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Past and future global transformation of terrestrial ecosystems under climate change

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Impacts of global climate change on terrestrial ecosystems are imperfectly constrained by ecosystem models and direct observations. Pervasive ecosystem transformations occurred in response to warming and associated climatic changes during the last glacial-to-interglacial transition, which was comparable in magnitude to warming projected for the next century under high-emission scenarios. We reviewed 594 published paleoecological records to examine compositional and structural changes in terrestrial vegetation since the last glacial period and to project the magnitudes of ecosystem transformations under alternative future emission scenarios. Our results indicate that terrestrial ecosystems are highly sensitive to temperature change and suggest that, without major reductions in greenhouse gas emissions to the atmosphere, terrestrial ecosystems worldwide are at risk of major transformation, with accompanying disruption of ecosystem services and impacts on biodiversity.
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CL IM ATE C HA NG E
Past and future global transformation
of terrestrial ecosystems under
climate change
Connor Nolan
1
,JonathanT.Overpeck
2,1
,JudyR.M.Allen
3
,PatriciaM.Anderson
4
,
Julio L. Betancourt
5
, Heather A. Binney
6
, Simon Brewer
7
,MarkB.Bush
8
,
Brian M. Chase
9
, Rachid Cheddadi
9
,MortezaDjamali
10
,JohnDodson
11,12
,
Mary E. Edwards
6,13
, William D. Gosling
14,15
, Simon Haberle
16
, Sara C. Hotchkiss
17
,
Brian Huntley
3
,SarahJ.Ivory
18
, A. Peter Kershaw
19
,Soo-HyunKim
17
,
Claudio Latorre
20
, Michelle Leydet
10
, Anne-Marie Lézine
21
, Kam-Biu Liu
22
,
Yao Liu
23
,A.V.Lozhkin
24
, Matt S. McGlone
25
, Robert A. Marchant
26
,
Arata Momohara
27
,PatricioI.Moreno
28
, Stefanie Müller
29
, Bette L. Otto-Bliesner
30
,
Caiming Shen
31
, Janelle Stevenson
32
, Hikaru Takahara
33
,PavelE.Tarasov
29
,
John Tipton
34
, Annie Vincens
35
, Chengyu Weng
36
,QinghaiXu
37
,
Zhuo Zheng
38
,StephenT.Jackson
39,1
*
Impacts of global climate change on terrestrial ecosystems are imperfectly
constrained by ecosystem models and direct observations. Pervasive ecosystem
transformations occurred in response to warming and associated climatic changes
during the last glacial-to-interglacial transition, which was comparable in
magnitude to warming projected for the next century under high-emission
scenarios.We reviewed 594 published paleoecological records to examine compositional
and structural changes in terrestrial vegetation since the last glacial period and
to project the magnitudes of ecosystem transformations under alternative future
emission scenarios. Our results indicate that terrestrial ecosystems are highly
sensitive to temperature change and suggest that, without major reductions in
greenhouse gas emissions to the atmosphere, terrestrial ecosystems worldwide are
at risk of major transformation, with accompanying disruption of ecosystem services
and impacts on biodiversity.
Terrestrial ecosystem function is governed
largelybythecompositionandphysical
structure of vegetation (13), and climate
change impacts on vegetation can potentially
cause disruption of ecosystem services and
loss of biodiversity (4,5). It is critical to assess
the likely extent of ecosystem transformation
as global greenhouse gas (GHG) emissions in-
crease (6) and to understand the full potential
magnitude of impacts should current GHG
emission rates continue unabated.
Ecosystem transformation generally involves
the replacement of dominant plant species or
functional types by others, whether recruited
locally or migrating from afar. Observations from
around the globe indicate that current climate
change may already be driving substantial changes
in vegetation composition and structure (3). Eco-
system change is accelerated by mass mortality
of incumbent dominants (7,8), and widespread
dieback events and other large disturbances are
already under way in many forests and wood-
lands (911), with further mortality events pre-
dicted under increasing temperatures and drought
(3,9,10,12). Replacement of predisturbance
dominants by other species and growth forms
has been widely documented (8,13,14). In addi-
tion, evidence is accumulating for geographic
range shifts in individual species, and climate
change is interacting with invasive species, fire
regimes, land use, and CO
2
increase to drive
vegetation changes in many regions (15,16).
