Download The impacts of climate change on terrestrial Earth surface systems

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PUBLISHED ONLINE: 14 OCTOBER 2012 | DOI: 10.1038/NCLIMATE1660
The impacts of climate change on terrestrial Earth
surface systems
Jasper Knight1* and Stephan Harrison2
National and international policy initiatives have focused on reducing carbon emissions as a means by which to limit future
climate warming. Much less attention has been paid by policymakers to monitoring, modelling and managing the impacts of
climate change on the dynamics of Earth surface systems, including glaciers, rivers, mountains and coasts. This is a critical omission, however, as Earth surface systems provide water and soil resources, sustain ecosystem services and strongly influence
biogeochemical climate feedbacks in ways that are as yet uncertain. We argue that there is a significant policy gap regarding
the management of Earth surface systems’ impacts under climate change that needs to be closed to facilitate the sustainability
of cross-national Earth surface resource use. It is also a significant challenge to the scientific community to better understand
Earth surface systems’ sensitivity to climate forcing.
T
he Intergovernmental Panel on Climate Change (IPCC)
showed in its fourth assessment report that different biophysical environments are likely to exhibit different responses to
ongoing climate change1. Generic properties of these environmental responses include spatial and temporal variability, nonlinearity,
feedbacks involving different biogeochemical processes and time
lags. The context for understanding these environmental responses
is brought into focus by many recent studies showing that, irrespective of future greenhouse-gas emissions, a mean surface temperature increase of 2 °C (over 1990 levels) is inevitable, and an
increase of 4 °C or more is not unlikely by 2100 (refs 2,3). Biosphere
responses to global climate change are relatively well understood.
Studies show that climate change alters biospheric systems’ internal dynamics and interconnectedness, for example, between plant
phenology and insect behaviour 4,5. Biome and species’ range shifts,
both laterally and by elevation, are now taking place in response to
climate forcing 6,7. The importance of such range shifts for ecosystem
structure and function, biodiversity, endemism and gene flow has
been widely recognized7, and there is much policy debate on management of biosphere responses to climate change8,9.
By contrast, Earth surface (or geomorphological) system
responses to climate change are poorly understood. Earth surface
systems in terrestrial environments tend to work over longer timescales and be more strongly affected by antecedent conditions and
nonlinearity than their biological counterparts10,11. Observations
of key variables that denote the response of Earth surface systems
to climate forcing — such as river discharge, permafrost temperature and slope mass movements — are generally of short (decadal)
duration and measured from specific field sites often using different methodologies12. These observations are also difficult to scale up,
temporally or spatially, in such a way that they are compatible with
analysis of satellite imagery and output from geophysical models and
global climate models (GCMs). For these reasons, and the false perception that Earth surface systems are not significantly vulnerable to
climate change, international discussion and policy has not focused
on understanding and managing climate impacts on these systems.
Here we outline the reasons why Earth surface system processes
and their feedbacks to the climate system and biosphere on different scales are a critical element of global responses to ongoing climate forcing, suggest the best tools to identify and predict changes
in Earth surface systems, and show why the challenges posed by
these processes and feedbacks require a considered and concerted
scientific and climate policy response.
Earth surface systems and their climate feedbacks
The roles of nonlinearity and hysteresis in Earth surface system
responses to climate forcing are difficult to parameterize in GCMs,
but are an emergent property of Earth systems’ behaviour. Such
behaviour can be validated through two different methodological
approaches. First, analytical techniques developed in the fields of
nonlinear dynamics and chaos theory show that time series exhibit
both bifurcations (emergence of bimodal or multimodal behaviour) and nonlinear forcing–response relationships that highlight the importance of hysteresis (system memory)13,14. Second,
much field morphostratigraphic and dating evidence from many
different Earth surface systems shows that systems’ responses to
climate forcing are strongly conditioned by antecedence and sediment supply, which cannot be accounted for by a linear forcing–
response model12,15. For example, studies in the Canadian Rockies
and Southern Alps show a mismatch between climate forcing (by
temperature and/or precipitation) and mass movement events16,17.
Where a linear forcing–response relationship is shown to exist, it is
present only over certain spatial and temporal scales and in certain
regions, and any critical precipitation threshold for mass movement
events, for example, are only locally applicable and cannot be taken
as a universal constant 15,18.
