506 research outputs found
State-dependence of climate sensitivity: attractor constraints and palaeoclimate regimes
Equilibrium climate sensitivity (ECS) is a key predictor of climate change.
However, it is not very well constrained, either by climate models or by
observational data. The reasons for this include strong internal variability
and forcing on many time scales. In practise this means that the 'equilibrium'
will only be relative to fixing the slow feedback processes before comparing
palaeoclimate sensitivity estimates with estimates from model simulations. In
addition, information from the late Pleistocene ice age cycles indicates that
the climate cycles between cold and warm regimes, and the climate sensitivity
varies considerably between regime because of fast feedback processes changing
relative strength and time scales over one cycle.
In this paper we consider climate sensitivity for quite general climate
dynamics. Using a conceptual Earth system model of Gildor and Tziperman (2001)
(with Milankovich forcing and dynamical ocean biogeochemistry) we explore
various ways of quantifying the state-dependence of climate sensitivity from
unperturbed and perturbed model time series. Even without considering any
perturbations, we suggest that climate sensitivity can be usefully thought of
as a distribution that quantifies variability within the 'climate attractor'
and where there is a strong dependence on climate state and more specificially
on the 'climate regime' where fast processes are approximately in equilibrium.
We also consider perturbations by instantaneous doubling of CO and
similarly find a strong dependence on the climate state using our approach.Comment: 32 pages, 10 figure
Extreme sensitivity and climate tipping points
A climate state close to a tipping point will have a degenerate linear
response to perturbations, which can be associated with extreme values of the
equilibrium climate sensitivity (ECS). In this paper we contrast linearized
(`instantaneous') with fully nonlinear geometric (`two-point') notions of ECS,
in both presence and absence of tipping points. For a stochastic energy balance
model of the global mean surface temperature with two stable regimes, we
confirm that tipping events cause the appearance of extremes in both notions of
ECS. Moreover, multiple regimes with different mean sensitivities are visible
in the two-point ECS. We confirm some of our findings in a physics-based
multi-box model of the climate system.Comment: 11 figure
Response maxima in modulated turbulence
Isotropic and homogeneous turbulence driven by an energy input modulated in
time is studied within a variable range mean-field theory. The response of the
system, observed in the second order moment of the large-scale velocity
difference D(L,t)=>~Re(t)^2$, is calculated for varying
modulation frequencies w and weak modulation amplitudes. For low frequencies
the system follows the modulation of the driving with almost constant
amplitude, whereas for higher driving frequencies the amplitude of the response
decreases on average 1/w. In addition, at certain frequencies the amplitude of
the response either almost vanishes or is strongly enhanced. These frequencies
are connected with the frequency scale of the energy cascade and multiples
thereof.Comment: 11 pages, 6 figure
The Mid-Pleistocene Transition induced by delayed feedback and bistability
The Mid-Pleistocene Transition, the shift from 41 kyr to 100 kyr
glacial-interglacial cycles that occurred roughly 1 Myr ago, is often
considered as a change in internal climate dynamics. Here we revisit the model
of Quaternary climate dynamics that was proposed by Saltzman and Maasch (1988).
We show that it is quantitatively similar to a scalar equation for the ice
dynamics only when combining the remaining components into a single delayed
feedback term. The delay is the sum of the internal times scales of ocean
transport and ice sheet dynamics, which is on the order of 10 kyr. We find
that, in the absence of astronomical forcing, the delayed feedback leads to
bistable behaviour, where stable large-amplitude oscillations of ice volume and
an equilibrium coexist over a large range of values for the delay. We then
apply astronomical forcing. We perform a systematic study to show how the
system response depends on the forcing amplitude. We find that over a wide
range of forcing amplitudes the forcing leads to a switch from small-scale
oscillations of 41 kyr to large-amplitude oscillations of roughly 100 kyr
without any change of other parameters. The transition in the forced model
consistently occurs near the time of the Mid-Pleistocene Transition as observed
in data records. This provides evidence that the MPT could have been primarily
a forcing-induced switch between attractors of the internal dynamics. Small
additional random disturbances make the forcing-induced transition near 800 kyr
BP even more robust. We also find that the forced system forgets its initial
history during the small-scale oscillations, in particular, nearby initial
conditions converge prior to transitioning. In contrast to this, in the regime
of large-amplitude oscillations, the oscillation phase is very sensitive to
random perturbations, which has a strong effect on the timing of the
deglaciation events
Vulnerabilities in first-generation RFID-enabled credit cards
Credit cards ; Radio frequency identification systems
Three-phase coexistence with sequence partitioning in symmetric random block copolymers
We inquire about the possible coexistence of macroscopic and microstructured
phases in random Q-block copolymers built of incompatible monomer types A and B
with equal average concentrations. In our microscopic model, one block
comprises M identical monomers. The block-type sequence distribution is
Markovian and characterized by the correlation \lambda. Upon increasing the
incompatibility \chi\ (by decreasing temperature) in the disordered state, the
known ordered phases form: for \lambda\ > \lambda_c, two coexisting macroscopic
A- and B-rich phases, for \lambda\ < \lambda_c, a microstructured (lamellar)
phase with wave number k(\lambda). In addition, we find a fourth region in the
\lambda-\chi\ plane where these three phases coexist, with different,
non-Markovian sequence distributions (fractionation). Fractionation is revealed
by our analytically derived multiphase free energy, which explicitly accounts
for the exchange of individual sequences between the coexisting phases. The
three-phase region is reached, either, from the macroscopic phases, via a third
lamellar phase that is rich in alternating sequences, or, starting from the
lamellar state, via two additional homogeneous, homopolymer-enriched phases.