Beyond observations of recent and ongoing
change, models indicate ecosystem transforma-
tion under climate projections for the 21st
century. These include dynamic global veg-
etation models (3,17), species distribution
models (18), and comparison of the multivariate
climate distance between biomes with that be-
tween modern and future climates (19). However,
the capacity for assessing the magnitudes of
ecosystem transformation under future climate
scenarios is limited by the difficulty of evaluat-
ing model performance against empirical records,
particularly when projected climate states are
novel (19,20).
Paleoecological records of past ecological re-
sponses to climate change provide an independent
means for gauging the sensitivity of ecosystems
to climate change. High-precision time-series
studies indicate that local and regional ecosystems
can shift rapidly, within years to decades, under
abruptclimatechange(2123), but sites with
such detailed chronologies are scarce. In this
study, we used published reports to compile
a global network of radiocarbon-dated paleo-
ecological records of terrestrial vegetation com-
position and structure since the Last Glacial
Maximum (LGM), ~21,000 years before the pres-
ent (yr B.P.) (24). Most postglacial warming
happened 16,000 to 10,000 yr B.P., although it
commenced earlier in parts of the Southern
Hemisphere (25,26). Global warming between
the LGM and the early Holocene (10,000 yr B.P.)
was on the order of 4 to 7°C, with more warm-
ing over land than oceans (26,27). These esti-
mates are roughly comparable to the magnitude
of warming that Earth is projected to undergo
in the next 100 to 150 years if GHG emissions
are not reduced substantially (28). The magni-
tudes of changes in vegetation composition and
structuresincethelastglacialperiod(LGP)
provide an index of the magnitude of ecosystem
change that may be expected under warming
of similar magnitude in the coming century
(29). Although the rate of projected future glob-
al warming is at least an order of magnitude
greater than that of the last glacial-to-interglacial
transition (26), a glacial-to-modern compari-
son provides a conservative estimate of the ex-
tent of ecological transformation to which the
planet will be committed under future climate
scenarios.
We reviewed and evaluated paleoecological
(pollen and macrofossil) records from 594 sites
RESEARCH
Nolan et al., Science 361, 920923 (2018) 31 August 2018 1of4
Fig. 1. Vegetation differences between
the LGP and the present. Each square
represents an individual paleoecological
site. The color density indicates the magnitude
of estimated vegetation change since the
LGP (21,000 to 14,000 yr B.P.). Background
shading denotes the estimated temperature
anomaly between the LGM 21,000 years
ago and today on the basis of assimilated
proxy-data and model estimates (27).
(A) Composition. (B) Structure.
on August 30, 2018 http://science.sciencemag.org/Downloaded from
worldwide (fig. S1), all drawn from peer-
reviewed published literature, to determine the
magnitude of postglacial vegetation change.
We adopted an expert-judgment approach in
which paleoecologists with relevant regional ex-
perience compiled published records (table S1);
reviewed the data, diagrams, and accompany-
ing papers; and inferred the composition and
structure of the glacial-age and Holocene veg-
etation at each site (24). For the purposes of
our analyses, we defined the LGP as the interval
between 21,000 and 14,000 yr B.P. Although
postglacial warming was under way in many
regions by 16,000 yr B.P. (25), continental ice
sheets were still extensive 14,000 yr B.P., and
some climate regimes remained essentially
glacialin nature, particularly in the Northern
Hemisphere (30). Extending the LGP window
to 14,000 yr B.P. provides a larger array of rec-
ords for the assessment, both in g lac iated an d
unglaciated terrains, and renders our analysis
more conservative (climatic and vegetation con-
trasts with the Holocene are likely to decrease
between 21,000 and 14,000 yr B.P.).
For each record, experts were asked to clas-
sify the magnitudes of compositional change and
structural change since the LGP as large, mod-
erate, or low and to provide detailed justifica-
tion for their judgments (24) (table S2). This
placed all the diverse records into a common
framework for comparison. For sites that experi-
enced moderate to large ecological change,
experts were also asked to assess the role of
climatechange(large,moderate,ornone)in
driving the observed vegetation change. For
each of these four judgments, experts were asked
to state their level of confidence as high, medium,
or low. In assessing the role of climate change,
experts were asked to focus specifically on wheth-
er climate change since the LGP was sufficient
to drive the observed changes, acknowledging
that other factors (e.g., human activity, postglacial
CO
2
increase, and megafaunal dynamics) may
have also played important roles. For sites with
a long history of human land use, experts used
Holocene records predating widespread land
clearance as a benchmark for comparison with
the LGP records.