The dynamics of Earth surface systems are also an integral component of global climate systems through their role in the carbon
cycle. Land-surface weathering and cation transport to the global
oceans19 drive the oceanic uptake of carbon dioxide, and are known
to have regulated carbon-cycle dynamics over long (106–107 years)
timescales20. Although GCMs integrate grid-scale biophysical processes across land and ocean surfaces, including carbon cycling,
the physical processes of catchment- to continental-scale weathering 21, and cation release and transport (including its volume) are
very poorly known, from both modelling and field studies22. A critical clue to these interactions, however, comes from Quaternaryto Holocene-age marine cores recovered adjacent to large river
mouths, where Al3+, Fe2+, Si2+ and other oceanward cation fluxes can
be explicitly linked to climate-induced weathering and/or tectonic
School of Geography, Archaeology and Environmental Studies, University of the Witwatersrand, Private Bag 3, Wits 2050, South Africa, 2College of Life
and Environmental Sciences, University of Exeter, Penryn, TR10 9EZ, UK. *e-mail: [email protected]
1
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NATURE CLIMATE CHANGE DOI: 10.1038/NCLIMATE1660
uplift of mountain belts inland23–25. This offshore evidence can also
be matched with sedimentological, geochemical and dating evidence from coastal sand dunes and estuaries into which fluvially
transported and wave-reworked sediments were deposited25,26. This
offshore and onshore evidence illustrates past tectonic–climate–
ocean geochemical and sediment system interlinks over millennial
and shorter timescales that are relevant to climate modelling, and in
the absence of significant anthropogenic forcing 27.
Climate sensitivity and uncertainty in model responses. GCMs
poorly represent feedbacks associated with Earth surface processes
where the physics is not well known. As such, although biogeochemical climate feedbacks can be parameterized within GCMs21,
there are few field data on other feedbacks that are available at scales
appropriate for model calibration. This remains the case despite
the fact that changes in biome structure and composition have a
direct impact on both geochemical (soil carbon storage and fluxes,
chemical translocation within soils and groundwater) and physical
properties (soil structure, soil erosion, mass movements, river sediment yield) of Earth surface systems12,28. Hitherto, field studies have
focused on Earth surface system responses to anthropogenic landuse change, with relatively little emphasis on the impacts of climate
change. As a result, the future dynamics of Earth surface systems as
a downstream response to climate-induced biome shifts are largely
unknown, and this must been seen as an issue of future policy relevance. It is likely, however, that increased aridity across many continental areas will increase land surface vulnerability to soil erosion
and loss, mass movements and associated geohazards29. In turn, this
will decrease the viability of agriculture in climatically marginal
areas, leading to a cycle of environmental deterioration that will be
continental in scale29,30. This interplay between ecological and physical processes under ongoing climate change is poorly understood
both from field trials and modelling, but such biomorphodynamic
studies represent an emerging area of interdisciplinary research31.
Understanding of these biomorphodynamic interrelationships is
critical for land-surface stability and geohazard management under
anthropogenic climate forcing, and for the sustainability of food,
water and ecological services32.
A measure of Earth system responses to climate forcing is that of
climate sensitivity, which refers to the equilibrium surface temperature response to doubled levels of pre-industrial atmospheric carbon
dioxide33. Estimates of climate sensitivity are an output from GCMs
and have a wide range of values, with median values reported by the
IPCC in the range 2.0–4.5 °C (ref. 33). Variations in climate sensitivity estimates broadly reflect the extent to which cloud physics and
carbon feedbacks, mainly through land-surface vegetation and soils,
are parameterized in the models34. It has been argued that climate
sensitivity is also a measure of biosphere sensitivity to forcing 35. The
proportionality between radiative forcing by atmospheric carbon
dioxide and climate sensitivity assumes that other atmospheric and
ocean feedbacks remain constant 34. As a result, this Charney climate
sensitivity, which includes ocean but not land-surface feedbacks, is
erroneously viewed as referring to equilibrium sensitivity alone36.
However, although Charney sensitivity may be appropriate over
some spatial and temporal scales, different parts of the Earth system (including different biomes and Earth surface process domains)
show highly variable and nonlinear responses to forcing by changes
in temperature, precipitation and other climatic variables on longer
timescales that are hidden when only their short-term net effect on
radiative forcing is considered37,38. This means that climate sensitivity is not a useful measure of the responses of individual Earth
surface systems to radiative forcing, for which alternative measures
are needed.
To calculate climate sensitivity in the absence of human activity
and under different climatic conditions, thereby validating the
robustness of sensitivity estimates, a number of studies have used
palaeoclimate data (for example, during the Last Glacial Maximum)
and considered that such calculations are meaningful when it comes
to considering the climate sensitivity of a globally warming world39,40.
