Temporal scales of ecosystem change, early warning indicators, regime shifts,
tipping points
Some notes by Kay Emeis
The largest temporal framework of reference with regard to recent ecosystem change is
the climatic transition from the last glacial to the present interglacial, the Holocene, that
started around 10.000 years ago. Major components of that climate transition are
changes in the geographical and seasonal patterns of insolation and melting of inland
ice, both causing the modern coastal zone to develop where it is now (the global sea
level rose by 120-130 m). Considering that many ecosystem processes are slow
(building of soils, filling of estuaries, delta building, reef buildup, it is relevant that
global sea level did not come to rest until approximately 6000 years ago, the birth of the
present-day coastal zone. That implies that many coastal features (sand bodies, deltas
and estuaries) have not reached equilibrium with regard to eustatic subsidence – the
Baltic Sea, for example, has rapidly rising northern shores and slowly subsiding
southern shores, which account for distinct coastal morphologies.
Main effects on global climate and ecosystem boundary conditions after the
deglaciation include changes in sea level (until appr. 6000 years ago), which effectively
created many of the nearshore ecosystems, and changes of net annual and seasonal
insolation patterns due to changes in Earth´s orbit around the sun. Other influences,
such as solar and volcanic activity, were of subordinate importance on the time scale of
several millennia. The modelled global effects of changing insolation were significantly
warmed sea surface temperatures in latitudes >30° N and >60°S, and dramatically
shifted vegetation patterns on land.
Figure 1: Global Eustatic sea level change since the last glaciation. Source: Ruddiman
(2004)(need to check the reference)
Figure 2: Synthesis of external (solar insolation) and internal changes (CO2
concentration, sea level for the last glacial-interglacial transition (Source: Ruddiman,
2004) (need to check the reference)
The inherently smooth changes in global insolation, however, are not imaged in
matchingly smooth regional (and global) paleoclimate archives. Rapid climate changes,
usually of a pattern with cooling poles and drier/wetter tropics, of global extent are
noted between 9000-8000, 6000-5000, 4200-3800, 3200-2500, 1200-1000, and since
600 calendar years BP ((Mayewski et al., 2004)). Most of these are attributed to
contractions of the low-latitude atmospheric circulation system, and associated changes
in moisture transport. Of all possible forcing mechanisms, Mayewski et al. (2004)
identified solar variability as the most likely modulator of insolation, and thus as the
underlying cause for rapid climate change.
Figure 3: General climate evolution in Europe over the last 10000 years (after
Schönwiese)
On shorter time scales of several centuries, climate has not been stable.
In a transient climate simulation of the last 500 years with a coupled atmosphere-ocean model
driven by estimated solar variability, volcanic activity and atmospheric concentrations of
greenhouse gases for the last centuries, Zorita et al. (2003) simulated a climate colder than
present conditions almost globally, and the degree of cooling was larger than most empirical
reconstructions (Figure 4)
.
The model simulates two clear minima of the global mean temperature around 1700 A.D. (the
Late Maunder Minimum) and around 1820 A.D. (the Dalton Minimum). The temperature trends
simulated after the recovery from these minima are as large as the observed warming in the 20th
century.
Hegerl et al., 2007 found that natural forcing, particularly by volcanism, explains a substantial
fraction of decadal variance in their reconstructions.
Additionally, most climate simulations show clear signals of decadal oscillations in sea surface
temperatures and various oceanographic parameters. At the temporal resolution of decades, such
decadal climate on ocean basin space scales variability comes into play. These are large-scale
atmospheric patterns of variability, and include such important features as the Pacific Decadal
Oscillation, the Arctic Oscillation and associated North Atlantic Oscillation, and the …..
Although the regionally integrated atmospheric temperature change associated with these
decadal oscillations is of order of tenths of degrees Celsius, the associated changes in wind
fields and oceanic currents impacts significantly on the physics, chemistry, and biology of
coastal areas (see figure 5: SST North Sea and Baltic Sea; Source: Alheit)
Figure 5: Winter season averages of North Atlantic Oscillation Index (NAO) and
average seas surface temperatures in the North Sea and Baltic Sea. The winter
temperature has a primer role in the development of, for example, zooplankton in the
subsequent spring and summer (Source: Alheit).