Our results indicate that the magnitude of
past glacial-to-interglacial warming was suffi-
cientatmostlocationsacrosstheglobetodrive
changes in vegetation composition that were
moderate (27% of sites) to large (71%), as well as
moderate (28%) to large (67%) structural changes
(Fig. 1 and table S3). These changes were par-
ticularly evident at mid- to high latitudes in the
Northern Hemisphere, as well as in southern
South America, tropical and temperate south-
ern Africa, the Indo-Pacific region, Australia,
Oceania, and New Zealand (Fig. 1A). Com-
positional change at most sites in the Neo-
tropics was moderate to large, but three sites
showed little or no compositional change, all
Nolan et al., Science 361, 920923 (2018) 31 August 2018 2of4
Fig. 2. Estimated temperature differences
for different categories of vegetation
response. Box plots of the estimated
mean annual temperature differences
between the LGM and today in each of
the three vegetation change categories
(low, moderate, and large) for
(A) composition and (B) structure.
Low vegetational changes are associated
largely with relatively small temperature
anomalies, whereas moderate and
large changes are associated
with larger post-LGM temperature
differences, indicating that the
magnitude of temperature change
plays an important role in the
magnitude of vegetation change.
The glacial temperature anomalies
are from data in (27). Analyses using
the TraCE-21ka simulation show
similar patterns (fig. S4).
1
Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA.
2
School for Environment and Sustainability, University of Michigan, Ann Arbor, MI 48109, USA.
3
Department of
Biosciences, University of Durham, Durham DH1 3LE, UK.
4
Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195, USA.
5
National Research Program, U.S.
Geological Survey, Reston, VA 20192, USA.
6
Geography and Environment, University of Southampton, Southampton SO17 1BJ, UK.
7
Department of Geography, University of Utah, Salt
LakeCity,UT84112,USA.
8
Department of Biological Sciences, Florida Institute of Technology, Melbourne, FL 32901, USA.
9
Centre National de la Recherche Scientifique, UMR 5554,
Institut des Sciences de lEvolution de Montpellier, Université Montpellier, Bat. 22, CC061, Place Eugène Bataillon, 34095 Montpellier, France.
10
Aix Marseille Université, Avignon
Université, CNRS, IRD, Institut Méditerranéen de Biodiversité et dEcologie, 13545 Aix-en Provence, France.
11
Palaeontology, Geobiology and Earth Archives Research Centre (PANGEA),
University of New South Wales, Sydney, NSW 2052, Australia.
12
State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences,
Xian 71002, Shaanxi, China.
13
College of Natural Sciences and Mathematics, University of AlaskaFairbanks, Fairbanks, AK 99775, USA.
14
Institute for Biodiversity and Ecosystem
Dynamics, University of Amsterdam, 1090 GE Amsterdam, Netherlands.
15
School of Environment, Earth and Ecosystem Sciences, The Open University, Walton Hall, Milton Keynes MK7
6AA, UK.
16
Department of Archaeology and Natural History, Australian National University, Canberra, Australia.
17
Department of Botany, University of Wisconsin, Madison, WI 53706, USA.
18
Department of Geosciences, Pennsylvania State University, State College, PA 16802, USA.
19
School of Earth, Atmosphere, and Environment, Monash University, Melbourne, VIC 3800,
Australia.
20
Departamento de Ecología, Institute of Ecology and Biodiversity (IEB), Pontificia Universidad Católica de Chile, Santiago, Chile.
21
Sorbonne Université, CNRS-IRD-MNHN,
LOCEAN/IPSL Laboratory, 4 Place Jussieu, 75005 Paris, France.
22
Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, LA 70803, USA.
23
School
of Informatics, Computing, and Cyber Systems, Northern Arizona University, Flagstaff, AZ 86011, USA.
