbes et al. analyzed annual ring growth for 168 stem slices of 2- to 3-cm thickness collected from 40 discrete individuals spread across 15 sample sites within an area of approximately 3 x 2.3 km, located at about 68°40’N, 58°30’E, to further examine this phenomenon.
The three scientists state they detected “a clear relationship with photosynthetic activity for upland vegetation at a regional scale for the period 1981–2005, confirming a parallel ‘greening’ trend reported for similarly warming North American portions of the tundra biome,” and they state “the standardized growth curve suggests a significant increase in shrub willow growth over the last six decades.” Additionally noting “the quality of the chronology as a climate proxy is exceptional,” Forbes et al. go on to state their findings “are in line with field and remote sensing studies that have assigned a strong shrub component to the reported greening signal since the early 1980s,” adding the growth trend agrees with the qualitative observations of nomadic reindeer herders, which suggest there have been “recent increases in willow size in the region.” They state their analysis “provides the best proxy assessment to date that deciduous shrub phytomass has increased significantly in response to an ongoing summer warming trend.”
Contemporaneously, Zhuang et al. (2010) used a process-based biogeochemistry model—the Terrestrial Ecosystem Model or TEM, which also employed a soil thermal model—to examine permafrost dynamics and their effects on the carbon dynamics of the Tibetan Plateau over the past century. This was done by “parameterizing and verifying” the TEM using existing real-world data for soil temperature, permafrost distribution, and carbon and nitrogen distributions throughout the region, and then extrapolating the model and its parameters to the whole of the plateau. The six scientists found, “during the 20th century, the Tibetan Plateau changed from a small carbon source or neutral in the early part of the century to a sink later.” They note “net primary production and soil respiration increased by 0.52 and 0.22 Tg C/year, respectively, resulting in a regional carbon sink increase of 0.3 Tg C/year,” so that “by the end of the century, the regional carbon sink reached 36Tg C/year and carbon storage in vegetation and soils is 32 and 16 Pg C, respectively.”
Zhuang et al. state the “increasing soil temperature and deepening active layer depth enhanced soil respiration, increasing the net nitrogen mineralization rate,” and “together with the [positive] effects of warming air temperature and rising CO2 concentrations on photosynthesis, the stronger plant nitrogen uptake due to the enhanced available nitrogen stimulated plant carbon uptake, thereby strengthening the regional carbon sink as the rate of increase in net primary production was faster than that of soil respiration.” Thus, they conclude “future warming will increase thawing of the permafrost, increase soil temperature and dry up soil moisture,” and “these physical dynamics may enhance [the] future strength of the regional carbon sink, since the rate of increase of net primary production is higher than that of soil respiration on the Tibetan Plateau.”
References
Forbes, B.C., Fauria, M., and Zetterberg, P. 2010. Russian Arctic warming and ‘greening’ are closely tracked by tundra shrub willows. Global Change Biology 16: 1542–1554.
Goetz, S.J., Bunn, A.G., Fiske, G.J., and Houghton, R.A. 2005. Satellite-observed photosynthetic trends across boreal North America associated with climate and fire disturbance. Proceedings of the National Academy of Sciences USA 102: 13,521–13,525.
Hallinger, M., Manthey, M., and Wilmking, M. 2010. Establishing a missing link: warm summers and winter snow cover promote shrub expansion into alpine tundra in Scandinavia. New Phytologist 186: 890–899.
Hudson, J.M.G. and Henry, G.H.R. 2009. Increased plant biomass in a High Arctic heath community from 1981 to 2008. Ecology 90: 2657–2663.
Jia, G.J., Epstein, H.E., and Walker, D.A. 2003. Greening of arctic Alaska, 1981–2001. Geophysical Research Letters 30: 31–33.
Jia, G.J., Epstein, H.E., and Walker, D.A. 2009. Vegetation greening in the Canadian Arctic related to decadal warming. Journal of Environmental Monitoring 11: 2231–2238.
Kimball, J.S., Zhao, M., Mcguire, A.D., Heinsch, F.A., Clein, J., Calef, M.P., Jolly, W.M., Kang, S., Euskirchen, S.E., McDonald, K.C., and Running, S.W. 2006. Recent climate-driven increases in vegetation productivity for the Western Arctic: evidence for an acceleration of the northern terrestrial carbon cycle. Earth Interactions 11: 1–23.
McBean, G., Alekseev, G., Chen, D., Forland, E., Fyfe, J., Groisman, P.Y., King, R., Melling, H., Vose, R., and Whitfield, P.H. 2005. Arctic climate: past and present. In Arctic Climate Impact Assessment: Scientific Report, 21–60. Cambridge, UK: Cambridge University Press.
Myneni, R.B., Keeling, C.D., Tucker, C.J., Asrar, G., and Nemani, R.R. 1997. Increased plant growth in the northern high latitudes from 1981 to 1991. Nature 386: 698–702.
Reynolds, M.K., Walker, D.A., and Maier, H.A. 2006. NDVI patterns and phytomass distribution in the circumpolar Arctic. Remote Sensing of Environment 102: 271–281.
Stow, D.A., Hope, A., McGuire, D., Verbyla, D., Gamon, J., Huemmrich, F., Houston, S., Racine, C., Sturm, M., Tape, K., Hinzman, L., Yoshikawa, K., Tweedie, C., Noyle, B., Silapaswan, C., Douglas, D., Griffith, B., Jia, G., Epstein, H., Walker, D., Daeschner, S., Petersen, A., Liming, Z., and Myneni, R. 2004. Remote sensing of vegetation and land-cover change in Arctic tundra ecosystems. Remote Sensing of Environment 89: 281–308.
Sturm, M., Racine, C., and Tape, K. 2001. Increasing shrub abundance in the Arctic. Nature 411: 546–547. Tape, K., Sturm, M., and Racine, C.H. 2006. The evidence for shrub expansion in northern Alaska and the Pan-Arctic. Global Change Biology 32: 686–702.
Verbyla, D. 2008. The greening and browning of Alaska based on 1982–2003 satellite data. Global Ecology and Biogeography 17: 547–555.
Zhang, K., Kimball, J.S., Hogg, E.H., Zhao, M.S., Oechel, W.C., Cassano, J.J., and Running, S.W. 2008. Satellite-based model detection of recent climate-driven changes in northern high-latitude vegetation productivity. Journal of Geophysical Research-Biogeosciences 113: G03033.
Zhou, L.M., Tucker, C.J., Kaufmann, R.K., Slayback, D., Shabanov, N.V., and Myneni, R.B. 2001. Variations in northern vegetation activity inferred from satellite data of vegetation index during 1981 to 1999. Journal of Geophysical Research 106: 20,069–20,083.
Zhuang, Q., He, J., Lu, Y., Ji, L., Xiao, J., and Luo, T. 2010. Carbon dynamics of terrestrial ecosystems on the Tibetan Plateau during the 20th century: an analysis with a process-based biogeochemical model. Global Ecology and Biogeography 19: 649–662.