Regime shift, thresholds and early warning indicators
Regime shifts are transitions from one quasi-stationary state of a system persisting for
several years and characterised by low-frequency variability to another stable state with a
transition period of one or two years. The availability of data that cover sufficiently long
time periods severely limits the unequivocal recognition of regime shifts in the past.
However, both theoretical and empirical observations suggest that increased variance of
one of several of the variables characterising ecosystems serves as an early warning
indicator for an impending threshold. Brock, W. A., and S. R. Carpenter. 2006.
Variance as a leading indicator of regime shift in ecosystem services. Ecology and
Society 11(2): 9. [online] URL: http://www.ecologyandsociety.org/vol11/iss2/art9/
http://www.resalliance.org/183.php: A threshold is defined here as a critical point
separating alternate regimes in ecological or social-ecological systems (ie a boundary
between the two regimes). When a threshold along a controlling variable in a system is
passed, the nature and extent of feedbacks change, such that there is a change in the
direction in which the system moves. A shift occurs when internal processes of the
system (rates of birth, mortality, growth, consumption, decomposition, leaching, etc.)
have changed such that the variables that define the state of the system begin to change
in a different direction, towards a different attractor. In some cases, crossing the
threshold brings about a sudden, large and dramatic change in the responding variables,
whilst in other cases the response in the state variables is continuous and more gradual.
Figure 6: Cartoon of concepts in (eco-)system states with quasi-stable regimes separated
by “energy” barriers
Although there are suggestions that regime shifts can be triggered by human action,
most reported regime shifts appear to be linked to underlying climatic changes.
A good example of a regime shift is an analysis of long-term monitoring data (1975 to
2004) in the German Bight (SE North Sea) by Schüter et al. (2002), and area strongly
influenced by land-ocean interactions via large river discharges. Using principal
component (PC) analysis, Schlueter et al. found that SST, air temperature, SST winter and
herring showed the highest positive correlation to the major mode of variability (first PC),
whilst phosphate, ammonium and some fish (cod and saithe) showed the highest negative
correlation with the first PC. A pronounced positive correlation was also found for the Gulf
Stream index, Secchi depth and salinity. This suggests that in the last decade, the German
Bight has been characterised by clearer and more marine waters. Changes in some
ecosystem variables (plankton and fish) are associated with changes in the hydroclimatic
forcing. However, not all biological variables showed pronounced shifts. Diatoms and C.
helgolandicus, for instance, did not show high correlation with the major mode of
variability (first PC), suggesting that the lower trophic levels of the German Bight
ecosystem are remarkably resilient. In fact, specific analyses of the data divided into three
different subsets (biological, climatic and chemical) characterise the climate of the German
Bight as highly dynamic also on short timescales (a few years) as compared to much
smoother biological and chemical components.
Sea Level Rise
The coastal zone in the sense of LOCZ (-200 m to +200m) has in the geological past
been shaped by global (eustatic) sea level variations and associated cycles of
transgressions and regressions. Reconstructions of (eustatic) sea level change over the
last few hundred thousand years suggest that the present sea level is near the maximum
sea level during the last 450.000 years.
Isostatic (regional) sea level varies widely depending on specific geological situations in
present-day coastal zones, but there is now unequivocal evidence for a small, but
crucial, eustatic sea level rise caused maily by thermal expansion of sea water (IPCC,
2007). Projections of sea level evolution until 2100 AD predict an increase of between
20 cm and 50 cm in global mean sea level, without (expected) regional effects of
increased storm surge height or frequency (IPCC, 2007).
Source: IPCC 2007
Aside from the consequences for coastal protection, the increase does not seem to be
dramatic, but may be expected to have severe consequences for extant tidal flats;
whether situated in estuaries, deltas, or on tidal coasts, these play important roles as
specific coastal habitats, biodiversity hot spots, and for mitigation of nutrient fluxes
from land to sea in the coastal zone.
In the geological past, the coastal zone was effectively a ramp that accommodated
rising/falling sea level by landward/seaward displacement of individual sedimentary and
coastal regimes. That degree of spatial freedom is effectively halved when the coast is
enforced by dikes (“habitat squeeze”). In the narrowing strip between an advancing sea
and a fixed coastline, the energy in the water increases and results in a general
coarsening of sediments: Fine particles and organic matter are less effectively retained,
and are instead advected to adjacent deeper water. Among other effects, this causes the
trophic state of tidal flat areas to decrease (more info on this in the Chesapeake Bay
Report distributed by Bill Dennison).