24
North-East Interdisciplinary Scientific Research Institute, Far East Branch
Russian Academy of Sciences, Magadan 685000, Russia.
25
Landca re Research, Lincoln 7640, New Zealand.
26
Department of Environment, York Institute for Tropical Ecosystems, University of York,
York YO10 5NG, UK.
27
Graduate School of Horticulture, Chiba University, Matsudo-shi, Chiba 271-8510, Japan.
28
Departamento de Ciencias Ecológicas, IEB and (CR)2, Universidad de Chile, Santiago,
Chile.
29
Institute of Geological Sciences, Freie Universität Berlin, D-12249 Berlin, Germany.
30
National Center for Atmospheric Research, Climate and Global Dynamics Laboratory, Boulder, CO 80307,
USA.
31
Yunnan Normal University, Key Laboratory of Plateau Lake Ecology and Global Change, Kunming, Yunnan 650092, China.
32
School of Culture, History, and Language, Australian National
University, Canberra, Australia.
33
Graduate School of Life and Environmental Sciences, Kyoto Prefectural University, Kyoto, 606-8522, Japan.
34
Department of Mathematical Sciences, University of
Arkansas, Fayetteville, AR 72701, USA.
35
Centre Européen de Recherche et dEnseignement des Géosciences de lEnvironnement (CEREGE), 13545 Aix-en-Provence, France.
36
School of Ocean and Earth
Science, Tongji University, Shanghai, China.
37
Institute of Nihewan Archaeology and College of Resource and Environmental Sciences, Hebei Normal University, Shijiazhuang 050024, China.
38
School of
Earth Science and Engineering, Guangdong Provincial Key Lab of Geodynamics and Geohazards, Sun Yat-Sen University, Guangzhou 510275, China.
39
Southwest Climate Adaptation Science Center, U.S.
Geological Survey, Tucson, AZ 85721, USA.
*Corresponding author. Email: stjackson@usgs.gov
RESEARCH |REPORT
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with medium to high confidence (fig. S2). Shifts
in vegetation structure were also moderate to
large at mid- to high-latitude sites, although a
few sites showed low change (Fig. 1B). The Neo-
tropics had nine sites with little or no structural
change (Fig. 1B), all with high-confidence assess-
ments (fig. S2). These sites have been occupied
by tropical forest ecosystems since the LGM,
although most have undergone moderate to
large compositional change (31,32). For nearly
all sites that experienced moderate or large eco-
logical change, climate change since the LGP
was judged to be sufficient to explain the ob-
servedchangeswithhighconfidence(tableS4).
Atmospheric CO
2
concentrations also increased
from 190 to 280 parts per million during the
deglaciation, interacting with and in some cases
modulating ecological responses to climate
change. However, CO
2
changes alone cannot
account for postglacial vegetation changes (sup-
plementary text).
Independently of the expert-judgment pro-
cess, we used the estimated anomaly in mean
annual temperatures between the LGM and
the present (preindustrial) as a proxy for the
overall magnitude of climate change since the
LGP (24). LGM temperature estimates were
derived using an assimilated datamodel in-
tegration (27). Low-change sites were largely
concentrated in regions where the estimated
temperature anomaly was relatively small (Fig. 1).
To explore this relationship further, we plotted
the frequency distribution of the difference
between estimated LGM and present-day mean
annual temperatures for individual sites in
each of the three ecological-response categories.
Nearly all sites with low compositional change
between the LGP and today are associated with
small estimated temperature anomalies (median,
2.4°C), whereas sites with moderate to high
compositional change have larger temperature
anomalies (Fig. 2A). Results for structural changes
are similar, although a greater number of sites
with low structural change include larger tem-
perature anomalies (Fig. 2B). This difference
is not surprising, because compositional change
in vegetation can occur without an accompany-
ing change in vegetation structure (Fig. 1). Europe
and eastern North America experienced un-
usually large temperature changes since the
LGM, owing to depressed temperatures near
the large ice sheets, and these regions show
substantial compositional and structural changes
since the LGP. However, results from other parts
of the globe indicate that widespread ecosystem
changesweredrivenbymuchsmallertemper-
ature changes (fig. S3). We repeated our anal-
ysis using the TraCE-21ka model simulations
(33,34), which yield a lower magnitude of LGP-
to-Holocene climate change (35); despite the
potential conservative bias, results for com-
positional and structural changes (fig. S4) were
similar to those in Fig. 2. Temperature dif-
ferences between the LGP and the present were
substantially greater for sites with large eco-
logical change than for those with low to mod-
erate change, by both paleoclimate estimates
(27,33) (table S2).
