This could drive an amplifying feedback, undermine biofuels strategy
Earth has done an ecological about-face: Global plant productivity that once flourished under warming temperatures and a lengthened growing season is now on the decline, struck by the stress of drought.
NASA-funded researchers Maosheng Zhao and Steven Running, of the University of Montana in Missoula, discovered the global shift during an analysis of NASA satellite data. Compared with a six-percent increase spanning two earlier decades, the recent ten-year decline is slight — just one percent. The shift, however, could impact food security, biofuels, and the global carbon cycle.
“We see this as a bit of a surprise, and potentially significant on a policy level because previous interpretations suggested that global warming might actually help plant growth around the world,” Running said.
“These results are extraordinarily significant because they show that the global net effect of climatic warming on the productivity of terrestrial vegetation need not be positive — as was documented for the 1980′s and 1990′s,” said Diane Wickland, of NASA Headquarters and manager of NASA’s Terrestrial Ecology research program.
That’s from a remarkable NASA news release today, “Drought Drives Decade-Long Decline in Plant Growth” (see narrated video below).
On Friday, the journal Science publishes the study itself, ” Drought-Induced Reduction in Global Terrestrial Net Primary Production from 2000 Through 2009” (subs. req’d), which found:
Terrestrial net primary production (NPP) quantifies the amount of atmospheric carbon fixed by plants and accumulated as biomass. Previous studies have shown that climate constraints were relaxing with increasing temperature and solar radiation, allowing an upward trend in NPP from 1982 through 1999. The past decade (2000 to 2009) has been the warmest since instrumental measurements began, which could imply continued increases in NPP; however, our estimates suggest a reduction in the global NPP of 0.55 petagrams of carbon. Large-scale droughts have reduced regional NPP, and a drying trend in the Southern Hemisphere has decreased NPP in that area, counteracting the increased NPP over the Northern Hemisphere. A continued decline in NPP would not only weaken the terrestrial carbon sink, but it would also intensify future competition between food demand and proposed biofuel production.
The notion that we could use a substantial amount of arable land or potable water for growing biomass to make energy was always dubious, as I’ve long argued (see “Are biofuels a core climate solution?“). If this study does in fact portend a long-term trend, then it further constrains the potential biofuel options.
The release contains an excellent video:
(Video Credit: NASA/Michelle Williams. This narrated video gives an overview of net primary production and the carbon cycle. High-resolution data from the Moderate Resolution Imaging Spectroradiometer, or MODIS, indicate a net decrease in NPP from 2000-2009, as compared to the previous two decades.)
The release itself continues:
Conventional wisdom based on previous research held that land plant productivity was on the rise. A 2003 paper in Science led by then University of Montana scientist Ramakrishna Nemani (now at NASA Ames Research Center, Moffett Field, Calif.) showed that global terrestrial plant productivity increased as much as six percent between 1982 and 1999. That’s because for nearly two decades, temperature, solar radiation and water availability — influenced by climate change — were favorable for growth.
Setting out to update that analysis, Zhao and Running expected to see similar results as global average temperatures have continued to climb. Instead, they found that the impact of regional drought overwhelmed the positive influence of a longer growing season, driving down global plant productivity between 2000 and 2009. The team published their findings Aug. 20 in Science.
“This is a pretty serious warning that warmer temperatures are not going to endlessly improve plant growth,” Running said.
The discovery comes from an analysis of plant productivity data from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite, combined with growing season climate variables including temperature, solar radiation and water. The plant and climate data are factored into an algorithm that describes constraints on plant growth at different geographical locations.
For example, growth is generally limited in high latitudes by temperature and in deserts by water. But regional limitations can very in their degree of impact on growth throughout the growing season.
Zhao and Running’s analysis showed that since 2000, high-latitude northern hemisphere ecosystems have continued to benefit from warmer temperatures and a longer growing season. But that effect was offset by warming-associated drought that limited growth in the southern hemisphere, resulting in a net global loss of land productivity.
