Deepest

Daisy Gilardini
Il parziale clamore suscitato dallo sconfinamento del progetto BEST (inizialmente votato ad una sorta di "audit" dei dataset termici globali, e quindi inserito nella sola detection) nell'ambito delle attribuzioni del climate change e dai relativi risultati dell'ultimo lavoro che corroborerebbero lo stato dell'arte sulle forzanti antropiche, fa da parziale spunto a questo breve post sulle relative novità in questo ambito.
Oggi è anche la deadline per l'ammissione dei papers che verranno considerati per la stesura del final draft - previsto per maggio 2013 - del primo basilare capitolo (WGI) del prossimo rapporto (AR5) dell'IPCC: da domani, siete fuori tempo massimo per questo appuntamento, casomai vi interessasse la cosa (vedi quiqui, cap. towards AR5).


Ecco, dunque, cinque importanti lavori di attribuzione che si affiancano a quello del gruppo Novim del progetto BEST, ad indiretta conferma che quest'ultimo lavoro - a parte l'etichetta di "ribelli" outsiders relativamente fuori dal mainstream attribuita agli autori - ha ben poco di nuovo, in tema. A parte, forse, il parziale ridimensionamento della forzante solare come main driver della prima impennata termica del XX secolo:  aspetto interessante su cui ci torneremo nella settimana dedicata al sole.

Qua e là avevo già accennato a questi lavori, qui li riassumo in post e evidentemente saranno una delle colonne portanti (insieme ad altri lavori usciti negli ultimi 6 anni e mezzo) nell'ambito delle continue, sempre più robuste, profonde e corroboranti conferme dell'importanza delle forzanti antropiche, rispetto ai rapporti precedenti.

✔ Huber M. & Knutti R.: Anthropogenic and natural warming inferred from changes in Earth’s energy balance, Nature Geoscience 5, 31–36 (2012) | doi:10.1038/ngeo1327, 2012

In questo primo lavoro, gli autori propongono un'altra variante alternativa di attribuzione, stavolta mediante analisi inerenti la conservazione dell'energia. Vediamo il bottomline, dall'abstract e a seguire dalle conclusioni:
The Earth’s energy balance is key to understanding climate and climate variations that are caused by natural and anthropogenic changes in the atmospheric composition. Despite abundant observational evidence for changes in the energy balance over the past decades, the formal detection of climate warming and its attribution to human influence has so far relied mostly on the difference between spatio-temporal warming patterns of natural and anthropogenic origin. Here we present an alternative attribution method that relies on the principle of conservation of energy, without assumptions about spatial warming patterns. Based on a massive ensemble of simulations with an intermediate-complexity climate model we demonstrate that known changes in the global energy balance and in radiative forcing tightly constrain the magnitude of anthropogenic warming. We find that since the mid-twentieth century, greenhouse gases contributed 0.85 °C of warming (5–95% uncertainty: 0.6–1.1 °C), about half of which was offset by the cooling effects of aerosols, with a total observed change in global temperature of about 0.56 °C. The observed trends are extremely unlikely (< 5%) to be caused by internal variability, even if current models were found to strongly underestimate it. Our method is complementary to optimal fingerprinting attribution and produces fully consistent results, thus suggesting an even higher confidence that human-induced causes dominate the observed warming.
It is thus extremely likely (> 95% probability) that the greenhouse gas induced warming since the mid-twentieth century was larger than the observed rise in global average temperatures, and extremely likely that anthropogenic forcings were by far the dominant cause of warming. The natural forcing contribution since 1950 is near zero. (...) Our results show that it is extremely likely that at least 74% (±12%, 1σ) of the observed warming since 1950 was caused by radiative forcings, and less than 26% (±12%) by unforced internal variability.

 ✔ Attanasio A., Pasini A., Triacca U.: A contribution to attribution of recent global warming by out-of-sample Granger causality analysis, Atmos. Sci. Let. 13: 67–72 (2012) | doi:10.1002/asl.365, 2012

Granger al servizio della fingerprint analysis. Lavoro già discusso qua:
The topic of attribution of recent global warming is usually faced by studies performed through global climate models (GCMs). Even simpler econometric models have been applied to this problem, but they led to contrasting results. In this article, we show that a genuine predictive approach of Granger analysis leads to overcome problems shown by these models and to obtain a clear signal of linear Granger causality from greenhouse gases (GHGs) to the global temperature of the second half of the 20th century. In contrast, Granger causality is not evident using time series of natural forcing.
Update 17/9: un aggiornamento di questo lavoro qui:
Pasini A., Triacca U., Attanasio A.: Evidence of recent causal decoupling between solar radiation and global temperature, Environ. Res. Lett. 7 (2012) | doi:10.1088/1748-9326/7/3/034020
The Sun has surely been a major external forcing to the climate system throughout the Holocene. Nevertheless, opposite trends in solar radiation and temperatures have been empirically identified in the last few decades. Here, by means of an inferential method—the Granger causality analysis—we analyze this situation and, for the first time, show that an evident causal decoupling between total solar irradiance and global temperature has appeared since the 1960s.