We also used our database of ecological change
since the LGM to assess the global distribution
of the probabilities of large compositional and
structural changes given GHG emission scenar-
ios [representative concentration pathways (RCPs)
2.6, 4.5, 6.0, and 8.5, each as simulated by the
Community Climate System Model version 4
(CCSM4)] (24,36). The range of LGM-to-present
temperature changes (Fig. 2) overlaps with the
range of temperature changes projected for the
coming century under these scenarios (Fig. 3A
and fig. S5). We quantified the relationship be-
tween temperature and ecological change by
using a logistic spline regression with ordered
categories (37). We fit models for compositional
and structural change by using the temperature
change since the LGM as the independent pre-
dictor variable. In both models, LGM-to-modern
temperaturechangeisasignificantpredictorof
ecosystem change (P< 0.001). We then used
these models to predict the risk of large change
for the future range of projected global temper-
ature changes (Fig. 3B) and to map the prob-
ability of large change under RCP 2.6 and RCP
8.5 (Fig. 3, C to F) at the end of the 21st
Nolan et al., Science 361, 920923 (2018) 31 August 2018 3of4
Fig. 3. Estimated vegetation change under future climate scenarios. (A) Box plots
of the estimated mean annual temperature differences between today and future climate
simulations for individual sites (as determined by using the nearest grid point). Most
sites show relatively small temperature change under the low-emission scenario (RCP 2.6),
with substantially greater change under the high-emission scenario (RCP 8.5). (B) Probabilities
of large changes in vegetation composition and structure as a function of temperature
change. (Cto F) Estimated probabilities of large compositional and structural changes by
the end of the 21st century (the average of the period from 2081 to 2100) under RCP 2.6
(C and E) and RCP 8.5 (D and F). Probabilities (B to F) are estimated from a logistic spline
regression model fit by using LGM-to-modern temperature change as a predictor variable
and observed LGP-to-modern vegetation changes (large versus not large) as the response
variable. Future temperature increases are calculated as an average for 2081 to 2100
under the model scenarios, minus an average for 1985 to 2005 from the CCSM4 historical
simulation. Analyses using the TraCE-21ka simulation show similar patterns (fig. S7).
RESEARCH |REPORT
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century (see fig. S6 for RCP 4.5 and RCP 6.0).
Under RCP 2.6, the probability of large compo-
sitional change is less than 45% over most of
the globe (Fig. 3C) and the probability of large
structural change is generally less than 30%
(Fig. 3E). By contrast, under the business- as-
usual emissions scenario, RCP 8.5, the proba-
bilities of large compositional change and large
structural change are both greater than 60%
(Fig. 3, D and F). Analyses using the TraCE-
21ka model yielded similar patterns (fig. S7).
Our study uses a single variable, mean annual
temperature, as a metric for the broader array
of climatic changes that can drive vegetation
change, and it compares vegetation and climate
states separated by 10,000 to 20,000 years. Fu-
ture climate change, like that in the past, will
be multivariate, involving shifts in seasonal tem-
peratures, seasonal precipitation, climate extremes,
and variability regimes. As mean annual tem-
perature increases, other ecologically important
variables will change, often in complex or counter-
intuitive ways (20,38,39), and ecological responses
will often be episodic or nonlinear (8,1315).
Although the temperature increases since the
LGP provide crude analogs for ongoing and
future climate changesfor example, boundary
conditions and forcings are different now
(26,40,41)our results nevertheless provide
concrete evidence that vegetation composition
and structure are sensitive to changes in mean
annual temperature of the magnitudes forecast
for the coming century and that vegetation
transformations will become increasingly exten-
sive as temperatures increase. Under the RCP
8.5 scenario, the rate of warming will be on the
order of 65 times as high as the average warm-
ing during the last deglaciation (26). Further-
more, the warming between the LGP and the
Holocene occurred within the range of previous
glacial and interglacial temperatures, whereas
projected future changes will exceed those ex-
perienced over the past 2 million years (26).