These incipient phases emerge with zero volume fraction. The four regions of
the phase diagram meet in a multicritical point (\lambda_c, \chi_c), at which
A-B segregation vanishes. The analytical method, which for the lamellar phase
assumes weak segregation, thus proves reliable particularly in the vicinity of
(\lambda_c, \chi_c). For random triblock copolymers, Q=3, we find the character
of this point and the critical exponents to change substantially with the
number M of monomers per block. The results for Q=3 in the continuous-chain
limit M -> \infty are compared to numerical self-consistent field theory
(SCFT), which is accurate at larger segregation.Comment: 24 pages, 19 figures, version published in PRE, main changes: Sec.
IIIA, Fig. 14, Discussio
Projections of the Transient State-Dependency of Climate Feedbacks
When the climate system is forced, e.g. by emission of greenhouse gases, it
responds on multiple time scales. As temperatures rise, feedback processes
might intensify or weaken. Current methods to analyze feedback strength,
however, do not take such state dependency into account; they only consider
changes in (global mean) temperature and assume all feedbacks are linearly
related to that. This makes (transient) changes in feedback strengths almost
intangible and generally leads to underestimation of future warming. Here, we
present a multivariate (and spatially explicit) framework that facilitates
dissection of climate feedbacks over time scales. Using this framework,
information on the composition of projected (transient) future climates and
feedback strengths can be obtained. Moreover, it can be used to make
projections for many emission scenarios through linear response theory. The new
framework is illustrated using the Community Earth System Model version 2
(CESM2).Comment: main text: 11 pages, 4 figures, 1 table Supporting Information: 14
pages, 17 figures, 1 table, 8 movie
Multivariate Estimations of Equilibrium Climate Sensitivity from Short Transient Warming Simulations
One of the most used metrics to gauge the effects of climate change is the
equilibrium climate sensitivity, defined as the long-term (equilibrium)
temperature increase resulting from instantaneous doubling of atmospheric
CO. Since global climate models cannot be fully equilibrated in practice,
extrapolation techniques are used to estimate the equilibrium state from
transient warming simulations. Because of the abundance of climate feedbacks -
spanning a wide range of temporal scales - it is hard to extract long-term
behaviour from short-time series; predominantly used techniques are only
capable of detecting the single most dominant eigenmode, thus hampering their
ability to give accurate long-term estimates. Here, we present an extension to
those methods by incorporating data from multiple observables in a
multi-component linear regression model. This way, not only the dominant but
also the next-dominant eigenmodes of the climate system are captured, leading
to better long-term estimates from short, non-equilibrated time series.Comment: Main Text (10 pages, 4 figures) plus Supporting Information (36
pages, 18 figures, 1 table
The relationship between the global mean deep-sea and surface temperature during the Early Eocene
EGU General Assembly, Vienna, Austria 23-27 May 2022, https://doi.org/10.5194/egushere-egu22-9897 Under continued high anthropogenic CO2 emissions, the atmospheric CO2 concentration around 2100 will be like that of the Early Eocene Climate Optimum (EECO, 56–48 Ma) hothouse period. Hence, reconstructions of the EECO climate give insight into the workings of the climate system under the possible future CO2 conditions. Our current understanding of global mean surface temperature (GMST) during the Cenozoic era relies on paleo-proxy estimates of deep-sea temperature (DST) combined with assumed relationships between global mean DST (GMDST), global mean sea-surface temperature (GMSST), and GMST. The validity of these assumptions is essential in our understanding of past and future climate states under hothouse conditions. We analyse the relationship between these global temperature indicators for the end-of-simulation global mean temperature values in 25 different millennia-long model simulations of the EECO climate under varying CO2 levels, performed as part of the Deep-Time Model Intercomparison Project (DeepMIP). The model simulations show limited spatial variability in DST, indicating that local DST estimates can be regarded representative of GMDST. Linear regression analysis indicates that GMDST and GMST respond stronger to changes in atmospheric CO2 than GMSST by factors 1.18 and 1.17, respectively. Consequently, the responses of GMDST and GMST to atmospheric CO2 changes are similar in magnitude. This model-based analysis indicates that changes in GMDST can be used to estimate changes in GMST during the EECO, validating the assumed relationships. To test the robustness of these results, other Cenozoic climate states besides EECO should be analysed similarly
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