In the summer 2010 Western Russia was hit by an extraordinary heat wave, with the region experiencing by far the warmest July since records began. Whether and to what extent this event is ...attributable to anthropogenic climate change is controversial. Dole et al. (2011) report the 2010 Russian heat wave was “mainly natural in origin” whereas Rahmstorf and Coumou (2011) write that with a probability of 80% “the 2010 July heat record would not have occurred” without the large‐scale climate warming since 1980, most of which has been attributed to the anthropogenic increase in greenhouse gas concentrations. The latter explicitly state that their results “contradict those of Dole et al. (2011).” Here we use the results from a large ensemble simulation experiment with an atmospheric general circulation model to show that there is no substantive contradiction between these two papers, in that the same event can be both mostly internally‐generated in terms of magnitude and mostly externally‐driven in terms of occurrence‐probability. The difference in conclusion between these two papers illustrates the importance of specifying precisely what question is being asked in addressing the issue of attribution of individual weather events to external drivers of climate.
Key Points
Former studies on the Russian heat wave 2010 are not contradictory
Russian heat wave 2010 likely attributable to anthropogenic climate change
This study investigates likely changes in mean and extreme precipitation over southern Africa in response to changes in radiative forcing using an ensemble of global climate models prepared for the ...Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). Extreme seasonal precipitation is defined in terms of 10-yr return levels obtained by inverting a generalized Pareto distribution fitted to excesses above a predefined high threshold. Both present (control) and future climate precipitation extremes are estimated. The future-to-control climate ratio of 10-yr return levels is then used as an indicator for the likely changes in extreme seasonal precipitation.
A Bayesian approach to multimodel ensembling is adopted. The relative weights assigned to each of the model simulations is determined from bias, convergence, and correlation. Using this method, the probable limits of the changes in mean and extreme precipitation are estimated from their posterior distribution.
Over the western parts of southern Africa, an increase in the severity of dry extremes parallels a statistically significant decrease in mean precipitation during austral summer months. A notable delay in the onset of the rainy season is found in almost the entire region. An early cessation is found in many parts. This implies a statistically significant shortening of the rainy season.
A substantial reduction in moisture influx from the southwestern Indian Ocean during austral spring is projected. This and the preaustral spring moisture deficits are possible mechanisms delaying the rainfall onset in southern Africa. A possible offshore (northeasterly) shift of the tropical–temperate cloud band is consistent with more severe droughts in the southwest of southern Africa and enhanced precipitation farther north in Zambia, Malawi, and northern Mozambique.
This study shows that changes in the mean vary on relatively small spatial scales in southern Africa and differ between seasons. Changes in extremes often, but not always, parallel changes in the mean precipitation.
We present a new method that allows a separation of the attribution of human influence in extreme events into changes in atmospheric flows and changes in other processes. Assuming two data sets of ...model simulations or observations representing a natural, or 'counter-factual' climate, and the actual, or 'factual' climate, we show how flow analogs used across data sets can provide quantitative estimates of each contribution to the changes in probabilities of extreme events. We apply this method to the extreme January precipitation amounts in Southern UK such as were observed in the winter of 2013/2014. Using large ensembles of an atmospheric model forced by factual and counterfactual sea surface temperatures, we demonstrate that about a third of the increase in January precipitation amounts can be attributed to changes in weather circulation patterns and two thirds of the increase to thermodynamic changes. This method can be generalized to many classes of events and regions and provides, in the above case study, similar results to those obtained in Schaller et al (2016 Nat. Clim. Change 6 627-34) who used a simple circulation index, describing only a local feature of the circulation, as in other methods using circulation indices (van Ulden and van Oldenborgh 2006 Atmos. Chem. Phys. 6 863-81).
This study investigates how climate change affects the daily extreme precipitation events that occur in the autumn in Cévennes mountain range (South of France). We use an ensemble of 10 EURO‐CORDEX ...model simulations with two horizontal resolutions (0.11° and 0.44°). Those data sets, after pooling all models together, are fitted by stationary generalized extreme value and empirical distributions for several periods to estimate a climate change signal in the tail of distribution of extreme rainfall. We find that the exceedance probability of a 1‐in‐100‐year event in the historical climate has increased by a factor of 2.5 ± 0.8 under the current climate. The results show that higher‐resolution simulations with bias adjustment provide a robust and confident increase in the intensity and likelihood of occurrence of the events in the current climate in comparison with the historical climate. These changes are in agreement with an observations‐based analysis in a previous study.
Plain Language Summary
This paper investigates the connection between autumn high precipitation events in the South of France and climate change. From an ensemble of regional climate simulations and observations, we show that the probability of exceeding 100‐year precipitation events has more than doubled, due to temperature increase.
Key Points
The attribution of extreme rainfall events in the South of France is made by using model simulations and observations
The change of extreme precipitation distribution is significant under climate change
We verify the Clausius-Clapeyron relation for precipitation events
Climate models are seen by many to be unverifiable. However, near-term climate predictions up to 10 years into the future carried out recently with these models can be rigorously verified against ...observations. Near-term climate prediction is a new information tool for the climate adaptation and service communities, which often make decisions on near-term time scales, and for which the most basic information is unfortunately very scarce. The Fifth Coupled Model Intercomparison Project set of co-ordinated climate-model experiments includes a set of near-term predictions in which several modelling groups participated and whose forecast quality we illustrate here. We show that climate forecast systems have skill in predicting the Earth's temperature at regional scales over the past 50 years and illustrate the trustworthiness of their predictions. Most of the skill can be attributed to changes in atmospheric composition, but also partly to the initialization of the predictions.
The year 2014 broke the record for the warmest yearly average temperature in Europe. Attributing how much this was due to anthropogenic climate change and how much it was due to natural variability ...is a challenging question but one that is important to address. In this study, we compare four event attribution methods. We look at the risk ratio (RR) associated with anthropogenic climate change for this event, over the whole European region, as well as its spatial distribution. Each method shows a very strong anthropogenic influence on the event over Europe. However, the magnitude of the RR strongly depends on the definition of the event and the method used. Across Europe, attribution over larger regions tended to give greater RR values. This highlights a major source of sensitivity in attribution statements and the need to define the event to analyze on a case‐by‐case basis.
Key Points
Human influences played a strong role in the record 2014 temperatures in Europe
An attribution result is very sensitive to the spatiotemporal definition of the event
The increase in risk due to climate change declines on average moving to smaller spatial scales
Tracing the upper ocean's "missing heat" Katsman, C. A.; van Oldenborgh, G. J.
Geophysical research letters,
July 2011, Letnik:
38, Številka:
14
Journal Article
Recenzirano
Over the period 2003–2010, the upper ocean has not gained any heat, despite the general expectation that the ocean will absorb most of the Earth's current radiative imbalance. Answering to what ...extent this heat was transferred to other components of the climate system and by what process(‐es) gets to the essence of understanding climate change. Direct heat flux observations are too inaccurate to assess such exchanges. In this study we therefore trace these heat budget variations by analyzing an ensemble of climate model simulations. The analysis reveals that an 8‐yr period without upper ocean warming is not exceptional. It is explained by increased radiation to space (45%), largely as a result of El Niño variability on decadal timescales, and by increased ocean warming at larger depths (35%), partly due to a decrease in the strength of the Atlantic meridional overturning circulation. Recently‐observed changes in these two large‐scale modes of climate variability point to an upcoming resumption of the upward trend in upper ocean heat content.
Key Points
An 8‐yr period without upper ocean warming is not exceptional
It is explained by more radiation to space (45%) and deep ocean warming (35%)
Recently‐observed changes point to an upcoming resumption of upper ocean warming
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