Although many ecological responses (e.g., species
migration, colonization, and succession) will
likely lag behind climate changes, ecosystem
transformations will often be accelerated by
disturbance and mortality events, land use, and
invasive species (715).
We therefore conclude that terrestrial veg-
etation over the entire planet is at substantial
risk of major compositional and structural
changesintheabsenceofmarkedlyreduced
GHG emissions. Much of this change could
occur during the 21st century, especially where
vegetation disturbance is accelerated or amp-
lified by human impacts (7). Many emerging
ecosystems will be novel in composition, struc-
ture, and function (42), and many will be
ephemeral under sustained climate change;
equilibrium states may not be attained until
the 22nd century or beyond. Compositional
transformation will affect biodiversity via dis-
integration and reorganization of communities,
replacement of dominant or keystone species,
pass-through effects on higher trophic lev-
els, and ripple effects on species interactions (16,43).
Structural transformation will have particularly
large consequences for ecosystem services (4),
including the achievement of nature-based
development solutions under the United Na-
tionsSustainable Development Goals (44).
Structural changes will also influence bio-
diversity, driving alterations in habitats and
resources for species at higher trophic levels.
Compositional and structural changes may
also induce potentially large changes to car-
bon sources and sinks, as well as to atmo-
spheric moisture recycling and other climate
feedbacks. Our results suggest that impacts
on planetary-scale biodiversity, ecological func-
tioning, and ecosystem services will increase
substantially with increasing GHG emissions,
particularly if warming exceeds that projected
by the RCP 2.6 emission scenario (1.5°C).
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ACKNO WLEDG MENTS
The paper benefitted from the thoughtful comments of
S. T. Gray and three anonymous reviewers. Funding:
This research was supported by the NSF (DEB-1241851,
AGS-1243125, and EAR-1304083) and by the Department
of the Interiors Southwest Climate Adaptation Science
Center. Research in northeast Siberia was funded by
the Russian Academy of Sciences, FEB (15-I-2-067),
and the Russian Foundation for Fundamental Research
(15-05-06420). Author contributions: S.T.J., C.N.,
and J.T.O. designed the project; all authors collected
data; C.N. analyzed data with advice from J.T.O., S.T.J.,
S.B.,and J.T.;andS.T.J.,C.N.,andJ.T.O.wrote the
paper with text contributions in the supplementary
materials from B.M.C., M.B.B., M.E.E., J.L.B., B.H., Y.L.,
and S.J.I. and further contributions from all authors.
Competing interests: Theauthorsdeclarenocompeting
interests. Data and materials availability: All data
are available in the main text or the supplementary
materials.
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References (4595)
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27 April 2017; resubmitted 24 April 2018
Accepted 30 July 2018
10.1126/science.aan5360
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Past and future global transformation of terrestrial ecosystems under climate change
Pavel E. Tarasov, John Tipton, Annie Vincens, Chengyu Weng, Qinghai Xu, Zhuo Zheng and Stephen T. Jackson
Momohara, Patricio I. Moreno, Stefanie Müller, Bette L. Otto-Bliesner, Caiming Shen, Janelle Stevenson, Hikaru Takahara,
Michelle Leydet, Anne-Marie Lézine, Kam-Biu Liu, Yao Liu, A. V. Lozhkin, Matt S. McGlone, Robert A. Marchant, Arata
Gosling, Simon Haberle, Sara C. Hotchkiss, Brian Huntley, Sarah J. Ivory, A. Peter Kershaw, Soo-Hyun Kim, Claudio Latorre,
Brewer, Mark B. Bush, Brian M. Chase, Rachid Cheddadi, Morteza Djamali, John Dodson, Mary E. Edwards, William D.
Connor Nolan, Jonathan T. Overpeck, Judy R. M. Allen, Patricia M. Anderson, Julio L. Betancourt, Heather A. Binney, Simon
DOI: 10.1126/science.aan5360
(6405), 920-923.361Science
, this issue p. 920Science
transformation in composition and structure.