However, it is likely that the very significant differences in carbon
dioxide values, albedo, ocean circulation, land-surface primary productivity and balance of carbon storage between the Last Glacial
Maximum and globally warming worlds mean that such comparisons of climate sensitivity cannot be usefully made. Estimates from
warm periods of the geological past, such as the Palaeocene–Eocene
Thermal Maximum, the Pliocene and Marine Oxygen Isotope
Stage 11 (~400 thousand years ago), are more suitable comparators
to the present day 41,42. Although climate sensitivity, by definition,
focuses on temperature conditions, hydrological cycle dynamics are
also important given their role in tropospheric radiative forcing and
through biosphere and Earth surface system feedbacks, which tend
to amplify temperature responses38. It is clear therefore that climate
sensitivity should consider in more detail such feedbacks and interactions over temporal and spatial scales that are appropriate to the
operation of different Earth surface system processes43–45, rather than
focus on Charney sensitivity alone. Such Earth-system sensitivity
is a better metric of climate forcing–response when variables such
as Earth surface systems, ice-sheet extent and vegetation cover are
allowed to vary. We now discuss how studies of Earth surface systems can be used to identify geomorphology sensitivity, a subset of
Earth-system sensitivity.
Defining geomorphological sensitivity. The concept of geomorphological sensitivity is a useful measure by which to consider
landscape-scale responses to climate forcing 46. It has the advantage over the narrow concept of climate sensitivity because it
measures the net and aggregated response to forcing of the entire
suite of geological and geomorphological processes that contribute to climate feedbacks and operates over spatial and temporal
scales longer than those over which Charney sensitivity is based.
Calculating geomorphological sensitivity does not require a priori
knowledge of the internal dynamics of Earth surface systems or
their scales of operation, feedbacks and time lags. As such, it is not
contingent on long time series of field observations of geomorphological systems, as is required to more accurately estimate climate
sensitivity 43,44. Geomorphological sensitivity can be defined in one
of two ways — as the equilibrium response to doubled carbon
dioxide of specific ‘types’ of Earth surface system such as glaciers
or rivers47,48, or as the equilibrium response of all those Earth surface systems that are present in one geographical area, as can be
demarcated by a watershed49. The first definition can be used to
establish the characteristic length scale of climate response of different physical environments and is the best definition for modelling and dating purposes14. The second definition describes the net
and aggregated response of slope and fluvial processes within river
basins, and is a more appropriate definition when considering the
implications of climate change on scales applicable to monitoring
and management 50,51.
Geomorphological sensitivity is a reflection of land-surface
instability, which is a transient response of the land surface to environmental disturbance and which, on a global scale, is driven principally by climate change over medium timescales (103–104 years)
with tectonics and other geological factors operating at longer timescales52. The physical manifestation of land-surface instability is the
dynamic behaviour (erosion, transport, deposition) of sediment
systems that are demarcated by systems’ boundaries such as river
watersheds or coastal sediment cells, and may include sediments
derived by weathering, volcanic, glacial, fluvial or other processes.
Sediment dynamics can be fairly accurately measured in the field
through gauged networks, using satellite imagery and LiDAR53,54, by
numerical modelling 55,56 and by radiometric dating 50. This diverse
and technologically informed toolbox means that sediment-system
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NATURE CLIMATE CHANGE DOI: 10.1038/NCLIMATE1660
a
b
Standardized
precipitation
–35.8
–35.6
–35.4
–35.2
–35.0
–34.8
–34.6
–34.4
18O per mil
3
2
1
0
–1
–2
–3
c
Avalanches
Rockfalls
1.5
1.0
0.5
0.0
Rockfall ‘rates’
d
Landslides
e
Floods
Iceland sea-ice cover
(weeks per year)
2 observations
30
20
10
0
1550
1600
1650
1700
1750
1800
1850
Years (AD)
Figure 1 | Variations in climate forcing and Earth surface system
responses during the period ad 1550–1850 in Europe, including the Little
Ice Age. a, Reconstructed central and eastern European precipitation
anomalies (blue line, left axis), displayed relative to standardized values.