 ✔ Gleckler P.J. et al.: Human-induced global ocean warming on multidecadal timescales, Nature Climate Change 2, 524–529(2012) | doi:10.1038/nclimate1553, 2012

Importante lavoro di attribuzione del GW oceanico su scala multidecennale:
Large-scale increases in upper-ocean temperatures are evident in observational records. Several studies have used well-established detection and attribution methods to demonstrate that the observed basin-scale temperature changes are consistent with model responses to anthropogenic forcing and inconsistent with model-based estimates of natural variability. These studies relied on a single observational data set and employed results from only one or two models. Recent identification of systematic instrumental biases in expendable bathythermograph data has led to improved estimates of ocean temperature variability and trends and provide motivation to revisit earlier detection and attribution studies. We examine the causes of ocean warming using these improved observational estimates, together with results from a large multimodel archive of externally forced and unforced simulations. The time evolution of upper ocean temperature changes in the newer observational estimates is similar to that of the multimodel average of simulations that include the effects of volcanic eruptions. Our detection and attribution analysis systematically examines the sensitivity of results to a variety of model and data-processing choices. When global mean changes are included, we consistently obtain a positive identification (at the 1% significance level) of an anthropogenic fingerprint in observed upper-ocean temperature changes, thereby substantially strengthening existing detection and attribution evidence.

✔ Notz D. & Marotzke J.: Observations reveal external driver for Arctic sea-ice retreat, GRL, 39, L08502 | doi:10.1029/2012GL051094, 2012

Lavoro di attribuzione della riduzione dei ghiacci artici marini, che fa il paio con il seguente lavoro pubblicato qualche anno prima:
The very low summer extent of Arctic sea ice that has been observed in recent years is often casually interpreted as an early-warning sign of anthropogenic global warming. For examining the validity of this claim, previously IPCC model simulations have been used. Here, we focus on the available observational record to examine if this record allows us to identify either internal variability, self-acceleration, or a specific external forcing as the main driver for the observed sea-ice retreat. We find that the available observations are sufficient to virtually exclude internal variability and self- acceleration as an explanation for the observed long-term trend, clustering, and magnitude of recent sea-ice minima. Instead, the recent retreat is well described by the super- position of an externally forced linear trend and internal variability. For the externally forced trend, we find a physically plausible strong correlation only with increasing atmospheric CO2 concentration. Our results hence show that the observed evolution of Arctic sea-ice extent is consistent with the claim that virtually certainly the impact of an anthropogenic climate change is observable in Arctic sea ice already today.

✔ Johannessen O.M.: Decreasing arctic sea ice mirrors increasing CO2 on decadal time scale, Atmos. and Oceanic Sci. Lett., 1, 51−56, 2008
Arctic sea ice is a keystone indicator of greenhouse-gas induced global climate change, which is expected to be amplified in the Arctic. Here we directly compare observed variations in arctic sea-ice extent and CO2 since the beginning of the 20th century, identifying a strengthening linkage, such that in recent decades the rate of sea-ice decrease mirrors the increase in CO2, with r ~ –0.95 over the last four decades, thereby indicating that 90% (r2 ~ 0.90) of the decreasing sea-ice extent is empirically “accounted for” by the increasing CO2 in the atmosphere. The author presents an empirical relation between annual sea-ice extent and global atmospheric CO2 concentrations, in which sea-ice reductions are linearly, inversely proportional to the magnitude of increase of CO2 over the last few decades. This approximates sea-ice changes during the most recent four decades, with a proportionality constant of 0.030 million km2 per ppmv CO2. When applied to future emission scenarios of the Intergovernmental Panel on Climate Change (IPCC), this relationship results in substantially faster ice decreases up to 2050 than predicted by IPCC models. However, departures from this projection may arise from non-linear feedback effects and/or temporary natural variations on interannual timescales, such as the record minimum of sea-ice extent observed in September 2007.

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