by Judith Curry
Natural internal climate variability is getting some welcome attention.
An in press article in the Bulletin of the American Meteorological Society reports on a 2012 Workshop in Taiwan that focused on understanding natural internal variability and multi-decadal climate changes:
Understanding Multi-decadal Climate Changes [link]
Chi-Pei Chang, Michael Ghil, Hung-Chi Kuo, Mojib Latif, Chung-Hsiung Sui and John M. Wallace
The 2012 National Taiwan University International Science Conference on Climate Change focused on two of the most difficult challenges in the study of climate change: 1) delineating the multidecadal and longer time scale variations in historical records that extend back only ~150 years, and 2) distinguishing between anthropogenically forced and natural variability.
ANTHROPOGENIC FORCING AND INTERNALLY GENERATED VARIABILITY
That there exists a large amount of free climate variability in the climate models suggests that significant portions of the observed multidecadal variability in the climate record could be inherently stochastic, i.e., attributable to sampling fluctuations associated with naturally occurring modes of variability. This is even true for ENSO, where coupled interactions enhance the variability at a distinct timescale, leading to a peak that stands out above the red background spectrum.
Regardless of the mechanisms that give rise to it, multidecadal climate variability modulates the rate of global-mean surface air temperature rise. Applying a “dynamical adjustment” to remove (or at least reduce) the contribution of these circulation changes to the global-mean temperature trend simplifies the space-time structure of the surface air temperature record and renders it more spatially and seasonally coherent. Results presented at the workshop suggested that the enhanced wintertime warming over high northern latitudes from 1965 to 2000 was mainly a reflection of unforced climate variability. Disregarding this dynamically induced component of the 20th century warming leads to around a 10% reduction in the inferred global climate sensitivity.
MULTIDECADAL TO CENTURY-SCALE CLIMATE VARIATIONS
Analyses of observed climate records during the 20th century, the last ice age, and the Holocene, in conjunction with climate modeling results, suggest that pronounced multidecadal tocentury-scale variability can be produced internally by a number of different mechanisms.
The observed sea-surface temperature (SST) variations and proxy climate records in the Southern Ocean (50°S-70°S) suggest the existence of pronounced global scale centennial variations, with the most recent maxima around the mid-1870s and mid-1970s. In one climate model, these long time scale variations originate from the slow accumulation of North Atlantic Deep Water in the Weddell Sea at mid-depth, which destabilizes the water column from below and eventually stimulates deep convection there. The accumulation of heat during the quiescent regime and its subsequent release to the atmosphere during the convective regime acts as a recharge oscillator in that model.
The Atlantic Meridional Overturning Circulation (AMOC) is another important source of global and regional scale multidecadal climate variability. Numerical simulations show that these variations are advected along interior pathways in the extratropical North Atlantic, reaching the subtropics several years later. Several independent fingerprints of AMOC variability were proposed at the meeting and new evidence was brought to light that the North Atlantic multidecadal SST mode known as the observed Atlantic Multidecadal Oscillation (AMO) may be linked to AMOC variations.
Modeling studies indicat that the AMOC weakens most at northern high latitudes in response to increasing greenhouse gas concentrations. The simulated AMOC weakening under anthropogenic forcing cannot be distinguished from natural AMOC variability in the record extending through just the first few decades of the 21st century, but the free and forced variability should become separable toward the middle of the century. Analysis of the 350-year-long Central England historical temperature record is suggestive of pervasive multidecadal variability that climate models suggest could be associated with variations in the strength of the AMOC.
The recent decrease of Arctic sea ice has attracted widespread media attention. This decrease is superimposed by a rich spectrum of variability of the Arctic sea ice. Strong variations with time scales of 50-120 years have been reported. It was suggested that the AMOC might be capable of influencing Arctic sea ice on this time scale through the inflow of Atlantic Water into the Arctic Ocean. It should be kept in mind, however, that while Arctic sea ice exhibited a record low in the last decade; Antarctic sea ice featured a record high. The role of global-scale unforced variability needs to be quantified in this context.
MATHEMATICAL THEORY RELATING TO CLIMATE CHANGE
Since the climate system may possess multiple equilibria, the stability of these equilibria and the transition dynamics between them matter. A systematic stability and transition theory for the oceanic thermohaline circulation has been developed. It is found that the transitions are crucially determined by basin size and geometry, as well as by the thermal and salinity Rayleigh numbers. Numerical results across a hierarchy of models suggest that both jumps and continuous transitions between climate equilibria are possible. In particular, the jump transitions may be associated with hysteresis phenomena.
The presentations included a review of fluctuation-dissipation theory (FDT) from statistical mechanics. This theory and its various generalizations allow one to calculate a system’s mean response to external forcing through the knowledge of appropriate correlation functions of its internal fluctuations. Such an approach may help one interpret the results derived from ensembles of numerical integrations with climate models.
The combined dynamics of low-frequency climate variability and forced climate change may be simplified by treating the faster processes as random noise, superimposed upon and interacting with the slower, nonlinear processes. Several presentations described how the theory of nonlinear, stochastically forced dynamical systems can provide insight into a wide range of time-varying climate phenomena.
An analysis based on observations and experiments with an atmospheric model coupled to a mixed layer ocean suggested that teleconnections are substantially stronger at multidecadal versus interannual timescales. In effect, the former is dominated by a global-scale “hypermode” which exhibits an equatorially-symmetric structure reminiscent of the El Nino-Southern Oscillation (ENSO).
The fact that climate variability involves such a wide range of time scales renders the separation of trends and cycles difficult. Several innovative spectral-analysis methods—including Empirical Mode Decomposition, the Multi-Taper Method, and Singular Spectrum Analysis—are being more widely used to study the trend and cycles on different time scales in climate records. When applied properly, these methods behave like data-adaptive temporal filters, and thus facilitate the differentiation between the century-long trends and multidecadal cycles. Their application to the historical record of global mean surface temperature and to several proxy records of local and regional temperatures substantiates the existence of human-induced global warming over the past century and also highlights the role of AMV in modulating the rate of global warming.
Most climate change meetings have tended to focus on the forced, thermodynamically induced variability of the climate system. In contrast, this meeting featured scientists who think outside of that box. The climate response to external forcing—especially on regional scales—is strongly influenced by dynamical processes in both the ocean and the atmosphere. Moreover, the existence of strong natural multidecadal to centennial variability makes the detection of anthropogenic climate change a challenge. The presentations at the workshop dealt with the full range of processes that contribute to forced and free (also referred to as unforced or internal) multidecadal climate variability. This broader framing of climate change science is required for quantifying the societal risks of future climate change, and for properly assessing the extent to which today’s weather, specifically the statistics of extreme weather events, is changing in response to human-induced climate change.
John Kennedy sent the Workshop link, that includes all of the presentations. This looks so rich, I’m expect I’ll be doing full posts on some of these.
JC comments: Well I had a tough time deciding what NOT to include in my excerpts, since all of this is music to my ears. Kudos to the National Taiwan University for hosting this workshop; dare I hope that this topic will be trending for workshops in the U.S. and Europe?
I do disagree with the following statement however:
Disregarding this dynamically induced component of the 20th century warming leads to around a 10% reduction in the inferred global climate sensitivity.
I regard this as THE key unknown, and I would not be surprised if it were significantly higher than 10%.
More of this kind of ‘outside that box’ thinking, please.