Dashed line indicates average values89. b, Palaeotemperature δ18O per mil
proxy, from the Greenland Ice Core Project ice-core record90, expressed as
decadal averages (red line, right axis; note inverted scale). c, Incidents of
avalanches, rockfalls, landslides and floods (black bars) per decade from
documentary records from Nordfjord, north-central Norway67. d, Rockfall
‘rates’ per decade from 1600s to 1840s from the Swiss Alps calculated
based on number of tree growth disturbances91. e, Average number of
weeks per year with sea ice around Iceland, averaged per 20-year period
from 1560s onwards (blue bars)92.
dynamics on different spatial scales and thus geomorphological
sensitivity can be interrogated using independent methodologies.
Some international initiatives, such as the PAGES 2k Network
(www.pages-igbp.org/workinggroups/2k-network), are focused on
evaluating such regional responses to climate forcing, which may be
a suitable spatial scale over which monitoring and management of
Earth surface systems can take place.
Evaluating geomorphological sensitivity
At present, monitoring and modelling of river sediment systems
offers the best hope of estimating geomorphological sensitivity 57,58.
Land-surface weathering and catchment erosion in response to climate forcing contributes sediment to fertile river floodplains and
coastal lowlands. Several studies from active mountain belts worldwide show that episodes of tectonic uplift result in renewed upland
incision, formation of mountain valley-fills, floodplain aggradation and shelf-sediment systems tracts59. In tectonically quiescent
areas, Quaternary–Holocene slope and valley terrace sequences
26
also show that river sediment supply and transport systems respond
to sub-Milankovitch-scale climate forcing 61. The geomorphological sensitivity of river systems can be estimated using their morphosedimentary and morphometric properties, including channel
patterns and sediment load, and changes in these properties over
time60. Geomorphological, sedimentary and radiometric dating can
be used in combination to quantify river responses to climate forcing over decadal to millennial timescales50,51. This approach can help
evaluate the responsiveness of geomorphological systems to climate
forcing on the longer timescales that are required for these systems
to attain quasi-equilibrium13,50.
Radiometric dating can also be used to identify the timing of
specific morphosedimentary fluvial forcing–response sequences. In
turn this can identify the spatial and temporal scales of operation
of the river system, its associated time lags and nonlinearities, and
estimates of sediment flux volumes in response to forcing 49,50. The
advantage of such a dated sediment-flux approach to understanding
a river system’s responses and feedbacks to climate forcing is that
measures of the aggregated, net response of the entire catchment
to forcing do not require detailed knowledge of the fluvial system’s
component parts50. This means that the sediment-flux approach is
useful on regional (basinal) scales and can be based on different
geophysical, morphosedimentary and dating evidence, and monitoring and modelling data61.
Studies of floodplain aggradation over historical times62,63 show
that fluvial sediment fluxes are highly sensitive to non-climatic
forcings including human-induced land-use changes50,64, which
can cause identifiable morphosedimentary basin-scale responses
within 2–5 years of forcing 62. We argue that the response of fluvial
and other Earth surface systems to ongoing climate change will be
similarly evident. We believe that more research on Earth surface
system responses to past climatic change will give us insights into
their future impacts and adaptation strategies. For instance, climate
cooling during the Little Ice Age in Europe (~ad 1550–1850) had
significant impacts on the sediment yields of mountain, fluvial and
slope systems65,66, particularly in marginal regions already predisposed to be climatically sensitive to changes in temperature and precipitation patterns, including their seasonality 66. In these examples,
such as in north Norway, Iceland, Greenland and the Swiss Alps,
significant changes in human activity followed a period of environmental change that was caused by climatic deterioration66,67 (Fig. 1).
Temperature and precipitation changes at this time resulted in complex nonlinear and time-lagged responses of different Earth surface
systems66. We argue that present conditions of climate change as exacerbated by human activity, which are already causing environmental
change worldwide, will result in similar human responses, which
may include land abandonment and island nation loss, large-scale
population migration and wholesale change in cultural practices68,69.
Set against this uniformitarian approach in the use of
morphosedimentary records to evaluate Earth surface system
responses to past climate forcing is the role of human activity in
contemporary Earth surface system modification. This includes
changes in land use and agriculture, river engineering and construction/mining 70,71 that have resulted in an exponential increase
in moved sediment volume during the Anthropocene70,72. Such
human intervention in Earth surface system dynamics suggests that
geomorphological sensitivity cannot be estimated uncritically from
modern analogues. Rather, morphosedimentary records from the
Little Ice Age and other periods of past rapid climate change may
yield better estimates of geomorphological sensitivity.