“This past decade’s net decline in terrestrial productivity illustrates that a complex interplay between temperature, rainfall, cloudiness, and carbon dioxide, probably in combination with other factors such as nutrients and land management, will determine future patterns and trends in productivity,” Wickland said.
Researchers are keen on maintaining a record of the trends into the future. For one reason, plants act as a carbon dioxide “sink,” and shifting plant productivity is linked to shifting levels of the greenhouse gas in the atmosphere. Also, stresses on plant growth could challenge food production.
“The potential that future warming would cause additional declines does not bode well for the ability of the biosphere to support multiple societal demands for agricultural production, fiber needs, and increasingly, biofuel production,” Zhao said.
“Even if the declining trend of the past decade does not continue, managing forests and croplands for multiple benefits to include food production, biofuel harvest, and carbon storage may become exceedingly challenging in light of the possible impacts of such decadal-scale changes,” Wickland said.
“A snapshot of Earth’s plant productivity in 2003 shows regions of increased productivity (green) and decreased productivity (red). Tracking productivity between 2000 and 2009, researchers found a global net decrease due to regional drought.” Credit: NASA Goddard Space Flight Center Scientific Visualization Studio.
Finally, the study itself concludes ominously:
Recent simulations from coupled climate-carbon cycle models have shown that there is a positive feedback between the carbon cycle and the climate system (29-31), and also that future biological carbon sinks could eventually level off and subsequently decline to zero…. [S]oil respiration largely follows a similar nonlinear temperature response in these models (31). These nonlinear amplification responses of water and carbon processes to warming are the major mechanisms responsible for the positive feedback between the carbon and climate systems.
Though the warming climate during this period continuously increased NPP over areas of high latitude and high elevations, these warming-benefited areas only account for 16% of the global total NPP and 24% of global vegetated land area. NPP over large land areas of lower latitude and altitude is negatively correlated with temperature (fig. S10A), mostly due to the warming-related increases in water stress and autotrophic respiration, especially for the SH (r = -0.94, p < 0.0001). Globally, interannual NPP is negatively correlated with air temperature over vegetated land (r = -0.64, p < 0.05) (table S3).
Over the past 10 years, large-scale periodic regional droughts and a general drying trend over the SH reduced global terrestrial NPP. Under a changing climate, severe regional droughts have become more frequent, a trend expected to continue for the foreseeable future (13, 33, 34). The warming-associated heat and drought not only decrease NPP, but also may trigger many more ecosystem disturbances (6, 35, 36), releasing carbon to the atmosphere (18, 37). Reduced NPP potentially threatens global food security and future biofuel production and weakens the terrestrial carbon sink. Continuous global monitoring of NPP will be essential to determining whether the reduced NPP over the past 10 years is a decadal variation or a turning point to a declining terrestrial carbon sequestration under changing climate.
And so we have another potential of an amplifying feedback in the carbon cycle.
This new study is the latest in a series of truly alarming scientific studies published just this year that suggest we may be closer to dangerous carbon-cycle tipping points and irreversible thresholds than anyone realized:
- Nature Stunner: “Global warming blamed for 40% decline in the ocean’s phytoplankton” — “Microscopic life crucial to the marine food chain is dying out. The consequences could be catastrophic.”
- 2010 Nature Geoscience study: Oceans are acidifying 10 times faster today than 55 million years ago when a mass extinction of marine species occurred.
- Stunner: Nature review of 20 years of field studies finds soils emitting more CO2 as planet warms — Biogeochemist: “… perhaps most likely explanation is that increasing temperatures have increased rates of decomposition of soil organic matter, which has increased the flow of CO2. If true, this is an important finding: that a positive feedback to climate change is already occurring at a detectable level in soils.”
- Science stunner: Vast East Siberian Arctic Shelf methane stores destabilizing and venting — NSF issues world a wake-up call: “Release of even a fraction of the methane stored in the shelf could trigger abrupt climate warming.”
Failure to takes strong and immediate action to reduce greenhouse gas emissions is increasingly looking like societal suicide.