Drivers Of Change In mid-Latitude weather Events (DOCILE)

The role of external drivers of climate change in mid-latitude weather events, particularly that of human influence on climate, arouses intense scientific, policy and public interest. In February 2014, the UK Prime Minister stated he "suspected a link" between the flooding at the time and anthropogenic climate change, but the scientific community was, and remains, frustratingly unable to provide a more quantitative assessment. Quantifying the role of climate change in extreme weather events has financial significance as well: at present, impact-relevant climate change will be primarily felt through changes in extreme events. While slow-onset processes can exacerbate (or ameliorate) the impact of individual weather events, any change in the probability of occurrence of these events themselves could overwhelm this effect. While this is known to be a problem, very little is known about the magnitude of such changes in occurrence probabilities, an important knowledge gap this project aims to address.

The 2015 Paris Agreement of the UNFCCC has given renewed urgency to understanding relatively subtle changes in extreme weather through its call for research into the impacts of a 1.5oC versus 2oC increase in global temperatures, to contribute to an IPCC Special Report in 2018. Few, if any, mid-latitude weather events can be unambiguously attributed to external climate drivers in the sense that these events would not have happened at all without those drivers. Hence any comprehensive assessment of the cost of anthropogenic climate change and different levels of warming in the future must quantify the impact of changing risks of extreme weather, including subtle changes in the risks of relatively 'ordinary' events.

The potential, and significance, of human influence on climate affecting the occupancy of the dynamical regimes that give rise to extreme weather in mid-latitudes has long been noted, but only recently have the first tentative reports of an attributable change in regime occupancy begun to emerge. A recent example is the 2014 floods in the Southern UK, which are thought to have occurred not because of individually heavy downpours, but because of a more persistent jet. Quantifying such changes presents a challenge because high atmospheric resolution is required for realistic simulation of the processes that give rise to weather regimes, while large ensembles are required to quantify subtle but potentially important changes in regime occupancy statistics and event frequency. Under this project we propose, for the first time, to apply a well-established large-ensemble methodology that allows explicit simulation of changing event probabilities to a global seasonal-forecast-resolution model. We aim to answer the following question: over Europe, does the dynamical response to human influence on climate, manifest through changing occupancy of circulation regimes and event frequency, exacerbate or counteract the thermodynamic response, which is primarily manifest through increased available moisture and energy in individual events?

Our focus is on comparing present-day conditions with the counterfactual "world that might have been" without human influence on climate, and comparing 1.5 degree and 2 degree future scenarios. While higher forcing provides higher signal-to-noise, interpretation is complicated by changing drivers and the potential for a non-linear response. We compensate for a lower signal with unprecedentedly large ensembles.

Event attribution has been recognised by the WCRP as a key component of any comprehensive package of climate services. NERC science has been instrumental in its development so far: this project will provide a long-overdue integration of attribution research into the broader agenda of understanding the dynamics of mid-latitude weather.

The proposed research clearly addresses a topic at the forefront of societal interest. Whenever an extreme climate event occurs politicians and the media inherently suspect a link to human induced climate change, but rarely have the means to back it up with scientific evidence. As such attribution statements need to be reliable, and easily accessible. This proposal will greatly increase our confidence in such statements, by bringing together, in a robust way, the event attribution community and the dynamics community to better understand the changing dynamics in attribution studies. To this end, there will be clear benefits on short and long scales.

Aside from the academic community, the following communities will feel a benefit:

Policy makers
One of the most important impacts of climate change is the possibility of enhanced extreme weather events. Many aspects of political decision making involve interpreting the scientific evidence presented. As such, reliable estimates of probabilistic event attribution are required in order to accurately inform these decisions. Research from this project will directly contribute to the reliability of probabilistic event attribution and therefore aid in political outcomes. This would be a foreseeable benefit over longer timescales (i.e. after the initial project finishes), as the reliability of event attributions improves over time, from the results of this project.

Risk assessors
Large scale atmospheric processes can strongly couple to finer scale weather features, which in extreme cases lead to "high impact" weather events. An example of this is the moisture feedback on blocking, which helps to develop the block, while also directly impacting on weather over Europe (e.g. Pfahl et al, 2015). Such processes are better represented in high-resolution models, and using the proposed model set up we will be able to use more accurate data information in risk assessment models. Clients of risk assessment companies (such as our project partner, JBA) often inquire about climate change risks on, e.g. flood activity, but are yet to explicitly buy an attribution study, citing concerns on the confidence of attribution statements. Our project directly addresses this concern, by focusing on improving known deficiencies with current attribution studies, and bringing together a unique mix of dynamicists and attribution scientists to work closely with partners at the interface between academia and the private sector (e.g. JBA, CEH, the Met Office).

The public
Extreme events are currently at the forefront of public concern; mainly as their coverage and linkage to climate change has increased in the recent years. For instance during the 2013/14 winter, the flooding event over the southern UK, and extremely cold temperatures over North America, both prompted huge media coverage. By increasing our understanding or both the underlying mechanisms, and the impacts of such extreme events, this project will clearly be attractive to media outlets and more generally the public community. It is expected that the benefits on these communities will be felt within the span of the project. The use of volunteer the computing paradigm has been shown to raise the awareness of this area of science and the societal importance of understanding impacts of climate change.