Policy gaps in the monitoring of climate change
Most current national and international strategies for monitoring climate change focus on measuring changes in climatic variables (temperature, precipitation) and terrestrial ecosystems using
in situ and remote-sensing techniques73. In mountain regions,
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NATURE CLIMATE CHANGE DOI: 10.1038/NCLIMATE1660
climatically triggered geohazards such as avalanches, rockfalls and
debris torrents are also routinely monitored74,75. The widespread
focus on terrestrial ecosystems comes about in particular because
these systems often change over seasonal to decadal time frames
that coincide with those of research funding and policy initiatives.
Furthermore, data from such studies can successfully inform on the
efficacy of biodiversity and ecosystem management policies, and
feed into development of future mitigation policy. Such ecological
data have also been used to develop gene flow, migration, succession and climax community models for ecosystem evolution under
climate change73,76. Many ecosystems worldwide are experiencing
measurable rates of change as a consequence of the combination of
climate change and human disturbance77.
By contrast, many Earth surface systems are not routinely monitored for their responses to climate change and human disturbance,
except where they impact on geohazards, river management and
coastal erosion. This is a critical policy omission at the national and
international scale, because Earth surface system dynamics contextualize and inform the overall landscape response to forcing, including that of the biosphere, and operate on transnational spatial scales
that are relevant for sustainable resource management 32,78. We argue
that such routine monitoring in areas with low human impact is
necessary for building up long-term data sets to monitor the effects
of ongoing change, to validate remote sensing-based observations
and to calculate geomorphological sensitivity, which is required for
targeted management strategies. Many mountain and Arctic sites
in which monitoring has taken place are small scale79 and may not
be representative of larger systems in which time-lag effects and
tectonic forcing are more significant controls on sediment morphodynamics80. As a result, present estimates of geomorphological
sensitivity and sedimentary system responses to forcing are probably
not representative of environments where climate changes have the
greatest potential to impact on agricultural sustainability, such as
semi-arid areas.
The failure of climate change mitigation through emissions
reductions and trading. At present, governments’ attempts to limit
greenhouse-gas emissions through carbon cap-and-trade schemes
and to promote renewable and sustainable energy sources are probably too late to arrest the inevitable trend of global warming 2,81.
Instead, there are increasingly persuasive arguments that government and institutional focus should be on developing adaption policies that address and help mitigate against the negative outcomes of
global warming, rather than carbon trading and cataloguing greenhouse-gas emissions. There are a number of reasons why we take this
viewpoint. (1) Earth-system feedbacks are an important component
of climate forcing, and therefore addressing greenhouse-gas emissions alone is an insufficient strategy for managing global warming.
(2) Different national and disciplinary (that is, from the perspective
of soils, forests, oceans and so on) schemes for calculating carbon
budgets use different methodologies and assumptions, and have a
different mix of natural and disturbed ecosystems and fossil-fuel
resources82–84. This means that carbon management schemes are
based on poorly constrained data and on budgetary sleights-ofhand that may have little relationship to the real world. (3) The
future impacts of global warming on land-surface stability and the
sediment fluxes associated with soil erosion, river downcutting and
coastal erosion are relevant to sustainability, biodiversity and food
security. Monitoring and modelling soil erosion loss, for example,
are also means by which to examine problems of carbon and nutrient fluxes, lake eutrophication, pollutant and coliform dispersal,
river siltation and other issues85. An Earth-systems approach can
actively inform on these cognate areas of environmental policy and
planning 86. (4) Earth surface systems’ sensitivity to climate forcing
is still poorly understood. Measuring this geomorphological sensitivity will identify those systems and environments that are most
vulnerable to climatic disturbance, and will enable policymakers
and managers to prioritize action in these areas. This is particularly
the case in coastal environments, where rocky and sandy coastlines
will yield very different responses to climate forcing, and where
coastal-zone management plans are usually based on past rather
than future climatic patterns87.
We note that a recent IPCC special report on extreme events
and disasters32 and the forthcoming fifth assessment report, due
2013–2014, include more explicit statements of the role of Earth
surface systems in responding to and influencing climate forcing 88.
However, monitoring of the response of these systems to climate
forcing requires decadal-scale data sets of instrumented basins and
under different climatic regimes worldwide. This will require a considerable international science effort as well as commitment from
national governments.
Received 10 April 2012; accepted 11 July 2012; published online
14 October 2012
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Acknowledgements
We thank B. Murray for comments.
Additional information
Reprints and permissions information is available online at www.nature.com/reprints.
Correspondence and requests for materials should be addressed to J.K.
Competing financial interests
The authors declare no competing financial interests.
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