Vulnerability is the extent to which people are at risk to suffer negative consequences from climate variability and extreme events or the effects of climate change. Vulnerabilities appear in different places: A small farmer family living in a flood-prone area can be called vulnerable to climate change, but so can be an economic sector such as rain-fed agriculture as a whole. Depending on the questions at hand, the vulnerability of such different elements of a society (exposure units) must be assessed.
In the IPCC's definition which is generally used in the climate change literature, vulnerability is composed of three components: exposure, sensitivity, and adaptive capacity. Exposure simply measures whether physical change is happening in a particular location, while sensitivity means the degree to which an exposure unit is actually at risk from that change. For example, rain-fed agriculture is more sensitive to changes in precipitation than irrigated agriculture. Finally, adaptive capacity determines whether people are able to adapt in order to mitigate damage or harness positive consequences.
It is important to note that sensitivity and adaptive capacity are not determined by biology alone. They are mediated by economic factors such as income, education, or credit availability, and sometimes even cultural or religious ideas. For example, communal water managers traditionally enjoy high social prestige in some communities in India, increasing the adaptive capacity of communities where these traditional structures are still intact.
Adaptation means to reduce vulnerability. In agriculture, this can be achieved by lowering exposure, such as building stronger flood defences or relocating, or by lowering sensitivity, such as moving to more drought-resistant crops. Adaptation can also mean building adaptive capacity: Providing a microcredit or subsidy scheme for individual farmers increases their adaptive capacity if the problem is a lack of funds for adaptation.
In any case, planning adaptation requires knowing what is the source of vulnerability. Vulnerability assessments explore the linkages between ecology and society in order to find out how and why climate change is actually harmful to people's livelihoods. They help reduce two key uncertainties about climate change: what to adapt to and how to adapt. They systematically employ different methods in order to deliver comparable assessments and decide the priorities of adaptation.
As the sources of vulnerability are diverse, different methods are suitable for different decisions. For example, a bottom-up assessment focuses on individual or local livelihoods and identifies exactly how people on the ground are aware of and vulnerable to climate change. A top-down assessment can be done on a much more aggregated level, aiming to find out how climate change affect an economic sector, such as irrigated agriculture, or show linkages between sectors via impact chains. Other types of assessment are used for evaluating adaptation policies or as a kick-starting strategy for community-based adaptation.
Practical Guidelines (External)
For a more in-depth literature on vulnerability assessments, see the references cited in the overview on AdaptationCommunity.  In the AdaptationCommunity knowledge base, you can find experiences of vulnerability assessments from different countries, some of which pertain specifically to the water sector. See also GIZ 2013.
- ↑ IPCC (2007): Impacts, Adaptation and Vulnerability. Fourth Assessment Report, Working Group II Report. http://www.ipcc.ch/publications_and_data/ar4/wg2/en/contents.html [accessed 17 April 2013]
- ↑ Hamill, Anne et al. (2013): Comparative analysis of climate change vulnerability assessments: Lessons from Tunisia and Indonesia. GIZ. Avaliable from AdaptationCommunity: https://gc21.giz.de/ibt/var/app/wp342deP/1443/?wpfb_dl=42
- ↑ AdaptationCommunity.net: Vulnerability Assessments. https://gc21.giz.de/ibt/var/app/wp342deP/1443/index.php/knowledge/vulnerability-assessment/
- ↑ GIZ (2013) Vulnerability Assessments: Experiences of GIZ with Vulnerability Assessments at the local level.
Birkmann, J., Wisner, B. (2005): Measuring the Un-Measurable - The Challenge of Vulnerability. Report of the Second Meeting of the UNU-EHS Expert Working Group on Measuring Vulnerability, 12-14 October 2005, Bonn, Germany.