(albeit more rapid) scenarios of warming. Without substantial mitigation efforts, terrestrial ecosystems are at risk of major
temperature changes of 4° to 7°C. They went on to estimate the extent of ecosystem changes under current similar
worldwide since the last glacial maximum 21,000 years ago. From this, they determined vegetation responses to
looked at documented vegetational and climatic changes at almost 600 siteset al.remains a challenge to predict. Nolan
Terrestrial ecosystems will be transformed by current anthropogenic change, but the extent of this change
Future predictions from paleoecology
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The 2012-2015 drought has left California with severely reduced snowpack, soil moisture, ground water, and reservoir stocks, but the impact of this estimated millennial-scale event on forest health is unknown. We used airborne laser-guided spectroscopy and satellite-based models to assess losses in canopy water content of California's forests between 2011 and 2015. Approximately 10.6 million ha of forest containing up to 888 million large trees experienced measurable loss in canopy water content during this drought period. Severe canopy water losses of greater than 30% occurred over 1 million ha, affecting up to 58 million large trees. Our measurements exclude forests affected by fire between 2011 and 2015. If drought conditions continue or reoccur, even with temporary reprieves such as El Niño, we predict substantial future forest change.
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Projections of ecosystem and biodiversity change for Africa under climate change diverge widely. More than other continents, Africa has disturbance-driven ecosystems that diversified under low Neogene CO2 levels, in which flammable fire-dependent C4 grasses suppress trees, and mega-herbivore action alters vegetation significantly. An important consequence is metastability of vegetation state, with rapid vegetation switches occurring, some driven by anthropogenic CO2-stimulated release of trees from disturbance control. These have conflicting implications for biodiversity and carbon sequestration relevant for policymakers and land managers. Biodiversity and ecosystem change projections need to account for both disturbance control and direct climate control of vegetation structure and function.
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Although disturbances such as fire and native insects can contribute to natural dynamics of forest health, exceptional droughts, directly and in combination with other disturbance factors, are pushing some temperate forests beyond thresholds of sustainability. Interactions from increasing temperatures, drought, native insects and pathogens, and uncharacteristically severe wildfire are resulting in forest mortality beyond the levels of 20th-century experience. Additional anthropogenic stressors, such as atmospheric pollution and invasive species, further weaken trees in some regions. Although continuing climate change will likely drive many areas of temperate forest toward large-scale transformations, management actions can help ease transitions and minimize losses of socially valued ecosystem services. Copyright © 2015, American Association for the Advancement of Science.
Chapter
Past Assessments The topics assessed in this chapter were last assessed by the IPCC in 2007, principally in WGII AR4 Chapters 3 (Kundzewicz et al., 2007) and 4 (Fischlin et al., 2007), but also in WGII AR4 Sections 1.3.4 and 1.3.5 (Rosenzweig et al., 2007). The WGII AR4 SPM stated "Observational evidence from all continents and most oceans shows that many natural systems are being affected by regional climate changes, particularly temperature increases," though they noted that documentation of observed changes in tropical regions and the Southern Hemisphere was sparse (Rosenzweig et al., 2007). Fischlin et al. (2007) found that 20 to 30% of the plant and animal species that had been assessed to that time were considered to be at increased risk of extinction if the global average temperature increase exceeds 2°C to 3°C above the preindustrial level with medium confidence, and that substantial changes in structure and functioning of terrestrial, marine, and other aquatic ecosystems are very likely under that degree of warming and associated atmospheric CO2 concentration. No time scale was associated with these findings. The carbon stocks in terrestrial ecosystems were considered to be at high risk from climate change and land use change. The report warned that the capacity of ecosystems to adapt naturally to the combined effect of climate change and other stressors is likely to be exceeded if greenhouse gas (GHG) emission continued at or above the then-current rate. 4.2. A Dynamic and Inclusive View of Ecosystems There are three aspects of the contemporary scientific view of ecosystems that are important to know for policy purposes. First, ecosystems usually have imprecise and variable boundaries. They span a wide range of spatial scales, nested within one another, from the whole biosphere, down through its major ecosystem types (biomes), to local and possibly short-lived associations of organisms. Second, the human influence on ecosystems is globally pervasive. Humans are regarded as an integral, rather than separate, part of social-ecological systems (Gunderson and Holling, 2001; Berkes et al., 2003).