Twenty-first century policymakers in the UK face a daunting array of challenges: an ageing society, the promises and threats for employment and wealth creation from artificial intelligence, obesity and public health, climate change and the need to sustain our natural environment, and many more. What these kinds of policy challenges have in common is complexity. Their implications spill over and transcend established boundaries between departments, policy domains, sectors and research disciplines.

These problems pose risks and opportunities that demand integration of insights from multiple bases of evidence, many of which are incomplete, requiring reliance on modelling and scenarios with all their inherent uncertainties. The challenges in question also interact in highly complex ways with one another - for example, climate disruption poses many risks to other systems in our natural and built environments, and for human and animal health. Finally, these kinds of policy challenges have implications and effects over the long term - reaching far into the future of our communities and environments and calling for policymaking with a perspective over decades, if not half-centuries and more. A case in point is the UK Government’s 25-Year Plan for the Environment, A Green Future: Our 25 Year Plan to Improve the Environment, published in January 2018. This ambitious document sets out a broad strategy “for improving the environment, within a generation, and leaving it in a better state than we found it”. The 25-Year Plan acknowledges the extreme complexity of the task it sets for integrated environmental policymaking. It sets out a range of goals and identifies many links with other policy domains and a very wide variety of stakeholders. The 25-Year Plan outlines a set of policy goals to be applied to highly complex systems that are co-evolving and whose stakeholders span many sectors, levels of activity and environments, and whose policy interventions are intended to have impacts extending over years and decades. It is a vision for policy whose ambition and implications point to the importance of an appropriate framework for appraisal and evaluation. The example of the 25-Year Plan highlights with particular force the significance of complex systems thinking and its implications for policymaking. As such, it can be taken as an example of a major shift in policy challenges, analysis, design and evaluation that cuts across many areas of policymaking and governance. In understanding and working with these challenges, complex systems thinking and complexity-appropriate tools for policy can be invaluable and are of great significance. This Annex will explain what complexity thinking is, what the features of complex systems are, and how new methodologies and tools can equip policymakers to work with unavoidable complexity. In particular, the annex highlights ways in which complexity appropriate evaluation strategies can be used to gather timely and rigorous information about the implementation of an intervention, making a positive contribution to enhancing the process of adaptive policy making.

1. Why complexity matters Key points:

 • Complex systems have characteristics that make their behaviour hard to predict and which present challenges to policy making and evaluation:

 • Complex systems may be in a state of continual change and may also resist change.

 • Context and history matter, the same intervention will often have different outcomes in different contexts, or if delivered in a slightly different way.

• Policy interventions in complex domains will often need to evolve over time in response to the way in which the system is adapting.

 • This highlights the importance of a continuous process of evaluation and learning, to enable flexible or adaptive management in complex, evolving environments.

 • Appreciation of how complexity can affect the policy process provides the opportunity to enhance effectiveness both in the design and delivery of the policy, and in its evaluation.

 • An appropriate evaluation strategy, in support of a learning or adaptive management approach, can help to track changes arising from a policy intervention over time, increase understanding of unexpected effects, and enable plans to be adapted if things take an unexpected course