Separate comprehensive studies published in the last couple of days in Nature Climate Change and Nature Geoscience find that from the northern tundra to the tropical forests, plant communities are showing substantial capacity to tolerate global warming.
The work builds on earlier research on the responsiveness to climate change of both tropical and Arctic plant communities. Given trajectories pointing to centuries of warming this is no reason to relax, of course. And, as you?ll read below from an expert on the relationship of climate to Arctic communities, there?s plenty of disruption coming. But it is important to weigh such work in calculating risks and benefits of various climate policies.
Both papers are by international teams of authors consisting of many of the best and brightest minds probing these two parts of the globe. I have queries out to some of both and will update this post as replies come in.
Here are links, abstracts and some excerpts:
The North
The paper in Nature Climate Change, ?Temperature and vegetation seasonality diminishment over northern lands,? pulls together a wide array of research, including the work by Bruce Forbes of the University of Lapland last year, on what I called ?pop-up forests? ? patches of rapidly-growing tundra shrubs.
Here?s an excerpt from the NASA news release (NASA funded the study; Boston University, the academic home of the two lead authors, has also posted a big cache of images and other background):
Vegetation growth at Earth?s northern latitudes increasingly resembles lusher latitudes to the south, according to a NASA-funded study based on a 30-year record of land surface and newly improved satellite data sets.
An international team of university and NASA scientists examined the relationship between changes in surface temperature and vegetation growth from 45 degrees north latitude to the Arctic Ocean. Results show temperature and vegetation growth at northern latitudes now resemble those found 4 degrees to 6 degrees of latitude farther south as recently as 1982.
?Higher northern latitudes are getting warmer, Arctic sea ice and the duration of snow cover are diminishing, the growing season is getting longer and plants are growing more,? said Ranga Myneni of Boston University?s Department of Earth and Environment. ?In the north?s Arctic and boreal areas, the characteristics of the seasons are changing, leading to great disruptions for plants and related ecosystems.??
However, researchers say plant growth in the north may not continue on its current trajectory. The ramifications of an amplified greenhouse effect, such as frequent forest fires, outbreak of pest infestations and summertime droughts, may slow plant growth.
Also, warmer temperatures alone in the boreal zone do not guarantee more plant growth, which also depends on the availability of water and sunlight.
?Satellite data identify areas in the boreal zone that are warmer and dryer and other areas that are warmer and wetter,? said co-author Ramakrishna Nemani of NASA?s Ames Research Center in Moffett Field, Calif. ?Only the warmer and wetter areas support more growth.?
This new science fits with how I described the Arctic in 2011: ?a?physical system that amplifies warm or cool jogs and a biological system attuned to such changes.?
At the end of this post you can read some thoughts from Bruce Forbes (who is an author) on the impact of projected shifts in coming decades on Arctic reindeer and the herders who depend on them.
The Tropics
The paper in Nature Geoscience, on the tropics, is ?Simulated resilience of tropical rainforests to CO2-induced climate change.? It is well summarized in Nature?s news article by Olive Heffernan, ?Tropical forests unexpectedly resilient to climate change.?
Here?s an excerpt from the abstract:
Using simulations with 22 climate models and the MOSES?TRIFFID* land surface scheme, we find that only in one of the simulations are tropical forests projected to lose biomass by the end of the twenty-first century?and then only for the Americas. When comparing with alternative models of plant physiological processes, we find that the largest uncertainties are associated with plant physiological responses, and then with future emissions scenarios. Uncertainties from differences in the climate projections are significantly smaller. Despite the considerable uncertainties, we conclude that there is evidence of forest resilience for all three regions.
[* The clunky acronym refers to this model: "Top-down Representation of Interactive Foliage and Flora Including Dynamics."]
Here?s one important line from the body of the paper:
Forest biomass carbon stocks in Asia and Africa are projected to be greater in year 2100 than at the present day, in all simulations. This is also true for the Americas/Amazon, except for the HadCM3 climate model [background on that model is here]. There is however a decreasing ability to sequester carbon in biomass; many pathways have a Cv? [vegetation carbon, including live roots] peak towards the end of the twenty-first century.
Swinging back to the north, here?s Bruce Forbes?s response to the question I posed to the authors of the paper on shifts in seasons at high latitudes. I asked them which findings were most troubling and which most encouraging:
1) Shifts in albedo [the amount by which surfaces reflect energy back to space] and feedbacks to the atmosphere potentially accelerating ongoing warming will probably be evident from structural changes within the tundra zone well befoere the coniferous forests migrate north.
2) There are interactions between increasing shrub cover and winter snow capture and spring melt that have strong implications for (1).
3) Increasing frequency and intensity of rain-on-snow and crusted snow events in reindeer and caribou grazing systems has the potential to significantly increase mortality in herds of animals upon which large numbers of people and wildlife depend.
4) The reduced seasonality (e.g. 3) will likely place additional stress on human populations already under pressure from oil and gas development.
5) The reduced seasonality means rivers and lakes freeze up later in autumn and melt out earlier in spring. We have seen this in Fennoscandia [click for background on this region] and northern Russia and it strongly affects both indigenous and non-indigenous livelihoods, like reindeer herding, tourism, oil and gas (winter road use), etc.
6) The reduced seasonality means insect harassment of reindeer begins earlier, which has implication for calf mortality as the exposure to intense harassment would begin before they reach their summer pastures on the arctic coast (Kara and Barents seas). Those shores are typically windy, whereas the inland areas by which they approach the coast are generally much less windy.
On the encouraging side:
1) The reindeer pastures of the tundra zone in west Siberia have been characterized as moderately to heavily ?overgrazed? for 5 to 6 decades, and accordingly the reindeer population has long been expected to crash. Yet with no supplemental feeding (unlike in Finland) the reindeer population has more than doubled since World War II from 300,000 to over 630,000 animals. And during the satellite era we find the productivity of the tundra zone has actually increased significantly (the pop-up forests paper and this paper). The west Siberian tundra is certainly becoming more steppe-like, patches of tall shrub woodlands interspersed with low shrub graminoid-dominated vegetation. In other words, a state change is afoot. So, I think it is time to reassess what is meant by ?overgazed? with less subjective metrics than outdated models of ?carrying capacity.?
2) High densities of reindeer managed by herders in northwestern Eurasia may be able to help check increases in shrub growth and height by concentrating grazing in existing low shrub areas before they grow taller. We have two large projects going on this now in Fennoscandia and western Siberia.
3) People and reindeer have been together through episodes of warming and cooling in the past 1,500 years or so. I suspect, but don?t have the solid evidence yet, that human agency is underestimated as a key factor in stucturing past and present tundra and forest-tundra ecosystems of northwest Eurasia. This is one reason we need the paleo data from terrestrial systems in my study regions.
As I explained above, I?ll add more thoughts here as they come in.
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